Dynamics of the Norwegian Margin

Dynamics o f the Norwegia n Margi n Geological Societ y Specia l Publication s Series Editors A. J . HARTLE Y R. E . ...

Author: Geological Society of London

31 downloads 952 Views 51MB Size Report

This content was uploaded by our users and we assume good faith they have the permission to share this book. If you own the copyright to this book and it is wrongfully on our website, we offer a simple DMCA procedure to remove your content from our site. Start by pressing the button below!
Report copyright / DMCA form

DOWNLOAD PDF

Dynamics o f the Norwegia n Margi n

Geological Societ y Specia l Publication s Series Editors A. J . HARTLE Y R. E . HOLDSWORT H

A. C . MORTO N M. S . STOKE R

It i s recommended tha t referenc e to al l or par t o f this book shoul d b e made i n one o f the followin g ways: N0TTVEDT, A . e t al . (eds ) 2000 . Dynamics o f th e Norwegian Margin. Geologica l Society , London . Special Publications , 167 . SUNDVOR, E. , ELDHOLM , O. , GLADCZENKO , T . P . & PLANKE , S . 2000. Norwegian-Greenland Se a thermal field. In: NOTTVEDT , A . e t al . (eds) Dynamics o f th e Norwegian Margin. Geologica l Society, London, Specia l Publications , 167 , 397-410 .

GEOLOGICAL SOCIET Y SPECIA L PUBLICATIO N NO . 16 7

Dynamics o f the Norwegia n Margi n

EDITED B Y

A. N0TTVED T

Norsk Hydr o Researc h Centre , Bergen , Norway

Co-editorial Boar d

BJ0RN T . LARSE N RO

Norsk Hydr o Exploration , Osl o Universit

SNORRE OLAUSSEN HARAL Saga Petroleum , Osl o Norwegia

Y H . GABRIELSE N y of Bergen

BJ0RN T0RUDBAKKE N 0RJA Saga Petroleum , Osl o Statoil

JAKOB SKOGSEI D University o f Osl o

2000 Published b y The Geologica l Society London

D BREKKE

n Petroleu m Directorate , Stavange r

N BIRKELAN D , Stavanger

THE GEOLOGICA L SOCIETY The Geologica l Societ y o f Londo n wa s founde d i n 180 7 an d i s th e oldes t geologica l societ y i n th e world . It receive d it s Roya l Charte r i n 182 5 for th e purpos e o f investigatin g th e minera l structur e o f th e Earth " an d i s no w Britain's nationa l societ y fo r geology . Both a learne d societ y and a professiona l body, th e Geologica l Societ y is recognized b y the Departmen t o f Trade an d Industry (DTI ) as the chartering authority for geoscience, abl e to awar d Chartere d Geologis t statu s upon appropriately qualified Fellows . Th e Societ y ha s a membershi p o f 8600 , o f who m abou t 150 0 liv e outsid e th e UK . Fellowship of the Societ y is open t o thos e holdin g a recognized honour s degree i n geology or cognate subjec t and wh o have a t leas t tw o years " relevan t postgraduate experience , o r wh o hav e no t les s tha n si x years' relevan t experienc e in geology o r a cognat e subject . A Fello w wit h a minimu m of five years' relevan t postgraduate experienc e in th e practice of geolog y ma y appl y fo r chartere d status . Successfu l applicants are entitle d to us e the designator y postnominal CGeo l (Chartered Geologist) . Fellow s o f th e Societ y ma y us e th e letter s FGS . Othe r grade s o f membershi p ar e availabl e to members no t ye t qualifyin g for Fellowship . The Societ y ha s it s ow n publishin g hous e base d i n Bath . UK . I t produce s th e Society' s internationa l journals. books an d maps , an d i s th e Europea n distributo r fo r publication s of th e America n Associatio n o f Petroleu m Geolo gists. (AAPG) . th e Societ y fo r Sedimentar y Geolog y (SEPM ) an d th e Geologica l Societ y o f Americ a (GSA) . Members o f th e Societ y ca n bu y book s a t considerabl e discounts . Th e publishin g Hous e ha s a n onlin e booksho p (http://bookshop.geolsoc.org.uk/. Further informatio n o n Societ y membershi p ma y b e obtaine d fro m th e Membershi p Service s Manager . Th e Geological Society . Burlingto n House , Piccadilly . Londo n W1 V OJU . UK . (Email : [email protected]: tel: +44(0)17 1 4349944) . The Society' s We b Sit e ca n b e foun d a t http://www.geolsoc.org.uk/ . Th e Societ y i s a Registere d Charity . number 210161 . Published b y Th e Geologica l Societ y from : The Geologica l Societ y Publishin g Hous e Unit 7 , Brassmil l Enterpris e Centr e Brassmill Lan e Bath BA13JN , U K (Orders: Tel . +4 4 (0)122 5 445046 Fax +44(0)122 5 442836 ) Online bookshop : http://bookshop.geolsoc.org.u k First publishe d 200 0 The publishers make no representation, express or implied . with regar d t o th e accurac y o f th e informatio n contained in this book an d canno t accep t an y lega l responsibilit y for any error s o r omission s that ma y b e made . © Th e Geologica l Societ y o f Londo n 2000 . Al l right s reserved. N o reproduction , cop y o r transmissio n o f thi s publication ma y b e made without written permission . N o paragraph o f thi s publication ma y b e reproduced , copie d or transmitte d sav e wit h th e provision s o f th e Copyrigh t Licensing Agency , 9 0 Tottenha m Cour t Road , Londo n W1P 9HE . User s registere d wit h th e Copyright Clearanc e Center. 2 7 Congres s Street , Salem , M A 01970 , USA : th e item-fee cod e fo r thi s publicatio n i s 0305-8719/00/$15.00 . British Librar y Cataloguin g i n Publication Dat a A catalogu e recor d fo r thi s boo k i s availabl e fro m th e British Library . ISBN 1-86239-056- 8 ISSN 0305-871 9 Typeset b y Aarontyp e Ltd , Bristol , U K Printed b y Th e Alde n Press , Oxfor d

Distributors USA AAPG Bookstor e PO Bo x 97 9 Tulsa OK 74101-097 9 USA Orders: Tel . + 1 91 8 584-2555 Fax + 1 91 8 560-2652 Email: [email protected] g Australia Australian Minera l Foundatio n Booksho p 63 Conyngham Stree t Glenside South Australi a 5065 Australia Orders: Tel . +6 1 8 8 379-044 4 Fax +6 1 8 8 379-463 4 Email: [email protected] u India Affiliated East-Wes t Pres s PV T Lt d G-l/16 Ansar i Road . Daryaganj . New Delh i 11 0 002 India Orders: Tel . +9 1 1 1 327-9113 Fax +9 1 1 1 326-0538 Japan Kanda Boo k Tradin g Co . Cityhouse Tam a 20 4 Tsurumaki 1-3-1 0 Tama-shi Tokyo 206-003 4 Japan Orders: Tel . +8 1 (0)42 3 57-7650 Fax +8 1 (0)42 3 57-7651

Contents

N0TTVEDT, A . Integrate d Basi n Studie s - Dynamic s o f th e Norwegia n Margin : a n 1 introduction Intra-plate riftin g an d basin formatio n CHRISTIANSSON, P., FALEIDE , J. I . & BERGE, A. M . Crusta l structur e i n the norther n Nort h 1 Sea: a n integrate d geophysica l stud y ODINSEN, T. , CHRISTIANSSON , P., GABRIELSEN , R. H. , FALEIDE , J. I . & BERGE , A . M . Th e 4 geometries an d dee p structur e o f th e norther n Nort h Se a rif t syste m TER VOORDE, M. , FARSETH , R . B. , GABRIELSEN, R. H . & CLOETHING, S. A. P. L . Repeated 5 lithosphere extensio n i n th e norther n Vikin g Graben: a couple d o r decouple d rheology ? ODINSEN, T. , REEMST , P. , VA N DER BEEK, P., FALEIDE , J. I . & GABRIELSEN , R. H . Permo - 8 Triassic an d Jurassi c extensio n i n the norther n Nort h Sea : result s from tectonostratigraphi c forward modellin g FOSSEN, H. , ODINSEN , T . FARSETH , R . B . & GABRIELSEN , R. H . Detachment s an d low - 10 angle fault s i n th e norther n Nort h Se a rif t syste m Basin fillin g RAVNAS, R. , N0TTVEDT , A. , STEEL , R . J . & WINDELSTAD , J . Syn-rif t sedimentar y 13 architectures i n th e norther n Nort h Sea NOTTVEDT, A. , BERGE , A . M. , DAWERS , N . H. , FaRSETH , R . B. , HAGER , K.-O. , 17 MANGERUD, G . & PUIGDEFABREGAS , C . Syn-rif t evolutio n an d pla y potentia l i n th e Snorre-H area , norther n Nort h Se a JORDT, H. , THYBERG , B. I. & N0TTVEDT, A. Cenozoic evolutio n of the central and norther n 21 North Se a with focu s o n differentia l vertica l movements o f the basi n floo r an d surroundin g clastic sourc e area s THYBERG, B. , JORDT, H. , BJORLYKKE , K. & FALEIDE , J. I . Relationship s betwee n sequenc e 24 stratigraphy, mineralog y and geochemistr y in Cenozoic sediment s of the northern Nort h Se a KYRKJEBO, R. , HAMBORG , M. , FALEIDE , J. I. , JORDT , H . & CHRISTIANSSON , P. Cenozoi c 27 tectonic subsidenc e fro m 2 D depositiona l simulation s of a regiona l transec t i n th e norther n North Se a basin Conjugate volcani c margin s SKOGSEID, J. , PLANKE , S. , FALEIDE , J . L , PEDERSEN , T. , ELDHOLM , O . & NEVERDAL , F . 29 NE Atlanti c continenta l riftin g an d volcani c margi n formatio n BREKKE, H. The tectoni c evolutio n of the Norwegian Se a continental margin, with emphasis 32 on th e V0rin g an d M0r e basin s MOGENSEN, T . E. , NYBY , R. , KARPUZ , R . & HAREMO , P . Lat e Cretaceou s an d Tertiar y 37 structural evolutio n o f the northeaster n par t o f th e V0rin g Basin , Norwegian Se a SUNDVOR, E., ELDHOLM , O. , GLADCZENKO , T. P . & PLANKE, S. Norwegian-Greenland Se a 39 thermal field ELDHOLM, O. , GLADCZENKO , J , SKOGSEID , J . & PLANKE , S . Atlanti c volcani c Margins : 41 a comparativ e stud y

5 1 9 3 5

3 9 9 5 3

5 7 9 7 1

VI

CONTENTS

Present stres s LINDHOLM, C . D. , BUNGUM , H. , HICKS , E . & VILLAGRAN , M . Crusta l stres s an d tectonic s 42 in Norwegia n region s determine d fro m earthquak e foca l mechanism s FEJERSKOV, M. , LINDHOLM , C . D. , MYRVANG , A . & BUNGUM , H . Crusta l stres s i n an d 44 around Norway : a compilatio n o f i n situ stres s observations FEJERSKOV, M . & LINDHOLM , C . D . Crusta l stres s in and aroun d Norway ; an evaluatio n of 45 stress generatin g mechanism s Index

9 1 1

469

Integrated Basi n Studie s - Dynamic s of the Norwegian Margin : an introduction ARVID N0TTVED T Norsk Hydro Research Centre, N-5020 Bergen, Norway Present address: Norsk Hydro Canada, lll-5th Avenue SW, Calgary AB, T2P 3Y6, Canada Several lithospheri c an d uppe r crusta l processe s interact i n th e formatio n o f rif t basin s an d evolution o f suc h basin s int o passiv e margins . Similarly, th e fillings of rif t basin s depen d o n a variety o f tectonic, morphologica l an d sedimen tary processes . Area s tha t hav e passe d throug h a complet e evolutio n fro m intra-cratoni c rift ing through breaku p an d passiv e margin forma tion offe r particula r opportunitie s t o stud y an d link th e differen t processe s o f basi n formatio n and filling . On e suc h area , tha t ha s a uniqu e combination o f a well-preserved rock recor d an d abundant data , i s th e Norwegia n Nort h Sea North Atlanti c margin , herei n referre d t o a s the Norwegia n margin . During post-Caledonia n time s th e Nort h Se a and mid-Norwegia n margi n underwen t severa l episodes o f lithospheri c extensio n (multi-phas e rifting), o f which th e lates t le d to crusta l break up an d accretio n o f oceani c crus t betwee n Norway an d Greenlan d nea r th e Paleocene Eocene transition . Prominent pre-breakup extensional episodes , i n lat e Permian-Triassic , lat e Jurassic, earl y Cretaceou s an d mid-Cretaceou s time, le d t o th e developmen t o f th e Nort h Se a rift system , th e larg e Cretaceou s V0rin g an d More sedimentar y basin s of f Norway, an d con jugate equivalent s off eastern Greenland . Lates t Cretaceous-Paleocene riftin g an d breaku p wer e accompanied b y large-scal e igneou s activity , developing th e presen t conjugat e volcani c mar gins of the North Atlantic . The Nort h Sea , i n particular , i s covered wit h an exceptionall y goo d geologica l an d geophysi cal industry an d academi c database , comprisin g both high-qualit y geophysica l profile s an d a large number o f industrial wells. About 3 0 years of active exploration i n the North Sea has le d to an advance d leve l o f understandin g o f th e geo logical evolutio n an d complexit y o f th e basin . Numerous paper s hav e bee n publishe d o n th e formation an d fillin g o f th e Nort h Se a intra cratonic rif t structure . The mid-Norwegia n margi n ha s a les s extensive, bu t increasing , industr y database . Never -

theless, th e V0rin g margi n i s amon g th e bes t studied volcani c margin s globall y du e t o a regional coverag e o f multichanne l seismi c lines, expanded sprea d profile s an d othe r geophysica l and geologica l data . I n addition , th e successfu l scientific drillin g throug h a sequenc e o f sea ward dippin g reflector s o n th e V0rin g Platea u has greatl y contribute d t o th e understandin g of th e margin . This settin g make s th e stud y regio n a n excellent laborator y i n whic h t o stud y progres sive rif t evolutio n an d it s inter-relationship with basin formatio n an d filling, including th e inter plays betwee n structura l evolution , erosion , sedimentation and magmatism, o n both regional and loca l scales. Consequently, the region allows research into fundamental earth processes, which also hav e direc t implication s fo r hydrocarbo n exploration an d assessment .

The Integrate d Basi n Studie s (IBS ) projec t The paper s an d researc h result s presente d her e have bee n prepare d a s par t o f th e Integrate d Basin Studie s projec t (Fig . l).Thi s projec t wa s funded unde r th e D G IIX , JOUL E I I pro gramme, wit h th e objectiv e t o stud y th e litho spheric an d uppe r crusta l processe s governin g the formatio n an d evolutio n o f extensiona l an d foreland basin s an d t o deciphe r th e rol e o f tectonics, sea-leve l and sedimentar y processes i n the filling of such basins. As part of this task, the project als o aime d a t studyin g the physica l laws of compactio n i n fine-graine d sediments . Base d on thes e results , a ne w generation o f descriptive as wel l a s numerica l basi n formatio n an d basi n fill models ha s bee n derived . Results fro m th e Integrate d Basi n Studie s project hav e bee n reporte d i n an extensiv e Final Report t o th e D G IIX , an d selecte d paper s prepared withi n Module s 1 an d 2 hav e bee n published i n tw o previou s Geologica l Societ y Special Publications , no s 13 4 and 156 .

From: NOTTVEDT , A . e t al. (eds ) Dynamics o f th e Norwegian Margin. Geologica l Society , London , Specia l Publications, 167 , 1-14 . 1-86239-056-8/OO/ S 15.00 © Th e Geologica l Societ y o f Londo n 2000 .

2

A. N0TTVED T

This boo k ha s bee n prepare d wit h th e inten tion t o giv e a representativ e pictur e o f th e sci entific spa n an d result s o f th e projec t Modul e 3 : Dynamics o f the Norwegia n Margin . I t includes papers prepare d b y Ph D student s funde d directly throug h th e project , a s wel l a s paper s written b y researcher s i n academi a an d industry that hav e bee n workin g in , o r closel y associate d with, th e project .

IBS Modul e 3: Dynamics of th e Norwegian Margin (IBS-DNM) The Integrate d Basi n Studie s projec t Modul e 3 , Dynamics o f the Norwegian Margi n (Fig . 2), was technically par t o f th e IB S project, bu t receive d funding directl y fro m th e Researc h Counci l o f Norway (RCN). Th e module was coordinated b y Norsk Hydro , an d contractua l arrangement s were mad e betwee n Nors k Hydr o an d EU , Norsk Hydr o an d RCN , an d Nors k Hydr o an d partners. Th e modul e wa s organize d int o thre e themes an d subordinat e topic s (Fig . 3) .

Research objectives The scop e o f the IBS-DN M projec t modul e wa s to analys e and mode l th e dynamics o f the basin s off mid-Norwa y an d i n th e norther n Nort h Sea , in order to establish a better understanding of the processes controllin g basi n formatio n an d fillin g and t o develo p ne w model s fo r multiphase , intraplate riftin g an d volcani c margin formation .

Fig. 1 . D G IIX : Joule I I Geoscienc e I I Programm e

Thematically, th e projec t modul e focuse d o n three relate d themes : (i ) Intra-plat e riftin g an d basin formation , (ii ) Basi n infil l an d (iii ) Conjugate volcani c margins. The dat a coverag e an d geological settin g resulte d i n a topica l focu s o n two geological provinces, the northern North Sea and th e Norwegian-Greenlan d Se a rifte d volca nic margins . Th e M0r e Basin , o n th e volcani c More margin , link s thes e tw o provinces . Geographically, th e tw o forme r researc h theme s (Intra-plate riftin g an d basi n formatio n an d Basin infill ) focuse d o n th e norther n Nort h Se a and th e Mor e Basin , whil e the latte r (Conjugate volcanic margins) primarily dealt with the More Voring margin s and thei r conjugates. Of particula r interes t in th e intra-plat e setting is th e kinematic s an d relativ e importanc e o f deeper versu s shallow crustal processes an d how 7 these contro l overal l subsidenc e patterns : th e erosional an d provenanc e histor y o f sedimen t supply areas ; an d th e architecture , composi tion an d histor y o f basi n fill . Th e wor k con centrated o n syn-rif t basi n fill, but include d als o post-rift strata . I t ha s bee n attempte d t o estab lish genera l an d couple d model s fo r rif t basi n formation an d sedimen t fillin g i n th e norther n North Sea . The Norwegian-Greenlan d Se a represent s complete extensio n an d thinnin g o f th e litho sphere. Therefore , th e margi n basin s bea r a n imprint no t onl y o f th e sam e event s a s th e intra-plate basins , bu t hav e als o undergon e a structural , magmati c an d depositiona l his tory reflectin g th e formatio n an d subsequen t

INTEGRATED BASI N STUDIE S development o f th e presen t volcani c passiv e margin. T o understan d th e basi n evolutio n an d tectono-magmatic histor y i t i s necessar y t o assess th e timin g an d magnitud e o f the differen t tectonic, magmati c an d subsidenc e episode s fo r the variou s basins . I n orde r t o determin e th e style o f deformatio n fo r eac h tectoni c episod e and t o quantif y th e magnitud e o f crusta l an d lithospheric extension , restoratio n o f th e struc turally define d extensional deformatio n throug h time wa s particularl y emphasized .

Database and methods The projec t databas e (Fig . 4 ) consiste d o f scientific dee p reflectio n an d refractio n seismi c data, larg e volume s o f industr y regiona l 2 D reflection seismi c data , 3 D seismi c data , wel l data, potentia l fiel d data , industr y specia l studies an d reports , i n additio n t o publishe d literature.

3

It is important, therefore , tha t th e project ha d access t o much o f the existing seismic, structura l and stratigraphi c framewor k i n the stud y regio n to defin e an d selec t th e optima l targe t area s fo r the detaile d studie s describe d i n th e variou s project topics . Th e industria l partner s wer e instrumental i n achievin g thi s objective. The wor k o n crusta l structur e an d basi n formation concentrate d aroun d seve n transect s from th e norther n Nort h Se a t o th e V0rin g margin (Fig . 4) , whereas th e topi c o n stres s field covers th e entir e Norwegia n continenta l shelf . The syn-rif t infil l studie s focuse d o n selecte d fault-blocks i n the northern Nort h Sea , wherea s the post-rif t (Cenozoic ) infil l studie s integrate d data fro m th e Danis h an d Norwegia n sector s of th e Nort h Sea , u p t o 62°N . Th e wor k o n erosion an d provenanc e include d th e norther n North Sea . Th e studie s o n volcani c margin s concentrated o n the mid-Norwegian margin , bu t comparative studie s wer e mad e wit h othe r vol canic margin s aroun d th e world .

Fig. 2 . IBS-DN M contractua l framewor k and involve d partners. RCN , Researc h Counci l o f Norway.

4

A. N0TTVED T subsidence an d ma y therefor e contribut e significantly t o th e stretchin g facto r a s calculate d from subsidenc e analysi s o f late r rif t episodes . A majo r questio n i s the relativ e contribution t o final extensio n contribute d b y eac h o f th e mai n episodes o f extension.

Key results. Th e dee p seismi c reflectio n surve y NSDP84 ha s forme d th e basi s fo r numerou s papers o n th e crusta l structur e an d basi n evolu tion o f th e norther n Nort h Se a rif t system . However integratio n with all available geophysical an d geologica l dat a fro m bot h offshor e an d onshore area s provide d som e interestin g ne w results. Reprocessing o f th e dee p reflectio n seismi c data resulte d i n enhance d dat a quality . B y integrating seismi c refractio n data , ES P dat a and gravity/magneti c data, an d combinin g thes e transects with high-quality commercial reflectio n seismics, i t ha s bee n possibl e t o ma p ou t th e structural outlin e o f th e grabe n geometr y o f the norther n Nort h Se a (Christiansso n e t al.. Odinsen e t al. (a)) , includin g linkag e betwee n upper an d mi d crusta l faults , faul t geometrie s of th e dee p crusta l level s an d structur e o f Fig. 3. IBS-DN M projec t structur e an d organization . the lowermos t crust . O f particula r interes t i s Past member s o f th e Steerin g Committe e includ e the improve d identificatio n of top basement , the Alv Orhei m (Statoil) . Jan Volse t (Statoil ) an d Ro n confirmation o f intr a uppe r mantl e dippin g J. Stee l (UB) . reflectors an d identificatio n o f a high-velocit y lower crusta l roc k body . New informatio n on th e level s of detachmen t and genera l geometrie s o f th e differen t faul t Theme 1 : Intra-plate Riftin g an d systems ha s als o bee n gaine d (Fosse n e t al.}. Basin Formation These investigation s revea l a mor e comple x interaction betwee n faul t system s tha n antici Topics 1.1: Crustal structure, pated earlier . The kinematic s of the faultin g was 1.2: Sedimentary basin formation and studied b y the use of analogue model s (Fosse n & Gabrielsen 1995) . In addition , th e confirmatio n 1.4: Tectonic modelling of th e existenc e o f 'grabe n units / whic h ar e The dynami c framewor k fo r Mesozoi c an d characterized b y shiftin g polaritie s alon g th e Cenozoic basi n developmen t i n th e norther n basin axi s o f th e Permo-Triassi c basi n axis , i s North Se a an d o n th e mid-Norwegia n margi n important (Faerset h e t al . 19950) . has bee n wel l documented ove r th e las t decade . Forward/backward numerica l modellin g o f However, ther e i s majo r uncertaint y a s t o th e crustal geometry , palinspasti c grabe n topogra nature o f th e dee p crusta l structur e an d it s phy an d therma l developmen t hav e resulte d i n relation t o th e sedimentar y cover . I n addition , better insigh t i n th e formatio n o f th e Nort h Se a the detaile d nature , significanc e and area l exten t rift (Odinse n e t al . (b) , te r Voorde et al.). Thes e of th e faultin g an d differentia l subsidenc e i n th e investigations conclud e tha t th e Permo-Triassi c post-rift interval s ar e poorl y known . stretching even t ha s bee n underestimate d i n For a complete understanding o f evolution o f many previou s work s i n th e area . Th e Permo the norther n Nort h Se a an d Mor e Basi n i t i s Triassic even t i s responsible for a more extensive essential t o conside r th e earlie r Permo-Triassi c extension tha n th e Lat e Jurassi c even t an d th e rift, whic h generate d th e structura l framewor k former even t als o resulte d i n extensio n o f a on whic h th e late r Jurassi c an d Cretaceou s rift s wider are a tha n di d th e latter . acted (Fig . 1.3) . Compactio n o f Permo-Triassi c The results fro m th e structural mappin g o f the basin fil l an d residua l Permo-Triassi c therma l M0re Basi n particularl y emphasize th e complexanomalies ma y enhanc e Cretaceous-Cenozoi c ity o f th e basin , wher e larg e basement-involved .

INTEGRATED BASI N STUDIE S rotated fault-block s have contribute d t o th e internal compartmentalization o f the basin. As for the Nort h Sea , th e M0r e Basi n wa s subjec t to multi-phas e extensio n (Grunnaleit e & Gab rielsen 1995) . Lithospheri c stretchin g probabl y commenced i n th e lat e Permian-earl y Triassic , followed b y a second episod e in the late Jurassic. In contrast t o the North Sea, however, early-midCretaceous riftin g an d successiv e Palaeogen e breakup strongly influenced furthe r development of the basin, resulting in a cumulative beta-facto r in excess of 3. Regional-scale antiforms along the northeastern basi n margin , basi n inversio n i n the S10rebot n Sub-basi n an d th e northeaster n More Basin, compressional reactivatio n of faults as see n o n th e nea r bas e Cretaceou s level , an d reverse drag and foldin g associated wit h faults of primarily extensiona l origin , sugges t tha t pro nounced multi-phas e lat e Mesozoic-Cenozoi c inversion ha s take n plac e (Mogense n e t al., Gabrielsen e t al 1999) . Relevance t o th e petroleum industry. Th e im proved model s fo r basi n developmen t an d faul t geometries obtaine d withi n thi s topi c provid e

5

new concept s t o th e industr y tha t ar e relevan t for th e understandin g o f san d distributio n i n general, an d i n searc h fo r th e subtl e tra p i n particular. I t is also expected tha t th e results will be o f importanc e i n basi n modelling , bot h o n the regiona l (crustal ) scale , an d fo r maturatio n and migratio n studie s on th e sub-regiona l scale. In addition , th e focu s o n th e pre-Jurassi c basi n development ha s opene d ne w perspective s tha t may b e o f importanc e fo r futur e exploration , particularly i n th e deepe r part s o f th e Nort h Sea Basin.

Topic 13: Present regional stress field Some rif t episode s i n th e norther n Nort h Sea More Basin-V0rin g Basi n correlate wit h break up an d initia l sea floor spreading in other basin s within th e Nort h Atlanti c rif t syste m an d the inter-relationship s betwee n pre-rif t struc tures (structura l fabric ) an d th e regiona l stres s field probabl y playe d a n importan t rol e i n structural evolution , b y rejuvenatio n o f inher ited structures .

Fig. 4. Schemati c outlin e o f the IBS-DN M stud y are a an d databas e (afte r Skogsei d e t al., this volume).

6

A. N0TTVED T

The orientatio n an d magnitud e o f the presen t stress fiel d ar e reflecte d i n bot h wel l dat a an d earthquake foca l mechanis m solutions . Th e us e of bot h i n situ measurement s an d earthquak e based data enables mappin g o f crustal stres s as a function o f depth . Mos t importan t ar e strai n markers fo r individua l tim e intervals . Regional an d loca l variation s i n stres s direc tions an d magnitud e yiel d informatio n o n th e intra-plate forces , plat e tectonic s an d recen t geological history . Thus , i t provide s impor tant inpu t t o th e tectoni c modelling . Couplin g of non-linea r lithospher e rheolog y t o a mode l involving stres s change s actin g on a basi n offer s the prospec t o f understandin g non-therma l sub sidence an d o f solvin g discrepancie s i n presen t crustal extensio n estimates . Key results. A databas e o f unprecedente d quality an d quantit y fo r th e regio n ha s bee n established, comprisin g stres s informatio n fro m a variet y of sources. Th e projec t ha s contribute d to th e compilatio n o f a n earthquak e foca l mechanism databas e tha t i s constantl y bein g expanded, an d whic h currentl y comprise s 10 9 solutions fo r northwester n Fennoscandi a an d Svalbard (Lindhol m et al., Lindhol m et al. 1995) . In addition , a tota l o f 34 5 borehol e breakou t observations an d 10 4 overcoring measurement s have bee n collecte d an d quality-assesse d int o a high-quality database (Fejerskov & Lindholm (a), Borgerud & Suav e 1995 , Fejersko v e t al . 1995 , G01ke e t al . 1995) . All thes e dat a wer e use d i n a join t inter pretation o f th e crusta l stres s fiel d i n Norwa y and adjacen t offshor e region s (Fejersko v & Lindholm (b)) . Regionally , th e analysi s ha s revealed a crusta l horizonta l compressiv e stres s with a dominatin g WNW-ES E direction(s Hm ax) in souther n Norway , whic h graduall y rotate s into N- S compressio n i n th e Barent s Se a region. Th e observe d stres s i s largely consisten t throughout th e uppe r crust , indicatin g tha t large-scale tectoni c mechanism s ar e th e mai n source. Thi s can , wit h a hig h degre e o f con fidence, be attribute d t o crusta l spreadin g alon g the mid-Atlanti c Ridge . Th e consisten t stres s pattern show s regiona l variation s probabl y caused b y regional stress generating mechanism s like th e continenta l margin-ridg e pus h an d sediment loading . Fault s tha t pertur b an d deviate th e regiona l stres s locall y als o hav e been observed . With importan t exception s i n loca l areas , the dominan t typ e o f faultin g i s foun d t o b e contractional and , t o som e extent , strike-slip , indicating tha t th e continenta l margi n i s subjec t to compression , a s als o indicate d b y i n situ

measurements onshore . Compressiv e differen tial horizonta l stresse s ar e foun d i n al l part s of th e margi n analysed , an d th e dat a indicat e that th e stres s regim e i n th e uppe r crus t ofte n alters with depth. A stress homogeneity through out th e brittl e crust is , however, wel l documen ted b y th e homogeneou s stres s direction s obtained fro m complementar y dat a (dee p earthquakes an d shallo w boreholes) . Relevance t o the petroleum industry. O n a shortterm basis , thi s topi c ha s provide d importan t information t o th e genera l understandin g o f the stres s fiel d alon g th e Norwegia n Margin , which i s o f considerabl e importanc e whe n entering int o ne w area s lik e th e V0rin g Basin . In addition , detaile d informatio n o n i n situ stresses i n severa l petroleu m field s i s expecte d to b e directly relevant to activities in those fields. In situ roc k stres s data ar e particula r importan t for th e plannin g and drillin g o f stabl e an d saf e wells. Th e stat e o f stres s i n an d aroun d th e hydrocarbon reservoi r i s also importan t fo r field development an d reservoi r management , an d the succes s o f hydrauli c fracturin g an d injec tion depend s o n knowin g ho w fracture s an d fluid front s propagat e unde r differen t stres s systems. Th e us e o f suc h dat a ma y o n a long term basi s reduc e cost s an d hel p optimiz e dril ling an d production . Theme 2 : Basi n Infil l

Topic 2.1: Syn-rift sediment architecture The stratigraph y and architectur e o f th e Trias sic-lower Jurassi c an d Cretaceous-Cenozoi c post-rift basina l infil l o f th e Nort h Se a ar e relatively wel l documented , wherea s th e Permo Triassic an d uppe r Jurassi c syn-rif t infil l ar e known onl y i n broades t outline . Detail s o f timing, styl e an d rate s o f th e event s tha t mak e up th e syn-rif t episodes , an d o f th e sedimen tary respons e t o thes e events, hav e onl y recentl y started t o emerge. Extensio n across th e norther n Viking Grabe n i s know n t o var y betwee n th e Tampen Spur , Vikin g Grabe n proper , Hord a Platform an d Sog n Grabe n segments . Thi s variability i n space , togethe r wit h tempora l variations (Permo-Triassic , Lat e Jurassic ) ha s considerable consequence s fo r th e composi tion o f syn-rif t infill . Thes e latera l change s an d related provenanc e an d sedimen t transpor t issues hav e bee n addresse d b y analysi s o f dif ferent pattern s o f syn-rif t stackin g an d b y studies o f th e relatio n betwee n rate s an d styl e of structura l events.

INTEGRATED BASI N STUDIE S Key results. A database of seismi c examples a s well as field analogues ha s been buil t to illustrate the variabilit y of syn-rif t architecture , involvin g integration o f data on rif t topography , erosiona l and drainag e patterns , relativ e se a leve l an d it s changing position , sedimen t transpor t processe s and facies and resultan t sand body geometry an d distribution (Marjana c 1994) . I n severa l suc h examples, th e spatia l an d tempora l evolutio n of reservoir san d facie s ha s bee n semi-quantita tively linke d t o th e structura l evolutio n o f th e parent half-grabe n an d neighbourin g footwal l sediment sourc e area s (Farset h e t al . \995b; Ravnas & Bondevik 1997 ; Nettvedt et al.). A synthesi s ha s bee n compile d o f variou s aspects o f th e three-dimensiona l geometr y o f syn-rift stratigraphi c architectura l element s an d their stackin g pattern, an d th e spatia l organiza tion o f potentia l facie s tract s an d stratigraphi c surfaces presen t i n marin e syn-rif t basi n fill s (Gabrielsen e t al . 1995 ; N0ttved t e t al . 1995 ; Ravnas e t al. \991a; Ravna s & Steel 1998) . This includes a n analysi s an d compariso n o f a number o f half-grabe n sub-basin s i n th e north ern Nort h Se a (Faerseth & Ravnas 1998 ; Ravnas et al.). The studie s hav e le d t o a classificatio n o f marine rift-basin s an d syn-rif t succession s i n terms o f sedimen t suppl y int o overfilled , sediment-balanced, sediment-underfille d an d sediment-starved basin s (Ravna s & Stee l 1997) . Dependent o n whethe r th e rift-basi n wa s over filled/sediment-balanced, sediment-underfille d or sediment-starved , a three-fold , sand-clay sand package , two-fol d sand-cla y packag e o r one-fold mud-pron e packag e constitute s th e syn-rift succession , respectively . I n case s wher e the rif t episod e wa s characterize d b y repetitiv e rift phases , result s sho w tha t th e successiv e rotational til t event s ar e ofte n separate d b y periods characterize d b y les s intens e faulting , the so-calle d intra-rif t quiescenc e o r relaxa tion stages . The projec t als o include d a stud y o f th e syn rift infil l o f th e Lusitania n Basi n i n Portuga l (Ravnas e t al . \991b). Relevance t o the petroleum industry. Th e results and interpretation s obtaine d withi n thi s topi c provide new insight and idea s for analysis of rift basin developmen t an d syn-rif t infill . Th e recog nition o f th e differen t type s o f syn-rif t sedimen tary architecture s provide s a ste p forwar d i n erecting predictiv e model s fo r th e analysi s o f half-graben syn-rif t sedimen t fill . I n addition , integrated detaile d analysi s o f rif t basi n struc tural evolution , high-resolutio n biostratigraphi c zonation, reworke d biozonation , basin-fil l facie s

7

and architectur e an d seismi c facies offe r a sensi tive too l fo r analysin g th e detaile d sequentia l basin development an d predictin g reservoi r san d distribution. Th e model s erecte d ma y serv e a s important reference s t o th e conceptua l trainin g of explorationists.

Topics 2.2: Post-rift sediment architecture and 2.3: Erosional episodes and provenance area Post-rift interval s commonly hav e a duratio n o f 50-100 Ma. Therma l relaxatio n followin g th e late Jurassi c rif t even t ha d almos t cease d an d the Nort h Se a basi n wa s i n a stat e clos e t o thermal equilibriu m during th e transitio n t o th e Cenozoic. Subsidenc e an d depositio n durin g the Cenozoic, therefore , wer e controlle d b y othe r factors. Th e Cenozoi c successio n i n th e Norwe gian Nort h Se a ca n b e divide d int o geneti c sequences o f 8-1 6 Ma duration , representin g major period s o f clastic wedge progradation an d retreat wit h respec t t o th e Norwegia n hin terlands. Th e clasti c wedge s ar e bounde d b y major floodin g surface s and reflec t variabl e subsidence rates . Th e characte r o f thes e variation s and o f the resultin g genetic sequences hav e bee n investigated and relate d to the uplift an d erosio n of th e surroundin g lan d areas . The sand/cla y rati o i n sedimentar y basin s i s commonly viewe d a s a functio n o f depositiona l energy. A facto r tha t tend s t o b e overlooke d is th e primar y provenanc e are a composition . A bette r understandin g o f th e processe s tha t cause change s i n th e compositio n o f th e sedi mentary sequenc e i n the Nort h Se a Basin therefore i s needed. Th e relativ e contributions o f th e Norwegian mainlan d an d Eas t Shetlan d Plat form a s sourc e area s ha s bee n evaluated , base d on analysis and descriptio n o f basin fill composition an d characteristics , mapping o f palaeodrai nage directions and transport routes , estimations of palaeo-provenanc e area s an d sourc e roc k lithologies, an d correlatio n o f basi n fill to prov enance area . Key results. Th e projec t ha s integrate d seismi c data fro m th e Danish secto r wit h the Norwegia n North Se a (Jord t e t al . 1995) . I t als o include s mineralogical an d geochemica l analyse s o f 160 0 samples o f cutting s an d core s fro m abou t 4 0 wells in th e Norwegia n secto r o f the Nort h Sea , and thu s represent s th e mos t comprehensiv e database currentl y available . Thes e dat a ar e important i n term s o f understandin g th e prove nance an d provid e a ne w basi s o n whic h t o

8

A. N0TTVED T

interpret direction s o f sedimen t transpor t an d areas o f tectoni c uplift . The seismostratigraphi c stud y show s tha t changes i n seismi c sequenc e geometr y occurre d in-phase wit h intra-continenta l stres s variation s on th e Europea n Platfor m an d tha t sedimen t supply an d differentia l tectoni c movement s i n the basin and i n the provenance area s controlle d Cenozoic depositio n i n the centra l an d norther n North Se a (Jordt e t al.). Seismi c sequences were generated largel y independent o f marke d globa l glacio-eustatic se a leve l falls . Generatio n o f seismic onla p appear s t o hav e bee n controlle d by sedimen t suppl y and basi n floo r topography . Thinning o f sedimentary strat a towards inclined surfaces result s i n marke d seismi c onla p an d apparent unconformabl e relationships . It is further show n that seismi c velocities in the North Se a Cenozoi c mudstone s var y system atically a s a functio n o f mineralogica l composi tion. A commonly observe d velocit y inversion at the base of the Pliocene and Pleistocen e sequenc e (Reemst e t al. 1996) has been foun d t o relate to a low conten t o f smectit e an d therefor e muc h faster compactio n o f thes e sediment s compare d to th e underlyin g smectite-rich mudstones (Thy berg e t al.}. Silic a cementatio n fro m biogeni c silica, as well as alteration o f opal-A to opal-CT, have bee n show n t o caus e abrup t increase s i n seismic velocities. It has been demonstrate d that pore pressure in the Tertiar y successio n i s governe d b y miner alogical composition . Thic k sequence s o f mud stones wit h hig h smectit e content , whic h typically hav e hig h specifi c surfac e an d lo w permeability, typicall y giv e rise t o overpressure , providing there are no sandy beds causing lateral drainage. Moreover , becaus e o f their high water content smectit e ric h mudstone s usuall y hav e lower densit y than th e overlyin g mudstones an d sandstones. Thi s densit y inversio n frequentl y caused diapirism . Finally, th e projec t ha s provide d result s contributing t o th e understandin g o f th e Cen ozoic uplif t o f Norwa y (Fjeldskaa r 1994 ; Stue vold & Eldhol m 1996) . Relevance t o th e petroleum industry. Thi s topi c has contribute d t o th e understandin g o f th e Cenozoic basi n formatio n an d filling in relation to th e uplif t an d erosio n histor y o f Souther n Norway. Thi s uplif t ma y strongl y influenc e secondary an d tertiar y migratio n an d trappin g of petroleum , particularl y i n area s clos e t o th e coast o f Norway . The presen t projec t i s on e o f th e firs t i n th e North Se a wher e seismi c stratigraph y ha s bee n related t o mineralogica l compositio n i n a sys -

tematic way . Understandin g th e regiona l variations i n th e velocity/dept h functio n an d th e processes responsibl e fo r thes e trend s i s impor tant whe n dept h convertin g seismi c profiles . The dat a o n mineralog y an d diagenesi s i s als o of valu e t o calibratio n o f seismi c respons e t o lithology, i n orde r t o understan d bette r th e information tha t ca n b e extracte d fro m seismi c attributes. The demonstrated relationshi p between mineralogical composition , diagenesi s an d overpres sure is expected t o b e of great interes t to drilling , particularly horizonta l drilling, and fo r handling of rock mechanica l problems durin g production . Moreover, th e present stud y ha s show n tha t th e North Se a Cenozoi c roc k propertie s canno t b e realistically represente d a s a simpl e functio n (i.e. linea r o r exponential ) of buria l depth . Thi s has importan t consequence s t o basi n modelling, as porosit y reductio n (compaction ) i s usuall y assumed t o b e a functio n o f overburde n o r effective stress .

Topic 2.4: Stratigraphic modelling Numerical modellin g i s becomin g increasingl y important i n th e understandin g o f sedimentar y basin deposition and filling. In particular, it helps evaluate the complex interplay between tectonics, eustacy, climate , erosion and sediment transport , and bette r constrai n th e boundar y condition s of geologica l interpretation s an d models . The overal l sedimentar y architectur e o f th e upper Jurassi c syn-rif t infil l i n th e Oseber g are a as wel l a s th e Cenozoi c infil l alon g on e o f the Nort h Se a regiona l profile s hav e bee n modelled, a s a functio n o f sedimen t input , sub sidence an d se a leve l fluctuation s t o obtai n additional informatio n abou t thei r interpla y and relativ e importance . Th e modellin g ha s used forwar d process-base d simulatio n pro grams of dynamic-slope type. Comparisons hav e been made betwee n observed Stratigraphi c architecture an d syntheti c Stratigraphic models where the mos t importan t controllin g factor s hav e been considered . Key results. Th e numerica l modellin g o f th e syn-rift infil l o f th e Oseber g are a demonstrat e the interactio n betwee n structura l evolution , sedimentation an d resultin g Stratigraphi c pat tern acros s th e rotatin g fault-block s (terVoorde et al . 1997) . Th e modellin g furthe r confirm s that th e sedimentar y architectur e o f th e Nort h Sea Cenozoi c infil l canno t b e explaine d b y sedi mentary processe s and/o r eustac y alon e (Kyrk jebe e t al.). Period s o f anomalou s subsidence .

INTEGRATED BASI N STUDIE S deviating fro m th e post-rif t therma l subsidence , are require d i n th e lat e Paleocen e an d lat e Miocene. However , som e o f th e Paleocen e subsidence ma y b e relate d t o th e initia l basi n form e. g exces s wate r dept h afte r lat e Cretac eous. Th e Miocen e event , o n th e othe r hand , includes basina l subsidenc e i n th e norther n North Sea , a s wel l a s sourc e are a uplif t i n th e Norwegian mainland , an d i s believe d t o repre sent a n intraplat e effec t couple d t o th e openin g of th e Nort h Atlantic . Relevance t o th e petroleum industry. Th e itera tive proces s betwee n interpretatio n an d model ling allow s th e geologis t t o bette r constrai n th e possible geologica l model s an d t o narro w i n on a les s numbe r o f likel y interpretations . Thi s type o f modellin g i s als o a n excellen t too l t o visualize th e comple x interactio n betwee n sedi ment supply , sea-leve l and tectonic s i n fillin g o f sedimentary basins .

Theme 3 : Conjugate Volcanic Margins Topics 3.1: Rift dimensions and duration of rifting, 3.2: Geodynamic modelling and 3.3: Comparative studies It ha s lon g bee n recognize d tha t th e forma tion o f th e norther n Nort h Atlanti c conti nental margin s wa s accompanie d b y exces s magmatic activity . Thi s cause d a sub-divisio n of rifted margin s int o volcani c and non-volcani c types. Unti l recently , th e volcani c margin s wer e thought t o b e an exceptiona l case . Recen t com parative studie s involvin g othe r rifte d margin s now sugges t tha t exces s magmatis m durin g breakup i s fa r mor e commo n tha n previousl y thought. I n fact , th e volcani c margi n migh t represent th e norma l evolutionar y cas e rathe r than bein g anomalous . The volcani c signatur e o f th e Nort h Atlanti c margin ha s bee n explore d b y seismi c reflectio n data an d scientifi c drilling , whereas th e tectoni c signature, an d thu s development, hav e to a large extent bee n hidde n belo w th e volcani c rocks . The project ha s involved geophysical-geologi cal mapping o f the V0ring, Mor e and conjugat e margins, i n orde r t o obtai n ke y dimension s and timin g o f th e lat e Cretaceous-Paleocen e rift episode , b y focusin g on : (1 ) width , styl e and timin g o f syn-rif t lithospheri c extension ; (2) exten t an d timin g o f syn-rif t regiona l uplift ; (3) extent , character , tim e o f emplacemen t and dimension s o f igneou s unit s (extrusives , intrusives, lowe r crusta l high-velocit y bodies) ;

9

(4) location o f continent-ocean transition ; (5) extent o f post-rift therma l margi n subsidence . Th e availability o f dat a als o of f Greenlan d offer s a possibility t o ma p an d mode l th e entir e rif t an d to compar e th e lat e Cretaceous-Paleocen e rif t dimensions wit h th e previou s rif t episodes . In addition , comparativ e studie s o f th e volcanic margi n formatio n i n th e Nort h Atlan tic, a s wel l a s globally , hav e bee n undertaken , with the objective to improv e our understandin g of tectono-magmati c volcani c margi n settin g and th e processe s governin g volcani c mar gin initiatio n an d development . I t ha s als o helped i n providin g a framewor k fo r analysi s of th e implication s o f volcani c margin s fo r erosion an d sedimentatio n o n loca l an d regiona l scales, an d fo r th e environmen t (palaeoceano graphy, palaeoclimate ) o n local , regiona l an d global scales . Key results. A numbe r o f crusta l transect s has bee n constructe d acros s th e Nort h Atlanti c margin b y us e o f dee p seismi c dat a acros s the margin . I t i s estimate d tha t th e conjugat e margins experience d som e 140k m o f crusta l stretching durin g th e Maastrichtian-Paleocen e rifting an d breakup , an d abou t 50-7 0 km of latera l displacemen t durin g Lat e Jurassic Cretaceous riftin g (Skogsei d 1994 ; Skogsei d & Eldholm 1995 ; Skogsei d e t al.). Th e Rockal l Trough has , however , experience d fa r mor e stretching, whic h i s interprete d t o b e relate d t o separate riftin g i n th e mid-Cretaceous . B y using the stretchin g estimates , palinspasti c map s hav e been constructed base d o n restorations t o 5 3 Ma pre-drift, 7 5 Ma pre-Cenozoi c breaku p an d 170 Ma Lat e Jurassi c pre-rif t plat e configura tions. Th e result s als o includ e a n evaluatio n o f the tectono-magmati c event s associate d wit h plume-lithosphere interactio n durin g rifting , with particula r focu s o n relativ e vertica l move ments an d provenanc e development . The tectoni c developmen t o f th e V0rin g an d More basin s i s controlle d b y tw o structura l trends, NE-S W an d NW-SE . I t i s suggeste d that th e NE-S W tren d wa s establishe d i n th e Paleozoic an d wa s activ e durin g al l subse quent tectoni c phases , wherea s th e NW-S E trend, probabl y reflectin g th e ol d Precambria n grain o f th e basement , controlle d th e tectoni c activity throughou t th e Cretaceou s an d Ceno zoic (Brekke) . Th e result s sho w tha t durin g th e Cretaceous an d Cenozoi c th e V0rin g Basi n wa s tectonically active , wit h repeate d phase s o f nor mal faultin g an d contractio n causin g large-scal e folding. Th e Mor e Basin , particularl y toward s the south , wa s overal l mor e tectonicall y quie t and experience d mainl y continuous subsidence .

10

A. N0TTVED T

Comparative studie s betwee n th e N E Atlan tic, th e Namibian , th e U S Atlanti c an d th e N W Australian margin s sho w tha t al l margin s see m to hav e ha d a protracte d rif t developmen t prio r to continental separation , tha t significan t crusta l thinning i s observe d adjacen t t o th e continent ocean boundary , an d tha t th e mai n puls e o f igneous activit y coincide s wit h th e tim e o f breakup (Eldhol m e t al. 1994 ; Eldhol m e t al.Planke & Eldhol m 1994) . O n th e othe r hand , i t is als o recognize d tha t exces s volcanis m ma y occur withou t a direc t lin k t o a n activ e mantl e plume, whic h ma y explai n th e larg e variet y i n tectono-magmatic developmen t o f volcanic margins worldwide. An extensiv e Norwegian-Greenland Se a thermal field data base named HEA T has been com piled, containin g al l publi c domai n dat a i n th e region; i.e . 436 hea t flo w value s (Sundvo r e t al.}. The hea t flo w o n oceani c crus t reveal s a clear , first-order hea t flow-crusta l ag e relationship , whereas continenta l slop e maxim a o n th e Mor e and Barent s Se a margin s contras t greatl y wit h the typica l lo w hea t flo w o f ol d oceani c an d thinned continenta l crust . Relevance t o th e petroleum industry. Th e un derstanding o f basi n geometries , rif t dimension s and subsidenc e histor y i s important t o explora tion companie s workin g offshor e mid-Norway . New informatio n o n magnitud e an d tempora l development o f intra-basinal vertical movements in combinatio n wit h palinspastic reconstructio n provides a too l fo r predictin g reservoi r facie s in th e oute r margi n basins . Suc h vertica l movements, includin g th e formatio n o f a lan d bridge fro m th e Charlie Gibb s Fractur e Zon e t o the S W Barents Sea margin, may hav e led to th e establishment o f larg e drainag e system s an d the probabilit y o f prospectiv e well-sorte d sedi ment sequence s o f generall y Lat e Cretaceou s Paleocene ag e i n th e adjacen t subsidin g parts o f the basins . In addition , th e geometrica l definitio n o f units o f igneou s material s a t crusta l level s has relevanc e fo r calculatio n o f heatflo w his tory, a s bodie s o f underplate d material s a t the bas e o f th e crus t ca n spik e heatflo w an d may hav e cause d maturatio n o f organi c mat ter tha t canno t b e predicte d b y presen t heat flow measurements . Th e compilatio n o f th e heat flo w databas e i s a n importan t too l i n this respect . The comparativ e wor k betwee n volcani c margins worldwid e i s importan t fo r th e under standing o f basi c geodynami c processe s an d tectono-magmatic developmen t i n general.

Concluding remark s The result s o f th e Integrate d Basi n Studies Dynamics o f th e Norwegia n Margi n projec t emphasize th e valu e o f academi a an d in dustry working together in order t o make signifi cant scientifi c progress . I t show s ho w publi c scientific grant s an d industr y fundin g ca n b e focused t o leverage investments put into research. It als o present s a mode l fo r ho w academi a an d industry researc h ca n b e effectivel y organize d into a singl e project. The achievement s o f th e Dynamic s o f th e Norwegian Margi n projec t hav e brough t ou r understanding o f thi s regio n forward , an d i t i s our hop e tha t th e result s presented i n thi s boo k will serv e as a n importan t referenc e for th e are a in th e year s to come. Th e projec t ha s als o estab lished a databas e tha t wil l serv e as a goo d basi s for continue d research o n the Norwegian margin and severa l ne w researc h project s hav e already been initiate d tha t buil d o n th e result s o f thi s project. I n addition , it has highlighte d some new : avenues o f researc h tha t ma y furthe r increas e our understandin g o f thi s margi n an d o f multi phase rif t evolutio n in general. Project staf f an d participants Direct project participation Principal Investigator s (PI) . Doctorat e Student s (PhD), Participatin g Scientist s (PS) , Master Student s (MS) an d Researc h Associate s (RA)

Theme 1: Intra-plate Rifting and Basin Formation Topics 1.1: Crust a! structure, 1.2: Sedimentary basin formation, 1.4: Tectoniic modelling PI: Prof . J . I . Faleid e (UO) . Prof. R . H . Gabrielse n (UB). Dr. W . Fjeldskaa r (RF) PhD: M.sci . P . Christiansso n (UO). C.sci. T . Odinse n (UB), C.sci. I . Grunnaleit e (UB) RA: C.sci . K . Lokn a (UB ) Industry: Dr . A . M . Berg e (Hydro). C.real. B . T. Larse n (Hydro). Prof. R.B . Faerseth (Hydro). Dr. A . Nottved t (Hydro). Dr. H . Fosse n (Statoil ) EU: Dr . P . Reems t (VU). Dr. P . va n de r Bee k (VU). M.sci. M . te r Voord e (VU). Prof. S . Cloetingh (VU) Topic 1.3: Present regional stress field PI: Prof . A . Myrvan g (NTH) . Prof. H . Bungu m (Norsar)

INTEGRATED BASI N STUDIE S PhD: Siv.ing PS: Dr MS: T

. M . Fejersko v (NTH ) . C . Lindhol m (Norsar ) . J0rgensen (NTH) , L. Borgeru d (NTH) , E. Svar e (NTH) , M. Villgra n (Norsar/UB) , E. Hick s (Norsar/UO ) Industry: Dr . T . H . Hansse n (Hydro) , C.real. B . T. Larse n (Hydro) , C.real. R . K . Bratl i (Saga) , Dr. L . N . Jense n (Statoil ) EU: Dr . M . Golke (UKa ) Theme 2: Basin Infill

Topic 2.1: Syn-rift sediment architecture PI: "Prof . R . J . Stee l (UB) , Dr. J . Underbil l (UEd ) PhD: C.sci . R . Ravna s (UB ) PS: Dr . P . Theriaul t (UB/Statoil) , D. Meller e (UB/Statoil) Industry: C.sci . K . Bondevi k (Hydro) , Prof. R . B . Faerseth (Hydro) , Dr. A . N0ttved t (Hydro) , J. Windelsta d (Statoil ) Topic 2.2: Post-rift sediment architecture, 2.3: Erosional episodes and provenance area PI: Prof . J . I . Faleid e (UO) , Prof. K . Bjorlykk e (UO ) PhD: M.sci . H . Jordt (UO) , C.sci. B.I . Thyber g (UO ) RA: Industry: Dr . P . va n Vee n (Hydro) , Dr. L . J. Skjol d (Hydro) , Dr. A . Ryset h (Hydro) , Dr. M . Ram m (Hydro ) Dr. A . N . N0ttved t (Hydro ) Topics 2.4: Stratigraphic modelling PI: C.real . M . Hambor g (IKU ) MS: R . Kyrkjeb o (NTH ) PhD: C.sci . R . Ravna s (UB ) Industry: C.sci . K . Bondevi k (Hydro) EU: M.sci . M . te r Voord e (VU) , Prof. S . Cloetingh (VU ) Theme 3: Conjugate Volcanic Margins Topics 3.1; Rift dimensions and duration, 3.2: Geodynamic modelling, 3.3: Comparative studies PI: Dr . J . Skogsei d (UO) , Prof. O . Eldhol m (UO ) MS: B . Flakstad (UO) , U. Byrkjelan d (UO) , F. Neverda l (UO) , S. Re n (UO) , E. Alvesta d (UO ) PhD: C.sci . T . Gladszenk o (UO ) PS: Dr . S . Planke (UO) , Dr. T . Pederse n (UO) , Prof. A . M . Myhr e Industry: C.rea l B . T. Larse n (Hydro )

11

Referees. Th e followin g person s kindl y serve d a s technical an d linguisti c referees fo r th e book : John Akselsen , Aril d Andresen , Kuve t Atakan , W . Scott Baldridge , Giovann i Bertotti , 0rja n Birkland , Eric Bogoslowski , Lar s Boldreel , Ros s Boutilier , Nicolas Chamot-Rooke , Sier d Cloetingh , Deni s Cou turier, Tom Dreyer , Richar d England , Doug Gardner , Rob Gawthorpe , Feli x Gradstein , Pa l Haremo , Jen s Havskov, William Helland-Hansen, Helg e Hjelmeland , Chuck Hurich, Erik P. Johannesen, Reida r Kanestrom, Ridvan Karpuz , Oddbjor n K10vjan , Joh n Knight , John Korstgard , Yngv e KristofTersen , Axe l Makurat , Ole J . Martinsen , Alai n Mascle , Joc k McCracken , Wojtec Nemec , Joha n P . Nystuen , Arvi d Nottvedt , Lars Norgard-Jensen , Nigel Platt, John Palmer , Sara h Prosser, Garr y Quinlan , Phi l D . Rice , Ja n Rivenass , Ellen Roaldset , Ala n Roberts , Yngv e Rundberg , Al f Ryseth, William Sassi, Ke n Saunders , Roge r Scrutton , Michel Seranne , Morte n Sparr e Andersen , Geral d Sullivan, Tor e Torske , Bj0r n T0rudbakken , Ja n Vol set, Erlin g Vagnes , Joh n Walsh , Marjori e Wilson , Graham Yielding and fou r anonymou s referees . Acknowledgements. Th e projec t wa s funde d b y th e European Unio n an d o f Norwa y Researc h Counci l under th e JOUL E I I researc h programm e (contrac t No. JOU2-C T 92-0110) . Nors k Hydro , Statoi l an d Saga provide d additiona l funding . Ar e B . Carlsson a t the Research Counci l o f Norway i s gratefully acknowledged fo r supportin g th e project . A s projec t leader , I am ver y gratefu l t o Bjor n T . Larse n fo r hi s involve ment i n th e projec t o n a n industr y client basis an d t o Vigdis Michelse n for he r secretaria l efforts . I als o ow e many thank s t o pas t an d recen t member s o f th e project Steerin g Committee , Ola v Eldholm , Ro y H . Gabrielsen, Snorr e Olaussen , Al u Orheim , Ton y Spencer, Ro n J . Stee l an d Ja n Volse t an d t o m y fellow colleague s o n th e Editoria l Board , Bjorn . T . Larsen, 0rja n Birkeland , Haral d Brekke , Ro y H . Gabrielsen, Snorr e Olaussen , Jako b Skogsei d an d Bjorn Torudbakken , withou t who m thi s boo k woul d never hav e com e t o light . Som e extende d thank s als o go t o th e man y scientist s wh o reviewe d th e manu scripts. However , th e principa l investigators , partici pating researcher s an d students , wh o showe d grea t efforts an d a remarkabl e spiri t o f co-operatio n throughout, o f cours e ar e th e ke y t o th e succes s of th e project . Finally , I woul d lik e t o pas s a wor d o f appreciation t o Bernar d Durand , Sier d Cloetingh , Cai Puigdefabrega s an d al l othe r scientist s i n th e Integrated Basi n Studie s project , fo r 3 year s o f stimulating co-operation .

Research contribution s Publications BLYSTAD, P. , BREKKE , H. , F^RSETH , R . B. , LARSEN , B. T. , SKOGSEID , J . & TORUDBAKKEN , B . 1995 . Structural elements of the Norwegia n continental shelf. Par t II : Th e Norwegia n Se a region . Nor wegian Petroleu m Directorat e Bulletin , 8.

12

A. N0TTVED T

BORGERUD, L . & SVARE , E . 1995 . In-situ stress fiel d o n the Norwegia n margin . In : FEJERSKOV , M . & MYRVANG, A . (eds ) Workshop o n Rock Stresses i n the North Sea. NT U Trondheim , 13-14.02.95 , pp.165-178. CLOETINGH. S. , SASSI , W. & TAS K FORC E TEAM \994a. The origi n o f sedimentar y basins : a statu s repor t from th e tas k forc e o f th e Internationa l Litho sphere Program . Marine an d Petroleum Geologv, 11, 659-683. CLOETINGH, S. , ELDHOLM , O. . LARSEN , B . T. , GABRIELSEN, R . H . & SASSI , W . (eds ) 1994/7 . Curren t state an d perspective s o f model s fo r extensiona l and inverte d basins . Tectonophysics, Specia l Vol ume, 240 . ELDHOLM, O . & THOMAS , E . 1993 . Scratchin g th e surface: Environmenta l impact o f volcanic margin formation. Earth an d Planetarv Science Letters, 117, 319-329 . ELDHOLM, O. , MYHRE , A . M . & THIEDE , J . 1994 . Cenozoic tectono-magmati c event s i n th e Nort h Atlantic: potentia l paleoenvironmenta l implica tions. In : BOULTER , M . C . & FISHER , H . C . (eds ) Cenozoic Plants and Climates o f the Arctic. NATO, ASI Series , 127 , Springer , Heidelberg , 35-55 . ELDHOLM, O. , SKOGSEID , J. , PLANKE , S . & GLADC ZENKO. T . P . 1995 . Volcanic margin concepts . In : BANDA. E. , TALWANI , M . & TORNE , M . (eds ) Rifted Ocean. Continent Boundaries. NAT O AS I Series Volume . Kluwer , Dordrecht , 1-16 . FEJERSKOV, M. , MYRVANG , A . M. , LINDHOLM , C . & BUNGUM, H . 1995 . In-sit u roc k stres s patter n o n the Norwegian continental shelf and mainland . In: FEJERSKOV, M . & MYRVANG , A . (eds ) Workshop on Rock Stresses i n th e North Sea. NT U Trondheim, 13-14.02.95 , pp. 191-201 . FJELDSKAAR, W. 1994 . The amplitud e and deca y o f the glacial forebulg e i n Fennoscandia . Norsk Geologisk Tidsskrift, 74 , 2-8 . FOSSEN, H . & GABRIELSEN , R . H . 1995 . Experimenta l modelling of extensiona l faul t systems . Journal o f Structural Geology, 18(5) , 673-687. FROSTICK, L . E . & STEEL , R . J . 19930 . Tectoni c signatures in sedimentary basin fills: an overview . International Association of Sedimentologists Special Publication, 20, 1-9 . FROSTICK, L . E . & STEEL , R . J . 19936 . Sedimentatio n in divergen t plate-margi n basins . International Association of Sedimentologists Special Publication, 20, 111-128 . F^RSETH, R . B . & RAVNAS . R . 1998 . Th e structura l configuration o f th e Oseber g Faul t Bloc k i n th e context o f th e norther n Nort h Se a structura l framework. Marine an d Petroleum Geology, 15 , 467-490. F/ERSETH, R . B. , GABRIELSEN , R. H . & HURICH , C . A . 19950. The influenc e of basement i n structuring o f the North Se a Basin offshore wes t Norway. Norsk Geologisk Tidsskrift, 75 , 2/3 , 105-119 . FÊRSETH, R . B. , SJ0BLOM , T . S. , STEEL , R . J. , LlLJE -

DAHL, T. , SAUAR , B . E . & TJELLAND , T . 19956 . Tectonic control s o n Bathonian-Volgia n syn-rif t successions o n th e Visun d Faul t Block , norther n North Sea . In : STEEL , R . J. , FELT , V . L. , JOHAN -

NESSEN, E . P . & MATHIEU . C . (eds ) Sequence Stratigraphy on the Northwest European Margin. Norwegian Petroleu m Societ y Specia l Publica tion. 5, 325-346 . GABRIELSEN, R . H . & STRANDENES . S. 1994 . Dynamic Basin Developmen t - A complet e geoscientifi c tool fo r basi n analysis . Proceedings Worl d Petro leum Congres s 1994 . 13-2 1 GABRIELSEN, R. H., GRUNNALEITE. I. & RASMUSSEN. E. 1997. Cretaceou s an d Tertiar y inversio n i n th e Bj0rn0yrenna Faul t Complex , south-western Barents Sea . Marine an d Petroleum Geologv. 14(2) . 165-178. GABRIELSEN. R . H. , ODINSEN . T . & GRUNNALEITE . I. 1999. Structurin g of th e norther n Vikin g Grabe n and th e Mor e Basin ; th e influenc e o f basemen t structural grain , an d th e particula r rol e o f th e M0re-Tr0ndelag Faul t Complex . Marine an d Petroleum Geology, 16 . 443-465. GABRIELSEN, R. H., STEEL, R. J. & NOTTVEDT. A . 1995. Subtle traps in extensional terranes: A model wit h reference t o th e Nort h Sea . Petroleum Geoscience. 1 , 223-235 . G0LKE. M. , COBLENTZ . S. , CLOETINGH . S . & FEJERS KOV. M . 1995 . Stres s syste m o f th e Norwegia n Continental Margi n - Par t I , In-sit u roc k stres s pattern o n th e Norwegia n continenta l shal f an d mainland. In : FEJERSKOV . M . & MYRVANG . A . (eds) Workshop o n Rock Stresses i n the North Sea. NTU Trondheim . 13-14.02.95 , pp. 250-274. GRUNNALEITE. I . & GABRIELSEN . R . H . 1995 . Th e structure o f the Mor e Basin . Tectonophvsics. 252 . 221-251. JORDT, H. . FALEIDE . J. I. . BJORLYKKE . K. & IBRAHIM, M. T . 1995 . Cenozoic stratigraph y o f th e centra l and norther n Nort h Se a Basin : tectoni c development, sediment distribution and provenance areas. Marine an d Petroleum Geology. 12 , 845-879. LINDHOLM, C . D. , BUNGUM . H. . VILLAGRAN . M . & HICKS, E. 1995. Crustal stress and tectonic s in Norwegian region s determined fro m earthquak e foca l mechanisms. In : FEJERSKOV . M . & MYRVANG . A . (eds) Workshop o n Rock Stresses i n the North Sea. NTU Trondheim , 13-14.02.95 , pp. 77-91 . MELLERE. D . & Steel, R . J . 1996 . Tidal sedimentatio n in Inner Hebrides half-grabens , Scotland : th e midJurassic Bearrerai g Sandston e Formation . /// : D E BATISTE, M . & JACOBS , P . (eds ) Geology o f Siliciclastic Shelf Seas. Geologica l Society . London, Specia l Publications , 117 . 49-79 . NOTTVEDT, A. , GABRIELSEN , R . H . & STEEL . R . J . 1995. Tectonostratigraph y an d sedimentar y archi tecture o f rif t basins , wit h referenc e t o th e northern Nort h Sea . Marine an d Petroleum Geology, 12 , 881-901. PLANKE, S . & ELDHOLM , O . 1994 . Seismi c respons e and constructio n o f seawar d dippin g wedge s o f flood basalts : V0rin g volcanic margin . Journal o f Geophysical Research. 99, 9263-9278. RAVNAS. R . & BONDEVIK . K . 1997 . Architecture an d controls o n th e Bathonian-Kimmeridgia n shal low-marine syn-rif t wedge s o f th e Oseberg-Brag e area, norther n Nort h Sea . Basin Research. 9 . 197-226.

INTEGRATED BASI N STUDIE S RAVNAS, R . & STEEL , R . J . 1997 . Contrasting style s of late Jurassi c syn-rif t turbidit e sedimentation : a comparable stud y o f th e Magnu s an d Oseber g areas, norther n Nort h Sea . Marine an d Petroleum Geology, 14 , 417-449. RAVNAS, R. & STEEL, R. J. 1998 . Architecture of marine rift-basin successions. AAPG Bulletin, 82,110-146. RAVNAS, R. , BONDEVIK , K. , HELLAND-HANSEN , W. , L0MO, L., RYSETH , A . & STEEL, R . J. 19970 . Sedimentation histor y as an indicato r of rif t initiation and development : Th e lat e Bajocian-Bathonia n evolution o f th e Oseberg-Brag e area , norther n North Sea. Norsk Geologisk Tidsskrift, 77,205-232 . RAVNAS, R. , HANSEN , J . W. , MELLERE , D. , NOTT VEDT, A., SJ0BLOM , T. S. , STEEL , R . J . & WlLSON,

R. C . L . 19976 . A marin e lat e Jurassi c syn-rif t succession i n th e Lusitania n Basin , wester n Portugal - tectoni c significanc e o f stratigraphi c signature. Sedimentary Geology, 114 , 237-266 . REEMST, P. , SKOGSEID , J . & LARSEN , B . T. 1996 . Base Pliocene velocit y inversion on th e easter n Vorin g margin - cause s an d implications . Global an d Planetary Change, 12 , 201-211 . SKOGSEID, J . 1994 . Dimension s o f Lat e Cretaceous Paleocene Northeas t Atlantic rif t derived from Cenozoic subsidence. Tectonophysics, 240 , 225-247 . SKOGSEID, J. & ELDHOLM , O . 1995 . Rifted continental margins of f mid-Norway . In : BANDA , E. , TAL WANI, M . & TORNE , M . (eds ) Rifted Ocean. Continent Boundaries. NAT O AS I Series Vol ume, Kluwe r Academic PUBLISHERS , pp. 147-153 . SKOGSEID, J. , ELDHOLM , O . & PLANKE , S . 1994 . Mesozoisk kontinenta l riftin g o g Kenozoisk mar gindannelse: dypseismik k o g skorpestruktu r p a V0ringmarginen. Geonytt, 21, 3-18 . STEEL, R . J . 1993 . Triassic-Jurassi c megasequenc e stratigraphy i n th e norther n Nort h Sea : rif t t o post-rift evolution . In : PARKER , J . R . (ed. ) Petroleum Geology o f Northwest Europe. Geologica l Society, London , 299-315 . STUEVOLD, L . M . & ELDHOLM , O . 1996 . Cenozoi c uplift o f Fennoscandi a inferre d fro m a stud y o f the mid-Norwegia n margin . Global and Planetary Change, 12 , 359-386.

TERVOORDE, M. , RAVNAS , R., F^RSETH, R. B . & CLOE-

tingh, S. 1997. Tectonic modelling of middle Jurassic syn-rift stratigraph y in the Oseberg-Brage area , northern North Sea. Basin Research, 9, 133-150. THERIAULT, P. & STEEL , R . J . 1995 . Aspects o f synrif t sedimentation i n th e Uppe r Jurassi c (Helmsdal e Boulder Beds) of the Inner Moray Firt h Basin . In: STEEL, R . J. , FELT , V . L. , JOHANNESSEN , E . P . & MATHIEU, C . (eds ) Sequence Stratigraphy o n th e Northwest European Margin. Norwegia n Petro leum Societ y Specia l Publication , 5 , 365-387.

Reports Reports liste d herei n ar e publi c an d ca n b e mad e available throug h th e Researc h Counci l o f Norwa y (RCN), Universit y o f Berge n (UB) , Universit y o f Oslo (UO ) an d Norwegia n Technica l Universit y i n Trondheim (NT U = NTH) .

13

FEJERSKOV, M. 1993 . Bergspenninger i Norge o g pa den norske sokkel . In : MYRVANG , A. , JOHANSEN , T. , HANSEN, A . & BERG , K . R . (eds ) Fjellsprengingsteknikk, bergmekanikk, geoteknikk. Osl o 1993 , 17pp. FEJERSKOV, M . 1994 . Breakout a s a tool fo r stress determination i n deep wellbores. NT H Repor t No. 1 , 15pp . FEJERSKOV, M. 1994 . Breakout interpretation. Methods and software used a t NTH. NT H Repor t No . 3 , 14pp. FEJERSKOV, M . 1994 . Breakout identification i n 7 wells near the Troll Field on the eastern flank of the northern Viking Graben. NTU Repor t No . 4, 39pp. FEJERSKOV, M . 1995 . Criteria for breakout identification based o n 4-arm oriented caliper logs. NT U Report No . 2 , 17pp . FEJERSKOV, M . 1995 . Breakout interpretation - 11 wells on the Visund Field, northern Viking Graben. NTU Repor t No . 5 , 39pp . FEJERSKOV, M. , i n pre p 1995 . Breakout interpretation in th e Tampen Spur area. NTU Repor t No . 6 . GLADCZENKO, T . P . & ELDHOLM , O . 1995 . LI P Database: Large Igneous Provinces - distribution and references. Geophys . Res. Group, Dep. Geol. , Univ. Oslo , Compute r Pgm./Databas e Doc . Ser . No. 15 , 5pp . HAMBORG, M. , KYRKJEBO , R . & RAVNAS , R . 1995 . Syn- and post-rift depositional modelling, northern North Sea. IK U repor t xxxxx . MARJANAC, L . T . 1994 . Reference data base for rift basins (syn-rift). Ui B Report , 41pp . NOTTVEDT, A. & IBS-DNM working group 1993 . IB S Module 3 - Dynamics of the Norwegian Margin. First Periodical Report - Project Description, June 1993, 28pp . NOTTVEDT, A . & IBS-DN M workin g grou p 1993 . Minutes of Meeting, IBS-DNM Project Seminar. Geilo, Novembe r 1993 , 57pp . NOTTVEDT, A. & IBS-DNM workin g group 1993 . IB S Module 3 - Dynamics of the Norwegian Margin. Second Periodical Report, December 1993, 24pp . NOTTVEDT, A . & IBS-DN M workin g grou p 1994 . Minutes of Meeting, IBS-DNM Project Seminar. Stavanger, Ma y 1994 , 81pp . NOTTVEDT, A. & IBS-DNM workin g group 1994 . IBSDNM Project Status Report, Ma y 1994 . NOTTVEDT, A. & IBS-DNM working grou p 1994 . IB S Module 3 - Dynamics of the Norwegian Margin. Third Periodical Report, June 1994, 32pp. NOTTVEDT, A. & IBS-DNM workin g group 1994 . IB S Module 3 - Dynamics of the Norwegian Margin. Fourth Periodical Report, December 1994, 49pp.N0TTVEDT, A . & IBS-DN M workin g group 1995 . IBS-DNM Project Status Report, February 1995. Three volumes . NOTTVEDT, A. & IBS-DNM workin g group 1995 . IB S Module 3 — Dynamics of the Norwegian Margin. Fifth Periodical Report, June 1995, 37pp . NOTTVEDT, A. & IBS-DNM workin g group 1994 . IB S Module 3 - Dynamics of the Norwegian Margin. Final Report, December 1995. PLANKE, S . 1993 . HEAT- Heat flo w data base program. Geophys . Res . Group , Dept . Geol. ,

14

A. N0TTVED T

Univ. Oslo , Compute r program/Dat a bas e Doc . Ser. No . 2 version 3.1, 10pp . PLANKE, S . 1993 . VELO - Seismic refraction!wideangle reflection velocity data base program. Geophys. Res . Group , Dept . Geol. , Univ . Oslo , Computer program/Dat a bas e Doc . Ser . No . 1 version 3.1 , 14pp. RAVNAAS, R. . HANSEN . J . W. . MELLERE , D. . NOTT VEDT, A. , SJ0BLOM , T . S . & STEEL , R . J . 1995 .

A marine to continental svn-rift succession: the Kimmeridgien Abadia and Lourinha Formations of the Santa Cruz area, Lusitanian Basin, Portugal. University o f Berge n Report . VANVEEN, P., ' SKJOLD . L . J . & RYSETH , A . E . 1994. A High-Resolution Stratigraphic Framework for the Paleogene i n th e Northern North Sea. Norsk Hydro Repor t R-055419 .

Theses BORGERUD, L . 1995 . Relations between Rock Stresses and Pore Pressure on the Norwegian Margin, 62-67 north - a study based on borehole breakouts. Diplom a thesis , Dep. of Geolog y an d Mineral Resource s Engineering , Norwegian Technical University, Trondheim . FEJERSKOV. M . 1996 . Determination o f in-situ rock stresses related to petroleum activities on the Norwegian continental shelf. Dr . ing . thesis . Department o f Geolog y an d Minera l Resource s Engineering, Norwegia n Technica l University , Trondheim, Norway . 162pp . FINSTAD, A . G . 1995 . Main structural elements o n th e Norwegian continental margin, 68-72"N and adja-

cent lan d areas : tempora l an d spatia l develop ment. Cand . scien t thesis , Dep . o f Geology . University o f Oslo . HOLMSEN. C . 1994 . Vndersokelse av kenoioiske o g mesoioiske sedimenter i Vikinggraben i relasjon til de n tertiare landhevingen. Cand . scient . thesis. Dep. o f Geology . Universit y o f Oslo . IBRAHIM. M . T . 1993 . Post-Jurassic basin fill i n th e northern North Sea. Cand. scien t thesis , Dep. of Geology, Universit y o f Oslo . JORDT, H . 1995 . The Cenoioic geological evolution o f the central and nor them North Sea based on seismicsequence stratigraphy. Dr . scien t thesis . Depart ment o f Geology, Universit y o f Oslo . JORGENSEN. T . 1994 . Determination an d Evaluation o f In-situ Rock Stresses a t th e Snorre Field. Diploma thesis, Dep . of Geolog y an d Minera l Resource s Engineering. Norwegia n Technica l University . Trondheim. KARLBERG. T . 1995 . En geofysisk undersokelse a v Hovgaardryggen. Cand . scien t thesis . Dep . o f Geology. Universit y o f Oslo . RAVNAS. R . 1996 . Variability o f syn-rift sedimentary architecture in marine rift-basins: examples from the middle-late Jurassic of the northern liking Graben, North Sea, and the Lusitanian Basin, western Portugal. Dr . scient . thesis . Department of Geology . Universit y o f Bergen . 210pp. SVARE, E . 1995 . Relations between Rock Stresses an d Pore Pressure on the Norwegian Margin, 62 67 north - a study based on leak-off tests and formation pressure. Diplom a thesis . Dep . of Geolog y and Minera l Resource s Engineering . Norwegian Technical University , Trondheim.

Crustal structur e i n the norther n North Sea : a n integrated geophysical stud y P. CHRISTIANSSON, 1 2 J . I . FALEIDE 1 & A . M . BERGE 3 1

2

Department of Geology, P.O. Box 1047 Blindern, N-0316 Oslo, Norway Present address: Norsk Hydro ASA, P.O. Box 200, 1321 Stabekk, Norway 3 Norsk Hydro Research Centre, P.O. Box 646, 5020 Sandsli, Norway Abstract: Thi s study focuses o n the deep structure o f the Viking Graben and adjacen t area s of th e norther n Nort h Se a (60-62°N) , an d it s implication s fo r th e amount , timin g an d nature of lithospheric extension. Two regiona l transects have been constructed base d o n a n integrated analysis of deep seismic reflection and refractio n data , gravity and magnetic data, and correlation s betwee n offshore an d onshor e geology. The shallow interpretation is based on high-qualit y conventional seismi c reflectio n dat a calibrate d agains t a larg e numbe r o f exploration wells . Th e ne w and partl y reprocesse d seismi c data , combine d wit h th e othe r geophysical data, make possible a better documentation of the crustal configuration, such as the pre-Jurassi c sedimen t distribution , basement an d Moh o relief , an d dee p faul t geom etries. A lower-crusta l bod y characterize d b y a n 8+kms" 1 velocit y an d a n averag e bul k density of 2.95 gcm~3 is present beneath the Horda Platform. This body probably represents a deep crustal root of partially eclogitized rocks that formed during the Caledonian orogeny. Heterogeneities withi n this bod y giv e rise to th e non-typica l velocity-density relation . Th e crust-mantle boundary is located a t th e bas e o f this body a t a depth o f 30-35 km and doe s not coincid e with the seismically defined Moho . Th e geometry of crustal thinning reflects th e cumulative effec t o f severa l post-Caledonian rif t phases . Result s sho w tha t Permia n riftin g affected a wid e area, fro m th e 0ygarde n Faul t Comple x t o th e Hutto n Fault .

Deep seismi c reflectio n dat a hav e forme d th e basis fo r numerou s paper s throug h th e las t decade, focusin g o n th e crusta l structur e an d basin evolutio n i n th e norther n Nort h Se a rif t system. However , poo r dat a quality , especiall y the lo w S/ N rati o a t depth , ha s le d t o man y model-driven interpretations. The NSDP84-line s were firs t describe d b y Gibb s (1987 10 km is indicated along Transect 1 (Fig. 11) . Th e crus t i s thickest i n th e east, clos e t o th e Norwegian coast , an d become s considerably thinne r west of the 0ygarden Faul t Zone. Th e Hord a Platfor m wa s relativel y littl e affected b y th e Lat e Jurassi c rifting , s o mos t o f this thinnin g mus t b e relate d t o Permia n an d earlier (Devonian? ) stretching . Thus , th e Per mian riftin g affecte d a wider area i n the norther n North Se a than did the Jurassic event. Transect 1 clearly show s tha t th e Permia n stretchin g mus t have bee n extensive , affectin g a n are a fro m th e 0ygarden Faul t Comple x t o the Hutton Faul t in the Eas t Shetlan d Basi n (Fig s 4 an d 11) . Thi s is supported b y correlatio n acros s pre-Jurassi c faults wit h mor e tha n 3 km o f vertica l displacement in some cases, and a basin much wider than the presen t Vikin g Grabe n underlai n b y a thinned crystallin e crust . However , whe n esti mating th e widt h o f th e Permia n rif t w e mus t correct fo r th e crusta l stretchin g relate d t o th e younger (Lat e Jurassic) event. Crustal thinnin g along Transec t 2 follow s th e same patter n a s fo r Transec t 1 , wit h maximu m thinning o f th e crus t unde r th e presen t Vikin g Graben (Fig . 11) . However , th e pre-Jurassi c basin geometr y i s her e uncertain . Ther e i s hardly roo m fo r bot h a thic k Permo-Triassi c

CRUSTAL STRUCTUR E I N TH E NORTHER N NORT H SE A and Devonian-?Carboniferous sequence beneath the deeply burie d Jurassi c strata . A thick Devo nian sequenc e i s probably presen t i n vie w o f it s existence bot h o n th e Eas t Shetlan d Platfor m in th e wes t an d th e Hord a Platfor m i n th e east. I f th e dee p basi n fil l consist s mainl y o f Devonian-?Carboniferous strata , th e main par t

35

of th e Permia n rif t basi n i s located furthe r east , on th e Hord a Platform . However , Permia n movements o n th e 'Wester n Margi n Fault ' (Figs 4 and 11 ) cannot b e ruled out . Another pronounce d differenc e i n crusta l structure betwee n the tw o transect s (Fig . 11 ) i s the shif t i n polarit y o f som e o f th e Lat e

Fig. 13 . (3 curves fo r crusta l Transects 1 and 2 . 1 , From crusta l thinnin g assuming an initia l crustal thicknes s of 36km; 2 , from revers e modelling (Transec t 1 Fjeldskaar e t al. 1999 ; Transect 2 , Christiansson 2000) ; 3, fro m forwar d modelling (Odinse n e t al . 19996) .

36

P. CHRISTIANSSON , J . I . FALEID E & A . M . BERG E

Palaeozoic rif t structure s beneat h th e easter n Horda Platform . Th e polarit y shif t i s probabl y connected t o transfe r faults running in east-west direction. Furthermore , Lat e Jurassi c riftin g appears t o b e more focuse d o n Transec t 2 compared wit h Transect 1 which gav e ris e t o a very thick sequenc e o f Uppe r Jurassi c strat a i n th e Viking Grabe n aroun d 60° N (Fig s 4 an d 11) . Most faul t geometrie s see m t o b e planar , although som e fault s sho w listri c geometrie s a t depth. Th e Western Margi n Fault , confinin g th e Viking Grabe n t o th e wes t o n Transec t 2 show similarities i n curvatur e an d di p t o th e Hutto n Fault an d it s deeper continuatio n o n Transec t 1 (Figs 4 and 11) . ,/3-curves derive d fro m crusta l thinnin g hav e been calculated fo r the two transects assumin g a constant initia l crustal thicknes s of 36 km before Permian riftin g (Fig . 13) . Ther e are , however , many uncertaintie s related t o thes e curves . Th e uncertainties i n identificatio n o f th e Moh o an d top crystallin e basement hav e bee n discusse d in previous chapters . The Moh o is identified along most o f the transects bu t uncertaintie s exis t with respect t o velocitie s use d i n th e dept h conver sion. To p crystallin e basemen t i s uncertai n particularly beneat h th e Vikin g Graben. W e d o not have control on the distribution of Devonian strata al l along th e transects . Therefore w e used the thicknes s of th e crystallin e part o f th e crust in ou r estimates . Strictl y speakin g thi s i s no t correct becaus e th e Devonian strat a shoul d hav e been considere d a s part o f the crus t involve d in Permian an d younge r stretching . Furthermore , the assumptio n o f a unifor m crus t o f constan t thickness i s questionable . Latera l thicknes s variations wer e probabl y presen t followin g th e extensional collapse of the Caledonides. We used the present-da y thicknes s o f 36k m eas t o f th e 0ygarden Faul t Zon e a s th e initia l crusta l thickness assumin g tha t Permia n an d younge r stretching di d no t affec t thi s area . Thi s i s no t quite true , a s w e know tha t structure s onshor e were reactivate d i n Permia n an d Jurassi c time s (Torsvik e t ai 1992) . All thes e uncertaintie s must b e kep t i n min d when analysing the /3 curves derived from crustal thinning. Despit e th e uncertainties , th e curve s reflect th e cumulativ e effec t o f severa l post Caledonian rif t events . I n Fig . 1 3 we compar e our j3 value s wit h thos e obtaine d b y forwar d modelling (Odinse n e t al. \999b) an d revers e modelling (Christiansson 2000 ; Fjeldskaa r e t al . 1999) of both transects . The curves are similar in shape bu t th e modelle d value s ar e sys tematically lowe r tha n thos e estimate d fro m crustal thinning . A carefu l analysi s o f th e vari ous /3 curve s i s beyon d th e scop e o f thi s paper .

For detail s an d discussion s w e therefore refe r t o the modellin g papers cite d above .

Summary an d conclusions Two regiona l transect s acros s th e norther n North Se a hav e bee n constructe d base d o n a n integrated analysis of deep seismic reflection an d refraction data , a s wel l a s gravit y and magnetic data. Th e shallo w part s ar e base d o n high quality conventiona l seismi c reflectio n dat a calibrated agains t a large number o f exploration wells. Th e ne w an d partl y reprocesse d seismi c data, combine d wit h the other geophysica l data, make possibl e a bette r documentatio n o f th e crustal configuration , such a s th e pre-Jurassi c sediment distribution, basement and Moho relief , and dee p faul t geometry . A lower-crusta l bod y characterize d b y a n 8+kms" 1 velocit y and a n averag e bul k density of 2.95gem" 3 i s presen t beneat h th e Hord a Platform. Thi s bod y probabl y represen t a dee p crustal roo t o f partially eclogitized rocks, which formed i n respons e t o Caledonia n collisio n and crustal thickenin g followed b y extensiona l collapse o f th e orogen . Th e non-typica l velocity density relatio n withi n th e lower-crusta l bod y may b e explaine d b y heterogeneitie s relate d t o the distributio n o f eclogites , whic h cause a bia s in th e seismi c velocitie s measure d fro m wide angle data. The crust-mantle boundar y is located at th e bas e o f thi s bod y an d doe s no t coincid e with th e seismically defined Moh o a t th e top . The geometr y o f crusta l thinnin g reflects th e cumulative effec t o f severa l post-Caledonia n rift phases . Stretchin g estimate s base d o n crus tal thinnin g have bee n compare d wit h estimates based o n othe r technique s suc h a s subsi dence analysis and forwar d tectonostratigraphic modelling. The interpretatio n presented her e woul d no t have bee n possibl e withou t th e integratio n o f various geologica l an d geophysica l dat a fro m both offshor e an d onshor e areas . This wor k wa s funde d b y th e Commissio n o f th e European Union and the Norwegian Research Council in th e framework o f the DGXII - Joul e Programme, sub-programme: Energ y fro m fossi l sources : Hydrocarbons, Integrate d Basin Studie s - Th e Dynamics o f the Norwegia n Margin . Nors k Hydr o ASA , Sag a Petroleum a.s . an d Statoi l a.s . provide d data fo r thi s study. W e ar e especiall y indebte d to J . E . Lie , who reprocessed th e NSD P lines. The paper also benefite d from discussion s with T . Andersen , P. T. Osmundsen, H. Austrheim , S . Plank e and J . Skogseid . The clos e co-operation withi n th e IB S project, particularly wit h R. Gabrielsen , A. N0ttved t and T . Odinsen , is highly

CRUSTAL STRUCTUR E I N TH E NORTHER N NORT H SE A appreciated. W e woul d lik e t o than k N . Chamote Rooke, Y . Kristofferse n an d C . Huric h fo r thoroug h and constructive review s of the manuscript. G . Farro w and S . Thompson improve d th e English .

References AKI, K . & RICHARDS , P . G . 1980 . Quantitative Seismology, Theory an d Methods, volume 1. Freeman, Sa n Francisco . ALLEN, P. A. 1981. Devonian lak e margin environments and processes , S E Shetland , Scotland . Journal o f the Geological Society, London, 138, 1-14 . ANDERSEN, T . B. , JAMTVEIT , B. , DEWEY , J . F . & SWENSSON, E . 1991 . Subductio n an d eductio n o f continental crust : majo r mechanis m durin g con tinent-continent collisio n an d orogeni c exten sional collapse , a mode l base d o n th e sout h Norwegian Caledonides . Terra Nova, 3, 303-310. AUSTRHEIM, H . 1987 . Eclogitizatio n o f lowe r crusta l granulites b y fluid s migratio n throug h shea r zones. Earth an d Planetary Science Letters, 81 , 221-232. 1990. Th e granulite-eclogit e facie s transition : a comparison o f experimenta l wor k an d a natura l occurrence i n th e Berge n Arcs , wester n Norway . Lithos, 25, 163-169 . 1994. Eclogitization o f the deep crust in continent collision zones. Comptes Rendus de I'Academic de s Sciences, Paris, 319 , 761-774 . & M0RK , M . B . E . 1988 . Th e lowe r continenta l crust o f the Caledonian mountai n chain : evidenc e from forme r dee p crusta l section s i n wester n Norway. In : KRISTOFFERSEN , Y . (ed. ) Progress in Studies o f th e Lithosphere i n Norway. Norge s Geologiske Unders0kels e Specia l Publication , 3 , 102-113. , ERAMBERT , M. & ENGVIK , A . K . 1997 . Proces sing o f crus t i n th e roo t o f th e Caledonia n continental collisio n zone : th e rol e o f eclogitization. Tectonophysics, 273 , 129-154 . BADLEY, M . E. , PRICE , J . D. , RAMBEC H DAHL , C . & AGDESTEIN, T . 1988 . Th e structura l evolutio n o f the norther n Nort h Vikin g Grabe n an d it s bear ing upon extensiona l modes o f graben formation . Journal o f th e Geological Society, London, 145 , 455-472. BARTON, P . & WOOD , R . 1984 . Tectonic evolutio n o f the Nort h Se a basin : crusta l stretchin g an d subsidence. Geophysical Journal o f th e Royal Astronomical Society, 79 , 987-1022. BEACH, A . 1985 . Som e comment s o n sedimentar y basin developmen t i n th e norther n Nort h Sea . Scottish Journal o f Geology, 21, 493-512. 1986. A dee p seismi c reflectio n profil e acros s th e northern Nort h Sea . Nature, 323 , 53-55 . , BIRD, T . & GIBBS, A. 1987 . Extensional tectonic s and crusta l structure : dee p seismi c reflection data from th e norther n Nort h Se a Viking Graben. In : COWARD, M . P. , DEWEY , J . F . & HANCOCK, P . L . (eds) Continental Extensional Tectonics. Geologi cal Society , London , Specia l Publications , 28 , 467-476.

37

BLUNDELL, D . J . 1990 . Seismi c image s o f continenta l lithosphere. Journal o f th e Geological Society, London, 147 , 895-913 . BOUNDY, T . M. , FOUNTAIN , D . M . & AUSTRHEIM , H . 1992. Structura l development an d petrofabric s o f eclogite facie s shea r zones , Berge n Arcs , wester n Norway: implication for deep crustal deformatio n processes. Journal o f Metamorphic Geology, 10 , 127-146. BRITTAN, J . & WARNER , M . 1996 . Wide-angle seismi c velocities i n heterogeneou s crust . Geophysical Journal International, 129 , 269-280 . BRUN, J . P . & TRON , V . 1993 . Developmen t o f th e North Vikin g Graben: inference s from laborator y modelling. Sedimentary Geology, 86, 31-51. CHRISTENSEN, N . I . & MOONEY , W . D . 1995 . Seismic velocity structur e an d compositio n o f th e con tinental crust : a globa l view . Journal o f Geophysical Research, 100 , 9761-9788 . CHRISTIANSSON, P . 2000 . Crustal structure an d basin evolution o f th e Northern North Sea. D r Scien t Thesis, Universit y o f Oslo. , BERGE, A. M . & FALEIDE, J. I. 19990 . Analysis o f Expanded Spread Profiles in the northern North Sea. Departmen t of Geology, Universit y o f Oslo , Internal Publication , 68 . DEEMER, S . J. & HURICH , C . A . 1991 . Comparison o f coincident high-resolutio n wide-apertur e an d CDP profilin g alon g th e southwes t coas t o f Norway. In : Continental Lithosphere: Deep Seismic Reflections. America n Geophysica l Union , Geodynamic Series , 22, 435-442. DIEBOLD, J . B . & STOFFA , P . L . 1981 . Th e trave l equation, tau-; ? mappin g an d th e inversio n o f common midpoin t data . Geophysics, 46, 238-254. DITCHA, E . 1998 . Pre-Tertiary evolution o f th e Stord Basin, northern North Sea. Cand . scient . thesis , University o f Oslo . DIXON, J . E. , FITTON , J . G . & FROST , R . T . C . 1981 . The tectoni c significanc e o f post-Carboniferou s igneous activit y i n th e Nort h Se a Basin . In : Petroleum Geology of the Continental Shelf. Heyden, London , 121-123 . DUNCAN, W . I . & BUXTON , N . W . K . 1995 . Ne w evidence fo r evaporiti c Middl e Devonia n lacus trine sediments with hydrocarbon sourc e potential on the East Shetlan d Platform, North Sea . Journal of th e Geological Society, London, 152 , 251-258 . ELDHOLM, O . & MUTTER , J . C . 1986 . Basi n structur e on th e Norwegia n margi n fro m analysi s o f digitally recorde d sonobuoys . Journal o f Geophysical Research, 91 , 3763-3783. EWING, J. 1963 . Elementary theory of seismic reflection and refraction measurements. In: HILL, M. N. (ed.) The Sea. Wiley-Interscience, New York, 3-19 . FICHLER, C . & HOSPERS , J . 1990 . Dee p crusta l structure o f th e norther n Nort h Vikin g Graben : results fro m dee p reflectio n seismi c an d gravit y data. Tectonophysics, 178 , 241-254 . FJELDSKAAR, W. , JOHANSEN , H. , TE R VOORDE , M. , CHRISTIANSSON, P . & FALEIDE , J . I . 1999 . Modelling o f crusta l thinnin g an d th e effect s o f possible intr a plat e stres s i n th e norther n Vikin g Graben. In : Basin Modelling. AAP G Memoir .

38


FLINN, D. 1985 . The Caledonide s o f Shetland. In: GEE , D. G . & STURT , B . A . (eds ) Th e Caledonide Orogen - Scandinavia an d Related Areas. Wiley , Chichester. FOSSEN, H . 1992 . The rol e o f extensiona l tectonics i n the Caledonide s o f sout h Norway . Journal o f Structural Geology, 14 , 1033-1046 . FÊRSETH, R . B. , GABRIELSEN , R. H . & HURICH , C . A . 1995. Influenc e of basemen t i n structurin g o f th e North Se a Basin , offshor e southwes t Norway . Norsk Geologisk Tidsskrift, 75 , 105-119 . GABRIELSEN, R . H . 1986 . Structura l element s i n gra ben system s an d thei r influenc e o n hydrocarbo n trap types . In: SPENCER , A. M . e t al. (eds) Habitat of Hydrocarbons on the Norwegian Continental Shelf. Graha m an d Trotman , London , 55-60 . , F^RSETH , R. , STEEL , R . J. , IDIL , S . & KLOVJAN , O. S . 1990 . Architectural styles of basinfil l i n th e northern Vikin g Graben. In : BLUNDELL . D . J . & GIBBS. A . D . (eds ) Tectonic Evolution o f th e North Sea Rifts. Clarendon , Oxford , 158-179 . GIBBS, A . 1987r/ . Linke d tectonic s o f th e norther n North Se a basin . In: BEAUMONT , C . & TANKARD , A. J . (eds ) Sedimentary Basins an d Basin Formation Mechanisms. Canadia n Societ y of Petroleu m Geologists, Memoir , 12 , 163-171 . 19876. Dee p seismi c profile s i n th e norther n North Sea . In : BROOKS , J . & GLENNIE . K . (eds ) Petroleum Geology of North West Europe. Graham an d Trotman , London , 1025-1028 . GILTNER, J . P . 1987 . Applicatio n o f extensiona l models t o th e norther n Vikin g Graben . Norsk Geologisk Tidsskrift, 67 , 339-352 . GLENNIE, K . W . 1984 . The structura l framewor k an d the pre-Permia n histor y o f th e Nort h Se a Area . ///: GLENNIE , K . W . (ed. ) Introduction t o th e Petroleum Geology o f th e North Sea, Blackwell Scientific, Oxford , 17-39 . GRIFFIN. W . L . & O'REILLY , S . Y . 1987 . I s th e continental Moh o th e crust-mantl e boundary ? Geology, 15 , 241-244. HANCOCK, J. M. 1990 . Cretaceous. In : GLENNIE, K . W . (ed.) Introduction t o th e Petroleum Geology o f th e North Sea. Blackwel l Scientific, Oxford , 61-73 . HARRISON, M . G . 1987 . A n interpretation o f deep seismic reflection profiles in the northern North Sea: implications for extension models. MS c thesis. University o f London , Roya l Hollowa y and Bed ford Ne w College . HARTZ, E . & ANDRESEN , A . 1995 . Caledonia n sol e thrust of East Greenland: a crustal scale Devonia n extensional detachment? Geology, 23 , 637-640. HEGNA. S . 1989 . Kinematiske metoder for bestemmelse av hastigheter fra marin-seismiske data med anvendelse i ostlige deler a v Loppahoyden. Cand . scient. thesis , Universit y of Oslo. HOLLIGER, K . 1987 . Crustal structure o f th e North Sea. Diploma thesis , ET H Zurich . & KLEMPERER , S . 1989 . A compariso n o f th e Moho interprete d fro m gravit y dat a an d fro m deep seismi c reflection dat a i n the northern Nort h Sea. Geophysical Journal, 97 , 247-258 . HOLLOWAY, S. . REAY , D . M. , DONATO , J . A . & BEDDOE-STEPHENS, B . 1991 . Distribution o f gran-

ite and possibl e Devonian sediment s in part o f the East Shetlan d Platform , North Sea . Journal o f the Geological Society, London, 148 , 635-638 . HOSPERS, J . & EDIRIWEERA , K. W . 1988 . Mappin g o f the to p o f th e crystallin e continental crust i n th e Viking Graben area , Nort h Sea . /// : KRISTOFFER SEN. Y . (ed. ) Progress i n Studies o f th e Lithospheres i n Norway. Norge s Geologisk e Under sokelse, Specia l Publication , 3 , 21-28. & 1991 . Dept h an d configuratio n o f th e crystalline basemen t i n th e Vikin g Graben area , northern Nort h Sea . Journal o f th e Geological Society, London, 148 , 261-265 . HOSSACK, J . R . 1984 . The geometr y o f listri c growth faults i n th e Devonia n basin s o f Sunnfjord , wes t Norway. Journal o f th e Geological Society, London, 141 , 629-638. ' HURICH, C . A . 1996 . Kinematic evolution of the lower plate during intracontinental subduction: an example from the Scandinavian Caledonides. Tectonics. 15,1248-1263. & KRISTOFFERSEN , Y . 1988 . Dee p structur e o f the Caledonid e oroge n i n souther n Norway : new evidenc e fro m marin e seismi c reflectio n profiling. In : KRISTOFFERSEN . Y . (ed. ) Progress in Studies o f th e Lithosphere i n Norway. Norge s Geologiske Undersokelse . Specia l Publication . 3 . 96-101. HYNES, A . & SNYDER . D . B . 1991 . Deep-crusta l mineral assemblage s an d potentia l fo r crusta l rocks belo w th e Moh o i n th e Scottis h Caledo nides. Geophysical Journal International. 123 . 323-339. IWAZAKI. T. , SELLEVOLL . M . A. . KANAZAWA . T. . VEGGELAND, T. & SHIMAMURAH, H. 1994 . Seismic refraction crusta l stud y alon g th e Sognefjord . south-west Norway , employin g ocean-botto m seismometers. Geophysical Journal International. 119,791-808. JACKSON, H . R. , FALEIDE . J . I . & ELDHOLM . O . 1990 . Crustal structur e o f th e sheare d southwester n Barents Se a continental margin. Marine Geology. 93, 119-146 . JOHNSON, H. , RICHARDS , P . C. , LONG . D. & GRAHAM . C. C . 1993 . United Kingdom Offshore Regional Report: the Geology of the Northern North Sea. British Geologica l Survey . HMSO. London . JORDT, H. , FALEIDE , J . L , BJORLYKKE . K . & IBRAHIM . M. T. 1995 . Cenozoic sequenc e stratigraph y o f the Central an d Norther n Nort h Se a basin. Tectoni c development, sedimen t distributio n an d prove nance areas . Marine and Petroleum Geology. 12. 845-879. ,, THYBERG , B . I . & NOTTVEDT , A . 1999 . Cenozoic evolutio n o f th e centra l an d norther n North Se a wit h focu s o n differentia l vertica l movements o f th e basi n floo r an d surroundin g clastic sourc e areas . This volume. KITTEROD, N.-O . 1986 . Hastighets variasjoner me d dypet, tolkningsprosedyrer for ESP og sonarboye data. Cand . scient . thesis. Universit y of Oslo . KLEMPERER, S . L . 1988 . Crustal thinnin g and natur e of extension in the norther n North Se a from dee p seismic reflectio n profiling . Tectonics, 1, 803-821.

CRUSTAL STRUCTUR E I N TH E NORTHER N NORT H SE A & HOBBS , R . 1991 . Deep Seismic Reflection Profiles around the British Isles, the BIRPS Atlas. Cambridge Universit y Press, Cambridge . & HURICH, C . A. 1990 . Lithospheri c structur e o f the Nort h Se a fro m dee p seismi c reflectio n profiling. In : BLUNDELL , D . J . & GIBBS , A . (eds ) Tectonic Evolution o f th e North Se a Rifts. Oxfor d University Press, Oxford , 37-6 3 & WHITE , N . 1989 . Coaxia l stretchin g o r litho spheric simpl e shea r i n th e Nort h Sea ? Evidenc e from dee p seismi c profilin g an d subsidence . In : TANKARD, A . J . & BALKWILL , H . (eds ) Extensional Tectonics and Stratigraphy of the North Atlantic Margins. America n Associatio n o f Pet roleum Geologists , Memoir , 46 , 511-522. KUSZNIR, N . J. & MATTHEWS, D. H. 1988 . Deep seismi c reflections an d th e deformationa l mechanis m o f the continenta l lithosphere . Journal o f Petrology, Special Lithosphere Issue , 66-87. & ZIEGLER , P . A . 1992 . Th e mechanis m o f continental extensio n an d sedimentar y basin for mation: a simpl e shear/pur e shea r flexural cantilever model . Tectonophysics, 215 , 117-131 . , MARSDEN , G . & EGAN , S . S . 1991 . A flexura l cantilever simpl e shear/pur e shea r mode l o f continental lithospher e extension : applicatio n t o the Jeann e d'Ar c basi n an d Vikin g Graben . In : ROBERTS, A . M. , YIELDING , G . & FREEMAN , B. (eds) Th e Geometry o f Normal Faults. Geologi cal Society , London , Specia l Publications , 56 , 41-60. , ROBERTS , A . M . & MORLEY , C . 1995 . Forwar d and revers e modelling o f rif t basi n formation . In: LAMBIASE, J . (ed. ) Hydrocarbon Habitat i n Rift Basins. Geologica l Society , London , Specia l Pub lications, 80 , 33-56. LEPICHON, X. , EWING , J . & HOUTZ , R . E . 1968 . Deep-sea sedimen t velocit y determinatio n mad e while reflectio n profiling . Journal o f Geophysical Research, 73, 2597-2614. LERVIK, K . S. , SPENCER , A . M . & WARRINGTON , G . 1989. Outline of Triassic stratigraphy and structur e in central an d northern Nort h Sea. In: COLLINSON, J. D. (ed.) Correlation in Hydrocarbon Exploration. Graham and Trotman, London, 173-189 . McGEARY, S . & WARNER , M . 1985 . Seismic profilin g the continental lithosphere . Nature, 317, 795-797 . MCKENZIE, D . P . 1978 . Som e remark s o n th e devel opment o f sedimentar y basins . Earth an d Planetary Science Letters, 40, 25-40. MENGEL, K . & KERN , H . 1992 . Evolutio n o f th e petrological and seismic Moho - implication s for the continenta l crust-mantl e boundary . Terra Nova, 4 , 109-116 . NORTON, M . 1986 . Lat e Caledonia n extensio n i n western Norway : a respons e t o extrem e crusta l thickening. Tectonics, 5, 195-204 . 1987. The Nordfjord-Sogn Detachment , W . Nor way. Norsk Geologisk Tidsskrift, 67 , 93-106. N0TTVEDT, A. , GABRIELSEN , R . H . & STEEL , R . J . 1995. Tectonostratigraphy an d sedimentar y architecture o f rif t basins ; with reference to th e north ern Nort h Sea . Marine an d Petroleum Geology, 12, 881-901.

39

ODINSEN, T. , CHRISTIANSSON , P., GABRIELSEN , R . H. , FALEIDE, J . I . & BERGE, A . M . 19990 . The geom etries an d dee p structur e o f th e norther n Nort h Sea rif t system . This volume. , REEMST , P. , VA N DER BEEK, P. , FALEIDE , J . I . & GABRIELSEN, R . H . 1999/7 . Permo-Triassi c an d Jurassic extensio n i n th e norther n Nort h Sea : results fro m tectonostratigraphi c forwar d model ling. This volume. OLAFSSON, I. , SUNDVOR, E., ELDHOLM, O. & GRUE, K . 1992. M0r e Margin : crusta l structure s fro m analysis of expande d sprea d profiles . Marine Geophysical Research, 14 , 137-162 . OSMUNDSEN, P . T . 1995 . Late-orogenic structural geology and Devonian basin formation in western Norway: a study from the hanging wall of the Nordfjord-Sogn detachment in the Sunnfjord region. Dr . scient . thesis, Universit y of Oslo . PINET, B . 1989 . Dee p seismi c profilin g an d sedimen tary basins . Bulletin d e l a Societe Geologique d e France, 8, 749-766. PLATT, N . H . 1995 . Structure s an d tectonic s o f th e northern Nort h Sea : ne w insight s fro m dee p penetration regiona l seismi c data . In : LAMBIASE , J. J . (ed. ) Hydrocarbon Habitat i n Rift Basins. Geological Society, London, Special Publications, 80, 103-113 . RESTON, T . J . 1990 . Shea r i n th e lowe r crus t durin g extension, not s o pure and simple. Tectonophysics, 173, 175-183 . SCOTT, D . L . & ROSENDAHL, B. R. 1989 . North Viking Graben: a n Eas t Africa n perspective . AAPG Bulletin, 73 , 155-165 . SELLEVOLL, M . & WARRICK , R . E . 1971 . A refractio n study of the crustal structure in southern Norway. Bulletin of the Seismological Society of America, 61, 457-471. SERANNE, M . 1992 . Devonia n extensiona l tectonic s versus Carboniferou s inversio n i n th e norther n Orcadian basin . Journal of th e Geological Society, London, 149 , 27-37 . & SEGURET , M . 1987 . Th e Devonia n basin s o f western Norway : tectonic s an d kinematic s o f a n extending crust . In: COWARD, M . P. , DEWEY , J . F . & HANCOCK , P . L . (eds ) Continental Extensional Tectonics. Geologica l Society , London , Specia l Publications, 28 , 537-548. STEEL, R . J. 1993 . Triassic-Jurassic megasequence stra tigraphy in the northern Nort h Sea: rift to post-rif t evolution. In : PARKER, J. R . (ed. ) Petroleum Geology of Northwest Europe. Proceedings of the 4th Conference. Geologica l Society , London, 299-315 . & RYSETH , A . 1990 . The Triassic-earl y Jurassi c succession i n th e norther n Nort h Sea : megase quence stratigraph y an d intra-Triassi c tectonics . In: HARDMAN , R . F . P . & BROOKS , J . (eds ) Tectonic Events Responsible for Britain's Oil and Gas Reserves. Geologica l Society , London , Spe cial Publications , 55 , 139-168 . , SIEDLECKA , A . & ROBERTS , D . 1985 . The Ol d Red Sandston e basin s o f Norwa y an d thei r deformation: a review . In : GEE, D. G . & STURT , B. A . (eds) Th e Caledonide Orogen - Scandinavia and Related Areas. Wiley , New York , 293-315 .

40


STEINHART, J . S . 1967 . Mohorovicic discontinuity . In: RUNCORN, K . (ed. ) International Dictionary o f Geophysics, Vol. 2. Pergamon, Oxford , 991-994 . STOFFA, P . L . & BUHL, P. 1979 . Two shi p multichannel seismic experiment s fo r dee p crusta l studies ; expanded sprea d an d constan t offse t profiles . Journal o f Geophysical Research, 84, 7645-7660 . ,, DIEBOLD , J . B . & WENTZEL , F . 1981 . Direct mappin g o f seismi c data i n th e domai n o f intercept tim e an d ray parameter - a plan e wave decomposition. Geophysics, 45 , 255-267. TORSVIK, T . H. , ANDERSEN , T . A. , EIDE , E . A . & WALDERHAUG, H . J . 1997 . Th e ag e an d tectoni c significance o f dolerit e dyke s i n western Norway . Journal o f th e Geological Societv, London, 154 , 961-973. , STURT, B . A., SWENSSON , E., ANDERSEN, T. B . & DEWEY, J . F . 1992 . Paleomagnetic datin g o f faul t rocks: evidenc e for Permia n an d Mesozoi c move ments and brittl e deformation alon g the Dalsfjord Fault, wes t Norway . Geophysical Journal International, 109 , 565-580 . UNDERBILL, J . R . & PARTINGTON , M . A . 1993 . Jurassic therma l domin g an d deflatio n i n th e Nort h Sea: implication s o f th e sequenc e stratigraphi c -

evidence. In : PARKER , J . R . (ed. ) Petroleum Geology of Northwest Europe. Proceeding of the 4t h Conference. Geologica l Society . London . 337-345. WERNICKE, B . 1985 . Uniform-sens e norma l simpl e shear o f th e continenta l lithosphere . Canadian Journal o f Earth Science, 22 , 108-125 . WHITE, N . 1989 . Natur e o f lithospheri c extensio n in the Nort h Sea . Geology. 17 . 111-114 . 1990. Doe s th e unifor m stretchin g mode l wor k in th e Nort h Sea ? In : BLUNDELL . D . J . & GIBBS . A. D . (eds ) Tectonic Evolution o f th e North Se a Rifts. Oxfor d Universit y Press. Oxford , 217-239 . & McKENZiE , D . P . 1988 . Formatio n o f th e 'steer's head ' geometr y o f sedimentar y basin s b y differential stretchin g o f th e crus t an d mantle . Geology, 16 , 250-253. ZERVOS, F . A . 1987 . A compilatio n an d regiona l interpretation o f th e norther n Nort h Se a gravity map. In : COWARD , M . P. , DEWEY , J . F . & HANCOCK, P . L . (eds ) Continental Extensional Tectonics. Geologica l Society , London , Specia l Publications, 28 , 477-493. ZIEGLER, P . A . 1982 . Geological Atlas o f Western an d Central Europe. Shel l International, Th e Hague .

The geometries and deep structure of the northern Nort h Sea rif t system TORE ODINSEN, 1'4 PETE R CHRISTIANSSON, 2'5 RO Y H . GABRIELSEN, JAN ING E FALEIDE 1

2

& ANKE R M . BERGE

1

3

Department of Geology, University of Bergen, Allegaten 41, N-5007 Bergen, Norway 2

Department of Geology, University of Oslo, P.O. Box 1047 Blindern, N-0316 Oslo, Norway 3

Norsk Hydro Research Centre, N-5020 Bergen, Norway 4

5

Present address: Statoil, N-5020 Bergen, Norway

Present address: Norsk Hydro ASA, P.O Box 200, N-1321 Stabekh, Norway Abstract: Th e enormou s quantit y o f commercia l reflectio n seismi c line s across th e Nort h Sea Basin have made th e area on e of the most thoroughl y studied continental setting s in the world. Furthe r insight in the deep architectur e of the crust is provided by c. 10000km deep reflection seismi c data . Unfortunately , thes e uniqu e database s hav e rarel y bee n combine d systematically to constrain possible tectonic models for the area. This paper i s built on a ful l integration o f high-qualit y commercia l line s (7stwt ) an d th e dee p (15stwt ) NSDP84- 1 and - 2 lines . Th e dee p line s hav e bee n post-stac k reprocesse d an d depth-converted . A numbe r o f dee p well s hav e provide d stratigraphi c contro l alon g th e lines . The overal l reflective pattern i n the lines divides the crust i n three, wit h a reflective upper an d lower crus t separated b y a les s reflectiv e middl e crust . Th e latera l change s i n reflectivit y matche s th e observed variatio n i n crusta l thickness , wher e th e thinnes t crust coincide s wit h th e Viking Graben are a wit h a tota l crusta l thicknes s o f 21-2 4 km, increasin g t o 30-3 6 km i n th e platform areas . Th e lowe r crust i s seen as a n undulatin g 4-1 Okm thic k ban d wit h shallow dipping reflections , with a Moh o tha t consist s o f reflection s with variable lateral thickness and amplitude , rathe r tha n on e single stron g reflection . The structura l analysi s show s tha t the crus t i s cut b y a numbe r o f larg e norma l fault s wit h varying geometries. I t i s assume d that som e o f thes e majo r fault s ar e long-live d feature s roote d i n ol d basemen t grains . The mos t spectacula r norma l fault s develope d durin g th e Permo-earl y Triassi c exten sional phase , bu t wer e ofte n reactivate d durin g th e Jurassi c extensiona l phase , an d wit h continued minor faul t movemen t int o the Cretaceous therma l cooling period. Integration of commercial an d dee p reflectio n seismi c section s show s tha t thre e detachmen t level s ar e present within the crust. These levels, which control changes in fault geometries , are believed to represen t latera l rheologica l interfaces combined wit h or intersecte d b y long-lived zones of weaknesses . Th e uppermos t leve l i s represente d b y supra-basemen t low-angl e norma l faults controlled by gravity and/or lithologica l changes during extension. An intra-basement (middle crust) leve l between 5 and 7 s (twt) coincides with decreasing dip o f the large r basi n bounding faults . Th e lowe r crus t i s th e deepes t detachmen t level , whic h probabl y exert s control o n th e geometri c change s o f th e upper-mantl e shea r zone s an d th e larges t crusta l normal faults .

The structura l framewor k for Mesozoi c t o Ter - nificanc e an d area l extent of the faultin g and it s tiary basi n developmen t i n th e norther n Nort h relatio n t o differentia l subsidence an d sedimen Sea (Fig. 1 ) has becom e increasingl y well docu - tatio n ar e stil l unde r debate . I n particular , th e mented durin g th e las t decad e (McKenzi e 1978 ; pre-Triassi c evolutio n i s debatable, a s th e thic k Sclater et al. 1986; Beach et al. 1987; Giltner 1987 ; sedimentar y layer s observe d withi n th e dee p Badleyetal. 1988 ; Gabrielsen e/#/. 1990; Roberts half-graben s o n reflectio n seismi c line s ar e no t et al. 1990) . Discrete phases of crustal extension, penetrate d b y wells. Data from th e southwestern with fault bloc k rotatio n an d sub-basi n develop- par t o f Norwa y an d sequenc e stratigraph y ment, ar e separate d b y therma l coolin g an d studie s indicat e tha t riftin g starte d i n Permia n broad basina l subsidenc e fro m Permia n t o Cre - tim e (Faerset h 1978 ; Torsvi k e t al . 1992 ; Steel taceous time . However , th e detaile d nature , sig - 1993) . I t i s als o acknowledge d tha t Devonia n

From: N0TTVEDT , A . e t al . (eds ) Dynamics o f th e Norwegian Margin. Geologica l Society , London , Specia l Publications, 167 , 41-57. 1-86239-056-8/OO/ S 15.00 © Th e Geologica l Societ y of Londo n 2000 .

42

T. ODINSE N E T AL .

Fig. 1 . Ke y ma p an d mai n structura l elements , norther n Nort h Se a (referenc e leve l i s base Cretaceous) . Area s that are primarily affected b y mid-Jurassic-early Cretaceous stretching in grey. Locations of transects 1 and 2 and semi-regional commercia l line s use d i n thi s paper ar e displayed .

(-Carboniferous?) sediments , whic h ar e wel l known fro m the UK sector , may cover basemen t in the deeper part s o f the northern Nort h Sea , a s suggested b y Platt (1995) , Fserseth e t al (19950 ) and Fsrset h (1996) . For a mor e complet e understandin g o f th e evolution an d interpla y betwee n th e differen t extensional phases , basi n geometr y an d subsi dence history , i t i s essential t o analys e thi s pre Jurassic evolution , whic h also encompasse s Caledonian an d post-Caledonia n structures . Compaction o f Permo-Triassi c basi n fil l an d residual Permo-Triassi c therma l anomalie s ma y enhance Triassi c an d Jurassi c subsidenc e an d may therefor e cause overestimatio n o f stretching estimates derive d fro m subsidenc e analysi s

assuming onl y on e rif t phas e (i n mid-Jurassic early Cretaceou s time) . T o succee d i n thi s evaluation, i t i s essential to perfor m a structural and geometrica l analysi s of sufficien t detai l an d completeness t o provid e a bas e fo r modelling . The presen t wor k i s an attemp t t o provid e thi s database, whic h ha s bee n used i n th e modellin g of tw o crusta l transects . Th e modellin g result s are presente d separatel y (Odinse n e t al . 2000) . To achiev e thi s aim , w e hav e combine d an d correlated th e dee p reflectio n seismi c line s NSDP84-1 an d - 2 ( I S s t w t ) (Fig s 2 a an d 3a ) with conventional (7 s twt) reflection seismi c lines to stud y th e structur e at differen t crusta l levels . This analysi s o f th e dee p crusta l structur e als o allows u s t o recogniz e th e pre-Jurassi c structure

Fig. 2 . (a ) Time-section o f crustal transec t 1 . Boxe s show the location s of figures referred to i n the text , (b ) Depth-converted crusta l transec t 1 . (Fo r location , se e Fig. 1. ) No vertica l exaggeration.

Fig. 3 . (a ) Time-section o f crustal transec t 2 . Boxe s sho w th e location s of figure s referre d t o i n th e text , (b ) Depth-converted crusta l transec t 2 . (For location , se e Fig. I. ) No vertica l exaggeration .

NORTH-SEA GEOMETRIE S AN D DEE P STRUCTUR E and particularl y th e architectur e o f th e lowe r crust. Lines NSDP84-1 and -2 (hereafter referred to a s transect 1 and transect 2 respectively) hav e been post-stac k reprocesse d an d depth-con verted (Fig s 2 b an d 3b) . Intersectin g well s have been use d t o establis h stratigraphi c contro l an d have provided velocit y data in the upper 3- 4 km of th e crust , wherea s analyse s o f expande d spread profile s (ESP ) hav e give n velocitie s a t deeper crusta l level s (Christansson e t al. 2000) . In th e first stage o f the analysi s th e geometr y o f the upper, middl e and lowe r crust, as well as the reflection Moho , hav e been determine d for transects 1 and 2 . Th e secon d stag e o f th e analysi s used th e observation s derive d fro m commercia l reflection seismi c lines.

Extensional models Following th e publicatio n o f dee p seismi c reflection dat a acquire d i n 198 3 an d 198 4 (Beach 1986 ; Beac h e t al . 1987 ; Gibb s 1987 ; Klemperer 1988) , three principal models (Fig. 4) for th e dee p structur e o f th e Nort h Se a Basi n have coexisted . Bot h th e symmetrica l pur e shear model , favoure d fo r th e Vikin g Grabe n by Giltne r (1987 ) and Badle y e t al . (1988) , an d

45

the inhomogeneou s shea r model , propose d b y Klemperer (1988 ) for thi s area , ma y b e see n a s developments o f th e McKenzi e (1978 ) model . The asymmetrica l simpl e shea r model , congru ent wit h th e mode l o f Wernick e (1985) , wa s proposed fo r th e Vikin g Grabe n b y Beac h (1986), Beac h et al. (1987) , Gabrielse n (1989 ) and Scot t & Rosendahl (1989) . Finally, differen t types of delamination models (Lister et al. 1986), which allo w fo r differentia l depth-dependen t extension (Royde n & Kee n 1980) , hav e bee n proposed fo r differen t stage s of the developmen t of th e norther n Nort h Se a b y Cowar d (1986) , Fossen et al. (2000) and Ter Voorde et al. (2000). Extensional models are generally based o n the interpreted geometrie s o f majo r crusta l faults . However, suc h model s als o hav e t o conside r rheology an d th e genera l layerin g o f th e conti nental crus t (Dunba r & Sawye r 1989) . Suc h layering result s in larg e jumps i n yiel d strength at laye r interfaces, and thereb y a vertical alteration o f stronger an d weake r intra-crustal zones . In a simpl e two-layere d mineralogica l model , where felsi c rock s dominat e th e upper crus t an d mafic rock s dominate th e lower crust, one would expect norma l fault s t o detac h a t th e weake r interface betwee n thes e tw o layers . However , a s shown b y Kuszni r & Par k (1987) , dependin g

Fig. 4. Thre e models for continenta l extension (afte r Liste r e t al . 1986 , fig. 1).

46


on th e crusta l composition , crusta l thickness , geothermal gradien t an d strai n rate , extensiona l faults ma y als o penetrat e throug h th e lowe r crust. I n a settin g suc h a s th e norther n Nort h Sea, whic h ha s undergon e severa l phase s o f tectonic activity , one would als o expect that pre existing weaknesse s withi n th e crust , suc h a s faults o r intrusions , pla y a majo r rol e durin g fault evolutio n (Dunba r & Sawye r 1989) . Thus, th e uppe r crus t i s deforme d b y brittl e localized faulting , wherea s th e lowe r crus t extends b y distribute d ductil e deformatio n con trolled b y non-Newtonia n power-la w cree p (Kusznir & Park 1987) . Evidenc e fro m expose d lower-crustal rock s an d recen t seismi c reflection studies sho w tha t ductil e deformatio n i s ver y heterogeneous an d localize d (Blundel l e t al. 1989; Blundel l 1990 ; Vilcott e e t a l 1993) . Thi s scale-relationship implie s tha t on e ha s t o dif ferentiate betwee n th e bul k ductil e (pur e shear) and localize d modes o f deformation when describing extensiona l mechanism s o f th e lower crust .

Geological setting The presen t crusta l architectur e i n th e norther n North Se a is generally accepte d t o b e a resul t of Permian-early Triassi c an d mid-Jurassic-earl y Cretaceous stretching , separate d b y therma l subsidence (Eyno n 1981 ; Gabrielse n e t al . 1990; Faerset h et a l 1995# ; Robert s e t a l 1995 ; Faerseth 1996) . I t i s als o anticipate d tha t bot h Precambrian an d Caledonia n structures , a s well as extensiona l collapse o f th e Caledonides , hav e influenced late r extensio n an d crusta l reconfi guration i n th e are a (Fros t 1987 ; Huric h & Kristoffersen 1988 ; Klempere r & Huric h 1990 ; Faerseth e t al \995a). Although th e detail s o f timing , significanc e and latera l exten t o f th e Permo-earl y Triassi c stretching ar e stil l a matte r o f debat e (Giltne r 1987; Gabrielsen e t al. 1990; White 1990 ; Faerseth et al . 1995a ; Robert s e t al . 1995) , recen t dat a suggest tha t thi s rif t phas e wa s mor e significan t than th e mid-Jurassic-earl y Cretaceou s exten sional event . Larg e tilte d faul t block s wit h throws o f th e orde r o f severa l kilometre s formed a 150k m wid e N- S oriente d basi n i n late Palaeozoi c time . Durin g th e therma l sub -

sidence tha t followe d th e rifting , faultin g occurred o n bot h margin s (Stee l & Ryset h 1990) a s a consequenc e o f interactio n o f latera l variations in thermal subsidence , sedimen t load ing, compactio n an d flexur e (Gabrielse n 1986 ; Badley e t al. 1988) . Several studie s hav e empha sized that thermal subsidence was still continuing when the mid-Jurassic rifting started . Hence, this must b e taken into account i n stretching calculations from subsidence analysis of the Cretaceou s post-rift sequenc e (Giltne r 1987 ; Gabrielsen et al. 1990; Roberts ^ al. 1995). The Jurassi c extensio n i n th e norther n Nort h Sea i s well constrained . Rotatio n o f majo r faul t blocks show s tha t riftin g wa s initiate d i n lat e Bajocian-early Bathonia n time , an d terminate d in earl y Ryazania n tim e (Ziegle r 1982 ; Leede r 1983; Badle y e t al . 1988 ; Ratte y & Haywar d 1993; Faerset h e t al . \995b). Th e Permo-earl y Triassic maste r fault s wer e partl y reactivated i n the Jurassic rifting , influencin g th e general structural patter n o f th e entir e basin, an d promotin g segmentation an d subsidenc e wit h opposin g polarities i n som e area s (Faerset h 1996) . Later , during th e rif t clima x i n lates t Jurassi c time , fault activit y wa s concentrate d a t fewe r fault s along th e grabe n margin . A s a result , th e in ternal grabe n relie f became mor e pronounce d a s the syste m developed a matur e grabe n topogra phy wit h platforms , sub-platforms , platfor m marginal high s an d a grabe n featur e sensu stricto, wit h a comple x centr e o f subsidenc e along it s axis . Durin g th e therma l coolin g tha t followed th e rifting , a n earl y to mid-Cretaceou s rapidly subsidin g basi n developed , accompanie d by mino r faul t movemen t alon g som e o f th e master fault s (Gabrielse n 1986) . Late Cretaceou s to Tertiar y modes t subsidenc e affectin g th e northern Nort h Se a i s believe d t o hav e bee n affiliated wit h openin g o f th e Nort h Atlanti c in early Tertiar y times .

General crusta l architecture, northern North Se a In bot h dee p reflectio n seismi c transect s (Fig s 2 and 3) , the geometry o f the upper reflectiv e crust is dominate d b y th e Cenozoic , relativel y flat lying an d unfaulte d post-rif t sequence , whic h

Fig. 5 . Reflectivit y patter n i n th e lowe r crus t fro m reprocesse d NSDP84- 1 an d 2 . (See Figs 2 a an d 3 a fo r location.) (a) Portion belo w Gullfaks faul t bloc k (transec t 1 ) showing a highl y reflectiv e lowe r crust wit h reflecto r sets arranged a s lenticula r zone s an d a discontinuou s Moh o reflectio n (arrows) , (b ) Portion belo w th e Hord a Platform (transec t 2 ) showing a diffus e lowe r crus t reflectivit y patter n an d n o well-define d Moh o (arrows) , (c) Portion belo w th e Hord a Platfor m (transec t 2 ) showing a discret e Moh o reflectio n (arrows ) an d a thi n reflective lowe r crus t above .

NORTH-SEA GEOMETRIE S AN D DEE P STRUCTUR E

47

48


unconformably overlie s the heavily faulted base ment an d it s Permia n t o lat e Jurassi c sedimen tary cove r (Jord t e t al. 1995) . Bot h transect s reveal asymmetrica l large-scal e extensiona l geo metries, wit h th e deepes t part s o f th e basi n situated i n th e westernmos t hangin g wal l o f the inne r wester n maste r faul t o f th e Vikin g Graben. Th e presen t depth-converte d dat a suggest tha t th e faul t geometrie s i n th e uppe r reflective crus t var y wit h positio n withi n th e basin an d tha t mor e tha n on e leve l o f detach ment ma y b e identified . I t i s particularly note d that a loca l detachmen t a t c . 6-7 km (4- 5 s twt) is observe d i n th e reflectio n seismi c data . Th e existence o f thi s supra-basemen t detachmen t on th e wester n shoulde r o f th e grabe n i s als o confirmed b y detaile d studie s o f Gullfak s faul t block (Fosse n 1989 ; Fosse n e t al . 2000) . Fo r the crustal-scal e structures , man y maste r fault s have planar o r slightl y curviplanar geometries in their uppe r parts , bu t see m t o flatte n a t a dept h of 12-1 5 km. It shoul d als o b e note d tha t ther e ar e differ ences betwee n th e tw o transects . Th e majo r easterly thro w a t basemen t level is spread across five major fault s i n the more northerly transect 1 (Fig. 2b) , whereas thi s thro w i s concentrated o n only on e maste r faul t i n the southerl y transect 2 (Fig. 3b) . Bot h transect s clearl y illustrat e pre Jurassic fault movement s as well as fault polarity shift (transec t 2), confirming the existence of preJurassic developmen t o f separat e N- S trendin g graben segment s (Gabrielsen et al. 1990 ; Faerseth et al . 19950) . Although th e middl e crus t i s characterized b y poor reflectivity , structura l observation s o f major significanc e ca n b e mad e i n th e repro cessed dee p seismi c reflectio n data , whe n thes e are combine d wit h an d correlate d wit h com mercial reflection seismi c lines. Thus, i t has bee n possible t o trac e som e maste r fault s int o th e more transparen t middl e crus t and , i n a fe w cases, t o th e to p o f th e muc h mor e reflectiv e lower crust . Althoug h les s clearl y displayed , similar geometrie s ar e als o indicate d belo w th e central par t o f th e mid-Jurassic-earl y Cretac eous Vikin g Graben . Th e fe w strong reflection s that ar e see n i n th e middl e crus t ar e primaril y low-angled. Mor e steepl y dippin g reflection s have bee n mappe d i n th e coast-paralle l dee p reflection seismi c line s by Faerset h et al. (1995a), who relate d thes e t o th e pre-lat e Mesozoi c structuring. The lowe r reflectiv e crus t i s see n a s a n undulating reflectiv e 4-1 0 km thic k ban d wit h primarily gentl y dippin g reflections , bot h i n sections transvers e t o th e strik e o f th e N- S oriented Permo-Triassi c an d Jurassi c basi n axi s

(Figs 2 and 3) , and alon g its strike in the Hord a Platform. Th e to p o f th e lowe r crus t shallow s from mor e tha n 20k m belo w th e platfor m margins t o 17k m beneat h th e presen t Vikin g Graben axis . Thi s reflectiv e zon e i s les s pro nounced belo w th e grabe n axis . I t i s also note d that th e reflection s i n th e lowe r crus t ar e no t always very prominent, an d ca n also appear a s a diffuse pattern . A t a large r scale , th e reflectiv e lower crus t i s undulating i n a long-wavelength low-amplitude mode , i n whic h th e shallowes t part reflect s th e mos t thinne d crust . I n detail , this undulatin g mod e i s repeate d b y reflecto r sets, ofte n arrange d a s lenticula r zones . Thi s is especially pronounced wher e the lowe r crust is most reflectiv e below the Gullfaks area (Fig . 5a). The genera l expressio n o f thes e zone s i s similar to th e anastomosin g shea r band s describe d b y Blundell e t al . (1989 ) and Blundel l (1990), wh o suggested tha t extensio n i n th e uppe r crus t i s accommodated b y norma l faultin g wherea s ductile shea r dominate s withi n the lowe r crust. The reflectiv e Moho i s generall y wel l defined in bot h transects . I n th e area s wher e th e lowe r crust i s more transparent , th e Moho is seen only as a diffus e patter n o f reflection s (Fig . 5b) . Ou r data sho w tha t th e Moh o consist s o f a se t o f reflectors wit h variabl e latera l thicknes s an d amplitude, rathe r tha n on e singl e stron g reflec tor. Thus , th e Moh o reflectio n i s displaye d partly wit h abrup t cut-off s o f th e reflectiv e band (Fig . 5a) , and partl y as a discrete reflection (Fig. 5c) . Below th e Hord a Platfor m th e reflection s o f the lowe r crus t spli t int o tw o separat e band s (Fig. 6) . A dedicated velocit y study has reveale d that th e body delineated by these two reflection s is characterized b y velocities typical for the uppe r mantle (S+kms" 1 ) (Christiansso n e t al . 2000) . However, result s from forwar d modellin g o f th e basin developmen t (Odinse n e t al . 2000 ) an d gravity modellin g (Christiansso n e t al . 2000), a s well a s th e thicknes s an d distributio n o f th e associated sedimentar y accumulatio n indicat e that th e bas e o f th e crus t woul d b e locate d at th e bas e o f th e lowe r reflectiv e band . Acknowledging thes e observations , th e entir e crustal thicknes s i s interprete d t o var y fro m c. 30km an d 36k m belo w th e platform s o n the grabe n flank s t o 21-2 4 km belo w th e axi s of th e bas e o f th e Jurassi c sequenc e i n th e Viking Graben . Beneath th e reflectiv e Moh o intra-mantl e reflectors ar e recorde d i n both transect s (Fig. 7). These features , dippin g 30-45 ° awa y fro m th e graben axis, are focused at the transition between the basin an d th e platform areas, an d follo w th e general patter n fo r intra-mantl e reflection s


49

Fig. 6. Portio n o f the lower crust below the Horda Platfor m (see Fig. 2a for location) showin g how the reflection s split int o tw o separat e band s (arrows ) envelopin g a mor e transparen t zone . (Fo r discussion of this pattern , see text.)

Fig. 7 . Portio n o f th e lowe r crust an d uppe r mantl e (see Fig. 2a for location ) showin g intra-mantle reflection s (the tw o lowe r arrows) .

described b y Beach (1986) and Klemperer (1988) . The interpretation o f the mantle structures offer s several alternative solutions, bu t th e coincidenc e between th e dippin g reflection s an d th e di p gradient o f th e Mon o ma y sugges t tha t th e re flections represent mantl e fault s o r shea r zones , similar t o thos e observe d fo r offshor e Britai n in the BIRP S dee p seismi c dat a (McGear y e t al. 1987; Blundel l 1990 ; Reston 1990) .

Large structura l feature s i n the norther n North Se a Although th e relationshi p betwee n th e forma tion o f th e reflectiv e pattern s an d th e tectoni c events in the norther n Nort h Se a is equivocal, a generally hel d assumptio n i s tha t repeate d crustal stretchin g ma y hav e modifie d the reflec tive architectur e substantiall y (Klemperer 1988 ;

50


Fig. 8 . Portio n o f transec t 1 (see Fig . 2 a fo r location ) showin g the fault-plan e reflectio n of th e principa l east-dipping faul t i n th e rif t syste m (se t o f arrow s t o th e left) . Part s of th e fault-plan e reflection of th e supra-crustal detachmen t belo w th e Gullfak s faul t bloc k (Dl ) ar e als o see n (se t o f arrows t o th e uppe r right).

Fig. 9 . Portio n o f transect 2 (see Fig. 3 b for location ) showing the fault-plan e reflection (arrows ) of the principal east-dipping faul t i n th e rif t system .

NORTH-SEA GEOMETRIE S AN D DEE P STRUCTUR E Blundell 1990 ; Klemperer & Hurich 1990) . It i s also accepte d tha t th e reflectio n Moh o corre sponds to the refraction Moho, whic h is the base of th e crus t i n th e are a (Barto n 1986) . One o f th e ke y factor s i n understandin g ho w the lowe r crus t deform s i n respons e t o litho spheric stretching , an d thereb y i n constrainin g possible tectonic models for the area, is to stud y the reflectio n from larg e norma l fault s from th e upper crus t an d it s change s wit h depths . I n previous studies , n o dee p line s crossin g th e North Se a Basi n sho w a singl e reflecto r identi fied a s a norma l fault , whic h ca n b e tracke d from th e uppe r crus t dow n t o o r belo w Moh o (Klemperer & White 1989 ; Blundel l 1990) . Ou r approach wa s to loo k specificall y a t th e deepes t penetrating norma l fault s an d t o analys e thei r fault plan e reflection , bot h o n th e commercia l lines an d o n th e dee p reprocessed lines . The geometr y o f th e principa l east-dippin g fault i n th e rif t syste m belo w Gullfak s i s illustrated i n Fig . 8 . Its wester n upper 3.5- 7 km (3-5 s twt) i s interpreted i n transec t 1 using cutoffs o f th e horizon s a s see n i n th e hangin g wall and footwall . From 7 km depth (5 s twt) the fault is seen a s a n almos t continuou s reflectio n down to c . 18km (8 s twt). It s uppe r par t i s character ized by a sub-planar, smoot h faul t plan e dippin g c. 55° (measure d i n depth-converte d section s with n o vertica l exaggeration) . It s dip graduall y decreases, definin g a sub-listri c geometr y whe n entering th e lowe r crust . Whe n th e di p o f th e fault reflectio n becomes les s tha n 10 ° it ca n n o

51

longer b e distinguishe d fro m othe r reflection s within th e lowe r crust . A simila r east-vergen t master faul t o f th e Mesozoi c Vikin g Graben i s observed i n transect 2 . But in contrast to what is seen i n transec t 1 , th e maste r faul t i n transec t 2 can only be tracked continuously fro m the upper crust int o the middle crust a t c . 8 s (twt) (Fig. 9). Assuming th e sam e geometr y fo r the tw o faults, the detachment-dept h i n transec t 2 may also b e c. 20 km. I t shoul d her e b e note d tha t th e dee p stratigraphy an d basemen t leve l i n transec t 2 is more uncertai n tha n i n transec t 1 . The interferenc e betwee n th e east-dippin g master faul t i n transect 1 and th e reflectiv e ban d of th e lowe r crus t ha s bee n a subjec t o f severa l papers (Beac h et al 1987 ; Klemperer 1988 ; Pinet 1989; Brun & Tron 1993) . In this study, the intra lower-crustal reflection s are sometime s found t o be aligne d wit h th e continuatio n o f th e east dipping maste r faul t (Fig . 8) . Althoug h thes e lower-crustal reflection s cannot b e confirmed t o represent a continuation o f the faul t i n the form of shea r zone s dow n t o th e Moho , ther e i s th e possibility tha n th e entir e crus t ma y hav e bee n offset b y th e maste r fault . West-dipping fault s dominat e th e easter n platform o f th e stud y area , especiall y t o th e north (transec t 1 ; Fig. 2b) . Th e mos t prominen t structure o f this set is the 0ygarden Faul t Zon e (Fig. 10) , whic h i s characterize d b y multipl e reactivation, an d whic h define s th e limi t o f th e rift syste m sensu stricto (Gabrielse n 1986 ; Gab rielsen et al. 1995 ; Nottvedt et al. 1995). The faul t

Fig. 10 . Portio n o f th e lin e SG8043-403 (see Fig. 1 for location ) showing the west-dippin g fault-plane, reflectio n (indicated wit h arrows ) o f the 0ygarden Fault.

52


Fig.11. Portion of the line SG8043 .01in the Horda Platform ( seeFig.1 for location )showing the low-angle

east-dipping fault-plane reflection (indicated with arrows).

Fig. 12 . Portio n o f th e lin e NVGTI-92-105 i n th e Hord a Platfor m (se e Fig . 1 for location ) showing three west-dipping fault s with different dips . Part s o f th e fault-plan e reflections are visibl e (indicated wit h arrows).

is clearly seen as an almost continuou s reflectio n from 2 to 8 s (twt) depth (Fig. 10) . The upper part of the fault , whic h cuts through th e sedimentary column, typicall y dip s a t 40-50° . I n th e base ment th e di p decrease s t o 20-30° . Th e detach ment leve l of this faul t i s seen t o b e near th e to p of the lower crust. I t is noted that discontinuou s westerly dippin g reflection s ar e see n belo w th e 0ygarden Faul t Comple x i n bot h transect s a t mid-crustal level s (Figs 2 an d 3) . Also, a se t o f east-dipping extensiona l fault s ar e see n i n th e Horda Platfor m are a i n transec t 2 . I n general , the di p o f th e fault s i n th e Hord a Platfor m varies, goin g fro m low-angle , curviplana r t o steep planar (Fig s 1 1 an d 12) . On th e wester n grabe n margin , th e fault s are curviplana r an d listri c wit h detachment s

situated shallowe r tha n th e major crusta l faults , so that th e shallower faults ar e transected by , or merge with , th e uppe r par t o f th e large r faults . This i s particularl y well documente d belo w th e Gullfaks an d Snorr e field s (Fosse n e l al 2000 ) (Figs 8 and 13) . Constraints on extensional model s fo r the northern Nort h Se a Tectonic model s ar e generall y base d o n th e interpreted geometrie s o f majo r norma l faults . The differen t model s proposed fo r th e northern North Se a have therefore le d to a debate o n th e geometric relation s an d th e natur e o f rift s i n general: planar rotationa l (Yieldin g et al. 1991) .


53

Fig. 13 . Portio n o f line NVGTI-92-105 across the Snorr e fault bloc k (see Fig. 1 for location ) showin g the fault-plan e reflectio n o f tw o east-dippin g intra-basemen t fault s wit h decreasin g di p a t depth s (indicate d with arrows) .

listric (Beac h e t al 1987 ; Gibb s 1987 ) an d combinations thereo f (Gabrielse n 1986 ; Spec k snijder 1987 ; Gabrielsen e t al . 1995 ) have bee n proposed. An y discussio n an d applicatio n o f such model s ha s t o tak e int o accoun t tha t th e area ha s experience d repeate d extensio n an d crustal reconfiguration , as i s seen i n th e presen t basin configuration , whic h show s considerabl e differences betwee n th e tw o transects . Beneat h the Hord a Platfor m i n transec t 1 , mos t o f th e Permo-early Triassi c maste r fault s di p toward s the mid-Jurassic-earl y Cretaceou s Vikin g Gra ben axis . In transec t 2 , it is seen that th e easter n part o f th e Hord a Platfor m i s influence d b y master fault s tha t di p awa y fro m th e grabe n axis, wherea s th e wester n par t i s characterize d by a serie s o f fault s wit h shiftin g polarities . It i s assume d tha t thi s contras t i n polarit y reflects th e primar y Permo-earl y Triassi c basi n configuration (Faerset h e t al . 19950 ; Faerset h 1996). Thi s ha s been confirme d b y restoration s recently published by Faerseth (1996). The width of th e Permo-earl y Triassi c basi n wa s c . 150km (transects 1 an d 2) . Thi s correspond s t o th e location o f th e are a o f thinne d crus t a s identi fied i n transec t 1 an d wher e modelle d stretch ing estimate s displa y mor e extensio n i n th e Permo-early Triassi c tha n i n th e mid-Jurassic early Cretaceou s (Robert s e t al . 1995 ; Odinse n et a l 2000) . In spit e o f som e difference s i n lower-crusta l reflectivity a s see n i n transect s 1 and 2 beneat h the platfor m areas , th e genera l impressio n i s

that th e signatur e o f th e lowe r crus t i s simila r between th e tw o transects . Thus , transec t 1 is characterized b y eastwar d crusta l thickenin g immediately belo w th e oute r limi t o f the Hord a Platform, wherea s a simila r thickenin g i s found further eas t in transect 2. The two transects show similarities bot h i n th e widt h o f th e are a wit h Moho shallowing , an d th e depth s t o th e Moh o below th e central par t o f the Viking Graben. The intra-mantl e reflection s observe d belo w the Hord a Platform , a s see n particularl y i n transect 1 , hav e a di p angl e o f 30-45° . Suc h features are frequently recorded withi n the upper mantle i n othe r extende d areas , on e exampl e being offshor e Britai n (McGear y e t al . 1987 ; Blundell 1990 ; Resto n 1990) . Althoug h i t ha s been propose d tha t the y may represen t magma , concentration o f fluids , o r diffractio n fro m th e lower crust , th e curren t assumptio n i s that the y are localized mantle faults (Blundell 1990; Klemperer & Hurich 1990 ; Reston 1993) . In transect 1 it i s noted tha t thes e mantl e reflection s coincide with shif t i n th e lower-crusta l reflectivity , whic h is a t th e transitio n betwee n th e thinne r crus t below th e Vikin g Grabe n an d th e thicke r crust below the Horda Platform (Fig. 2). This, as well a s th e observe d geometr y o f th e feature , makes i t reasonabl e t o sugges t a mantl e fault . The coincidenc e betwee n th e locatio n o f th e mantle fault, th e lateral thinning of the crust an d the high-velocit y (8+kms" 1 ) bod y belo w th e Horda Platfor m (transec t 1 ) ma y hav e signifi cance. Althoug h thi s mus t b e subjecte d t o

54


further study , this observation ma y indicate tha t the high-velocit y bod y i s a long-live d (Caledo nian?) featur e (Christiansson e t al. 2000) . Figure 1 4 summarize s th e mai n structura l observations fro m th e seismi c reflectio n line s in the area. This conceptual mode l i s based mainl y on transect 1 but also has relevance to transect 2. The model show s that, although th e most o f the faults ar e linea r t o curvilinear , listri c an d mor e complex geometrie s ar e also observed. Th e latter geometries ma y b e a resul t o f pre-existing base ment structure s influencin g renewe d faulting . In th e Hord a Platfor m area , fo r example , i t i s indicated tha t listri c faults are linke d t o o r situ ated abov e stee p plana r norma l fault s (Badle y et al. 1988 ; Gabrielsen 1986) . It is acknowledged that th e shiftin g polarit y o f th e Permo-earl y Triassic (o r older? ) basi n syste m beneat h th e Horda Platfor m impart s geometri c problem s that hav e no t ye t bee n solve d (Fig s 2 an d 3) . The resolution in the present data is not sufficien t to solv e th e proble m o f cross-cuttin g relation s between th e major variably dipping faults . Three level s of detachments (Dl, D2 and D3 ) are present within the crust, as shown in Fig. 14.

The supra-basemen etachment (Dl ) i s found within th e sedimentar y column , exemplifie d within th e Gullfak s faul t bloc k (Fig . 8) , bu t i s also observe d elsewher e i n th e are a (Gabrielse n et al . 1995) . Accordin g t o Fosse n e t ai (2000) , Dl i s situate d directl y abov e basement , an d i s probably linke d to the larger Gullfaks fault com posing a ramp-flat-ramp geometr y a t depth . The intra-basemen t detachmen t leve l (D2 ) i s frequently see n i n th e commercia l line s within the middl e crystallin e crust . Th e di p relation s and th e fac t tha t thes e feature s ca n b e easil y followed alon g strik e sugges t tha t the y ar e no t side-reflections o r nois e fro m processing . The y also generate deeper multiple s in the commercial lines. The D 2 level is similar on bot h side s of the Viking Graben . As state d above , continuatio n betwee n th e master fault s o f th e uppe r crus t an d structure s in th e lowe r crust-uppe r mantl e i s viable . Therefore th e possibilit y of a n easterl y dipping simple shea r zon e exists . Thi s i s supporte d b y recent studie s showin g tha t change s betwee n coupled an d decouple d mode s o f deformatio n are i n best agreemen t wit h the observe d patter n

Fig. 14 . Conceptua l model wit h special reference t o transec t 1 summarizes the mai n structura l observations from th e dee p an d commercia l lines. (Note th e thre e detachment levels (Dl, D 2 an d D3 ) an d th e possibl e lin k between th e mantle faul t an d th e principa l east-dipping normal faul t throug h th e lowe r crust.) Two possibl e scenarios are illustrated for the deformation o f the lower crust during movement of the largest normal fault. Als o displayed ar e stee p planar fault s i n th e footwal l o f th e rif t system . (See tex t for furthe r information.)


55

of subsidenc e (Te r Voord e e t al. 2000) . How - by rheological interface s within the crust, but are ever, a t leas t tw o type s o f contacts betwee n th e probably als o influence d b y pre-existin g loca upper-crustal faul t an d tha t o f the upper mantle lized zone s o f weaknesses , suc h a s fault s an d can b e depicted . I n eithe r case , th e intr a lowe r shear zones. Hence, the fault geometrie s suppor t crust wil l the n b e th e deepes t detachmen t leve l • tha t decoupling occurre d a t mor e that on e level, (D3) a s show n i n Fig . 14 . and tha t th e principal decouplin g surfac e can be The principal flattenin g levels D2 and D 3 can traced int o the lower crust, and possibly throug h easily b e explained a s a consequenc e o f changes the lowe r crust an d int o th e upper mantle . in rheolog y a t depth , wher e flattenin g occur s The deformatio n o f th e uppe r an d middl e within th e weake r intra-crusta l layers . I n a crust i s localize d alon g a numbe r o f norma l simple layere d mode l base d o n verticall y chan - faults, whic h favour s bul k simpl e shear . Th e ging mineralogical composition o f the crust, one geometries o f th e fault s ar e variable , with steep should expec t tha t al l large normal fault s would planar, listri c an d composit e geometrie s bein g detach a t th e weake r interface , fo r example , observed. Th e muc h weake r lowe r crus t i s within th e middl e crust . I n reality , an y crus t i s characterized b y bul k pur e shear , accommo much mor e heterogeneous tha n this, especially a dated b y anastomosin g ductil e shea r zone s continental crus t tha t ha s undergon e repeate d separated b y elongate d roc k bodies . A n east heating an d cooling . On e als o ha s t o tak e int o dipping an d low-angl e ban d o f reflector s i s account th e long-live d zone s o f weaknesses , recorded withi n th e predominantl y transparen t which probabl y hav e a n impac t o n th e faul t upper mantle . Althoug h othe r possibilitie s ar e geometries. I t i s therefor e no t surprisin g tha t evaluated, thes e feature s ar e believe d t o repre some large r listri c faults see m t o detac h within sent fault s o r shea r zones . the middl e crust wherea s other s ente r th e lower crust. I t i s als o notice d (Fig . 14 ) that som e o f This wor k wa s funde d b y th e Commissio n o f th e the intra-mantl e reflection s ar e situate d i n th e European Unio n an d th e Norwegia n Researc h Coun continuation o f large-scal e crustal faults . I f this cil in the framework of Integrated Basi n Studies - Th e o f th e Norwegia n Margin . Nors k Hydr o means tha t thes e ar e crustal-scal e shea r zones , Dynamics a.s., Saga Petroleu m a.s . and Statoi l a.s . provided data this implie s tha t (upper ) crusta l an d mantl e and researc h effort s fo r thi s study . W e ar e especiall y faults ma y b e couple d throug h th e lowe r indebted to J. E. Lie, who reprocessed th e NSDP lines. crust. This can best be explained by a lower crust Reviews an d discussion s b y H . Fossen , a s wel l a s composed o f shea r lense s (Blundel l 1990 ) com- linguistic corrections by H. Brekk e and J. McCrachen , parable wit h lowe r metamorphi c panel s a s dis - are ver y muc h appreciated . W e woul d lik e t o than k played i n the Basin-and-Range province . In thi s H. Hjelmelan d an d a n anonymou s reviewe r for thor model th e lowe r crus t deform s b y bul k pur e ough and very constructive criticism . The authors hav e shear (Allmendinge r e t al. 1987; Hamilton 1987) . honoured thei r suggestions on al l major points . Summary

References

The presen t crusta l architectur e o f the norther n North Se a is the resul t of a long-lasting tectonic history of crustal reconfiguration. Of the several tectonic phase s tha t hav e affecte d th e area , th e Permo-Triassic an d especiall y th e mid-Jurassi c to earl y Cretaceou s extensio n ar e bes t known . The Permo-early Triassic norma l fault s ar e seen to be frequently reactivate d durin g the Jurassicearly Cretaceou s stretching . I t i s also observe d that som e o f th e larg e fault s hav e bee n activ e during the thermal cooling periods that followe d the tw o stretchin g phases. The conceptual model show s that both supra and intra-basemen t detachmen t level s ar e pre sent withi n th e uppe r (Dl) , middl e (D2 ) an d lower (D3) crust, respectively. The Dl leve l may partly b e controlled b y gravity and/or litholog y during th e Jurassic-earl y Cretaceou s phas e (Fossen e t al. 2000). D 2 an d D 3 may b e caused

ALLMENDINGER, R: W., HAUGE, T. A., HAUSER, E. C, POTTER, C . J . & OLIVER , J . 1987 . Tectoni c heredity an d th e layere d lowe r crus t i n th e Basi n and Range Province , wester n Unite d States . In : COWARD, M. P. , DEWEY , J . F . & HANCOCK, P . L . (eds) Continental Extensional Tectonics. Geologi cal Society , London , Specia l Publications , 28 , 233-246. BADLEY, M . E. , PRICE , J . D. , RAMBECH-DAHL , C . & AGDESTEIN, T . 1988 . Th e structura l evolutio n o f the northern Vikin g Graben and it s bearing upo n extensional mode s o f basi n formation . Journal o f the Geological Society, London, 145 , 455-472. BARTON, P. J. 1986 . Comparison of deep reflection an d refraction structure s i n th e Nort h Sea . In : BARAZANGI, M . & BROWN , L . (eds ) Reflection Seismology: a Global Perspective. America n Geo physical Unio n Series , 13 , 297-300. BEACH, A . 1986 . Som e comment s o n sedimentar y basin developmen t i n th e norther n Nort h Sea . Scottish Journal o f Geology, 22 , 1-20 .

56


, BIRD , T . & GIBBS, A . 1987 . Extensional tectonic s and crusta l structure : dee p seismi c reflection data from th e norther n Nort h Se a Vikin g graben . In : COWARD, M . P. , DEWEY , J . F . & HANCOCK , P . L . (eds) Continental Extensional Tectonics. Geologi cal Society , London , Specia l Publications , 28 , 467-476. BLUNDELL, D . J . 1990 . Seismi c image s o f continenta l lithosphere. Journal o f th e Geological Society, London, 147 , 895-913 . , RESTON , T . J. & STEIN, A. M . 1989 . Deep crustal structural control s o n sedimentar y basi n geome try. America n Geophysica l Unio n Geophysica l Monograph Series , 48 , 57-64. BRUN, J.-P . & TRON , V . 1993 . Developmen t o f th e North Vikin g Graben: inferenc e from laborator y modelling. Sedimentary Geology, 86 , 31-51 . CHRISTIANSSON, P. , FALEIDE , J . I . & BERGE , A . M . 2000. Crusta l structur e i n th e Nort h Sea ; a n integrated geophysica l study . This volume. COWARD, M . P . 1986 . Heterogeneou s stretching , simple shea r an d basi n development . Earth an d Planetary Science Letters, 80, 325-336. DUNBAR, J . A . & SAWYER , D . A . 1989 . Ho w pre existing weaknesse s control th e styl e of continental breakup . Journal o f Geophysical Research, 94 , 7278-7292. EYNON, G . 1981 . Basi n developmen t an d sedimenta tion i n the Middl e Jurassic of the norther n Nort h Sea. In : ILLING , L . V. & HOBSON , G . D . (eds ) Th e Petroleum Geology of the Continental Shelf of North West Europe. Heyden , London , 196-204 . FOSSEN, H . 1989 . Indicatio n o f transpressiona l tec tonics i n th e Gullfak s Oi l Field , norther n Nort h Sea. Marine an d Petroleum Geology, 6, 22-30 . , ODINSEN , T. , FÊRSETH , R . B . & GABRIELSEN, R. H . 2000 . Detachment s an d low-angl e faults in the norther n Nort h Se a rif t system . This volume. FROST, R . E . 1987 . Th e evolutio n o f th e Vikin g Graben tilte d faul t bloc k structures : a compres sional origin . In: BROOKS , J . & GLENNIE , K . (eds) Petroleum Geology of North West Europe. Graham an d Trotman , London , 1009-1024 . F^RSETH, R. B. 1978. Mantle-derived Iherzolite nodules and megacryst s fr im Permo-Triassic dykes, Sunnhordland, wesi. Norway . Lithos, 11, 23-35. 1996. Interactio n o f Permo-Triassi c an d Jurassi c extensional fault-block s durin g th e developmen t of th e norther n Nort h Sea . Journal o f th e Geological Society, London, 153 , 931-944. , GABRIELSEN , R . H . & HURICH , C . A . 1995a . Influence o f basement i n structuring of the Nort h Sea Basi n offshor e southwes t Norway . Norsk Geologisk Tidsskrift, 75 , 105-119 . , SJOBLOM , T . S. , STEEL , R . J. , LILJEDAHL , T. , SAUAR. B . E . & TJELLAND , T . 1995/7 . Tectoni c controls o n Bathonian-Volgia n syn-rif t succes sion o n th e Visun d Faul t Block , norther n Nort h Sea. In : STEEL , R . J. , FELT , V. , JOHANNESSEN , E. & MATHIEU , C. (eds) Sequence Stratigraphy o n the Northwest European Margin. Norwegia n Petro leum Society , Special Publication , 5, 325-346. GABRIELSEN, R . H . 1986 . Strucural elements in graben systems an d thei r influenc e o n hydrocarbo n tra p

types. In : SPENCER , A . M . e t al. (eds ) Habitat o f Hydrocarbons on the Norwegian Continental Shelf. Graham and Trotman, London , 55-60 . 1989. Reactivation of faults on the Norwegian continental shel f and it s implication s for earthquak e occurrence. In : GREGERSEN , S . & BASHAM , P . (eds) Causes and Effects o f Earthquakes a t Passive Margins and in Areas with Post-glacial Rebound on both Sides o f th e North Atlantic. Elsevier, Amsterdam, 69-92 . , FÊRSETH , R . B. , STEEL , R . J. , IDIL . S . & KLOVJAN, O. S . 1990 . Architectural styles of basin fill in the norther n Vikin g Graben. In: BLUNDELL. D. J . & GIBBS , A . D . (eds ) Tectonic Evolution o f the North Se a Rifts. Clarendon , Oxford , 158-179 . , STEEL , R . J . & NOTTVEDT , A . 1995 . Subtl e traps in extensiona l terranes : a mode l wit h referenc e t o the Nort h Sea . Petroleum Geoscience, 1, 223-235 . GIBBS, A . D . 1987 . Deep seismi c profiles in th e north ern Nort h Sea . In : BROOKS , J . & GLENNIE , K . (eds) Petroleum Geology of North West Europe. Graham an d Trotman , London , 1025-1028 . GILTNER, J . P . 1987 . Applicatio n o f extensiona l models t o th e norther n Vikin g Graben . Norsk Geologisk Tidsskrift, 67 , 339-352 . HAMILTON, W . 1987 . Crusta l extensio n i n th e Basi n and Rang e Province , southwester n Unite d States . In: COWARD . M . P. , DEWEY , J . F . & HANCOCK . P. L. (eds ) Continental Extensional Tectonics. Geological Society , London , Specia l Publications . 28, 155-176 . HURICH, C . A . & KRISTOFFERSEN , Y . 1988 . Dee p structure o f th e Caledonid e Oroge n i n souther n Norway: ne w evidenc e fro m marin e seismi c profiling. In : KRISTOFFERSEN , Y . (ed. ) Progress in Studies o f th e Lithosphere i n Norway. Norge s Geologiske Undersokelse . Specia l Publication , 3 . 96-101. JORDT, H. , FALEIDE , J. L , BJORLYKKE , K . & IBRAHIM , M. T. 1995 . Cenozoic sequence stratigraphy of the central an d norther n Nort h Se a Basin : tectoni c development, sedimen t distributio n an d prove nance areas . Marine an d Petroleum Geology. 12 , 845-879. KLEMPERER, S . L . 1988 . Crustal thinnin g and natur e of extension in the northern North Se a from dee p seismic reflection profiling. Tectonics, 7, 803-821. & HURICH , C . A . 1990 . Lithospheri c structur e of the Nort h Se a fro m dee p seismi c profiling . In : BLUNDELL, D . J . & GIBBS , A . D . (eds ) Tectonic Evolution o f th e North Se a Rifts. Clarendon . Oxford, 37-63 . & WHITE , N . J . 1989 . Coaxia l stretchin g o r lithospheric simpl e shea r i n th e Nort h Sea ? Evidence fro m dee p seismi c profilin g an d sub sidence. In : TANKARD , A . J . & BALKWILL . H . R . (eds) Extensional Tectonics an d Stratigraphy o f the North Atlantic Margins. American Associatio n of Petroleu m Geolog y Memoir , 46, 511-522 . KUSZNIR, N . J . & PARK , R . G . 1987 . The extensional strength o f th e continental lithosphere : it s depen dence o n geotherma l gradient , an d crusta l com position an d thickness . In : COWARD , M . P. , DEWEY, J . F . & HANCOCK, P . L . (eds) Continental

NORTH-SEA GEOMETRIE S AN D DEE P STRUCTUR E Extensional Tectonics. Geologica l Society , Lon don, Specia l Publication , 28, 35-52. LEEDER, M . R . 1983 . Lithospheri c stretchin g an d North Se a Jurassi c clasti c sourc e lands . Nature, 305, 510-514 . LISTER, G . S. , ETHERIDGE , M . A . & SYMONDS , P. A . 1986. Detachmen t faultin g an d th e evolutio n o f passive continental margins. Geology, 14, 246-250. MCGEARY, S. , CHEADLE , M . J. , WARNER , M . R . & BLUNDELL, D . J . 1987 . Crusta l structur e o f th e continental shel f aroun d Britai n derive d fro m BIRPS dee p seismi c profiling . In : BROOKS , J . & GLENNIE, K . (eds ) Petroleum Geology o f North West Europe. Graha m an d Trotman , London , 33-41. McKENZiE, D . 1978 . Som e remark s o n th e develop ment o f sedimentar y basins . Earth an d Planetary Science Letters, 40, 25-32. N0TTVEDT, A. , GABRIELSEN , R . H . & STEEL , R . J . 1995. Tectonostratigraph y an d sedimentar y architecture o f rif t basins ; wit h referenc e t o th e northern Nort h Sea . Marine an d Petroleum Geology, 12 , 845-879. ODINSEN, T., REEMST , P. , VANDE R BEEK , P. , FALEIDE , J. I . & GABRIELSEN , R. H . 2000 . Permo-Triassi c and Jurassi c extensio n in the northern Nort h Sea : results fro m tectonostratigraphi c forwar d model ling. This volume. PINET, B . 1989 . Dee p seismi c profilin g an d sedimen tary basins . Bulletin d e l a Societe Geologique d e France, 8, 749-766. PLATT, N . H . 1995 . Structur e an d tectonic s o f th e northern Nort h Sea : ne w insight s fro m deep penetration regiona l seismi c data . In: LAMBIASE , J. J . (ed. ) Hydrocarbon Habitat i n Rift Basins. Geological Society , London, Specia l Publications , 80, 103-113 . RATTEY, R . P . & HAYWARD , A . B . 1993 . Sequenc e stratigraphy o f a faile d rif t system : th e Middl e Jurassic t o Earl y Cretaceou s basi n evolutio n o f the Central an d Norther n Nort h Sea . In : PARKER, J. R . (ed. ) Petroleum Geology o f Northwest Europe: Proceedings o f th e 4t h Conference. Geo logical Society , London , 215-249 . RESTON, T . J . 1990 . Th e lowe r crus t an d th e exten sion o f th e continenta l lithosphere : kinemati c analysis of BIRPS deep seismi c data. Tectonics, 9, 1235-1248. 1993. Evidenc e fo r extensiona l shea r zone s i n th e mantle, offshor e Britain , an d thei r implication s for th e extensio n o f th e continenta l lithosphere . Tectonics, 12 , 492-506. ROBERTS, A. M., YIELDING, G. & BADLEY, M. E . 1990. A kinematic model for th e orthogonal openin g of the Lat e Jurassi c Nort h Se a Rif t System , Den mark-Mid Norway. In: BLUNDELL, D. J. & GIBBS, A. D . (eds ) Tectonic Evolution o f th e North Se a Rifts. Clarendon , Oxford, 180-199 . ,, KUSZNIR , N . J. , WALKER , I . & DORN LOPEZ, D . 1995 . Quantitative analysi s of Triassi c

57

extension i n th e norther n Nort h Sea . Journal o f the Geological Society, London, 152 , 15-26 . ROYDEN, L . & Keen, C . E. 1980 . Rifting processes and thermal evolutio n o f th e continenta l margi n o f eastern Canad a determine d fro m subsidenc e curves. Earth an d Planetary Science Letters, 51 , 343-361. SCLATER, J . G. , HELLIGER , S. J. & SHOREY , M. 1986 . An analysis of the importance of extension in accounting for the post-Carboniferous subsidence of th e North Se a basin. Universit y o f Texa s Institute fo r Geophysics, Interna l Report . SCOTT, D . L . & ROSENDAHL, B. R. 1989 . North Viking Graben: a n eas t Africa n perspective . AAPG Bulletin, 73 , 155-165 . SPEKSNJIDER, A. 1987 . The structura l configuration of Cormorant Bloc k I V i n contex t o f th e norther n Viking Graben structural framework . Geologic en Mijnbouw, 65 , 357-379 . STEEL, R . & RYSETH , A . 1990 . Th e Triassic-earl y Jurassic successio n i n th e norther n Nort h Sea : megasequence stratigraph y an d intra-Triassi c tectonics. In : HARDMAN , R . F . P . & BROOKS , J . (eds) Tectonic Events Responsible for Britain's Oi l and Ga s Reserves. Geologica l Society , London , Special Publications , 55, 139-168 . 1993. Triassic-Jurassic megasequence stratigraphy in th e Norther n Nort h Sea : rif t t o post-rif t evolution. In: PARKER, J. R . (ed. ) Petroleum Geology of Northwest Europe: Proceedings of the 4th Conference. Geologica l Society, London , 299-316 . TER VOORDE, M., F^RSETH, R. B. , GABRIELSEN, R. H . & CLOETINGH , S . A. P . L . 2000 . Repeate d litho spheric extensio n i n the norther n Vikin g Graben: a couple d o r a decouple d rheology ? This volume. TORSVIK, T . H. , STURT , B . A. , SWENSSON , E. , ANDERSEN, T . B . & DEWEY , J . F . 1992 . Palaeo magnetic datin g o f faul t rocks : evidenc e fo r Permian an d Mesozoi c movement s an d brittl e deformation alon g th e Dalsfjor d Fault , wes t Norway. Geophysical Journal International, 109 , 565-580. VILCOTTE, J . P. , MELOSH , J., SASSI, W. & RANALLI , G . 1993. Lithospher e rheolog y an d sedimentar y basins. Tectonophysics, 226 , 89-95 . WERNICKE, B . 1985 . Uniform-sens e norma l simple shear o f th e continenta l lithosphere . Canadian Journal o f Earth Sciences, 22 , 108-125 . WHITE, N. J. 1990 . Does the uniform stretchin g mode l work i n th e Nort h Sea ? In : BLUNDELL , D. J . & GIBBS, A. D . (eds ) Tectonic Evolution of th e North Sea Rifts. Clarendon , Oxford , 217-240 . YIELDING, G. , BADLEY , M . E . & FREEMAN , B . 1991 . Seismic reflection s fro m norma l fault s i n th e northern Nort h Sea . In : ROBERTS , A . M. , YIELD ING, G . & FREEMAN , B . (eds ) Th e Geometry o f Normal Faults. Geologica l Society , London , Special Publications , 56 , 79-89. ZIEGLER, P . A . 1982 . Geological Atlas o f Western an d Central Europe. Shel l Internationale , The Hague .

This page intentionally left blank

Repeated lithosphere extension in the northern Vikin g Graben : a couple d o r a decoupled rheology ? M. TE R VOORDE, 1 R . B . F^RSETH, 23 R. H . GABRIELSEN 3 & S . A . P . L . CLOETINGH 1 1

Institute of Earth Sciences, De Boelelaan 1085, 1081 HV Amsterdam, the Netherlands 2 Norsk Hydro, Exploration, P.O. Box 200, N-1321 Stabekk, Norway 3 Department of Geology, University of Bergen, Allegaten 41, N-5007 Bergen, Norway Abstract: Th e Lat e Permian-Earl y Triassi c an d Jurassic-Cretaceou s rif t event s that influ enced th e structur e o f th e norther n Vikin g Grabe n ar e examined . Here , a 2 D forwar d numerical model including faults i s applied o n seismi c line NVGT88-04, whic h crosses th e basin i n th e E— W direction . W e concentrat e o n (1 ) th e amoun t o f Jurassi c basi n subsi dence that can be explained by thermal contracting resulting from th e Permo-Triassic event, (2) the spatia l connection between the tw o rif t event s and (3 ) the apparen t mod e o f flexur e (coupled v . decoupled ) o f th e lithosphere . Modellin g result s indicate tha t post-rif t subsidence a s a resul t o f Permo-Triassi c riftin g ha s practicall y ceased a t th e onse t o f Jurassi c rifting. Th e position of the rif t axi s is shown to migrate with time, from th e Horda Platfor m in Permo-Triassi c tim e t o th e presen t Vikin g Graben centr e i n Jurassic-Cretaceou s time . The majo r differenc e betwee n the consequence s o f a couple d versu s a decouple d mod e o f flexure i s that th e latte r cause s a large r amoun t o f fault-bloc k rotation. O n thi s basis , we suggest a decouple d mod e o f flexur e fo r th e Permo-Triassi c rif t phase , a couple d mod e o f flexure for th e Earl y Jurassic rift phase , and a decoupled mod e o f flexure again for th e Lat e Jurassic period .

The Viking Graben i s part o f the series of linked half-grabens tha t compos e th e Nort h Se a sedimentary basi n (Beac h e t al. 1987 ; Badley e t al. 1988; Faerset h et al . 19950 ; Christiansso n e t al . 2000; Odinsen e t al. 2000; Fig. 1) . This basin was formed durin g severa l extensional events following Caledonian collision. The first event occurred in Devonia n tim e (e.g . Hossac k 1984 ; McCla y et al. 1986 ; Andersen & Jamtveit 1990 ; Fossen & Rykkelid 1992 ) and affecte d a n area that extends far beyon d th e late r margin s o f th e rif t system . This wa s followe d b y th e pronounce d Permo Triassic(?) an d Jurassic-Cretaceou s rif t phases . As wel l contro l i s spars e fo r th e pre-Triassi c sequences, th e ag e of the olde r o f these events is still a matter o f debate (see Table 1) . However, a Late Permian-Early Triassi c ag e is suggested by the occurrenc e o f Permia n sediment s i n th e southern Viking Graben (Lervi k et al. 1989 ) and in the Uns t Basi n (Johns & Andrews 1985 ; Platt 1995), b y palaeomagneti c datin g o f faul t rock s in th e southwester n par t o f Norwa y (Torsvi k et al . 1992) , an d b y th e occurrenc e o f Permia n dykes i n th e southwes t Norwegia n coasta l are a (Faerseth et al. 1976 ; Faerseth 1978; Furnes et al . 1982). Additiona l mino r Lat e Triassi c event s were recognize d b y Morto n e t al . (1987 ) an d

Gabrielsen et al. (1990), and a phase of increased subsidence occurre d i n lat e Cretaceous time . In spit e o f almos t 3 0 years o f researc h i n th e Viking Graben area, severa l problems remai n t o be solved: • Becaus e of overprint of later structuring , the Permo-Triassic rif t stag e wa s identifie d only with difficulty i n seismic sections available for the earl y studies of th e norther n Nort h Sea . Because o f this and th e limite d well control , the relative magnitudes of the Permo-Triassi c and Jurassic-Cretaceous events in the Viking Graben hav e bee n disputed . Som e worker s (e.g. Whit e 1990 ; Lippar d & Li u 1992 ) have argue d tha t Jurassi c riftin g wa s domi nant. However , Whit e (1990 ) cam e t o thi s conclusion b y focusing on th e Eas t Shetland terrace are a i n th e west , wherea s majo r Triassic riftin g seem s t o hav e occurre d i n the east, below the Horda Platform. Lippar d & Li u (1992 ) inferre d non-unifor m thin ning (/3 Crust < Anantie ) i n th e Jurassi c Hord a Platform, t o obtai n th e bes t fi t betwee n observed an d calculate d basi n subsidence . Others (e.g . Giltne r 1987 ; Marsde n e t al . 1990) derived from numerica l modelling that

From: NOTTVEDT , A . e t al . (eds ) Dynamics o f th e Norwegian Margin. Geologica l Society , London , Specia l Publications, 167 , 59-81. l-86239-056-8/00/$15.0 0 © Th e Geologica l Societ y of Londo n 2000 .

60

M. TE R VOORD E E T AL .

Fig. 1 . Ma p o f Vikin g Grabe n an d it s surroundings , showin g th e locatio n o f th e seismi c line s NVGT88-04. NSDP84-01 an d NSDP84-02 . the majo r par t o f th e extensio n occurre d during th e Permo-Triassi c phase , an d tha t the basi n subsidenc e o f th e secon d phas e i s for a larg e par t du e t o therma l contractio n resulting fro m th e firs t event . Giltner (1987 ) inferred thi s fro m a ID , unifor m extension model, includin g effects o f a finite rif t phase , whereas Marsde n e l al . (1990 ) used a 2 D depth-dependent extensio n model , assumin g instantaneous stretching . Althoug h th e im portance o f th e Permo-Triassi c even t i s now firmly established, b y new , deep-penetratio n regional seismi c line s (Plat t 1995) , improve d interpretations o f dee p seismi c reflectio n data (Christiansso n e t al . 2000 ) an d basi n modelling studie s (Robert s e t al . 1993 , 1995; Odinsen e t al . 2000) , th e amoun t o f exces s heat Inherited ' b y th e Jurassi c even t i s stil l not wel l established . Another matte r o f debat e i s th e positio n of th e Triassi c rif t axis . I t i s ofte n state d (e.g. Lippar d & Liu 1992 ; Yieldin g et al. 1992) that the Triassic rift axi s coincided with that of the Jurassic-Cretaceous basin. Badley et al . (1984, 1988 ) suggested that mos t o f th e major fault s that were active during the latter episode wer e reactivate d basement-involved

faults o f th e firs t rif t episode , an d tha t greatest stretchin g an d subsidenc e occurre d above th e Permia n rif t axis . However , they also observe d occasiona l basemen t fault s i n places unaffecte d b y th e Permia n rif t stage . On th e othe r hand , Faerset h (1996 ) argue d that the thickness distribution of the sediment pile suggest s tha t th e tw o observe d riftin g stages may have little spatial connection. Th e effects o f the Permo-Triassic rif t phas e can b e recognized t o th e eas t belo w th e Hord a Platform (e.g . Stee l & Ryset h 1990 ; Sneide r et al. 1995), whereas no unequivocal evidence for th e rif t i s found below the Jurassic riftin g axis. Therefore, Faerseth (1996) proposed that the Permo-Triassi c extensio n maximum ma y have bee n locate d i n th e easter n par t o f th e basin, with the main rif t situate d beneat h th e present Horda Platform . This is supported b y observations o f othe r dominantl y Triassi c basins in the eastern North Sea , e.g. the Egersund Basin , th e Hor n Grabe n an d th e Ast a Graben (Lervi k e t al . 1989) , which ar e als o found eas t o f th e Jurassi c rift axis . Several numerica l basi n modellin g studies have been carried out o n the northern Viking Graben (e.g . Giltne r 1987 ; Marsde n e t al .

LITHOSPHERE EXTENSIO N I N TH E VIKIN G GRABE N

61

Table 1 . Dating o f th e major rift events, a s proposed o r used by various workers

Reference

First rif t phas e

Second rif t phas e

Giltner (1987 ) Gabrielsen e t al. (1990) Marsden e t al . (1990) (modelled a s instantaneous events: White (1990 ) Lippard & Liu (1992 )

248-2 13 Ma* ?Late Palaeozoic-Scythian 250-2 13 Ma 230 Ma Triassic 260-235 Maf o r 225-200 Maf Early Triassi c 250 Ma Kungurian-Anisian (261-236 MaJ) 261-236 Mat

169-98 Ma* Bathonian-Ryazanian Bathonian-Ryazanian 170 Ma) 160-100 Ma

Roberts et al . (1995) (modelled a s instantaneous events: Odinsen et al . (2000) This study

156-131 Mat Mid- Jurassic-Earliest Cretaceou s 170 Ma ) Callovian-Ryazanian (165-141 Mat) 165-141 Mat

* Time scal e o f Harlan d e t al . (1982). t Time scale of Haq e t al. (1987). tTime scal e o f Harlan d e t al . (1990). 1990; Robert s e t al . 1995 ; Odinse n e t al . 2000). In most o f these studies an estimate of the lithosphere rigidity, often expresse d a s an 'effective elasti c thickness ' (EET) , wa s mad e to calculate th e flexural response o n the mass redistribution as a result of the extension. The EET value s estimate d b y differen t worker s vary considerably, from 1.5 km (Roberts et al. 1995) t o ±4 5 km (Odinse n e t al . 2000). Thi s discrepancy i s ofte n attribute d t o th e possi bility tha t th e crus t i s decouple d fro m th e mantle, becaus e o f vertica l variation s i n the strengt h o f th e lithosphere . However , little attentio n ha s bee n pai d s o fa r t o th e influence o f thi s strengt h distributio n o n the extensio n mechanism. The purpos e o f thi s pape r i s t o examin e th e nature o f th e variou s episode s o f riftin g i n th e Viking Graben , thereb y focusin g on th e follow ing questions: • Ho w much post-rif t subsidenc e was induced by the excess heat, remnant fro m th e Permo Triassic stretchin g event ? An d ho w di d thi s eventually influence the later developmen t o f the basin? • I s there an y spatial connectio n betwee n the rift events ? Wher e wa s th e regio n o f max imum Permo-Triassi c extension , an d di d this influenc e th e positio n o f th e Jurassic Cretaceous basin ? • I s the lithosphere likely to be in a 'decoupled' state, an d wha t doe s thi s mea n fo r th e flexural response o n extension ? Whereas i n earl y numerica l modellin g studie s in th e Vikin g Grabe n (e.g . Badle y e t al . 1988 ;

White 1990 ) statement s abou t th e olde r rif t phase wer e derived fro m studie s on th e amoun t of extensio n an d subsidenc e o f th e latter , Rob erts e t al . (1995 ) focuse d particularl y o n th e pre-Jurassic event , usin g a combine d flexura l backstripping an d forwar d modellin g method . Following thes e workers , w e investigat e th e Permo-Triassic rif t phas e a s wel l a s th e younge r ones. We concentrate o n the northern par t o f the Viking Grabe n (60°20 /-61°N), usin g a depth converted versio n o f seismi c lin e NVGT88-0 4 (Figs 1 an d 2) . A 2D , non-unifor m numerica l model, includin g fault s an d incorporatin g th e effect o f a finit e perio d o f extension , i s used . The mode l allow s fo r bot h couple d an d decoupled behaviou r (Te r Voord e e t al . 1998) . As th e effec t o f compactio n durin g buria l i s not include d in th e model , w e first calculate the decompacted thicknesse s o f th e sedimen t pack ages. Thes e thicknesse s ar e use d t o constrai n the modelling.

Geological structure Seismic lin e NVGT88-04 i s situated betwee n th e deep seismi c reflectio n line s NSDP84-0 1 an d - 2 published previousl y (Beac h e t al . 1987 ; Gibb s 1987) and recentl y reprocessed and reinterprete d (Christiansson e t al . 2000 ; Odinse n e t al . 2000 ; Fig. 3) . Th e thre e line s togethe r provid e a n outline o f th e cross-sectiona l structur e o f th e northern Vikin g Graben. Heavil y faulted Meso zoic synrif t sediment s ar e unconformabl y over lain b y a Cenozoic , almos t unfaulte d post-rif t sequence. Pre-Jurassi c sediment s ca n clearl y

Fie 2 (a ) Seismic lin e NVCJTKK-0 4 Positio n o f th e lin e i s indicated i n Fig . I . Vortica l axis : two-wa y trave l lim e (ms|. (b) Depth-converted interpretation . 0FZ , 0ygarden Faul t Zone- HBS . Hil d Bren t Statfjor d Fault ; ()s , Oseher g Fault ; Br . Brag e Fault ; BrF , Brag e Fas t Fault ; 0y . 0ygarde n Fault .


63

Fig. 3 . Depth-converte d interpretatio n o f line s NSDP84-0 1 an d NSDP84-02 . Afte r Christiansso n e t al. (2000). Position o f th e line s i s indicated i n Fig . 1 .

be observe d belo w th e Hord a Platform , an d were recently also recognize d furthe r to th e west (Platt 1995 ; Faleid e e t al . 2000) . Th e Permo Triassic even t extende d ove r a wide r are a tha n the Jurassi c rif t (e.g . Gabrielsen e t al. 1990 ; Faerseth e t al . \995a; Robert s e t al . 1995 ; Odinse n et al . 2000) .

intervals (pre-Jurassic , earl y Jurassic , lat e Jur assic an d Cretaceous ) wer e extracted , an d th e decompacted thicknesse s wer e calculate d (Bald win & Butle r 1985 ; Dykstr a 1987 ) (Fig . 4) . A s the Bajocia n to p o f th e Bren t grou p i s clearl y visible o n th e seismi c profiles, we choose thi s t o be th e boundar y betwee n th e earl y Jurassi c an d the lat e Jurassic. Fo r th e compaction correctio n we use d th e porosity-dept h relatio n

Decompaction of th e sediments From lin e NVGT88-04 , th e compacte d thick nesses o f th e sediment s fo r fou r seperate d tim e

where <j) i s porosit y an d z i s depth . W e choos e 0o = 0.56 an d c = 0.39 km"1 , a s suggeste d b y

64


Fig. 4. Sedimen t thicknes s as measured alon g profile NVGT88-04 , subdivided into fou r dept h intervals . Dashed lines, befor e decompaction; continuou s lines , after decompaction . Sclater & Christie (1980 ) for a n average d sand shale litholog y in th e Nort h Sea . Subsequently , the reconstructe d profile s for eac h tim e interval were obtaine d b y correctin g fo r th e horizonta l fault component s a t eac h leve l (Fig . 5) . I f th e sedimentation rat e wa s hig h enoug h t o avoi d the basi n t o becom e underfilled , th e resultin g profiles represent the basin subsidence during the given time interval. This was not alway s the cas e during th e histor y o f th e stud y area. I n Cretac eous an d earl y Tertiar y time , fo r example , th e water dept h wa s probably mor e tha n 50 0 m an d may eve n hav e increase d t o a maximu m o f 1000m in the central part o f the basin (Nelso n & Lamy 1987 ; Bertram & Milto n 1988) . As wate r depths ar e not include d i n the numerica l model , we regard basin subsidence derived for suc h time intervals as underestimated.

Basin subsidence , subdivide d int o fou r time-intervals

Interval top basement-top Triassic (Fig. 5a) The most strikin g feature s in this profil e are th e large, wedge-shape d basin s a t th e easternmos t part o f th e profile , beneat h th e presen t Hord a Platform. Thes e wedges are thinnin g t o th e west and sho w th e larges t subsidenc e o f th e Permo Triassic basi n syste m in th e stud y area. Decom pacted sedimen t thicknesse s u p t o 6.2k m ar e derived. T o th e west , beneat h th e presen t rif t axis, a sediment pile with an approximatel y even thickness o f 4. 5 km i s found . However , becaus e of th e relativel y poo r qualit y o f th e seismi c image beneat h th e Bren t Grou p a t thi s depth , this estimat e shoul d b e regarde d a s uncertain.


65

Fig. 5. Decompacte d sedimen t thicknes s along profile NVGT88-04 , subdivide d int o four interval s and correcte d for subsequen t faulting . Fault s A, B , C and D ar e describe d i n the text . We interprete d fou r major , westerl y dippin g faults beneat h th e Hord a Platform , wherea s the faul t patter n wes t o f faul t 'C ' (th e Brag e East Fault ) seem s t o b e dominate d b y easterly dipping faults . Thi s i s emphasize d b y th e sedi mentary wedge s slightl y increasing i n thicknes s towards th e wes t abov e th e Oseber g structure .

Interval top Triassic-top Brent (Fig. 5b) At thi s time , subsidenc e wa s shifted toward s the present rif t axis , reachin g a maximu m basi n

depth o f 3.7k m i n a basi n o f les s tha n 35k m width. Th e positio n o f th e presen t rif t axi s als o corresponds t o th e sit e wher e th e faul t polarit y shifted fro m easterly dipping faults i n the west to westerly dippin g fault s i n th e east . Thi s implie s that thi s sit e has move d fro m th e Brag e are a i n Permo-Triassic tim e t o th e presen t rif t axi s in Earl y Jurassi c tim e i n thi s par t o f th e basin . However, variation s alon g strik e i n th e relatio n between th e Permo-Triassi c an d Jurassi c centr e of extension exist , possibly influence d by (deacti vated?) transfe r faults (Faerset h 1996) .

66


It should b e noted that considerable faultin g is derived fo r thi s period . Thi s implie s tha t th e early Jurassi c extensiona l episod e ma y b e mor e important tha n might be expected base d o n lines farther t o th e south , wher e thi s sequenc e i s seen as a genera l progressiv e thickenin g t o th e wes t with onl y ver y limited faul t control .

Interval top Brent-base Cretaceous (Fig. 5c) Although th e positio n o f riftin g i n thi s perio d coincided wit h th e Earl y Jurassi c basi n centre , activity alon g east-dippin g fault s i n th e wes t became more pronounced, reflectin g initiatio n of the Shetlan d Platform . Faul t rotatio n bot h eas t and wes t o f th e rif t axi s appear s t o hav e le d t o footwall uplif t an d erosion . Th e maximum basin depth wa s 5.2km , measure d i n th e strongl y westerly thinnin g sedimen t wedg e bounde d b y fault 4 B\ Th e shallowes t poin t o f thi s wedg e lies ±17 km west of the deepest point , at a dept h of 2.1km .

Interval base Cretaceous-base Tertiary (Fig. 5d) Except fo r a slight offset alon g tw o west-dippin g faults a t th e Hord a Platform , th e effec t o f faulting i s hardl y noticeabl e i n th e Cretaceou s sequence. Instead , subsidenc e i n a wide r are a i s observed. Maximu m sedimen t thickness , alon g the rif t axis , is measured t o b e 3.8km . Possibly , thermal subsidenc e an d sedimen t loading , com bined wit h a genera l ris e in se a level, resulted i n gradual buria l o f Jurassi c faul t block s durin g this period . Although we acknowledge the fact that the top of the basement i s difficult t o constrain fro m line NVGT88-04, an d i s therefore no t undisputed , a thick Triassi c sedimen t pil e wa s als o suggeste d by Christiansso n e t al. (2000), who reinterprete d the deep structure of the northern Viking Graben from dee p reflectio n seismi c lines and gravimetric and magneti c data (Fig. 3). Bearing this in mind, some preliminary conclusions can be drawn. The Permo-Triassic rif t phas e affecte d a wide r are a than di d th e subsequen t Jurassic-Cretaceou s event. Thi s i s consistent wit h result s o f Faerseth (1996). Also , Odinse n e t al . (2000) , usin g th e interpretation o f Christiansso n e t al . (2000) , derived a regiona l stretchin g distributio n in th e Permo-Triassic compare d wit h th e mor e loca lized thinnin g i n Jurassi c time . Th e are a o f strongest subsidenc e associate d wit h the Permo Triassic even t i s positione d t o th e eas t o f th e

location o f majo r Jurassi c extension . Thi s i s also consisten t wit h a shif t o f th e rif t axi s to th e west afte r Triassi c times , whic h i s reflecte d i n the reversa l o f faul t polarit y i n tim e wes t o f th e Oseberg area . The faul t polaritie s derive d fro m lin e NVGT88-04 ar e no t necessaril y representativ e for th e whol e Vikin g Graben area , a s th e faul t polarity shift s alon g strik e (Gabrielse n e t al . 1990; Faerseth et al . 19950 ; Faerset h 1996) . However, a shif t o f th e faul t polarit y i n tim e i s supported b y wha t w e regard a s th e mos t likel y interpretations o f line s furthe r t o th e sout h (e.g. NVGT88-02) . I n thi s area , east-dippin g faults withi n the Vikin g Graben see m t o repre sent maste r fault s i n th e Permo-Triassi c perio d as wel l a s i n th e earl y stag e o f Jurassi c rifting , but ar e i n place s cross-cu t b y younge r west dipping fault s durin g th e lat e stag e o f Jurassi c rifting (Faerset h 1996) .

Lithosphere rheology One o f th e critica l factor s i n a flexura l basi n model i s th e presume d lithospher e rheology . In mos t models , th e lithospher e i s assume d t o react o n a loa d i n the sam e wa y a s a thi n elastic plate floating on a viscous fluid. The thickness of this (imaginary ) plate , th e so-calle d 'effectiv e elastic thickness ' (EET ) determine s th e flexural response o f th e lithospher e t o loading : a smal l EET yield s a large-amplitude , short-wavelength response, wherea s a larg e EE T lead s t o small amplitude, long-wavelengt h flexure . Th e EE T can thu s b e estimate d b y considerin g i t a s a factor o f th e 'respons e function 1 describin g th e lithosphere deformation caused b y loading. This approach ha s bee n followe d fo r th e Vikin g Graben b y variou s workers , resultin g in supris ingly larg e variation s i n estimate s o f th e EET . Odinsen e t al. (2000) derived an EE T determined by th e 450 :C isotherm , whic h i n thei r mode l corresponds t o a dept h o f c . 45 km, usin g a kinematic numerica l mode l develope d b y Koo i (1991). O n th e othe r hand , Kuszni r et al. (1991). Roberts e t al . (1995 ) an d Te r Voord e e t al . (1997), usin g flexural models fo r footwal l uplift , arrive at EET values between 1. 5 and 6 km in the same area . W e propos e tha t thi s discrepanc y might b e associate d wit h th e assumptio n o f a 'coupled" versus a 'decoupled ' lithospher e rheology, the latter of which is characterized b y a very weak an d ductil e lower crust (i.e . Kusznir et al . 1991; Te r Voord e e t al . 1998) . The continenta l plate may contai n dippin g o r sub-horizontal weak ductile zones, which cannot support significan t bendin g stresses . I t i s argued


67

Fig. 6 . Strengt h profile s for a typica l continental lithospher e i n a n extensiona l regime , assumin g a 2 0 km thic k upper crust , a 15k m thic k lowe r crust , an d a tota l lithospher e thicknes s of 125km , fo r variou s lithospher e rheologies. (a) and (b ) assume a 'dry' rheology, the upper crust consistin g of dry quartzite an d th e lower crust of diabase, an d (c ) assumes a 'wet ' rheology , th e uppe r crus t consistin g of wet quartzite an d th e lowe r crus t o f diorite. A zer o por e pressur e wa s assume d i n (a) , an d a hydrostati c por e pressur e i n (b ) and (c) .

(e.g. L e Picho n & Chamot-Rook e 1991 ; Burov & Diamen t 1995 ) tha t th e appearanc e o f suc h zones i n th e lowe r crus t permit s mechanica l decoupling o f th e uppe r crus t fro m th e mantle . A redistributio n o f load s i n th e uppe r crust , for exampl e a s a resul t o f extensiona l faulting , would the n b e compensated fo r b y elastic deformation i n th e uppe r crus t exclusively , whereas the lowe r crus t woul d adjus t b y ductil e flow . This ca n explai n th e ver y lo w (1.5-6 km) effec tive elasti c thicknesse s obtaine d i n modellin g studies where the basi n topograph y wa s used a s the mai n constrainin g factor . Figure 6 show s strengt h profile s fo r th e tensional regim e i n a typica l continenta l crus t before extension . Th e profile s ar e constructe d from th e commonl y used , laboratory-derive d strength equation s (e.g . Goetz e & Evan s 1979 ; Brace & Kohlstedt 1980 ; Carter & Tsenn 1987 ; see Appendi x 1) . Th e depth-strengt h curve s demonstrate clearl y th e effec t o f th e strai n rat e on th e crusta l configuration . I f strai n rate s ar e higher than 1CT 13 s"1 the shallowest weak ductile layer occur s a t th e bas e o f th e lowe r crust , o r perhaps eve n deeper . Fo r lowe r strai n rates , a n additional ductile zone appears a t the base of the upper crust . A full y decoupled mod e o f flexure is likely t o be an exceptional case, becaus e th e rate of lower

crustal flo w i s generall y no t hig h enoug h t o compensate fo r th e entir e upper-crusta l defor mation (Buc k 1988 ; Te r Voord e e t al 1998) . Therefore, par t o f th e compensatio n probabl y mostly occurs b y deep, very low viscosity mantle material causin g th e lithospher e t o b e i n a 'partly decoupled ' mod e (se e Te r Voord e e t al . 1998). However , t o mak e th e effect s o f th e assumption o f couple d v . decouple d behaviou r on th e modellin g result s a s clea r a s possible , we wil l conside r onl y th e end-member s o f th e decoupled-coupled spectru m i n thi s study. The numerica l model We use a forward, 2D difference model , describ ing th e lithospheri c deformatio n resultin g fro m extension, an d includin g the effec t o f faultin g i n the uppe r crust . A finit e syn-rif t phas e i s con sidered (Waltha m 1989 , 1990) . Thi s i s o f majo r importance whe n calculatin g th e post-rif t ther mal subsidence , a s stressed , fo r example , b y Ter Voord e & Cloetingh (1996) . A presentatio n of th e mode l i s give n i n Fig . 7 . Followin g Kusznir e t al . (1991) , th e mode l i s divided int o an uppe r laye r of faultin g wit h vertica l shea r i n the deforming hanging wall, and a lower layer of more distributed deformation. In both layer s the

68


Fig. 7 . Schemati c representatio n o f the model. Extensio n is achieved b y distributed deformation (i.e. deformation along faults ) i n th e uppe r crust , an d b y distribute d thinnin g in th e lowe r lithosphere.

condition o f volum e conservatio n i s satisfied . Extension rate s ca n b e varie d pe r tim e interva l and pe r fault . Th e fault s ar e assume d t o flatte n into th e detachmen t tha t form s th e boundar y between th e tw o layers . Th e dept h o f thi s detachment i s an importan t factor , a s i t ha s th e same functio n i n th e mode l a s a "neckin g depth' (e.g. Braun & Beaumont 1989 ; Weissel & Karne r 1989; Koo i e t al. 1992) . Th e neckin g dept h i s defined a s the level that remain s horizontal in the absence o f isostati c forces , an d i s a decisiv e factor fo r th e existenc e an d amoun t o f footwal l uplift (e.g . Te r Voord e & Cloetingh 1996) . Temperatures ar e calculate d fro m th e hea t transfer equation :

where T i s temperatur e ( C) , / i s tim e (s) , K i s thermal diffusivit y (nrs" 1 ), v i s velocity , F i s heat productio n (Wm~ 3 ), p i s densit y (kgm~ 3 ) and C i s specific heat (J C C - 1 kg" 1 ). The equatio n is solved usin g a finite difference method o n a rectangula r grid . Therma l proper ties o n eac h gri d poin t ar e obtaine d b y interpo lation from th e second, movin g grid representin g the extending basin. Hea t transfe r during a s well as afte r th e deformatio n i s calculated . Flexure i s calculate d fro m th e thin-plat e approximation (e.g . Bodine e t al . 1981) :

where u - i s deflectio n (m) , D i s rigidit y ( N m) , Arompi i s densit y o f th e materia l underneat h th e plate (kgm~ 3 ), p COmP2 i s densit y o f th e materia l above th e plate (kgm~ 3 ), g is gravitational acceleration (ms~ 2 ) an d q i s vertical loa d (Nm~ 2 ). The vertica l loa d i s calculate d b y vertica l integration o f th e densit y contrast s cause d b y

deformation o f th e crust , temperatur e change s and th e mas s o f deposited sediments , relativ e to that o f th e undeforme d crust :

where a i s thermal expansio n coefficien t ( K ] ). For th e couple d mod e o f flexure , w e assum e the deflectio n u - t o b e constan t i n eac h vertica l column throug h th e whole lithosphere. signifyin g that th e crust an d mantl e are forge d togethe r b y the lowe r crust . Hence , th e integratio n interva l for calculatin g th e vertica l loa d q comprises th e whole lithosphere . p c ompi i n equatio n (3 ) i s the asthenospher e density , an d p ComP2 th e sedi ment density. For th e decoupled mod e of flexure, the deflectio n u - is assumed t o b e different fo r th e crust an d mantle . Fo r th e uppe r crust , q i s calculated b y integrating from th e surfac e t o th e lower crust , p ComPi i s take n a s th e lower-crusta l density, p comp2 a s th e sedimen t density , an d th e rigidity i s assume d t o b e lo w ( E E T < 6 k m ) . This i s differen t fro m earlie r studie s usin g lo w EET value s (e.g . Kusznir e t al . 1991 ; Roberts et al . 1995) , wher e th e influenc e o f decouplin g on th e compensatio n material s i s no t take n into account . The therma l feature s o f th e mode l hav e bee n described i n mor e detai l b y Te r Voord e & Bertotti (1994) . th e structura l feature s b y Te r Voorde & Cloeting h (1996) , an d th e effect s o f decoupling hav e bee n discusse d b y Te r Voord e et al . (1998) .

Model parameters The mode l parameter s w e used i n thi s stud y ar e summarized i n Tabl e 2 . Th e Moh o dept h i s assumed t o b e 35km , base d o n th e crusta l thickness observe d belo w th e platfor m areas . The detachmen t dept h i s assume d t o b e 18km .

LITHOSPHERE EXTENSIO N I N TH E VIKIN G GRABE N Table 2 . Parameters used for th e modelling Parameter

Value

Deviation used fo r sensitivity tes t

EET (coupled ) EET (decoupled ) Density sediment s Density uppe r crus t Density lowe r crus t Density asthenosphere Moho dept h Detachment dept h Young's modulus Poisson rati o

450°C isother m 1.5-6 km 1900 kg m-3 2700 kg m"3 2950 kg m-3 3300 kg m-3

see Fig. 8 see Fig . 8 ilOOkgm- 3 ±50kgm~ 3 ±50 kgm- 3 -

35km 18km 7 x 10 10 Nm 0.25

±2 km ±5 km -

based o n observation s o f Fosse n e t al. (1998). They describe d th e flattening of normal fault s a t a maximu m dept h between 1 8 and 20km , in the Tampen Spu r are a o n th e wester n flan k o f th e Viking Graben . Although Fosse n e t al . (1999 ) suggeste d tha t these flattene d fault s ar e inherite d mechanica l weak zones in the basement, relate d to Devonia n extensional structure s o r Caledonian thrusts , th e detachments coul d also be linked to ductile shear zones in the middle an d lowe r crust an d thu s be explained b y temperature - an d pressure-con trolled change s i n th e lithospher e rheolog y (e.g. Fosse n & Gabrielse n 1996) . Th e differen t detachments ma y have different causes , and ma y also have been initiated at different stage s during rifting. Then , trul y low-angl e structure s migh t also develo p b y flattenin g o f originall y steepe r fault structures , durin g repeate d phase s o f stretching, an d perhap s ac t a s detachments i n a later stag e (Fosse n e t al . 2000) . Althoug h th e flattening o f th e fault s wit h dept h ma y hav e no unequivoca l explanation , i t ma y indicat e the existence and positio n o f ductile zones in the lower crust . I n th e cas e o f th e Vikin g Graben , the maximu m observe d dept h o f flattenin g should the n als o b e interprete d a s th e dept h o f mechanical decoupling . Model sensitivit y an d constraints The overal l stat e o f flexur e depend s o n th e strength o f the layer(s) , the densit y and amoun t of sediments , th e dept h o f th e Moho , an d th e depth o f neckin g durin g lithospheri c extension . To obtai n a n impression of the model sensitivit y to these variable s fo r th e Vikin g Grabe n con figuration, w e teste d th e effect s o f varyin g th e EET, th e densitie s an d th e Moh o depth , usin g

69

parameters an d deviation s give n in Table 2 an d the sam e faul t configuratio n a s i n Fig . 5 . Because, in our model , a change in necking level implies a chang e i n faul t shapes , effect s o f varying thi s leve l o n th e resultin g flexura l respons e are difficul t t o tes t independently. However, i t is possible t o calculat e whic h chang e i n sedimen t density would b e equivalent t o a change of 1 km in th e leve l o f necking (see Appendix 2) , and w e used thi s analog y t o estimat e th e mode l sensi tivity fo r th e leve l o f necking. The deviation s i n densitie s an d Moh o dept h cause maximum variations in the resulting basin subsidence o f 60 m fo r th e couple d cas e an d 185m fo r th e decouple d case . A reasonabl e variation o f 5k m i n neckin g dept h ca n b e compared wit h a variatio n i n sedimen t densit y of —14 8 kgm~3 (i n the cas e o f a deepe r neckin g level) o r 262kgm~ 3 (i n th e cas e o f a shallowe r necking level) for th e couple d cas e (Appendi x 2, equation (6)) , leading to a maximum variatio n in basin subsidenc e o f 100m . Fo r th e decouple d case th e leve l o f neckin g ha s n o influenc e (Appendix 2 , equation (6)) . In Fig . 8 the effect s o f changes i n EE T value s are show n fo r th e simulatio n o f th e Permo Triassic tim e interva l (i.e . Fig . 5a) . Th e figur e shows clearl y that th e choic e o f th e EE T i s o f limited influenc e fo r th e couple d scenario , bu t might be important i n the case o f decoupling. In the modelling , w e use d a n EE T define d b y th e 450°C isother m fo r couple d flexure , an d th e EET givin g the bes t fit for decouple d flexure . Finally, effect s o f reasonabl e error s i n th e estimated fault shape s and deformation mechan ism ma y b e o f th e orde r o f 1 km (Whit e e t al . 1986; Whit e & Yieldin g 1991 ; Withjac k & Peterson 1993) . In previou s studie s o n th e Vikin g Grabe n (Marsden e t al. 1990 ; Roberts e t al. 1995 ; Odinsen e t al . 2000) , modellin g result s and (seismic ) data normall y fit within 1000m . Base d o n this , on the estimated sensitivit y of the model, an d o n uncertainties in the seismi c data, fo r exampl e as a resul t o f possibl e error s i n chose n velocit y models, w e regar d a fi t o f th e mode l wit h th e data withi n 1000 m a s a goo d approximation . Modelling result s The numerica l mode l wa s use d t o simulat e th e sequential basin configuration s shown i n Fig . 5 , in order to relate the amount o f basin subsidence to th e amount o f extension along various faults, and t o th e strengt h an d mod e o f flexur e o f the lithosphere . I n th e model , sediment s ar e assumed t o fill the basi n u p t o th e surface . Thi s

70


Fig. 8. Modellin g result s fo r interva l to p basement-to p Triassic , assumin g differen t value s fo r th e EET. (a) Decouple d rheology ; (b ) coupled rheology .

is a realistic approach, as long as the basin is not underfilled a t th e en d o f th e modelle d episode . The modellin g parameter s w e used ar e summar ized i n Tabl e 2 .

Interval top basement-top Triassic (Fig. 9) Triassic sediment s ar e suppose d t o hav e bee n deposited i n lacustrine environments, dominate d by alluvia l fan s alon g th e grabe n margin s an d with fine r fluvia l o r lacustrin e sediment s i n th e lows' (e.g . Stee l & Ryset h 1990) . A t th e en d o f Triassic tim e th e norther n Vikin g Grabe n wa s approximately a t se a level . The sedimen t thick ness show n i n Fig . 5 a might thu s b e interprete d as th e tota l basi n subsidenc e durin g thi s tim e interval. W e focuse d especiall y o n th e flank s o f the graben , becaus e o f th e poo r seismi c resolu tion in the basin centre. Consequently , th e muc h better fi t o f th e mode l beneat h th e platfor m areas tha n beneat h th e rif t axi s (Fig . 9a ) reflect s the highe r confidenc e o f th e observe d sedimen t thickness i n tha t area . To obtai n th e bes t modellin g result , a faul t configuration wa s require d wit h a chang e i n fault polarit y on th e flanks of the Viking Graben and o f th e Brag e are a (Fig . 9a) . Th e thinnin g of the mappe d sequenc e i n th e Oseber g are a towards th e eas t i s the n explaine d b y extensio n along th e east-dippin g faul t 'A' , boundin g th e Viking Grabe n i n th e west , wherea s th e Viking Graben centr e subside d furthe r alon g th e west dipping faul t W B', 25k m farthe r t o th e east . Th e

major extensio n occurre d o n th e Hord a Plat form, provide d tha t th e to p Triassi c interpreted in th e grabe n centr e i s correct . Two scenario s wer e modelle d fo r thi s tim e interval, on e assumin g a couple d rheolog y wit h an EE T determine d b y th e 45 0 C isother m (corresponding t o a n initia l dept h o f 4 3 km i n the model) , th e othe r assumin g a decouple d rheology wit h a n EE T o f 1.5km . A reasonabl e fit between the modelled basi n configuration and the on e interprete d fro m th e seismi c lin e coul d be obtaine d fro m bot h assumption s (Fig . 9b) . but wit h differen t value s fo r th e amoun t o f extension. I n th e cas e o f a couple d configura tion, th e lithospher e i s to o stron g t o sho w a significant flexura l respons e o n th e extension , and th e basemen t subsidenc e i s almos t entirel y due to crustal movements along the faults. How ever, if we adopt a decoupled mod e o f extension, the mode l show s a syn-rif t uplif t o f th e area , leading t o a smal l amount o f subaeria l footwal l erosion, followe d by thermal subsidence (Fig. 9b and c) . The modelle d uplif t an d subsidenc e patterns for th e decouple d scenari o agre e wit h th e findings o f Robert s e t al. (1995) , an d thos e o f the couple d scenari o wit h finding s o f Odinse n et al . (2000) . Robert s e t al . (1995 ) modelle d th e pre-Jurassic riftin g stag e i n th e Hord a Platfor m region wit h a flexural cantilever model, usin g an EET o f 1.5km , an d foun d tha t a stag e o f foot wall uplif t an d erosio n precede d a stag e o f overall therma l subsidence . O n th e othe r hand , the results of Odinsen et al. (2000), who use d th e


71

Fig. 9. Modellin g results for interva l to p basement-to p Triassic. (a ) Modelled faul t configuration ; dashe d lin e indicates basi n dept h a s derive d fro m profil e NVGT88-04 . (b ) Long dashes, modellin g resul t fo r couple d rheology; shor t dashes , modellin g resul t fo r decouple d rheology ; continuou s line , basi n dept h a s derive d fro m profile NVGT88-04 . (c ) Modelled basi n dept h fo r decouple d rheology , a t (fro m to p t o bottom ) 0 , 30 , 74 and oo Ma afte r th e en d o f rifting . coupled approach , d o no t indicat e suc h uplift . According t o Robert s e t al. (1995) , th e seismi c expression o f th e bette r image d Triassi c faul t blocks beneat h th e Hord a Platfor m give s th e impression o f bevellin g at th e fault-bloc k crests. This ca n b e notice d i n interpretation s of Beac h et al . (1987 , fig. 2), Robert s e t al . (1993, fig. 6a) and Odinse n e t al . (2000 , fig s 2 an d 5) , an d supports th e decouple d scenari o a s th e mos t likely one . The tota l amount s o f extensio n use d i n ou r model ar e 2 3 km fo r th e couple d scenari o an d 34.5km fo r th e decouple d scenario , signifyin g an averag e (3 valu e o f 1.1 8 fo r th e couple d case , and 1.2 9 for th e decouple d cas e (measure d ove r the entir e profile) . Majo r extensio n i n Permo Triassic tim e took plac e on the Horda Platform, the tota l modelled amoun t o f extension being 14 or 22 % wes t o f th e Brag e fault , an d 2 2 or 38 % east o f it , fo r th e couple d o r decouple d case , respectively.

Roberts e t al . (1995 ) reporte d a f t valu e o f 1.34 in the Horda Platform area , alon g the sam e section a s in the present study , whereas Odinsen et al . (2000 ) calculate d a n averag e /3 facto r o f 1.27 fo r thei r norther n transect , an d 1.1 9 fo r their souther n transect . The y derive d f t factor s for th e Hord a Platfor m o f 1.3 3 an d 1.39 , bu t they di d no t indicat e exactl y over whic h width this was measured . Figure 9c shows the syn-rift uplif t an d thermal subsidence with time for the decoupled scenario . Indicated ar e the top o f the basement a t th e end of rifting, 3 0 Ma late r (i.e. latest Triassic), 74 Ma later (i.e . whe n th e secon d majo r rif t phas e i s assumed t o start ) and afte r tota l therma l relaxation. In contrast t o earlier proposals (e.g. Giltne r 1987; Marsde n e t al. 1990 ; Roberts e t al . 1995), but i n accordance wit h Odinsen et al. (2000), the present mode l suggest s tha t therma l subsidenc e as a resul t o f Permo-Triassi c riftin g canno t have ha d muc h influenc e on th e Jurassi c event .

72

M. TE R VOORD E ET AL .

(It shoul d b e note d tha t therma l subsidenc e fo r the couple d cas e woul d hav e eve n les s effect. ) Researchers wh o state d tha t a n inherite d ther mal anomaly influence d Jurassic riftin g use d I D thermal calculation s (e.g . Giltne r 1987) , instan taneous Triassi c riftin g (Marsde n e t al. 1990 ; Roberts e t al . 1995) , and/o r derive d thei r statements fro m studie s o f th e Jurassi c even t alone (Giltne r 1987). Interval top Triassic-top Brent Group (Fig. 10) The to p o f th e Bren t Grou p horizo n i s dia chronous o n a regiona l scale , an d i s situated i n the Bathonia n (Middl e Jurassic) sequence i n the northern Vikin g Graben . Th e lowe r Jurassi c sequence consist s predominantl y o f marin e shales, whereas the Middle Jurassic sequence was deposited i n non-marine to paralic environments (Fjellanger e t al . 1996 ; Ravna s e t al . 1999) . Th e Tarbert Formatio n o f th e uppermos t Bren t Group consist s o f shallo w marin e sandstones . As a n approximation , th e profil e show n i n Fig. 5 b might thu s b e regarde d t o represen t th e total basin subsidence during early Jurassic time, until mid-Bathonia n time . This tim e interva l could b e simulate d wit h a much les s complicate d faul t configuratio n than that o f th e Permo-Triassi c phase . Th e positio n of th e rif t axi s was i n th e Vikin g Graben centre ,

coinciding wit h th e sit e o f majo r subsidenc e a s well a s th e positio n o f shif t i n faul t polarit y along th e profil e (Fig . lOa) . However , th e subsidence in the basin centre might be overestimated, because, again , th e seismi c data a t thi s depth in the basi n centr e d o no t allo w fo r a decisiv e solution. Compare d wit h the earlie r rift phase , a change i n faul t di p wa s impose d betwee n th e Oseberg an d Brag e are a (faul t k CT). Figur e 1 0 shows th e modellin g results for th e couple d an d the decouple d case . Fo r th e decouple d case , a n EET o f 6k m wa s used . Th e couple d scenari o gives result s tha t ar e mor e consisten t wit h th e observed basi n configuration , a s hardl y an y tilting o f th e faul t block s i s observed . Fo r thi s reason, therma l subsidenc e relate d t o thi s phase is assume d t o b e negligible . T o mode l th e observed amoun t o f subsidence, modelled offset s along th e fault s ha d t o b e se t u p t o 2 km larger than th e observe d offsets , suggestin g a n over estimation o f th e amoun t o f extension , and th e existence o f a n additiona l source o f basi n subsidence, differen t fro m fault-relate d extension . As show n b y th e modellin g results of th e to p basement-top Triassic interval, only a small part of the additional subsidence (i.e. less then 350m) can b e explained by remnant thermal subsidence caused b y Permo-Triassi c rifting , whic h wa s not include d i n th e modellin g results. Anothe r explanation fo r basi n subsidenc e migh t b e th e existence of a 'proto-rift stage ' (Gabrielsen 1986; Nottvedt e t al . 1995) . Nottved t e t al . (1995 )

Fig. 10 . Modellin g result s fo r interva l to p Triassic-to p Brent , (a ) Modelle d faul t configuration ; dashe d lin e indicates basi n dept h a s derive d fro m profil e NVGT88-04 . (b ) Long dashes , modellin g resul t fo r couple d rheology; shor t dashes , modellin g resul t fo r decouple d rheology ; continuou s line , basi n dept h a s derive d fro m profile NVGT88-04 .

LITHOSPHERE EXTENSIO N I N TH E VIKIN G GRABE N proposed a n idealized , three-stage mode l fo r rif t evolution, involvin g a proto-rift stage, a syn-rif t stage and a post-rift stage . The proto-rift stag e is characterized b y thermally induced domal uplift , or b y depositio n i n a wide , slowl y subsidin g basin with only minor faul t activity . The syn-rif t stage describes the phase of active stretching an d block rotation , an d th e post-rift stage consists of asymptotically decreasing subsidence, caused b y thermal contraction . Interferenc e between post rift subsidenc e relate d t o th e Permo-Triassic rif t phase an d proto-rif t subsidenc e relate d t o th e Jurassic rif t phas e (Stee l 1993 ; Nottved t e t al. 1995) migh t for m a n alternativ e sourc e fo r th e extra subsidenc e observed . The total amount o f extension in the modelled scenario (Fig . lOa ) i s 7.3km , yieldin g an aver age ( 3 value o f 1.05 . Th e modelle d /3 reache s a value o f 1.1 6 i n th e basi n centre . Apparently , a mino r extensiona l even t ha s occurre d i n earliest Jurassic time, as argued earlier by R0e & Steel (1985 ) an d Gabrielse n e t al . (1990) . Th e

73

magnitude o f thi s event cannot b e derived fro m the modelling results, because o f the large uncertainty i n the basi n depth . This earl y Jurassi c extension , combine d wit h the additiona l subsidence , cause d th e deposi tional environmen t t o conver t fro m continenta l to marine , implyin g tha t th e creatio n o f ne w accommodation spac e outpace d sedimen t sup ply. Subsequently , a s sedimentatio n continued , the depositiona l environmen t passe d t o non marine again , an d i n lates t Bajocian-earlies t Bathonian time the first rotational movements of the secon d majo r rif t phas e commence d (Faer seth e t al . 19956 ; Ravna s & Bondevik 1999). Interval top Brent Group-base Cretaceous (Fig. 11) The Uppe r Jurassi c sediment s i n th e Vikin g Graben consis t mostl y o f marin e shales . Th e shales o f th e Bathonian-Oxfordia n Heathe r

Fig. 11 . Modellin g result s for interva l top Brent-bas e Cretaceous , (a ) Modelle d faul t configuration ; dashed lin e indicates basi n dept h a s derive d fro m profil e NVGT88-04 . (b ) Long dashes , modellin g resul t fo r couple d rheology; shor t dashes , modellin g resul t fo r decouple d rheology ; continuous line , basi n dept h a s derive d fro m profile NVGT88-04 . (c) Modelled basi n depth fo r decoupled rheology , at (fro m top t o bottom) 0 and o o Ma afte r the en d o f rifting .

74


formation ar e overlai n b y th e organic-ric h Draupne Formation (equivalen t t o the Kimmer idge Cla y Formation) , an d bot h ar e interfin gered b y sandston e unit s (e.g . Ravna s e t al. 1999). Base d o n environmenta l interpretations , the wate r dept h a t th e en d o f thi s episod e (Ryazanian) i s estimate d t o b e 200 m o n th e margins t o 300 m i n th e grabe n centr e (Badle y et al . 1988) , althoug h thi s migh t b e a ver y con servative estimat e (e.g . Marsde n e t al . 1990 ; Yielding e t al . 1992) . The modellin g result s ar e show n i n Fig . 11 . The EE T use d fo r th e decouple d cas e wa s 6 km. Broadly th e sam e faul t configuratio n coul d b e used a s fo r th e earl y Jurassi c phase , bu t wit h a few mor e activ e faults . Th e basi n no w becam e asymmetric, deepenin g t o the east in this part of the graben . However , thi s varie s alon g strik e (see, e.g. Odinsen e t al. 2000). The footwal l uplif t in th e wes t i s bes t explaine d b y usin g th e decoupled approach , a s show n i n Fig . l i b . This approac h yield s a n averag e /3 facto r ove r the whole transect o f only 1.0 7 (and a maximum of 1.2) . Together wit h th e earl y Jurassi c phase , this make s a Jurassi c ( 3 of 1.12 . Thi s i s slightl y lower tha n th e values found fo r Jurassic stretch ing b y Odinse n e t al . (2000), wh o obtaine d 1.1 5 for th e norther n transec t an d 1.1 9 fo r th e southern transect , o r b y Robert s e t al . (1993), who reporte d a ( 3 of 1. 3 i n th e basi n centre , of 1.1 5 in th e Eas t Shetlan d Basi n an d west ern Hord a Platfor m an d o f 1.0 5 i n th e easter n Horda Platform . Tha t th e 3 valu e w e derive d is relativel y lo w ca n b e relate d t o th e fac t tha t our mode l assume s a zer o wate r depth , whic h is a n underestimatio n fo r thi s tim e interval .

Figure l i e display s th e basi n configuratio n immediately afte r stretchin g a s wel l a s afte r thermal subsidence . Fro m this figure, it is evident that th e therma l anomal y cause d b y th e extension alon e canno t b e responsibl e fo r th e sub sequent Cretaceou s subsidence . Thi s ca n partl y be explaine d b y th e fac t tha t th e basi n wa s underfilled at the end of this rift phase, which was not include d i n th e modelling . Th e modelle d amount o f extension , an d thu s th e therma l anomaly, i s therefore probabl y a n underestima tion. Durin g Cretaceou s time , muc h o f the sedi mentation wa s accommodated b y th e infillin g o f previous rif t bathymetry , causin g additiona l subsidence becaus e o f sedimen t loading . Thi s effect i s furthe r exaggerate d b y th e compactio n of earlie r deposite d sediment s (Yieldin g e t al . 1992), whic h i s no t take n int o accoun t i n th e present model .

Modelled Moho configuration To compar e th e modelle d Moh o dept h wit h the observations, we superimposed th e Moh o defor mation o f th e thre e tim e inteval s on eac h other , the resul t of whic h is show n in Fig . 12. The 'observed Moho ' i n Fig . 1 2 is derive d b y inter polation betwee n th e Moh o reflector s observe d in seismi c line s NSDP84-01 an d - 2 (e.g . Chris tiansson e t al . 2000 ; Odinse n e t al . 2000) . although w e acknowledge tha t th e Moh o i s no t unequivocally image d east o f the Viking Graben (see Fig. 3) . Starting from a constant initia l value of 3 5 km, th e modelle d Moh o dept h i s too larg e compared wit h th e dept h derive d fro m th e

Fig. 12 . Moh o uplif t a s a consequence o f rifting . Dashe d line , modelled Moh o dept h resultin g from thre e stages of rifting ; continuou s line , Moho dept h alon g lin e NVGT88-04, a s derive d fro m interpolatio n betwee n interpretations o f lin e NSDP84-0 1 an d NSDP84-02 .

LITHOSPHERE EXTENSIO N I N TH E VIKIN G GRABE N observations. Th e maximu m difference , belo w the Vikin g Grabe n axis , i s 6.5km . Thi s i s i n agreement wit h findings o f Odinsen e t al. (2000), who suggeste d tha t th e Permia n pre-rif t Moh o depth migh t hav e varie d fro m ±3 5 km o n th e flanks to les s tha n 30k m i n th e basi n areas . T o obtain th e bes t fi t fo r th e subsidenc e data , Lippard & Li u (1992 ) neede d t o assum e a n initial crusta l thicknes s varyin g fro m 28k m i n the centr e t o 30-3 2 km o n th e flank s o f th e Graben. Thi s pre-Permia n variatio n i n Moh o depth i s likely to b e caused b y Devonian crusta l

75

thinning, impose d o n th e thickene d crus t o f the Caledonia n Orogen y (Anderse n & Jamtvei t .1990; Fosse n & Rykkelid 1992) .

Mode o f flexur e i n the Vikin g Grabe n End-members: the coupled versus the decoupled mode An importan t resul t o f thi s modellin g stud y i s that i t i s har d t o discriminat e betwee n th e

Fig. 13 . Mod e diagram s fo r th e lithosphere , indicatin g the possibilit y of a decouple d rheolog y in Moh o depth-Moho temperatur e spac e (Spadin i & Podladchikov 1996) , for thre e differen t lithospher e rheologies . Boundaries between the ductile and brittle lower crust are given for strain rates of (from left t o right) 10~ 17, 10~ 16, 10~15, 10~ 14 an d 10~ 13 s" 1. Blac k dot s indicate th e assume d pre-rif t (r 0) and post-rif t (t\) Moh o dept h (a t th e position o f major Moh o uplift ) i n th e Vikin g Graben, a s derive d from Fig . 11. Th e ligh t grey arrow s indicate the chang e i n th e stat e o f th e lithospher e in cases o f instantaneou s rifting an d ver y slow rifting .

76


coupled an d th e decouple d mod e o f flexur e o n the basis of the fit between modelling result s an d seismic dat a alone (e.g. Fig . 9) . This might b e an explanation fo r th e larg e variation s betwee n EET value s use d fo r previou s modellin g studie s of th e Vikin g Graben. Nevertheless , w e showe d that th e presence or absenc e of footwall uplif t i s a goo d indicato r fo r th e mod e o f flexure : th e suspected presenc e o f erode d faul t block s i n th e Permo-Triassic phas e an d o f footwal l uplif t i n the lat e Jurassic phas e canno t b e explained wit h the couple d approac h (eve n thoug h w e use d a rather dee p neckin g level) , wherea s th e absenc e of footwall uplift i n early Jurassic tim e is impos sible t o reconstruc t b y decouple d flexure . Although caution shoul d b e taken when interpreting th e result s (w e modelle d onl y th e end members o f th e coupled-decouple d spectrum , whereas th e 'partl y decoupled ' mod e i s mos t likely to occur), th e best result s were obtained b y assuming a decoupled mod e o f flexura l respons e in th e Permo-Triassi c phase , a couple d mod e o f flexure in early Jurassi c time , an d a change bac k to decouple d flexur e i n lat e Jurassi c time . Th e choice o f th e mod e o f flexur e i s important , a s it ca n hav e a stron g influenc e o n th e derive d amount o f extension . Fo r th e Permo-Triassi c phase, fo r example , w e derive d a d facto r o f 1.18 fo r th e couple d cas e an d o f 1.2 9 fo r th e decoupled case . A prerequisit e for decouple d behaviou r i s th e existence o f a ductil e lowe r crust . Figur e 1 3 shows whic h Moh o dept h an d temperatur e will caus e th e manifestatio n o f a ductil e lowe r crust accordin g t o th e rheologica l laws . Th e curves wer e derive d b y calculatin g th e dept h a t

which brittl e deformatio n give s wa y t o ductil e deformation (Spadin i & Podladchikov 1996) , i.e. where cr b i s th e brittl e strengt h an d a d i s th e ductile strengt h neede d t o caus e deformatio n a t a give n strai n rat e (see Appendix 1) . The param eters use d are give n in Tabl e 3. The Moh o depths an d temperature s o n th e lef t sid e ar e associated wit h a n entir e brittl e crust , wherea s the condition s o n th e righ t sid e o f th e curve s yield a ductile lowe r crust. I f we assume th e bol d line t o b e th e pre-rif t geother m i n th e norther n Viking Graben , an d th e blac k do t a t t Q t o indicate th e stat e o f th e pre-rif t Moho , w e ca n conclude fro m th e figur e tha t th e lowe r crus t i s at least partly ductile, and that th e first condition for a decouple d mod e o f flexur e i s fulfilled . However, the uplift o f mantle material combined with syn-rif t coolin g wil l eventuall y resul t i n a shift fro m th e decouple d t o th e couple d stat e of the lithospher e (Spadin i & Podladchiko v 1996) . which agree s wit h ou r modellin g result s fo r th e Permo-Triassic an d th e Earl y Jurassi c phases . The chang e bac k t o th e decouple d stat e i n Bathonian tim e cannot b e explained in this way. or b e relate d t o th e chang e i n extensio n (an d thus strain ) rate . I n fact , a t lowe r strai n rates , decoupled behaviou r is more likel y to occur than at hig h strai n rate s (e.g . Bru n & Tro n 1993 ; Ter Voord e e t al. 1998 ; see Figs 6 and 13) . However, a therma l even t coul d explai n th e weaken ing o f th e lithospher e an d a shif t t o decouple d behaviour. A Mid-Jurassi c pre-rif t therma l dom e i n th e so-called 'tripl e junction' o f th e Nort h Se a rif t i s

Table 3 . Parameters used for construction of strength profiles

Upper crus t Lowe

r crus t

Quartzite (w) Quartzite (d) Diorit

e (wet ) Diabas e (dry )

£ p (kJmol-') 172. 6 Ap (Pa~"s-' ) 1.26 x 10~ 13 n 1. 9 £d (kJmor 1 ) -4d(s~ l ) crD (GPa ) R (J(molK)- 1 ) Thermal diffusivit y sediment s Thermal diffusivit y crus t Thickness heat producin g laye r Heat productio n

134 21 6.03 x 1(T 24 1.2 2.72 2.

_ _ -

2 27 6 6 x ID' 16 3.1 6 x 1(T 20 4 3.0 5

Mantle olivine 510 7 x 10~ 14 3.0 535 5.7 x 10 " 8.5

8.314J (molK)- 1 0.75 x lO^nrs- 1 l . O x 10- 6 m 2 s-' 15km 2.3 x 10- 6 Wm- 3

Material constant s fo r quartzite , diorite , diabas e an d olivin e ar e adopte d fro m Tsen n & Carter (1987) .

LITHOSPHERE EXTENSIO N I N TH E VIKIN G GRABE N well established (e.g . Ziegler & Van Hoorn 1989; Hendrie e t al. 1993 ; Underbil l & Partingto n 1993), evidenced b y basin-wide subaerial erosio n (the 'Mid-Cimmerian ' unconformity), th e extrusion o f the Rattra y volcani c rocks, and floodin g events o n basi n margins . Thi s therma l dom e is interprete d t o b e th e effec t o f a short-lived , anomalously ho t asthenospheri c puls e (Hendrie et al . 1993 ) o r transien t mantl e plum e (Underbill & Partington 1993) . However, althoug h th e timing of this event is correct t o explain the shif t to decouple d behaviou r i n Bathonia n time , th e location migh t b e too fa r awa y (i.e. ±250 km t o the south ) fro m th e norther n Vikin g Grabe n to hav e suc h a pronounce d effect .

The partly decoupled mode of flexure As mentione d above , i n thi s stud y w e onl y look a t th e end-member s o f th e spectru m o f decoupled-coupled mode s o f flexure . Never theless, th e mos t probabl e mechanis m t o occu r is th e partl y decouple d mod e o f flexure . I n thi s case, th e viscosit y o f th e lowe r crus t i s to o high t o allo w fo r a n isostati c compensatio n level i n th e lowe r crust . Therefore , a s a firs t approximation, th e lithospher e wil l reac t i n a coupled mode , an d onl y short-wavelengt h lateral variatio n i n th e loa d i s compensate d b y lower-crustal flow. In general , modelle d surfac e deflection s obtained b y assumin g a partl y decoupled lithosphere diffe r fro m th e fully couple d mod e in tha t an extr a short-wavelengt h componen t ca n b e observed. I n contrast , th e differenc e fro m th e fully decouple d cas e i s that th e long-wavelength uplift componen t disappear s (Te r Voord e e t al . 1998). Translatin g thi s t o Fig s 9-1 1 raise s th e suggestion o f partl y decoupled flexure.

Conclusions The evolutio n o f th e Vikin g Grabe n ha s bee n studied usin g a numerical model, constrained by seismic lin e NVGT88-04 . Althoug h th e impor tance o f th e rif t phas e i n Permo-Triassi c tim e has bee n affirme d b y th e modellin g results , th e post-rift subsidenc e i s shown t o hav e practically ceased a t th e onse t o f Jurassi c rifting . Hence , no significan t amoun t o f basi n subsidenc e i n the Jurassi c phas e ca n b e ascribe d t o therma l contraction resultin g fro m th e Permo-Triassi c event. Th e Jurassi c subsidenc e shoul d therefor e be explained entirely by a new extensional event,

77

possibly precede d b y th e 'proto-rift ' stag e a s defined b y Nottved t e t al . (1995). The position o f the rift axi s is shown to change with time . Th e Permo-Triassi c rif t axi s wa s positioned beneat h th e present Horda Platform, whereas th e Jurassi c rif t axi s wa s positione d beneath th e present Vikin g Graben centre . Thi s result migh t be dependent o n th e positio n alon g strike (Faerset h 1996) . Fo r example , Odinse n et al . (2000 ) foun d fo r lin e NDSP84- 1 tha t th e maximum (3 factors fo r bot h th e Permo-Triassi c and th e Jurassi c phase s wer e i n th e presen t Viking Grabe n axis , wherea s fo r th e mor e southern lin e NSDP84-0 2 th e maximu m ( 3 factor i n th e Permo-Triassi c phas e wa s o n th e Horda Platform . To ou r knowledge , this basin modelling study is th e firs t on e tha t attempt s t o discriminat e between th e couple d an d th e decouple d mod e of flexure . Th e majo r differenc e betwee n thes e two mode s i s th e amoun t o f fault-bloc k rota tion, which is larger whe n a decouple d rheolog y is assumed. The coupled and decouple d mode of flexure are sometime s har d t o discriminat e o n the basi s o f basin geometrie s alon e (e.g . Fig . 9) , which ma y explai n th e larg e variatio n i n EE T values foun d i n th e literature . Nevertheless, the modelling result s sugges t a decouple d mod e o f flexure i n th e Permo-Triassi c phase , a couple d mode o f flexur e i n earl y Jurassi c time , an d a change bac k t o decoupled flexure in Lat e Juras sic time . Subsequen t researc h o n thi s topi c should focu s o n furthe r constraint s o n th e mode o f flexure, such as the lower-crustal thickness an d material , an d th e pre-rif t geotherm . We woul d lik e t o than k R . Ravna s fo r bringin g u s together, an d T . Odinse n fo r discussion s abou t th e development o f th e Vikin g Graben . Review s b y R. Boutilier and D. Waltham were greatly appreciated. This researc h wa s supporte d b y th e IB S (Integrate d Basin Studies ) project , par t o f th e Joul e I I researc h programme funde d b y th e Commissio n o f Europea n Communities (Contract JOU2-CT92-0110). This paper is Publicatio n 98100 1 o f th e Netherland s Researc h School of Sedimentary Geology.

Appendix 1 The brittl e yiel d strengt h i s give n b y Byerlee' s law (Byerle e 1978 ; Brace & Kohlstedt 1980) :

where p i s th e density , g i s th e gravitationa l acceleration, z i s the depth , a = 0.75, A =0 for zero por e pressure , an d A = 0.4 fo r hydrostati c pore pressure . Ductil e deformatio n i s assume d

78


to occu r b y power la w cree p (Kirb y 1983) :

where e is strain rate, A, n and E p ar e empiricall y derived materia l constants , R i s th e ga s con stant an d T i s th e absolut e temperature . Fo r olivine with a strengt h exceeding 200 MPa, duc tile deformatio n i s describe d b y Dor n cree p (Goetze & Evans 1979 ; Tsenn & Carter 1987) :

Parameters ar e give n i n Tabl e 3 .

Appendix 2 The dept h o f necking is the leve l of zero vertical motion i n th e absenc e o f isostati c forces . Thi s level determines the ratio between thinning of the upper crust, where crustal material i s replaced by sediments, and thinnin g of the lower lithosphere, where crusta l materia l i s replace d b y mantl e material. Therefore , th e dept h o f neckin g i s a decisive facto r fo r th e ne w loa d distributio n caused b y crustal extension. For a give n amount of thinning , the chang e i n loa d A P cause d b y a variation i n neckin g dept h A N i s equa l t o th e change i n loa d A P cause d b y a variatio n i n sediment densit y Ap s. Ap s ca n b e calculate d as follows.

Fig. A2 . Thinnin g o f th e crus t aroun d neckin g level s N] o r W 2, assumin g the depressio n o f th e surfac e i s equal (i.e . 3 is different) fo r bot h cases. Reference loa d P I cause d b y thinning around necking leve l A^ , usin g sediment density ps] (see Fig. Al):

where g i s gravitational acceleration (ms~2 ), 3 is a stretchin g factor , p u i s densit y o f th e uppe r crust (kgm~ 3 ), p \ i s densit y o f th e lowe r crus t (kgirT 3 ), p m i s densit y o f th e mantl e (kgrrr 3 ) and M i s the Moh o dept h (km) . AP cause d b y thinnin g around N 2 instea d o f around TV, :

AP cause d b y usin g sedimen t densit y p instead o f p\:

2

This yield s the followin g equivalenc e betwee n &N(=N\ - N 2) an d A p ( =pi - p 2):

Fig. Al . Thinnin g of th e crus t aroun d neckin g level s N] o r A r2, assuming (3 is equal (i.e. the depressio n of th e surface i s different) fo r bot h cases .

If w e assum e th e depressio n o f th e surfac e a s a result of crustal thinning to be constant (as it can

LITHOSPHERE EXTENSIO N I N TH E VIKIN G GRABE N be observed), a change in neckin g depth implies a chang e in th e j3 factor (se e Fig . A2) . The n

and

which is the equatio n to be used for the cas e of the Vikin g Graben .

References ANDERSEN, T. B . & JAMTVEIT , B. 1990 . Uplift o f dee p crust durin g erogeni c extensiona l collapse : a model base d o n fiel d studie s i n th e Sogn Sunnfjord regio n o f wester n Norway . Tectonics, 9, 1097-1112 . BADLEY, M . E. , EGEBERG , T . & NIPEN , O . 1984 . Development o f rif t basin s illustrate d b y th e structural evolutio n o f th e Oseber g structure , Block 30/6 , offshor e Norway . Journal o f th e Geological Society, London, 141 , 639-649 . , PRICE , J . D. , RAMBEC H DAHL , C . & AGDE STEIN, T . 1988 . Th e structura l evolutio n o f th e northern Vikin g Grabe n an d it s bearin g upo n extensional mode s o f grabe n formation . Journal of th e Geological Society, London, 145 , 455-472 . BALDWIN, B . & BUTLER , C . O . 1985 . Compactio n curves. AAPG Bulletin, 69, 622-626. BEACH, A., BIRD , T . & GIBBS , A . D . 1987 . Extensiona l tectonics an d crusta l structure : dee p seismi c reflection dat a fro m th e norther n Nort h Se a Viking Graben . In : COWARD , M . P. , DEWEY , J . F. & HANCOCK, P. L. (eds) Continental Extensional Tectonics. Geologica l Society , London , Specia l Publications, 28, 467-476. BERTRAM, G . T . & MILTON , N . J . 1988 . Reconstructing basi n evolutio n fro m sedimentar y thickness: the importance of paleobathymetric control, with reference t o th e Nort h Sea . Basin Research, 1 , 247-259. BODINE, J . H. , STECKLER , M. S . & WATTS, A. B . 1981 . Observations of flexure and rheology of the oceanic lithosphere. Journal o f Geophysical Research, 86 , 3695-3707. BRACE, W . F . & KOHLSTEDT , D . L . 1980 . Limit s on lithospheri c stress impose d b y laborator y experiments. Journal o f Geophysical Research, 85 , 6248-6252. BRAUN, J . & BEAUMONT , C . 1989 . A physica l explanation o f th e relatio n betwee n flan k uplift s and th e break-unconformit y a t rifte d continental margins. Geology, 17 , 760-764. BRUN, J . P & TRON , V . 1993 . Developmen t o f th e North Vikin g Graben: inferences fro m laborator y modelling. Sedimentary Geology, 86, 31-51 . BUCK, W. R . 1988 . Flexural rotation of normal faults . Tectonics, 7, 959-973.

79

BUROV, E . & DIAMENT , M . 1995 . The effectiv e elasti c thickness (r e) o f continenta l lithosphere : wha t does i t reall y mean ? Journal o f Geophysical Research, 100 , 3905-3927 . BYERLEE, J . D . 1978 . Frictio n o f rocks . Pure an d Applied Geophysics, 116, 615-626 . CARTER, N . L . & TSENN, M . C . 1987 . Flow propertie s of th e continenta l lithosphere . Tectonophysics, 136, 27-63 . CHRISTIANSSON, P. , FALEIDE , I . J . & BERGE , A . M . 2000. Crustal structure in the northern North Sea : an integrate d geophysical study . This volume. DYKSTRA, 1987 . Compactio n correctio n fo r buria l history curves : applicatio n t o Lopatin' s metho d for sourc e roc k maturatio n determination . Geobyte, Nov . 87 , 16-23 . FALEIDE, J. I . et al. 2000. Deep basi n geometries in the northern North Sea ; constraints from gravity and magnetic data . FJELLANGER, E. , OLSEN , T . R . & RUBINO , J . L . 1996 . Sequence stratigraph y an d palaeogeograph y o f the Middl e Jurassi c Bren t an d Vestlan d deltai c systems, Norther n Nort h Sea . Norsk Geologisk Tidsskrift, 76 , 75-106. FOSSEN, H . & GABRIELSEN , R. H . 1996 . Experimental modelling o f extensiona l fault system s by us e o f plaster. Journal of Structural Geology, 18, 673-687. & RYKKELID , E. 1992 . Post-collisional extension of th e Caledonid e oroge n i n Scandinavia : struc tural expression s an d tectoni c significance . Geology, 20 , 737-740. , ODINSEN , T. , F^RSETH , R . B . & GABRIEL SEN, R . H . 2000 . Detachment s an d low-angl e faults i n th e norther n Nort h Se a rif t system . This volume. FURNES, H., MITCHELL, J. G., ROBINS, B., RYAN , P. D . & SKJERLIE , F . 1982 . Petrography an d geochem istry of peralkaline, ultrapotassi c syenit e dykes of Middle Permia n age , Sunnfjord , Wes t Norway . Norsk Geologisk Tidsskrift, 62 , 147-159 . FÊRSETH, R . B . 1978 . Mantle-derived Iherzolit e xenoliths an d megcryst s fro m Permo-Triassi c dykes , Sun-nhordland, Wester n Norway . Lithos, 11 , 23-35. 1996. Interactio n o f Permo-Triassi c an d Jurassi c extensional fault-block s during th e developmen t of th e norther n Nort h Sea . Journal o f th e Geological Society, London, 153 , 931-944 . , GABRIELSEN , R . H . & HURICH , C . A . 19950 . Influence o f basement in structuring of the Nort h Sea basin , offshor e southwes t Norway . Norsk Geologisk Tidsskrift, 75 , 105-119 . , MCINTYRE , R . M . & NATERSTAD , J . 1976 . Mesozoic alkalin e dyke s i n th e Sunnhordlan d region, wester n Norway : ages , geochemistr y an d regional significance . Lithos, 9, 332-345. , SJOBLOM , T . S. , STEEL , R . J. , LILJEDAHL , T. , SAUAR, B . E . & TJELLAND , T . 19956 . Tectoni c controls o n Bathonian-Volgia n syn-rif t succes sions on th e Visund Fault Block , northern Nort h Sea. In : STEEL , R . J. , FELT , V. , JOHANNESSEN , E. & MATHIEU , C. (eds) Sequence Stratigraphy o n the Northwest European Margin. Norwegia n Petro leum Society , Specia l Publication , 5 , 325-346.

80


GABRIELSEN, R . H . 1986 . Structura l element s i n graben system s an d thei r influenc e on hydrocar bon tra p types . In : SPENCER , A . M . e t al. (eds ) Habitat of Hydrocarbons on the Norwegian Continental Shelf. Graha m an d Trotman , London , 55-60. , R€RSETH , R . B. , STEEL , R . J. , IDIL , S . & KL0VJAN, O. S . 1990. Architectural styles of basin fill in the norther n Vikin g Graben. In: BLUNDELL , D. J . & GIBBS , A . D . (eds ) Tectonic Evolution o f th e North Se a Rifts. Clarendon , Oxford , 158-179. GIBBS, A . 1987 . Deep seismi c profile s i n th e Norther n North Sea . In: BROOKS , J . & GLENNIE , K . W . (eds) Petroleum Geology o f North-West Europe. Graham an d Trotman , London , 1925-1928 . GILTNER, J . P . 1987 . Application s o f extensiona l models t o th e Norther n Vikin g Graben . Norsk Geologisk Tidsskrift, 67 , 339-352 . GOETZE, C. & EVANS , B . 1979 . Stress and temperatur e in th e bendin g lithospher e a s constraine d b y experimental rock mechanics. Geophysical Journal of th e Roval Astronomical Society, 59 , 463-478. HAQ, B . U./HARDENBOL , J . & VAIL , P . R . 1987 . Chronology o f fluctuatin g se a level s sinc e th e Triassic (25 0 millio n year s ag o t o present) . Science, 235, 1156-1166 . HARLAND, W . B. , ARMSTRONG , R . L. , Cox , A . V. , CRAIG, L . E. , SMITH . A . G . & SMITH, D . G . 1990 . A Geologic Time Scale. Cambridg e Universit y Press, Cambridge . , Cox, A . V. , LLEWELLYN , P. B. , PICKTON , C . A. , SMITH, A . G . & WALTERS , R . 1982 . A Geological Time Scale. Cambridg e Universit y Press , Cam bridge. HENDRIE, D . B. , KUSZNIR , N . J . & HUNTER , R . H . 1993. Jurassi c extensio n estimate s fo r th e Nort h Sea 'tripl e junction' fro m flexura l backstripping : implications fo r decompressio n meltin g models . Earth and Planetary Science Letters, 116, 113-127 . HOSSACK, J . R . 1984 . The geometr y o f listri c growth faults i n th e Devonia n basin s of Sunnfjord , Wes t Norway. Journal o f th e Geological Society, London, 141 , 629-638 . JOHNS, C . R . & ANDREWS , I . J . 1985 . The petroleu m geology of the Unst Basin , North Sea. Marine and Petroleum Geology, 2 , 361-372 . KIRBY, S . H . 1983 . Rheolog y o f th e lithosphere . Review o f Geophysics an d Space Physics, 21 . 1458-1487. Kooi, H . 1991 . Tectonic modelling o f extensional basins: the role of lithosphere flexure, intraplate stress an d relative sea-level change. Ph D thesis , Vrije Universiteit , Amsterdam . . CLOETINGH , S . & BURRUS, J. 1992 . Lithospheri c necking an d regiona l isostac y a t extensiona l basins, 1 . Subsidence an d gravit y modelling with an applicatio n t o th e Gul f o f Lion s margi n (SE France) . Journal of Geophysical Research, 97, 17553-17571. KUSZNIR. N . J. , MARSDEN , G . & EGAN , S . S . 1991 . A flexural cantilever simple-shear/pure-shea r mode l of continenta l lithospher e extension : application s to th e Jeann e d'Arc-Basin , Gran d Bank s an d

Viking Graben , Nort h Sea . In : ROBERTS . A . M. . YIELDING, G . & FREEMAN, B . (eds) Th e Geometry of Normal Faults. Geologica l Society , London . Special Publications , 56 , 41-60. LE PICHON , X . & CHAMOT-ROOKE , N . 1991 . Exten sion o f continenta l crust . In : MULLER . D . W. . MCKENZIE, J . A . & WEISSERT . H . (eds ) Controversies in Modern Geology: Evolution of Geological Theories in Sedimentology, Earth History and Tectonics, Academic , London , 313-338 . LERVIK, K . S. , SPENCER , A . M . & WARRINGTON . G . 1989. Outlin e of Triassic stratigraphy and structure in th e central and northe n North Sea . In : COLLINSON, J . (ed. ) Correlation in Hydrocarbon Exploration. Graham an d Trotman, London, 173-189 . LIPPARD, S . & Liu, G. 1992 . Tectonic modelling of th e northern Nort h Se a usin g progra m RIFT . In : LARSEN. R . M. , BREKKE . H. . LARSEN . B . T . & TALLERAAS , E . (eds ) Structural an d Tectonic Modelling and its Application to Petroleum Geology. Norwegia n Petroleu m Society . Specia l Pub lication, 1 , 43-54. MARSDEN, G. . YIELDING . G. , ROBERTS . A . M . & KUSZNIR, N . J . 1990 . Applicatio n o f a flexura l cantilever simple-shea r pure-shear mode l o f con tinental extension on th e formatio n o f the north ern Nort h Se a basin . In : BLUNDELL , D . J . & GIBBS, A . D . (eds ) Tectonic Evolution of th e North Sea Rifts. Clarendon . Oxford , 236-257 . MCCLAY, K . R. , NORTON. M . G.. CONEY. P . & DAVIS. G. H . 1986 . Collaps e o f th e Caledonia n oroge n and th e Old Red Sandstone. Nature. 323. 147-149 . MORTON. N. . SMITH , R . M. . GOLDEN . M . & JAMES . A. V. 1987. Comparative stratigraphic study of the Triassic-Jurassic sedimentatio n an d basi n evolu tion i n th e norther n Nort h Se a and northwes t of the Britis h Isles. In: BROOKS, J . & GLENNIE. K . W . (eds) Petroleum Geology o f North-West Europe. Graham an d Trotman. London . 697-710 . NELSON. P . H . H . & LAMY . J . M . 1987 . The Mor e West Shetland s area : a review . /// : BROOKS . J . & GLENNIE . K . W . (eds ) Petroleum Geology of North-West Europe. Graha m an d Trotman . London. 775-784 . NOTTVEDT. A. , GABRIELSEN . R . H . & STEEL . R . J . 1995. Tectonostratigraphy an d sedimentar y architecture o f rif t basins , with reference t o th e northern North Sea. Marine and Petroleum Geology, 12. 881-901. ODINSEN. T.. REEMST , P.. VA N DER BEEK. P. . FALEIDE . J. I . & GABRIELSEN . R. H . 2000 . Permo-Triassi c and Jurassi c extension in the northern Nort h Sea : results fro m tectonostratigraphi c forwar d modelling. This volume. PLATT. N . H . 1995 . Structur e an d tectonic s o f th e northern Nort h Sea : ne w insight s fro m dee p penetration regiona l seismic data . In : LAMBIASE . J. J . (ed. ) Hydrocarbon Habitat i n Rift Basins. Geological Society. London. Special Publications . 80. 103-113 . RAVNAS. R . & BONDEVTK . K . 1999 . Architectur e an d controls o n Bathonian-Kimmeridgia n shallow marine syn-rift wedge s of the Oseberg-Brage area , northern Nort h Sea . Basin Research, in press.

LITHOSPHERE EXTENSIO N I N TH E VIKIN G GRABE N HELLAND-HANSEN, W. , L0MO , L. , RY SETH, R . & STEEL , R . J . 1999 . Sedimentatio n history as an indicato r of rif t initiatio n and development: th e late Bajocian-Bathonian evolutio n of the Oseberg-Brag e area , norther n Nort h Sea . Norsk Geologisk Tidsskrift, i n press . ROBERTS, A. , YIELDING, G. , KUSZNIR , N., WALKER , I . & DORN-LOPEZ , D . 1993 . Mesozoi c extensio n i n the Nort h Sea : constraint s fro m flexura l back stripping, forwar d modellin g an d faul t popula tions. In: PARKER, J. R . (ed.) Petroleum Geology of Northwest Europe: Proceedings of the 4th Conference. Geologica l Society , London, 1123-1136 . ,, , & 1995 . Quantitativ e analysis o f Triassi c extensio n i n th e norther n Viking Graben. Journal of th e Geological Society, London, 152 , 15-26 . R0E, S . L . & STEEL , R . J . 1985 . Sedimentation , sealevel ris e and tectonic s at th e Triassic-Jurassi c boundary (Statfjor d Formation) , Tampe n Spur , northern Nort h Sea . Journal o f Petroleum Geology, 8 , 163-186 . SCLATER^ J . G . & CHRISTIE , P . A . F . 1980 . Conti nental stretching: a n explanation o f the post-mid Cretaceous subsidenc e o f th e Nort h Se a basin . Journal o f Geophysical Research, 85, 3711-3739 . SNEIDER, J . S. , D E CLARENS, P . & VAIL , P . R . 1995 . Sequence stratigraph y o f th e Middl e t o Uppe r Jurassic, Vikin g Graben , Nort h Sea . In: STEEL , R. J., FELT , V., JOHANNESSEN , E . & MATHIEU , C . (eds) Sequence Stratigraphy o n th e Northwest European Margin. Norwegia n Petroleu m Society , Special Publication , 5 , 167-197 . SPADINI, G . & PODLADCHIKOV , Y . 1996 . Spacin g o f consecutive norma l faultin g i n th e lithosphere : a dynamical mode l fo r rif t axi s migratio n (Tyr rhenian Sea). Earth and Planetary Science Letters, 144, 21-34 . STEEL, R . 1993 . Triassic-Jurassic megasequence strati graphy i n the Northern Nort h Sea : rif t t o post-rift evolution. In: PARKER, J. R . (ed. ) Petroleum Geology of Northwest Europe. Proceedings of the 4th Conference. Geologica l Society , London, 299-315. & RYSETH , A . 1990 . The Triassic-earl y Jurassi c succession i n th e norther n Nort h Sea : megase quence stratigraph y an d intra-Triassi c tectonics. In: HARDMAN , R . P . F . & BROOKS , J . (eds ) Tectonic Events Responsible for Britain's Oil and Gas Reserves. Geologica l Society , London , Spe cial Publications , 55 , 139-168 . TER VOORDE , M . & BERTOTTI , G . 1994 . Therma l effects o f norma l faultin g durin g rifte d basi n formation. 1 : A finit e differenc e model . Tectonophysics, 240 , 133-144 . & CLOETINGH , S . 1996 . Numerical modelling of extension i n faulted crust : effects o f localized an d regional deformatio n o n basi n stratigraphy . In : BUCHANAN, P . G . & NIEUWLAND , D . A . (eds ) Modern Developments in Structural Interpretation, Validation an d Modelling. Geologica l Society , London, Specia l Publications , 99 , 283-296. , RAVNAS , R. , F^RSETH , R . & CLOETINGH , S . 1997. Tectoni c modellin g o f th e middl e Jurassi c synrift stratigraph y i n th e Oseberg-Brag e area ,

81

northern Vikin g Graben . Basin Research, 9 , 133-150. , VAN BALEN, R . T. , BERTOTTI , G . & CLOETINGH , S. A . P . L . 1998 . Th e influenc e o f a stratifie d rheology o n th e flexura l respons e o f th e litho sphere t o (un)loadin g b y extensiona l faulting . Geophysical Journal International, 134 , 721-735 . TORSVIK, f. H. , STURT , B . A., SWENSSON , E. , ANDER SEN, T . B . & DEWEY , J . F . 1992 . Palaeomagneti c dating o f faul t rocks : evidenc e fo r Permia n an d Mesozoic movement s an d brittl e deformatio n along th e extensiona l Dalsfjor d Fault , wester n Norway. Geophysical Journal International, 109 , 565-580. TSENN, M . C . & CARTER , N . L . 1987 . Upper limits of power la w cree p o f rocks . Tectonophysics, 136 , 1-26. UNDERBILL, J . R . & PARTINGTON , M . A . 1993 . Jurassic therma l domin g an d deflatio n i n th e North Sea : implication s o f th e sequenc e strati graphic eveidence . In : PARKER , J . R . (ed. ) Petroleum Geology of Northwest Europe: Proceedings o f th e 4t h Conference. Geologica l Society , London, 337-345 . WALTHAM, D . 1989 . Finit e differenc e modellin g o f hanging wal l deformation . Journal o f Structural Geology, 11 , 433-437. 1990. Finit e differenc e modellin g o f sandbo x analogues, compaction and detachment free deformation. Journal of Structural Geology, 12,375-381. WEISSEL, J . K . & KARNER , G. D . 1989 . Flexural uplif t of rif t flank s du e t o mechanica l unloadin g o f th e lithosphere durin g extension . Journal o f Geophvsical Research, 94 , 1 3 919-13 950. WHITE, N . J. 1990 . Does the uniform stretching model work i n th e Nort h Sea ? In : BLUNDELL , D . J . & GIBBS , A . D . (eds ) Tectonic Evolution o f th e North Se a Rifts. Clarendon , Oxford , 217-240 . & YIELDING , G . 1991 . Calculatin g norma l faul t geometries a t depth : theor y an d examples . In : ROBERTS, A . M. , YIELDING , G . & FREEMAN , B . (eds) Th e Geometry o f Normal Faults. Geolog ical Society , London , Specia l Publications , 56 , 251-260. , JACKSON , J. A . & MCKENZIE , D . P . 1986 . Th e relationship betwee n th e geometr y o f norma l faults an d tha t o f th e sedimentar y layer s in thei r hanging walls . Journal o f Structural Geologv, 9 , 789-795. WITHJACK, M . O . & PETERSON, E . T. 1993 . Predictio n of normal-faul t geometrie s - a sensitivit y study. AAPG Bulletin, 11 , 1860-1873 . YIELDING, G. , BADLEY , M . E . & ROBERTS , A . 1992 . The structural evolution of the Brent Province. In: MORTON, A . C. , HASELDINE , R . S. , GILES , M . R . & BROWN , S . (eds ) Geology o f th e Brent Group. Geological Society, London, Specia l Publications , 61, 27-40. ZIEGLER, P . & VA N HOORN , B . 1989 . Evolutio n o f North Se a rif t systems . In : TANKARD , A . J . & BALKWILL, H . R . (eds ) Extensional Tectonics and Stratigraphy of the North Atlantic Margins. American Associatio n o f Petroleu m Geologists , Memoir, 46, 471-500.

This page intentionally left blank

Permo-Triassic an d Jurassic extension in the norther n Nort h Sea: results fro m tectonostratigraphi c forward modelling TORE ODINSEN,

14

PAU L REEMST,

JAN ING E FALEIDE 1

3

25

PETE R VA N DE R BEEK,

& RO Y H . GABRIELSEN

26

1

Department of Geology, University of Bergen, Allegaten 41, N-5007 Bergen, Norway 2

Faculty of Earth Sciences, Vrije Universiteit, De Boelelaan, 1081 HV Amsterdam, The Netherlands ^Department of Geology, University of Oslo, P.O. Box 1047 Blindern, N-0316 Oslo, Norway 4 5

Present address: S tat oil, N-5020 Bergen, Norway

Present address: Geologica AS, P.O. Box 8034, N-4029 Stavanger, Norway 6

Present address: Universite Joseph Fourier, 15 rue Maurice Gignoux, 38031 Grenoble Cedex, France

Abstract: W e have undertaken 2D forward modelling across th e northern North Sea, based on reprocessed , interprete d an d depth-converte d dee p reflectio n seismi c line s NSDP84- 1 and - 2 (15stwt) an d refractio n data . Tw o separat e stretchin g phases , Permo-Triassi c an d Jurassic, ar e recognized . Th e cumulativ e stretchin g i s consistent wit h th e observe d crusta l structure an d th e overal l basi n configuration , as reproduce d b y forwar d modelling . Good agreement betwee n observe d an d modelle d to p basemen t level , and crusta l thickness below the platfor m area s ar e particularl y emphasized . Crustal-scal e modellin g indicate s tha t crustal thicknes s varied acros s th e norther n Nort h Se a at th e onse t o f th e Permo-Triassi c rifting, fro m c. 35km i n th e platfor m area s t o les s than 30k m i n th e interio r o f th e basin . This ma y b e ascribe d t o Devonian(-Carboniferous? ) crusta l stretching . Thinnin g o f th e crust ha s progressivel y bee n narrowed , fro m post-Caledonia n extensiona l collapse , t o les s regional Permo-Triassi c basins , an d finall y developmen t o f th e Vikin g Graben are a i n th e Jurassic-early Cretaceou s time . Mos t o f th e Permo-Triassi c stretchin g occurre d betwee n the 0ygarden Fault Zon e t o the east and th e Shetland Platfor m (souther n transect) and th e Hutton Faul t alignmen t t o th e west . Th e widt h o f th e Permo-Triassi c basi n wa s c . 120125km, with calculated /3 mean between 1.3 8 an d 1.40 . Permo-Triassi c /3 mean estimates across the present Horda Platform vary between 1.3 3 and 1.39 . The Jurassic /3 mean estimates for the same are a var y betwee n 1.0 8 and 1.13 . Acros s th e Vikin g Graben , Permo-Triassi c /3 mean varies betwee n 1.2 8 (southern transect ) an d 1.4 1 (northern transect) . Thi s i s lowe r tha n estimates fo r th e Jurassi c /3 mean, whic h amount s t o 1.5 3 and 1.42 . Permo-Triassic an d Jurassic /3 mean estimate s acros s th e Eas t Shetlan d Basi n ar e 1.2 9 and 1.11 , respectively . Lithospheric therma l evolutio n reflects th e genera l difference s betwee n Permo-Triassic an d Jurassic stretching , wit h a muc h wide r therma l perturbatio n durin g th e forme r an d a focusing an d latera l migratio n toward s th e eas t o f th e pea k therma l elevatio n durin g th e latter. Ther e ar e stil l uncertaintie s related t o th e degre e o f (de)couplin g between th e uppe r crust an d uppe r mantl e durin g th e Permo-Triassi c an d th e Jurassi c rif t phases . Thes e uncertainties ar e relate d t o th e interpla y between age, strain rate , crusta l rheology , crustal thickness an d long-live d zones o f weaknesses.

The norther n Nort h Se a ha s bee n extensivel y phase . A previous rif t phas e is best recognized in described i n the literature as an extensiona l sedi- reflectio n seismi c line s whic h cros s th e Hord a mentary basin , whic h forme d durin g repeate d Platform , a s deepl y buried , rotate d larg e faul t lithospheric stretching . A s a resul t o f hydro - blocks . Becaus e o f th e lac k o f wel l dat a fro m carbon exploratio n o f th e area , muc h attentio n syn-rif t sediment s associate d wit h thes e struc has focuse d o n rotated faul t block s tha t evolve d tures , th e age of this earlier even t i s still a matte r during th e mid-Jurassi c t o earl y Cretaceou s rif t o f debate , an d bot h Permia n (Eyno n 1981 ; From: N0TTVEDT , A . e t al. (eds ) Dynamics o f th e Norwegian Margin. Geologica l Society , London , Specia l Publications, 167 , 83-103. 1-86239-056-8/00/S15.0 0 © Th e Geologica l Societ y of Londo n 2000 .

84


Badley e t al. 1988 ; Gabrielsen e t al. 1990 ; Faer seth e t al . 1995 ) and Triassi c (Beac h e t al . 1987 ; Giltner 1987 ; Robert s e t al . 1995 ) age s hav e been proposed . Several studies have also focused on the crustal and lithospheri c configuration across th e north ern North Sea, as interpreted fro m a deep seismic reflection surve y NSDP8 4 an d othe r dee p line s (Beach 1986 ; Beach e t al . 1987 ; Klemperer 1988 ; Klemperer & Huric h 1990 ; Brun & Iron 1993) . Limitations i n th e dat a qualit y hav e permitte d several interpretation s o f th e crusta l an d litho spheric configuration. It ha s bee n suggeste d tha t lithospheric stretchin g wa s governe d b y sym metrical pur e shea r (Giltne r 1987 ; Badle y e t al . 1988), decouple d symmetrica l shea r (Klemp -

erer 1988) , decoupled asymmetrica l shea r (Res ton 1990) , o r asymmetrica l simpl e shea r (Beac h 1986; Beac h e t al . 1987 ; Gabrielsen 1989) . Modelling o f basi n formatio n i n th e Nort h Sea wa s initiate d b y th e wor k o f McKenzi e (1978), wh o use d th e are a a s on e o f hi s typ e examples. Hi s widel y recognize d mode l fo r lithospheric stretchin g ha s bee n followe d b y a number o f relate d studies . Earl y work s wer e concentrated o n th e Central Graben area , where ID subsidenc e analysi s was carrie d ou t (Sclater & Christi e 1980 ; Woo d 1981 ; Wood & Barto n 1983). Mor e recently , acces s t o conventiona l seismic reflectio n dat a an d wel l dat a move d attention toward s th e norther n Nort h Se a (Fig. 1) . Suc h dat a facilitate d calculatio n o f

Fig. 1 . Ke y ma p an d mai n structura l elements, northern North Sea (referenc e leve l i s base Cretaceous). Area s that ar e primaril y affecte d b y Jurassic-early Cretaceou s stretching ar e show n i n grey . Location s of transect s 1 and 2 are shown .

NORTH SE A PERMO-TRIASSI C AN D JURASSI C EXTENSIO N stretching estimate s fro m subsidenc e an d sum mation o f fault heave (Giltner 1987 ; Badley et al. 1988). Som e worker s als o hav e focuse d o n stretching estimate s lookin g a t crusta l thin ning derive d fro m dee p seismi c reflectio n line s (Beach e t al . 1987 ; Klemperer 1988) . The dee p lines have not previousl y been subjected to mor e advanced quantitativ e studie s suc h a s forwar d or revers e modelling. The presen t stud y i s base d o n th e frequentl y used dee p (15stwt ) seismi c reflectio n line s NSDP84-1 and - 2 (transect 1 and 2 of the present study), whic h hav e bee n post-stac k reprocesse d and redisplaye d (Figs 2 a an d 3a ) an d dept h converted (Fig s 2 b an d 3b) . Detaile d strati graphic an d structura l architectur e hav e bee n interpreted fro m high-qualit y conventional seismic line s (7stwt) . T o obtai n optima l velocit y information, velocitie s from well s were used fo r the uppermos t 3- 4 s o f th e section , wherea s refraction velocitie s fro m expande d sprea d pro files (ESPs) were applied to the deeper parts of the transects. Th e dee p basi n geometr y i s furthe r constrained by gravity and magnetic data (Christiansson et al. 2000). We studied the thermal an d tectonic evolution in the northern Viking Graben by forwar d modelling , integrating Permo-Trias sic an d mid-Jurassic-earl y Cretaceou s litho spheric stretching . Thre e mai n topic s wer e th e specific subjec t of our stud y of transects 1 and 2 : to estimat e an d compar e Permo-Triassi c an d Jurassic stretching ; t o evaluat e th e modelle d crustal configuratio n with respec t t o th e strati graphy, an d th e basemen t an d Moh o reliefs ; t o evaluate lithospheric thermal evolution. The stud y extend s previou s studie s i n tha t we full y integrat e commercia l an d dee p seismi c lines t o bette r constrai n th e structura l architec ture an d basemen t topography . Ou r modellin g approach allow s u s t o trac k th e therma l an d mechanical evolutio n o f th e lithosphere , a s wel l as basi n stratigraphy , throug h multipl e finiterate riftin g phases , thu s allowin g meaningfu l predictions o f stratigraph y t o b e made. W e emphasize tha t thi s pape r i s par t o f continuin g work i n th e Nort h Se a Basin , an d th e reade r will therefor e benefi t fro m additiona l readin g of Christiansso n et al. (2000) , Fosse n et al. (1999), Odinse n e t al . (2000 ) an d Te r Voord e et al . (2000) . Th e wor k o f Te r Voord e e t al . (2000) is especially relevant as they take som e of the presen t modellin g result s furthe r int o a new modelling tool . Seismic dat a Transect 1 ha s a lengt h o f 270k m an d run s NW-SE. Fro m SE , th e transec t crosses th e

85

0ygarden Faul t Zon e an d Hord a Platform . It continue s acros s th e interio r o f th e Vikin g Graben an d Eas t Shetlan d Basin , onto th e Eas t Shetland Platfor m (Fig . 2) . Transect 2 i s 270k m lon g an d run s E- W (Fig. 3) . A t th e easter n en d o f transec t 2 i t crosses th e 0ygarde n Faul t Zon e an d Hord a Platform, befor e i t passe s acros s th e Vikin g Graben, whic h is both deeper and narrower than in transec t 1 furthe r north . Fro m th e Vikin g Graben, transec t 2 passe s ont o th e Shetlan d Platform. Unfortunately , velocit y dat a fro m ESPs wer e no t availabl e alon g transec t 2 . Thi s forced u s t o calibrat e th e dee p velocit y profil e with data fro m transec t 1 . Furthermore, th e preJurassic stratigraphy and depth t o basemen t ar e not wel l constrained beneath th e western part o f the Hord a Platform . In bot h transects , th e uppe r reflectiv e crus t is dominated b y Cenozoic, relativel y flat-lying and unfaulted post-rif t sequence s (Jord t et al. 1995) , unconformably coverin g heavil y faulte d Meso zoic syn-rif t sediments . Bot h transect s revea l a n asymmetrical pattern , wit h th e deepes t part s o f the basi n situate d t o th e west , i n th e hangin g wall o f th e inne r wester n maste r faul t o f th e Viking Graben . The middl e crus t i s characterize d b y poo r reflectivity. Thi s i s i n stron g contras t t o th e undulating reflective lower crust. Although there are som e difference s i n th e reflectiv e patter n o f the lower crust in the two transects, the principal architecture i s consistent . Als o th e widt h an d area o f Moh o shallowing , an d th e depth s o f Moho, ar e simila r i n transect s 1 and 2 (Odin sen e t al . 2000) . Reflection s belo w Moh o ar e recorded in bot h transects . Thes e feature s dip away fro m th e grabe n axi s an d ar e focuse d a t the transitio n betwee n strongl y thinne d crus t and th e thicke r crus t belo w th e platfor m areas . They follow th e general pattern fo r intra-mantle reflections a s describe d b y Klempere r (1988) . Modelling procedur e an d parameters Early modellin g studie s o f th e norther n Nort h Sea (Giltner 1987 ; White 1990 ) were based on the McKenzie (1978 ) model . Thes e model s predic t the first-order characteristics of the basin, but d o not mode l fault-controlle d syn-rif t stratigraph y because they fail to incorporate th e mechanics of rifting i n a self-consisten t manne r (Koo i e t al . 1992). Alternatively , shorter-wavelength model s have bee n applie d tha t explicitl y tak e motio n along rif t borde r fault s int o accoun t (Marsde n et al. 1990; Roberts e t al. 1993, 1995). The short wave-length model s requir e ver y lo w flexura l

Fig. 2. (a ) Post-stac k reprocesse d transec t 1 in time . (For location , see Fig . 1. ) (b) Depth-converte d an d interprete d transect I . Questio n mark s (? ) refer t o uncertaintie s o f the Permo-Triassi c sequenc e an d basemen t level . N o vertica l exaggeration .

Fig. 3. (a ) Post-stack reprocesse d transec t 2 in time. (For location , se e Fig. 1. ) (b) Depth-converted an d interprete d transect 2. Question mark s (?) refer to uncertaintie s of the westwar d continuation of th e Permo-Triassi c sequenc e an d basemen t level . N o vertica l exaggeration.

88


rigidities o f th e lithospher e (effectiv e elasti c thickness (EET ) 1.5- 6 km) an d ar e i n apparen t agreement wit h a gravit y stud y o f th e cen tral North Se a by Barton & Wood (1984) , which suggested loca l isostati c compensation . Th e latter study , however , calculate d th e isostati c response o f an initially unflexed an d undeforme d plate t o post-rif t loadin g b y sediment s an d thermal contraction . A s discusse d b y Koo i e t al. (1992) , suc h a treatmen t o f isostati c com pensation i s internally inconsistent and i s bound to yiel d very low elastic thicknesses . I n addition. Barton & Wood (1984 ) di d no t tak e th e gravity effect o f sedimen t compactio n int o account , another facto r that wil l lead t o an underestimat e of lithospheri c strength (Cowi e & Karner 1990) . A mor e recen t seismi c an d gravit y stud y o f the centra l Nort h Se a b y Hollige r & Klemp erer (1990 ) indicate s significan t departures fro m local isostas y ove r th e basin , suggestin g tha t th e lithosphere ha s retaine d flexura l rigidity . Thi s conclusion i s consisten t wit h inference s fro m dynamic model s o f riftin g (Brau n & Beaumon t 1989; Bass i e t al . 1993 ; Buro v & Diament 1995 ) that unde r moderat e amount s o f stretching , th e lithosphere wil l retai n significan t strength. Ebin ger e t al . (1991 ) an d Va n de r Bee k (1997 ) hav e shown that rifting i n the East African and Baikal rifts, whic h i n man y way s provid e present-da y

analogues t o riftin g i n th e norther n Nort h Sea , is controlle d b y a stron g lithospher e wit h EE T of c . 30 km. We us e a 2 D forwar d mode l (Koo i e t al . 1992). whic h adopt s a finit e strengt h o f th e lithosphere durin g stretching . Th e kinematic s of stretchin g an d th e flexura l isostati c respons e are controlle d b y th e dept h o f neckin g (Z neck ) (Fig. 4 ) (Brau n & Beaumon t 1989 ; Weisse l & Karne r 1989 ; Koo i e t al . 1992) . A surfac e depression wit h a dept h (S) i s then give n by

where 3 i s a variabl e stretchin g factor. In thi s manner, fault-controlle d syn-rif t strati graphy ca n b e simulate d withou t havin g t o revert t o extremel y low flexura l rigiditie s (Koo i et al . 1992 ; Spadin i e t a l 1995) . Th e applie d Zneck is constrained b y seismic data an d tria l and error, an d considere d robust . Th e mode l i s ru n by dividin g th e crusta l an d subcrusta l litho sphere, wit h a constan t pre-rif t latera l thicknes s (35 km), into a number o f 5 km wide boxes. Eac h box i s assigne d a crusta l an d a subcrusta l stretching facto r (Royde n & Kee n 1980) . Th e crustal an d subcrusta l factor s are assume d t o be equal i n ou r model . I t incorporate s latera l hea t flow an d finit e stretchin g (Cochra n 1983) . Th e

Fig. 4 . Schemati c illustratio n o f th e lithospheri c neckin g (afte r Koo i e l al . 1992 ; modifie d fro m Brau n & Beaumont 1989) . The leve l o f necking (Z neck) is defined a s th e leve l o f n o vertica l motio n i n th e absenc e of isostatic forces . A shallo w leve l o f neckin g (bottom ) create s a surfac e depressio n tha t i s shallower tha n th e isostatically compensate d basi n dept h (CD ) an d result s i n a downwar d stat e o f flexure. Conversely, fo r a dee p level o f neckin g (top ) a n upwar d loa d act s o n th e lithosphere , resultin g i n flexura l supporte d rif t flanks .

NORTH SE A PERMO-TRIASSI C AN D JURASSI C EXTENSIO N thermal stat e an d subsequen t subsidenc e o r uplift ar e calculate d usin g a finit e differenc e scheme. Thi s basemen t subsidenc e i s inverte d to th e correspondin g stratigraph y b y infil l o f sediments and calibrate d b y an estimated waterdepth profile . Th e flexura l strengt h o f the litho sphere i s controlle d b y th e 450° C isotherm , which rise s durin g therma l perturbatio n a s a consequence o f stretching . Wit h thi s approac h the flexura l strengt h o f th e crus t an d mantl e are alway s coupled . Thi s i s obviousl y a simpli fication of the elastic response as the model does not allo w a further investigatio n of the interplay between strain rate, age, composition an d thick ness of the crust. As shown by Burov & Diament (1995) an d Cloeting h & Buro v (1996) , thes e factors contro l th e lithospheri c strength , whic h may chang e totall y before , durin g an d afte r rifting. The subject has been further investigated by Te r Voord e e t al (2000) , wh o teste d bot h end-members (couple d o r decoupled) , an d w e will consider that stud y at th e end o f this paper. Compaction o f sediments is calculated using a standard porosity-dept h relationshi p (f(z) = 0.58e~° 41r) fo r al l sediment s (Sclate r & Christi e 1980). Thi s i s a simpl e approach wher e a sand / shale rati o o f 40/6 0 reflect s th e 'averag e lithol ogy' i n th e compactio n scheme . Kyrkjebo e t al . (2000) teste d th e effect s o f varyin g lithology on compaction by backstripping the Cretaceous and Cenozoic sequence s alon g transec t 2 assumin g 100% o f shal e an d 100 % o f sand , usin g th e porosity-depth relatio n o f Sclate r & Christi e (1980). Th e differenc e betwee n shal e an d san d volumes (2 D cross-sectiona l area ) wa s o f th e order o f 0-20%. Hence , th e sensitivit y analysis indicated tha t th e uncertaintie s related t o com paction-decompaction model s d o no t under mine our regiona l conclusions . The mode l als o account s fo r change s i n se a level. We used a long-term sea-leve l curve base d

on Komin z (1984) . For specifie d stages i n basi n evolution, tectoni c subsidenc e an d crusta l struc ture ca n b e examined . I n addition , th e forwar d model ca n predic t a likel y positio n o f seismi c horizons, whic h ar e otherwis e difficul t t o track , for instanc e in the Viking Graben an d belo w the western part of the Horda Platfor m in transect 2. The mode l parameter s applie d i n th e presen t study ar e show n i n Tabl e 1 . The timin g of events responsible fo r th e pres ent structura l framewor k i n the northern Nort h Sea ha s bee n debate d i n th e literature . O n th e basis of published an d ou r ow n data, tw o major stretching phase s wer e use d i n th e modelling , namely Artinskia n t o Ladina n (261-23 6 Ma), referred t o a s Permo-Triassic, an d Bathonia n t o Berriasian (165-14 1 Ma), referre d t o a s Juras sic (tim e scal e o f Harlan d e t al . (1990)) . I t i s acknowledged tha t th e initiatio n o f th e firs t phase i s especiall y controversial , a s n o well s have penetrated th e thick pre-Triassic sediment s observed withi n th e dee p half-graben s i n th e Norwegian par t o f th e norther n Nort h Sea . However, age dating in the southwestern part o f Norway indicat e Permia n age s (Faerset h 1978 ; Torsvik e t al. 1992) , which is in accordance wit h seismic studie s (Badle y e t al . 1988 ; Gabrielse n et al. 1990) . Permo-Triassic riftin g ha s als o been suggested fo r th e adjacen t Uns t Basi n (John s & Andrews 1985) . W e hav e chose n 26 1 Ma a s initiation o f Permian riftin g bu t realiz e that thi s age i s uncertain . Precise datin g of the initiatio n o f the Jurassi c rift phas e ha s als o bee n subjec t t o disagreemen t (Gabrielsen e t al . 1990) . According t o Helland Hansen e t al . (1992), increased subsidenc e rates and relativ e sea-level rise date back as far as late early Bajocia n time , althoug h significan t sub basin formatio n probabl y date s t o Kimmerid gian time in many areas (Steel 1993) . This make it reasonabl e t o assum e tha t mos t o f th e hea t

Table 1 . Parameters used i n th e tectonostratigraphic modelling (after Bodine e t al . 1981; Steckler 1981; Kooi e t al . 1992) Symbol

Definition

Value

•^neck

Depth o f neckin g (regiona l istostasy ) Isotherm describin g th e effectiv e elasti c thicknes s (EET ) Initial effectiv e elasti c thicknes s Initial lithospheri c thicknes s Initial crusta l thicknes s Coefficient o f therma l expansion Temperature o f asthenospher e Thermal diffusivit y Mantle densit y a t surfac e condition s Crustal densit y a t surfac e condition s Water densit y

18km 450°C 44km 130km 35km 3.4 x KT^C- 1 1333°C 7.8xlO-7m2s-' 3.33 gem"3 2.80gcirr3 1.03gcm~ 3

Te EET L C a Tasth

K m c c

89

90


was introduced during Bajocian-Callovian time . Accepting th e complexitie s i n precis e datin g o f the norther n Nort h Se a rif t phase s (N0ttved t et al. 1995) , it i s important t o not e tha t reason able change s o f th e age s w e hav e use d her e will no t chang e th e stretchin g result s o r th e crustal model .

Both phase s ar e characterize d b y regional , E-W oriente d extensio n associated wit h normal faulting an d syn-rif t sedimentatio n (e.g . Faerseth et al. 1995), and followe d by thermal cooling and sediment loadin g (e.g . Giltne r 1987 ; Gabrielse n et al . 1990 ; Whit e 1990 ; Robert s e t al . 1995) . Figure 5 summarize s th e stratigraphy , relativ e

Fig. 5. Tectono-sedimentologica l events , norther n Nort h Se a (modified fro m Gabrielse n e t al . 1990 ; Nottvedt et al . 1995) . Tim e scal e fro m Harlan d e t al . (1990).

NORTH SE A PERMO-TRIASSI C AN D JURASSI C EXTENSIO N Table 2 . List o f ages assigned t o th e horizons Stratigraphy Sea floor 0 Base Pliocen e 5 Near to p Oligocen e 2 Inter lowe r Oligocen e 3 Base Oligocen e 3 Base Eocen e 5 Base Tertiar y 6 Inter Cenomania n 9 Inter lowe r Cretaceou s (Ryazanian ) 14 Base Cretaceou s 14 Upper middl e Jurassic 16 Lower Jurassi c 20 Lower middle Triassic 23 Lower Triassi c 244 Upper lowe r Permia n 26

Age (Ma )

5 1 5 6 5 5 1 4 5 0 6 1

sea-level elevation and th e tectonic event s of the northern Nort h Sea . Seismi c horizons (Tabl e 2 ) are calibrate d t o well s along th e transects , an d the age s o f th e horizon s ar e correlate d t o th e time scal e o f Harland e t al (1990) . Stratigraphic modellin g an d crustal thickness estimates A clarificatio n o n ou r us e o f 'Vikin g Graben ' may b e neede d i n th e following , a s previou s workers als o hav e applie d thi s ter m fo r th e whole northern Nort h Se a rift system . We make a distinctio n betwee n th e muc h mor e regiona l northern Nort h Se a an d th e presen t c . 60 km wide Viking Graben, wher e most o f the Jurassi c rifting an d th e post-Jurassic subsidence occurred (Fig. 1) . When th e Permo-Triassi c Vikin g Gra ben and th e Jurassic Viking Graben ar e referre d to, thi s i s fo r compariso n o f stretchin g magni tudes i n th e sam e area , i.e . wit h respec t t o th e present 60k m wid e Vikin g Graben . Thi s als o goes for the Horda Platfor m and Shetlan d Basin and othe r areas . Figures 6 an d 7 sho w th e modelle d an d observed stratigraphy , an d th e basemen t sub sidence of transects 1 and 2 . The Cretaceous an d Tertiary stratigraph y i s identicall y reproduce d during modelling . Also , th e Jurassi c strat a ar e generally in very good agreement, although some discrepancies occu r i n area s tha t hav e bee n up lifted an d eroded . Thi s i s eviden t fo r th e Gull faks are a an d th e wester n ri m o f th e Hord a Platform i n transec t 1 and toward s th e 0ygar den Faul t Zon e i n both transects . Th e modelled and observe d Triassi c strat a ar e mostl y i n concert, althoug h a misfi t o f 300-40 0 m occur s

91

locally belo w th e Gullfak s bloc k i n transec t 1 and below the Horda Platform i n transect 2 . The good agreemen t a t to p basemen t leve l i s em phasized, a s previou s definition s of thi s surface have been affecte d b y considerable uncertaintie s (Klemperer 1988 ; Fichle r & Hosper s 1990 ; Marsden e t al . 1990) . A notio n o f th e apparen t thickness variatio n o f th e Jurassi c an d pre Jurassic sediment distributio n fro m th e Gullfaks block t o th e Vikin g Grabe n (Fig . 6 ) i s neces sary. Thi s portio n o f transec t 1 ha s been * backstripped b y Fosse n e t al . (2000) , an d a t Rogaland Research , an d th e result s hav e bee n consistent whe n decompactio n o f th e sediment s is take n int o account . In area s wher e interpretation o f the stratigra phy is uncertain or no interpretation is available, the model can sugges t a first-order stratigraphy. This i s eviden t i n transec t 2 withi n th e deepes t parts o f th e Vikin g Grabe n an d th e wester n Horda Platform , wher e th e position s o f th e Triassic an d pre-Triassi c horizon s ar e suggeste d by th e forwar d model. Here , to p basemen t level is furthe r constraine d b y gravimetri c an d mag netic data (Christiansso n e t al . 2000). Calculated tectoni c an d basemen t subsidenc e is demonstrated fo r differen t position s along th e modelled transect s (Fig s 6 a an d 7a) . Th e base ment subsidenc e curve s quantif y th e relativ e importance o f th e tw o rif t phases , an d sho w that th e subsidenc e associate d wit h th e Permo Triassic stretchin g was generall y larger tha n fo r Jurassic stretching . I t i s onl y withi n th e Viking Graben tha t Jurassi c subsidenc e outpace d th e Permo-Triassic event . Th e tota l basemen t sub sidence reache s a maximu m o f 12-1 4 km within the deepes t par t o f th e Vikin g Graben . Th e calculated tectonic subsidence shows the flexural and therma l response , wit h it s hanging-wal l subsidence an d footwal l uplif t i n respons e t o fault movement . Exact estimate s o f th e present-da y crusta l thickness ar e importan t durin g forwar d model ling. Thi s constrain s no t onl y th e fina l strati graphic an d crusta l model , bu t als o influence s the parameter s use d durin g modelling. Figure 8 displays th e modelle d crusta l structur e o f transects 1 and 2 . Th e observe d an d modelle d crustal thicknes s ar e i n concer t belo w mos t o f the platform areas. This is important as these are the area s wher e th e crusta l thicknes s i s bes t constrained. Belo w th e basi n area s i n bot h transects, an d belo w th e wester n Hord a Plat form i n transect 2 modelled crust i s thicker than is observe d (locall y u p t o 9 km). According t o Christiansso n e t al . (2000) , refraction dat a (ESP ) clos e t o transec t 1 an d gravity data , sho w tha t th e bas e o f th e crus t

92


Fig. 6 . (a ) Modelle d stratigraph y of transec t I . Boxe s a t to p sho w calculate d tectoni c (dotted lines ) an d basement (continuou s lines) subsidenc e alon g th e modelle d transect , (b ) Observed stratigraph y of transec t I . (Note vertica l exaggeration. )

coincides wit h th e bas e o f th e reflectiv e lowe r crust i n bot h transects . A particula r proble m exists belo w th e easter n par t o f th e Vikin g Graben i n transec t 1 an d belo w th e Hord a Platform i n transec t 2 wher e th e Moh o split s into tw o separat e reflectiv e band s (Fig s 2 and 3 ) (Odinsen e l ai 2000) . I n transec t 1 th e up per ban d deepen s fro m 2 0 t o 2 8 km. an d th e lower ban d deepen s fro m 2 4 to 36k m t o th e SE. Here, th e uppe r ban d delineate s velocitie s o f 6+kms" 1 abov e an d 8+kms" 1 belo w (Chris tiansson e t al. 2000) , whic h correspond s t o th e seismic definitio n o f th e Moho . However , thi s

solution wil l plac e the Moh o a t th e sam e or eve n shallower leve l belo w th e norther n Hord a Plat form tha n underneat h th e Vikin g Graben . implying a tota l crusta l stretchin g of abou t th e same orde r o f magnitud e fro m th e Vikin g Gra ben to the 0ygarden Faul t Zone . However , bot h sediment distributio n an d subsidenc e curve s contradict this . Thus , modellin g result s sho w that bul k stretchin g decrease s eastward s fro m the Vikin g Graben. whic h woul d b e compatibl e with a n increas e i n th e crusta l thickness . Hence , when th e bul k modelle d stretchin g is inverted to Moho depth , on e finds that th e modelled Moh o

NORTH SE A PERMO-TRIASSI C AN D JURASSI C EXTENSIO N

93

Fig. 1 . (a ) Modelle d stratigraph y of transec t 2 . Proposed Permo-Triassi c sequenc e emplace d fro m th e Horda Platfor m t o th e Vikin g Graben. Boxe s at to p sho w calculate d tectonic (dotte d lines ) and basemen t (continuous lines ) subsidence alon g th e modelle d transect , (b ) Observed stratigraph y o f transec t 2 . (Note vertical exaggeration. )

corresponds t o th e lowe r o f th e tw o reflectiv e bands (Fig . 8a) . I t i s unclear ho w fa r sout h th e 50km wid e and 5-1 0 km thic k bod y continues , as n o velocit y dat a ar e availabl e sout h o f transect 1 . However , i t i s note d tha t th e sam e lower-crustal spli t ca n b e interprete d fro m th e crustal reflectivit y patter n i n transect 2 (Odinsen et al 2000) , an d tha t th e modelle d Moh o leve l (Fig. 8b ) coincides with that o f transect 1 . Christiansson e t al . (2000 ) obtaine d th e sam e result s •

from gravit y modelling o f th e crusta l transects . They als o addresse d th e natur e o f thi s bod y i n more detail. Stretching distribution and thermal evolution Estimating th e stretchin g distributio n i n th e northern Nort h Se a ha s bee n th e subjec t o f many paper s i n recen t year s (Beac h e t al . 1987;

94


Fig. 8. (a ) Modelle d present-da y crusta l structure , an d observe d an d modelle d Moh o depth s fo r transec t 1 . Top o f high-velocit y bod y shoul d b e note d (fro m Christiansso n e t al. 2000). (b ) Modelled present-da y crusta l structure, an d observe d an d modelle d Moh o depth s fo r transec t 2 . It shoul d b e noted tha t th e modelle d Moho leve l coincide s wit h th e bas e o f th e reflectiv e lowe r crus t (simila r t o transec t 1) . Giltner 1987 ; Badley e t al . 1988 ; Marsde n e t al . 1990; White 1990 ; Roberts e t al . 1993 , 1995 ; Ter Voorde e t al. 2000), and hav e yielde d a rang e o f magnitudes fo r th e specifi c areas . Th e larg e variations ar e dependen t o n th e metho d use d (summation o f faul t heaves , forwar d modelling , backstripping, measurement s o f crusta l thin ning), th e cross-sectiona l area , an d th e inter pretation fo r tha t specifi c area . Thi s als o indicates ther e ma y no t b e a uniqu e /3 solution , as ou r understandin g an d simpl e measuremen t of stretching will never b e able t o reflec t th e tru e dynamics o f a n extendin g crust . Recognizing the genera l limitations of stretching estimates , th e presen t forwar d mode l com pares th e magnitud e o f th e Permo-Triassi c an d Jurassic rif t phase s (an d th e coincidental therma l evolution) i n on e mode l run , withou t havin g t o change metho d o r approac h durin g modelling . The 3 value s ar e sample d eac h 5k m alon g th e transects, an d mirro r th e crusta l stretchin g i n a 'spiky' manne r simila r t o Marsde n e t al . (1990 , figs 2.1 5 an d 2.16) . However , unlik e Marsde n et al . (1990 ) an d other s (Robert s e t al . 1993 , 1995; Ter Vord e e t al. 2000), we do no t decoupl e

the lowe r fro m th e uppe r crust , althoug h w e likewise assum e a ductil e deformatio n mechan ism i n th e lowe r crust . Hence , th e presen t 3 profiles ar e no t presente d a s smoot h pur e shea r profiles, a s the y ar e mean t t o resembl e the fault controlled topograph y wher e laterall y adjacen t flexural loads ar e incorporated .

Late early Permian-early mid-Triassic stretching (Artinaskian (261 Ma)-Ladinian (2 36 Ma)) Figure 9 show s th e modelle d latera l stretch ing distributio n alon g th e tw o transects . Th e 3 distribution fo r Permo-Triassi c riftin g indicate s that stretchin g wa s distributed i n a broad basin , delineated t o th e eas t b y th e 0ygarde n Faul t Zone and t o th e west by the Hutto n Faul t align ment i n transec t 1 and th e Shetlan d Platfor m i n transect 2 . Th e basi n i s define d b y a serie s o f large tilte d faul t blocks , whic h ar e bes t image d below the Horda Platform an d the Tampen Spu r area. Th e block-boundin g norma l fault s hav e a

NORTH SE A PERMO-TRIASSI C AN D JURASSI C EXTENSIO N

95

Fig. 9 . (a ) Modelle d latera l stretchin g distribution o f transec t 1 . (b) Modelled latera l stretchin g distribution of transec t 2 . spacing o f 15-2 0 km an d throw s u p t o 4- 5 km (Faerseth e t al, 1995) . Th e orientatio n o f th e faults wa s primaril y th e sam e a s fo r th e Juras sic faults, c . N-S belo w the Horda Platfor m an d the Vikin g Graben , changin g t o NNE-SS W below th e Shetlan d Basin . The modellin g procedur e implie s tha t a n initial Permian crust of lateral uniform thickness existed. W e believ e tha t 35k m i s probabl y a reasonable initia l thicknes s i f w e conside r th e crust i n adjacen t area s tha t ar e littl e affecte d b y the Permo-Triassi c an d Jurassi c stretching . This is also in accordance with other studies (Sellevoll 1977; McCla y e t al . 1986 ; Kinc k e t al . 1991) . Furthermore, test s hav e show n tha t changin g the initia l value fro m 35k m t o 3 0 or 40k m wil l have som e impac t o n th e modelle d stretchin g estimates, bu t wil l no t alte r th e relativ e importance o f th e tw o rif t phases . Tabl e 3 give s an overvie w o f th e modelle d mea n stretchin g values fo r individua l semi-regiona l area s alon g the tw o transects . The modelle d /? mean fo r th e Permo-Triassi c stretching i s 1.2 7 (transec t 1) , an d 1.1 9 (tran sect 2) . I f w e conside r onl y th e interio r o f thi s basin /3 mean become s 1. 4 in transec t 1 (from th e 0ygarden Faul t Zon e t o the Hutton alignment), and 1.3 8 in transect 2 (from the 0ygarden Fault Zone t o th e East Shetlan d Platform) . Th e width of thi s basi n ma y hav e bee n c . 120-125 km

when riftin g ende d i n earl y mid-Triassi c time . Anean fo r th e Hord a Platfor m i s 1.3 3 an d 1.3 9 in transect s 1 an d 2 respectively . Thes e value s are ver y simila r t o thos e obtaine d b y Robert s et al. (1995), who analysed an east-west oriented profile acros s th e Hord a Platform . Thei r pro file wa s locate d sout h o f transec t 1 . Th e /? mean values acros s th e 'Permo-Triassic ' Vikin g Gra ben ar e 1.4 1 i n transec t 1 an d 1.28(? ) i n tran sect 2 (Table 3) . Table 3 . Modelled (3 mean estimates fo r th e PermoTriassic and Jurassic rift phases Area Transect I Horda Platfor m Viking Grabe n Western Eas t Shetland Basi n East Shetlan d Basin Whole transec t Transect 2 Horda Platfor m Viking Graben Shetland Platfor m Whole transec t

Permo-Triassic Jurassic riftin g rifting mea n (3 mean (3 1.33 1.41 1.14

1.08 1.42 1.13

1.29

1.11

1.27

1.15

1.39 1.28(?) 1.0(?) 1.19

1.13 1.53 1.03(?) 1.19

96


Permo-Triassic /3 mean fo r th e Eas t Shetlan d Basin i s 1.29 . Here , th e Hutto n alignmen t ha d a simila r statu s t o th e 0ygarde n Faul t Zon e o n the eas t flan k o f th e grabe n system . Model ling result s indicat e a basemen t subsidenc e o f c. 1 km i n the hangin g wal l of th e Hutto n align ment (Fig . 6a) . Th e are a northwes t o f th e Hut ton alignmen t i s considerably les s influence d by Permo-Triassic stretching , wit h a /3 mean reduce d to 1.14 . The modellin g doe s no t recor d an y Permo Triassic stretchin g history fo r th e Shetlan d Plat form. Here , Tertiar y sediment s unconformabl y overlie a thick sequence o f Palaeozoic sediments , which agai n ar e locall y drape d b y a ver y thi n Permo-Triassic sequence . Th e thick pre-Permia n sequence i s supporte d b y gravimetri c an d mag netic dat a (Christiansso n e t aL 2000 ) Accordin g to Plat t (1995) , thes e Palaeozoi c sediment s ar e most likel y o f Devonian(-Carboniferous? ) age . The absenc e o f Mesozoic strat a o n th e Shetlan d Platform suggest s tha t erosio n ha s occurred , initiated b y footwal l uplif t o f th e boundin g fault syste m (Robert s & Yieldin g 1991) . Ou r

modelling analysi s also show s tha t th e Shetlan d Platform becam e elevate d (an d eroded ) durin g rifting. I t i s uncertai n ho w larg e a volum e o f sediment wa s erode d durin g lon g period s o f elevation, an d conversel y ho w muc h sedimen t accumulated durin g th e intervenin g periods o f deposition. Jurassic stretching (Bathonian (165 Ma)Berriasian (141 Ma)) Overall Jurassi c stretchin g amounts t o approxi mately th e sam e i n transec t 1 (/3mean i s 1.15 ) an d transect 2 (/3mean is 1.19) . Although this is similar to th e Permo-Triassi c result s fo r transec t 2 i t is much lowe r fo r transec t 1 (Table 3) . The Jurassic 3 mQan across th e Horda Platfor m amounts t o 1.0 8 along transec t 1 and 1.1 3 alon g transect 2 . I n othe r words , Jurassi c stretchin g for th e Hord a Platfor m are a wa s substantially less tha n th e Permo-Triassi c phas e (1.3 3 an d 1.39) for the same area. Estimate d Jurassi c 3mcan across the Viking Graben i s 1.42 in transect 1 and

Fig. 10 . Modelle d therma l evolution of th e lithospher e along transect 1 from th e Triassi c period t o present . The bas e o f th e lithospher e is described b y th e 130 0 C isother m (contou r interval i s 10 0 C). Latera l EET valu e is displayed.

NORTH SE A PERMO-TRIASSI C AN D JURASSI C EXTENSIO N 1.53 i n transec t 2 . Thi s i s highe r tha n th e les s constrained calculate d Permo-Triassi c values . Anean i n th e Eas t Shetlan d Basi n i s 1.11 , whic h is muc h les s tha n th e Permo-Triassi c stretchin g of 1.29 . Stretchin g acros s the Shetlan d Platfor m (transect 2 ) is merely 1.03 . Modelled thermal evolution of transect 1

Modelled therma l evolutio n an d th e EE T valu e in transec t 1 ar e show n i n Fig . 10 . Maximu m thermal perturbatio n a t th e en d o f th e Permo Triassic rif t phas e affecte d a muc h wide r par t o f the lithosphere tha n Jurassic extensio n (Fig s lO d and f) . Th e latera l exten t o f th e riftin g i s expected t o b e mirrore d i n th e post-rif t sequence s i n Figs 6 and 7 ; these figures demonstrate tha t th e thermal subsidenc e tha t followe d th e firs t phas e produced a broade r an d mor e uniforml y dis tributed post-rif t sequenc e tha n th e latte r phase . It i s als o note d tha t ther e i s a shif t toward s the eas t an d a focusin g o f th e pea k therma l elevation fro m th e Permo-Triassi c t o th e Jur assic rif t phase . The EET , whic h i s controlle d b y th e 450° C isotherm, starte d ou t wit h a n initia l valu e o f 44 km. A t th e en d o f Permo-Triassi c stretching , EET varie d fro m 4 2 km t o 3 0 km belo w th e platform an d basi n areas, respectivel y (Fig. lOf) . At th e onse t o f th e Jurassi c stretching , EE T is increase d t o 38k m belo w th e basi n area s (Fig. lOe) . EE T wa s reduce d afte r th e Juras sic stretching, to 40 km below the platform areas, and 30k m belo w th e Vikin g Graben . Thes e estimates are very close to those derived from th e Baikal an d Eas t Africa n rift s (EE T c. 30km) (Ebinger e t al. 1991 ; Va n de r Bee k 1997) . Several studie s hav e emphasize d tha t pos t Permo-Triassic therma l subsidenc e wa s no t ended whe n Jurassi c riftin g starte d (Giltne r 1987; Gabrielse n e t al . 1990 ; Robert s e t al . 1995), suggestin g tha t th e Jurassi c norther n North Se a wa s a hybri d basi n affecte d simulta neously b y therma l subsidenc e an d renewe d initiation o f stretching . Latera l heat-flo w mod elling o f th e temperatur e structur e for transec t 1 supports thi s conclusion (Fig . lOe) . The residua l Permo-Triassic hea t a t th e onse t o f th e Jurassi c rift phas e indicate s tha t ther e wa s a potentia l for mino r therma l subsidenc e befor e therma l equilibrium. The elevate d therma l perturbation , an d th e reduced EE T (fro m 4 0 km t o 3 8 km), fro m Paleocene t o Eocen e tim e (Fig s lO b an d c) , is a response t o introductio n o f a mino r stretchin g value o f 1.02-1.0 3 durin g Paleocen e time . Thi s elevates th e temperatur e fiel d slightl y an d in -

97

creases th e subsidenc e i n Paleocen e an d Eocen e time, a s recorde d i n th e wel l dat a an d seismi c cross-sections. Th e additiona l post-Paleocen e thermal coolin g gav e enoug h accommodatio n space t o reproduc e th e observe d stratigraph y without introducin g unrealisticall y larg e wate r depths. Introducin g thi s (3 valu e wil l obviousl y contribute to the thinning of the crust. Although this crusta l thinnin g i s speculative , i t wil l no t have an y impac t o n th e crustal-scal e modellin g that i s addresse d below . Wit h respec t t o th e modelled Permo-Triassi c an d Jurassi c stretchin g results, th e Paleocen e value s are withi n th e limi t of uncertaintie s o n th e regiona l scal e tha t i s studied here . It i s noted tha t recen t studie s see m t o confirm that a mil d therma l even t occurre d i n th e northern Nort h Se a Basi n i n earl y Tertiar y time (Hall & White 1994 ; Nadin & Kuznir 1995 ; Nadin e t al . 1995) , an d variou s explanation s fo r this departur e fro m McKenzies " (1978 ) post-rif t subsidence mode l hav e bee n offered . Accordin g to Bertra m & Milto n (1989 ) an d Nadi n & Kusznir (1995 ) n o othe r mechanis m i s require d for th e formatio n o f Tertiar y accommodatio n space tha n post-Jurassi c therma l subsidenc e 'buffered' b y a transien t Paleocen e uplif t event . On th e othe r hand , Hal l & White (1994 ) argue d that a rapi d increas e i n water-loaded subsidenc e occurred a t th e beginnin g of Tertiar y time . Thi s increase, u p t o 1 km i n amplitude , i s anomalou s in tha t i t i s no t predicte d b y th e lithospheri c stretching model , unles s a n additiona l phas e of Tertiar y stretchin g i s invoked . Ther e is , however, onl y limite d evidenc e fo r sufficien t Tertiary norma l stretching . Th e present-da y thermal statu s show s tha t th e lithospher e ha s almost reache d therma l equilibrium . Hence , th e EET show s a n almos t unifor m valu e o f 4 2 km (Fig. lOa) . Comments o n erosion , wate r dept h an d tectonic uplif t The proble m o f reproducin g area s tha t hav e been elevate d abov e se a leve l an d erode d i s present i n al l stratigraphi c modelling . On e wa y of dealin g wit h thi s proble m usin g th e 2 D for ward modellin g tool i s to decreas e th e stretching values an d thereb y elevat e th e specifi c areas . This wil l reduc e th e stratigraphi c misfi t tha t appears i n area s tha t ha s bee n uplifte d an d eroded. W e hav e chose n t o d o thi s fo r th e Gullfaks bloc k an d the western rim of the Horda Platform i n transect 1 and clos e t o th e 0ygarden Fault Zon e i n bot h transects . I t i s emphasize d that thi s reductio n o f th e Jurassi c stretchin g

98


values (3 and TiO 2 content s (Fig . 8) ; mea n value s oi f Fe2O3 an d TiO 2 ar e 8. 7 an d 1.17wt % respec tively. Th e averag e Mg O conten t i s 2.6wt% . Carbonate-cemented interval s ar e also rathe r characteristic (Fig s 9 an d 10) , an d som e o f th e high iro n conten t ca n b e relate d t o anker ite layer s confirme d b y XRD . Relativel y hig h metal concentration s ar e see n i n thi s sequenc e (Fig. 11) , an d th e averag e value s for Ni , C u an d Co ar e onl y lowe r tha n thos e o f CSS- 2 an d CSS-3, an d ar e muc h highe r tha n i n th e upper most sequences . A n increas e i n th e concen tration o f thes e trac e element s clos e t o th e

CSS-l-CSS-2 boundary , especiall y wit h respec t to nicke l and zinc , can be see n (Fig . 11) . Thi s is not foun d i n wel l 26/4-1 , however , a s CSS- 1 contains mainl y san d an d sample s analyse d ar e not fro m tuff-beds . Th e silic a conten t i n th e shales mainl y varie s betwee n 4 5 an d 60wt% , which i s clos e t o typica l value s fo r mafi c rock s (e.g. Eart h 1951) . However , i n th e sand-ric h units o f CSS- 1 i n well s 26/4- 1 an d 24/6-1 , a higher silic a content i s observed . Paleocene sediment s underlyin g th e Balde r Formation ma y hav e a relativel y hig h smectit e content, a s well a s high Fe , Mg , Cu , N i an d Z n content, indicatin g mafi c sourc e rocks . Thes e sediments ma y b e derive d fro m basi c basemen t rocks o r erosio n o f Mesozoi c sediment s fro m surrounding areas . A suppl y o f volcaniclasti c material fro m th e N W ma y als o explai n thi s distribution. The content o f chlorite an d a weak increase i n illit e i n CSS- 1 (Fig s 4 an d 5 ) ca n be attributed t o diageneti c transformation s wit h smectite a s th e precurso r mineral , o r th e disso lution o f roc k fragments . Top CSS- 1 probabl y indicate s a condense d section, wit h hig h concentration s o f MnO , carbonate an d a hig h concentratio n o f fine grained 'expandable ' clay s an d sometime s P 2O5 near th e CSS-l-CSS- 2 boundary . Thes e dat a indicate period s o f lo w clasti c sedimen t suppl y

Fig. 4. Cla y minera l distribution of some wells in the norther n Nort h Sea . Th e seismi c sequence stratigraphi c units are characterized b y different cla y mineral composition and particularl y wit h respec t t o th e smectit e content . A lo w smectit e an d hig h kaolinil e conten t i n th e Lowe r Oligocen e sequenc e (CSS-3 ) i n easter n wel l 30/3- 3 ma y indicate a mor e proxima l fades . Locatio n o f well s is show n i n Fig . 1 .

Fig. 5 . Cla y mineral distributio n o f well s 24/6- 1 an d 26/4-1 , correlated wit h seismic section CNST82-1 0 an d th e Cenozoi c seismi c sequence stratigraphi c framework Depths ar e relativ e t o mea n se a leve l accordin g to Jord t e t al. (1995) . Locatio n o f well s an d seismi c line i s shown i n Fig . 1 .

Fig. 6. Feldspa r conten t analysed by XRD an d relate d to the seismic sequence stratigraphic framework. Locatio n of well s is shown i n Fig . 1 . Clasti c feldspa r ma y b e use d a s a n indicato r o f provenanc e an d weathering . Th e K-feldspar conten t i s low in all wells except i n the Pliocen e sediments, where it reflects a colder climate. The albit e content i s also lo w i n Miocen e an d olde r sediment s excep t i n wel l 26/4-1 . I t i s possible tha t albit e to som e extent ma y b e authigeni c i n th e Eocen e sediment s wit h a volcani c source , mrkb . metre s belo w Kell y bushing.

SEQUENCE STRATIGRAPH Y I N CENOZOI C SEDIMENT S

253

and a starved depositional environmen t (Fig. 3). The enrichmen t o f manganese, phosphoru s an d zinc i s therefore probably du e t o a hig h relative organic productivity , particularl y o f siliceou s and calcareou s organisms , thoug h S0rense n & Nielsen (19810-c ) have interpreted th e presenc e of rhodochrosit e i n Paleocen e sediment s a s a result o f the suppl y o f Mn durin g volcanism . The Balde r Formation , correspondin g t o th e upper part of sequence CSS-1 , contains siliceou s shales and numerou s tuffaceous zone s and layers (Jacque & Thouveni n 1975 ; Malm e t al. 1984) . These interbedde d shale s and tuf f layer s contain a hig h amoun t o f microcrystallin e quart z an d have a ver y lo w conten t o f detrita l mineral s (Malm e t al . 1984) . Explosive volcanis m in th e British an d th e Faeroe-Greenlan d Tertiar y volcanic province s hav e bee n regarde d a s th e source fo r th e volcani c ashe s foun d widesprea d in th e Nort h Se a (Mal m e t al . 1984 ; Kno x & Morton 1988 ) and onshore Denmark (Spjeldnae s 1975; Nielse n & Heilmann-Clausen 1988) .

Eocene sequence (CSS-2)

Fig. 6 (continued}

Eocene (CSS-2 ) sediment s consis t mostl y o f smectite i n th e stud y are a (Fig s 4 an d 5) . Petrographical observation s sho w ver y lo w quartz content . Silic a content ha s a mea n value of 50 wt% (Fig . 7 ) and th e XRD dat a show also little quart z (mainl y les s tha n 20wt% ) an d feldspar (0-1 0 wt%) . Magnesium has an average value o f 2.5wt % MgO . Th e Eocen e mudrock s are ric h i n aluminiu m (15wt % A\ 2O^), iro n (9.5 wt% Fe 2O3) an d titaniu m (1.14wt % TiO 2). This i s typica l o f basi c volcani c sediments , an d the dry bul k compositio n i s not fa r fro m that o f Icelandic basalt s (e.g . Barth 1951 ) and indicate s that volcaniclasti c materia l represent s a domi nant par t o f th e sedimentatio n i n CSS-2 . Th e high conten t o f th e trac e metal s suc h a s nickel , cobalt, zin c an d coppe r (Fig . 11 ) i s als o con sistent wit h a volcani c origin. Eocene sediments are sodiu m ric h an d thi s ma y possibl y b e be cause of the presence of authigenic albite formed in volcani c sediments . Enrichmen t o f phos phorus and/o r manganes e i s als o see n clos e t o the CSS-2-CSS-3 boundary o r within the CSS-2 sequence in som e wells , th e bes t examples being wells 24/6-1 and 34/7-6 . The fine-graine d CSS- 2 sediments an d indication s o f lo w rate s o f non volcanic clasti c sedimen t suppl y indicat e a rela tively dee p marin e facie s during CSS- 2 time s in the Nort h Se a Basin. The Paleocen e an d Eocen e sediment s i n th e North Sea are highly smectitic (Figs 4 and 5) , and

254

B. I . THYBER G ET AL .

Fig. 7 . Distributio n o f some majo r element s (SiO2 + A1 2O3 and Na 2O + K 2 O) based on XRF analyse s of cuttings and relate d t o th e seismi c sequence stratigraphi c framework . Locatio n o f well s is shown i n Fig . 1 . (Not e th e lo w silica conten t an d hig h aluminiu m content i n th e smectite-ric h Eocen e sediment s (CSS-2). )


Fig. 7 (continued)

255

256

Fig. 7 (continued)

B. I . THYBER G E T AL .

SEQUENCE STRATIGRAPH Y I N CENOZOI C SEDIMENT S this has also by previous workers been related t o a volcani c sourc e (Nielse n 1974 ; Karlsson e t al . 1979; Sorense n & Nielse n 1981a-c ; Rund berg 1989 ; Pearso n 1990 ; Hugge t 1992) . Th e quartz content woul d b e expected t o b e higher if erosion o f the adjacent landmasse s wa s the main source o f th e basi n sediments . Th e cla y mineral analysis an d th e trac e an d mai n elemen t dat a also sho w that thi s is not th e case. A higher ratio of kaolinite-illit e + smectite i n sandstone s i n well 24/6-1 indicates this well to be located closer to th e sedimen t sourc e tha n th e othe r well s i n the stud y are a (Ziegle r 1982 ) (Figs 3 and 5) .

Lower Oligocene sequence (CSS-3) The cla y minera l distributio n (Fig s 4 an d 5 ) shows tha t smectit e remain s th e mai n cla y mineral componen t i n th e Lowe r Oligocen e sedi ments (CSS-3). However, the content of kaolinite is higher, particularly in wells 30/3-3 and 24/6-1, which represen t a mor e proxima l facie s clos e to a n easter n an d wester n source , respectivel y (Fig. 1) . Well 26/4-1 , locate d mor e i n th e dista l part o f th e basin , ha s a correspondingl y highe r content o f smectit e compare d wit h kaolinite . Thus, th e regiona l cla y minera l distributio n i n CSS-3 tim e is also controlle d b y the basi n development an d sedimen t transpor t directio n a s indicated i n Fig . 3 . In wel l 30/3- 3 a pronounce d shift i n cla y minera l assemblage s nea r th e CSS-2-CSS-3 boundary occurs, fro m a smectiterich sedimen t t o a cla y mineral composition en riched i n kaolinite above th e sequence boundar y (Fig. 4) . However , Rundber g (1989 ) indicated a higher smectit e conten t i n wel l 30/3- 3 i n th e depth interval corresponding t o CSS- 3 i n Fig. 4. The K-feldspa r an d plagioclas e conten t i s broadly similar and low throughout the sequence (Fig. 6) . Th e silic a conten t ha s value s clos e t o 59wt% an d A1 2O3 conten t clos e t o 13wt% . A sligh t increas e i n silica , an d correspondin g decrease in alumina content, is usually seen close to th e transitio n fro m Eocen e t o Oligocen e sediments (e.g . wel l 30/3- 3 (Fig . 7b ) an d 34/7- 2 (Fig. 7d)) , thoug h thi s i s not alway s the case , a s illustrated b y wel l 34/7- 1 (Fig . 7c) . Th e sedi ments i n th e lowe r par t o f CSS- 3 i n wel l 34/7-1 have silic a conten t o f abou t 48wt% , whic h indicates a basi c composition . I n wel l 24/6- 1 (Fig. 7a) , th e highe r silic a conten t i s du e t o a more sand y litholog y compared wit h th e under lying section . Th e iro n conten t i n th e investi gated well s varie s widely , betwee n 4. 4 an d 8.27wt% Fe 2O3, an d i s enriche d i n smectiti c mudstones (Fig . 8) . Thi s variatio n i s als o exhibited b y titanium . Aluminiu m an d titaniu m

257

contents are , however , lowe r than i n the Eocen e sediments (Fig . 8) . The trac e meta l value s (Ni , Co , C u an d Zn ) are almos t alway s lowe r tha n i n th e under lying CSS- 2 (Eocene ) sediments , bu t ar e stil l relatively hig h (Fig . 11) . However , th e presenc e of smectite , an d th e iron , titaniu m an d trac e element (Ni , Co, C u and Zn ) contents indicate a high volcani c componen t i n th e CSS- 3 (Lowe r Oligocene) sediments . Continue d inpu t o f vol canic ash fall s o r reworkin g of volcanic material is probabl y th e explanatio n fo r thes e observe d trends. Th e sedimentatio n rat e increase d durin g Early Oligocen e time , probabl y i n respons e t o Oligocene uplifts an d erosio n of soft Eocen e an d Paleocene sediments . Thi s also explain s the hig h sedimentation rate s foun d b y Jordt e t al . (1995, 1999) fo r thi s period . N i an d Z n sulphide s ar e easily oxidize d when th e sediment s ar e exposed , and wil l quickl y b e depleted . Th e reduce d amount o f the trac e metal s therefor e ma y b e a n indication o f reworked volcanic material. In well 34/7-1, trac e elements , iro n an d titaniu m values are simila r t o thos e obtaine d fro m CSS- 2 sequence (Fig. 1 Ic). The highe r content o f kaolinite (Fig . 4 ) may indicat e more sedimen t suppl y from land . Extensivel y reworke d Earl y Paleo cene and Earl y Eocene as h fall s an d lavas , yielding glass-ric h sediments , thu s occu r throughou t Late Eocen e t o Miocen e tim e (Hugge t 1992) , and Pearso n (1990) interpreted sediment s from a volcanic sourc e a s fa r u p a s Lowe r Oligocene .

Lower-Upper Oligocene sequence (CSS-4) Smectite i s a majo r componen t i n th e cla y fraction i n well s 34/7-1 , 34/7-2 , 34/7-6 , 34/7- 2 (Fig. 4 ) and 26/4- 1 (Fig. 5) . Kaolinite i s also a n important constituen t i n thes e sediments , parti cularly i n th e proxima l part s o f th e basi n (wel l 24/6-1, Fig . 5 ) identifie d b y progradatio n fro m the wes t in CSS- 4 tim e (Figs 1 and 5) . However, in wel l 34/7- 1 thes e sediment s hav e lowe r smec tite an d highe r illit e content s (Fig . 4) . Th e feld spar conten t i s approximatel y th e sam e a s i n the underlyin g Oligocene an d Eocen e sediment s (Fig. 6) , wit h respec t t o bot h plagioclas e an d K-feldspar. Th e sand y litholog y i n wel l 24/6- 1 has a relativel y hig h silic a conten t (Fig . 7a) ; otherwise th e silic a conten t i s abou t 50wt% . Differences i n lithology and facie s ar e als o indi cated i n variable s suc h a s aluminiu m an d iro n contents, wit h the highes t iro n conten t foun d i n well 34/7-6 , which als o ha s th e highes t accumu lation o f smectit e (Fig . 4) . Th e lowes t iro n content i s i n wel l 24/6-1 . Otherwis e averag e iron conten t fo r CSS- 4 i s abou t 4.4-4.9wt %

258


Fig. 8 . Distributio n o f som e majo r element s (Fe 2O3 + Mg O an d TiO 2) base d o n XR F analyse s o f cuttings and related t o th e seismi c sequenc e stratigraphi c framework . Location o f wells is shown i n Fig. 1 . (Note th e hig h iron content o f th e smectite-ric h mud.)


Fig. 8 (continued]

259

260

Fig. 8 (continued)


Fig. 9. Relationship s between seismic stratigraphic units, velocity and cla y mineralogy in well 30/3-3. Location o f well and seismi c line is shown in Fig. 1 . (Note changes i n velocity nea r th e boundar y betwee n seismi c units CSS- 3 and CSS-4 , probabl y cause d b y th e difference s i n smectit e content. )

Fig. 10 . Relationship s betwee n seismi c slruligruphi c units , velocit y an d cla y mineralog y i n wel l 34/7-1 . Locatio n o f wel l and seismi c lin e is shown i n Fig . I . I t shoul d h e noted tha i th e Pliocen e sediments, which arc mineralogicall y i m m a t u r e with lo w smectite content, sho w a stron g increas e i n velocity as a resul t of progressiv e compaction . This i s no t th e cas e wit h th e Eocen e an d Oligoccn e smectite-ric h sediments .

SEQUENCE STRATIGRAPH Y I N CENOZOI C SEDIMENT S Fe2O3. Th e hig h Ca O conten t i n wel l 30/3- 3 is due t o carbonate-cemente d intervals , a s indi cated b y XRD (Fig . 9) . Both clos e t o th e CSS-3-CSS- 4 sequenc e boundary an d withi n th e CSS- 4 sequenc e ther e is a n upwar d decreas e i n Fe , T i an d th e trac e metals Ni, Co , C u an d Zn , an d increas e in silica content (Fig s 7 , 8 an d 11) . Th e trac e metal s Co, Ni , C u an d Z n hav e generall y slightl y lower value s i n th e CSS- 4 tha n th e underly ing CSS-3 (Fig . 11) ; this finding also reflect s tha t CSS-4 ha d a simila r sedimen t compositio n t o that of CSS- 3 throughou t the Oligocen e epoch . However, th e mos t pronounce d reductio n i n trace metal s suc h a s Co , Ni , C u an d Z n ca n b e seen in well 34/7-1 (Fig. 1 Ic), which also shows a decreasing smectit e content clos e t o th e CSS-3CSS-4 boundar y (Fig s 4 and 10) . The highes t concentratio n o f biogenic silic a is found i n the basa l par t of sequence CSS- 4 (wells 30/3-3, 34/7- 1 an d 34/7-6) . Th e biogeni c silic eous facie s i s mainly dominated b y diatoms, bu t sponge spicule s ar e als o commo n an d radio laria ar e present a s minor component s (Thyber g et al 1999) . Thyberg e t al . (1999) , on th e basi s of detaile d diato m flor a i n CSS-4 , considere d that th e siliceou s sediment s ma y no t represen t the same stratigraphi c horizons , indicate d b y the presence o f diatoms bot h in Eocene i n the northernmost studie d wel l (36/1-2) , an d a low diatom content, mostl y fragments , i n CSS- 5 (Lowe r Miocene) i n wel l 26/4-1 . A n exampl e o f th e diatom assemblage s an d commonl y observe d 'silica aggregates ' i s shown i n Fig . 12 . The sequenc e CSS- 4 ha s a ric h diato m flor a with the dominanc e of Paralia species , Paralia thybergii (Stabel l 1996) , possibl y indicatin g a nearshore settin g compare d wit h th e dominan t Paralia sulcata foun d i n coasta l setting s i n general an d i n the Nort h Se a during the presen t (Stabell 1985 ; Stabell & Lange 1990) . Diatomac eous facie s interbedde d wit h glauconiti c facie s (Fig. 13 ) therefore sugges t that shallower marin e conditions prevaile d i n CSS- 4 tim e tha n i n earlier Cenozoi c time . Poorl y sorted , imma ture clasti c an d angula r grain s observe d i n wel l 34/7-1 also indicate a short-distance transport of clastic material , whic h interfinger s wit h shallow marine biogeni c and glauconiti c sediments . Thi s input o f clasti c fragment s indicate s structura l local high s within , o r clos e to , th e basin. Rund berg (1989 ) suggeste d a n easter n sourc e fo r th e sediments, o n th e basi s o f a sand y prograda tional syste m in the uppe r par t o f the Oligocene sequence i n th e Aga t area . An eve n sedimen t thicknes s i n th e norther n North Se a Basin has been interprete d t o indicat e low clasti c suppl y fro m th e lan d an d a reduce d

263

topographic relie f onshor e (Jord t e t al . 1995 , 1999). Thi s i s supporte d b y th e stud y o f th e sediment compositio n i n th e CSS- 4 sequence , which indicate s sediment-starve d depositio n i n the basin . Thus , th e mineralog y o f CSS- 4 an d the aggradin g seismi c reflectio n patter n bot h indicate reduce d clasti c sedimen t supply . Th e basin configuratio n during Lat e Oligocen e tim e was an important facto r controlling the regional distribution o f siliceous sediments, couple d wit h organic productivity , lo w dissolutio n rate , p H and a reduce d clasti c sedimen t supply .

Lower Miocene sequence (CSS-5) A pronounced upwar d shif t i n the mineralogica l and geochemical compositio n occur s clos e to the base o f CSS-5 (Fig s 4 and 5) . Chlorite increase s up-section, concomitan t wit h a reductio n i n th e concentration o f smectit e i n th e cla y fractio n (Fig. 4) . Smectite-ric h sediment s are , however , characteristic i n wel l 34/7-6 , where CSS- 5 sedi ments hav e simila r cla y mineralog y t o th e underlying CSS- 4 (Fig . 4) . The smectit e conten t in this well, however, seems to decrease upwards , as wel l a s i n th e CSS- 5 sequenc e i n wel l 30/3-3 . In th e southernmos t wells , 24/6- 1 an d 26/4- 1 (Fig. 5) , upwar d enrichmen t o f kaolinit e an d reduction o f smectit e close t o th e bas e o f CSS- 5 is als o observed . I n thre e wells , 34/7-1 , 34/7-2 and 34/7-6 , th e plagioclas e conten t i n Lowe r Miocene sediment s i s relativel y hig h (Fig . 6 ) compared wit h that i n the underlying sequences . This is , however , no t foun d i n wel l 30/3- 3 (Fig. 6b ) or wel l 26/4- 1 (Fig. 6a) . The variation s in K 2 O/Na 2 O rati o als o reflec t thes e regiona l variations wit h respect t o th e feldspars. The silic a conten t i s high, wit h a mea n valu e of 75wt% , an d th e A1 2O3 conten t i s clos e t o 6.85 wt% i n CSS- 5 (Fig . 7) . The Ti , M g an d F e contents ar e generall y lo w (Fig . 8) , wit h mea n values o f 0. 4 wt% TiO 2, 1. 0 wt% Mg O an d 4.3wt% Fe 2O3, respectively . Trace metal s (Co, Ni , Z n an d Cu ) ofte n sho w a wea k reductio n i n concentratio n clos e t o th e base o f th e Lowe r Miocen e sequenc e (Fig . 11) . However, thi s i s no t observe d i n wel l 26/4- 1 (Fig. lib) . Lo w value s sugges t tha t volcani c influence i s low i n thes e sediments . Eastward decreasin g kaolinit e content i n wells 24/6-1 an d 26/4- 1 (Fig . 5 ) indicates tha t clasti c sediments wer e source d fro m th e Eas t Shetlan d Platform i n CSS- 5 time , i n accordanc e wit h th e seismic data markin g outbuilding fro m th e west in the area aroun d 60°N , into the Viking Grabe n (Fig. 1) .

264


Fig. 11 . Distributio n of trace elements (Co, Ni , Cu an d Zn ) base d o n XRF analyse s of cuttings. Location o f wells is shown i n Fig. 1 . (Note the relativel y high content s o f most of these element s i n the partly volcani c Eocen e an d Oligocene sediment.)


Fig. 11 (continued)

265

266


Fig. 12 . Scannin g electron micrograph s o f th e mos t commo n diatom s i n Uppe r Oligocen e sediment s in well 30/3-3 .

Middle-Upper Miocene sequence (CSS-6 and CSS-7) CSS-6 is mapped onl y in the southern part o f the Norwegian Nort h Sea , an d thin s belo w seismi c resolution i n a northwar d direction . Som e bio stratigraphic investigation s hav e indicate d tha t sediments o f CSS- 6 ag e ar e absen t i n th e northern Nort h Se a (e.g . Eidvi n & Rii s 1992) ; however, othe r studie s (e.g . Gradstei n & Ba'ck strom 1996 ) have indicated tha t CSS- 6 sediment s actually ar e presen t there . Th e northwar d thin ning o f CSS- 6 indicate s tha t CSS- 6 i s absen t in th e norther n Nort h Sea ; therefore , w e hav e grouped possibl e CSS- 6 sediment s togethe r with CSS-5 . The bas e o f CSS- 7 i s characterize d b y a pronounced downla p surfac e generated b y sediments buildin g ou t fro m th e eas t i n th e centra l North Sea . Th e thicknes s o f CSS- 7 i s clos e t o seismic resolutio n farthe r t o th e north , bu t biostratigraphic evidenc e fro m quadran t 3 4 (Steurbaut e t al. 1991 ; Eidvi n & Rii s 1992 ) indicates tha t i t i s presen t locally . W e have , however, no t bee n abl e t o distinguis h betwee n these sediments of CSS-7 age and th e underlying CSS-5 o n the seismic data in the northern Nort h

Sea. However , gamma-ra y lo g respons e i n th e investigated well s ofte n show s a marke d shif t in th e uppe r par t o f th e Miocen e succession , suggesting a subdivisio n int o tw o sequence s (Fig. 10) . W e us e thi s shif t i n lo g respons e t o indicate th e bas e o f CSS-7 . W e hav e als o use d gamma-ray an d soni c lo g data , i n additio n t o biostratigraphic data , t o identif y th e to p o f th e Miocene sequence ; thi s i s often recognize d b y a marked upwar d increas e i n gamma-ra y lo g response (Fig s 9 an d 10 ) and i n som e well s b y an increas e i n th e seismi c velocity causin g pro nounced velocit y inversio n a t th e bas e Pliocen e (Fig. 10) . W e hav e n o mineralogica l an d geochemical dat a fro m CSS- 7 i n thi s study.

Pliocene (CSS-8) and Pleistocene sequences (CSS-9 and CSS-10) The CSS- 8 sequenc e ha s bee n investigate d i n wells 30/3-3 , 34/7-1 , 34/7- 2 an d 34/7-6 . W e therefore hav e only mineralogical an d geochem ical dat a fro m th e northernmos t well s (Fig . 1) . The littl e Pleistocene sedimen t analyse d appear s to be similar to the mineralogical composition of the Pliocen e sediments .


267

Fig. 13 . Glauconit e san d grain s fro m wel l 34/7- 1 indicating slow sedimentation , possibl y o n a submarin e high .

The Pliocen e an d Pleistocen e sequence s ar e 'coarse-grained mudstones ' wit h littl e smectit e and enrichmen t o f chlorite, illit e an d t o a certain degree kaolinit e (Fig . 4) . Muc h o f th e kaolinit e found ma y b e take n a s a n indicatio n o f th e degree o f reworkin g o f olde r sediments , a s very little kaolinit e woul d hav e bee n produce d whe n the climate was cold an d erosion rate s were high. Much o f wha t i s recorde d a s illit e b y XR D i n these layer s i s actuall y mic a derive d fro m rela tively unweathere d basemen t rocks . A relativel y high chlorite content i n the sediments is evidence of limite d weatherin g i n a col d climat e i n a region undergoin g glacia l erosion . In th e lowe r Tertiar y sequence , muc h o f th e chlorite ma y b e presen t a s authigeni c mineral s formed b y th e alteratio n o f volcani c glas s an d smectite o r basi c roc k fragments , and ca n there fore b e a possible sourc e fo r CSS- 8 sequences . However, th e iro n conten t i n CSS- 8 i s normally relatively lo w compare d wit h tha t i n th e lowe r part o f the Cenozoi c sectio n (Fig . 8c-f) , a s was also observed b y Rundberg (1989) . Chlorite with a lo w iron and hig h Mg content ma y represent a metamorphic sourc e (Curti s e t al. 1985) . Th e Pliocene an d Pleistocen e sediment s hav e prob ably bee n derive d fro m metamorphi c basemen t rocks fro m wester n Norway . Karlsso n e t al .

(1979) hav e als o reporte d detrita l chlorit e i n a study o f wel l 2/11- 1 i n th e centra l Nort h Sea . The highes t silic a conten t i s foun d i n th e Pliocene and Pleistocene sediment s and , couple d with lo w alumin a (Fig . 7d-f) , give s als o stron g indications tha t Pliocen e an d Pleistocen e sedi ments hav e a differen t compositio n fro m thos e of th e lowe r Tertiar y sequence . Generally , th e highest conten t (>20% ) o f plagioclase i s almost always limite d t o CSS- 8 (Fig . 6c-e) . Lowe r amounts o f K-feldspa r (generall y . Globa l cycle s o f relativ e changes o f se a level . In : PAYTON . C . E . (ed. ) Seismic Stratigraphy - Applications to Hydrocarbon Exploration. America n Associatio n o f Petro leum Geologist s Memoir . 26 , 83-98. WEAVER. C. E . 1989 . Clays, Muds an d Shales. Elsevier . Amsterdam. ZIEGLER, P . 1982 . Geological Atlas o f Western an d Central Europe. Elsevier . Amsterdam.

Cenozoic tectoni c subsidenc e fro m 2 D depositiona l simulation s o f a regional transec t in the norther n North Se a basi n RUNE KYRKJEB0, 1 MARTI N HAMBORG, 2 JA N ING E FALEIDE, 3 HENRIK JORDT 3 & PETE R CHRISTIANSSON 3 1

Geological Institute, University of Bergen, AI legal en 41, N-5007 Bergen, Norway 2 SINTEF, Petroleum Research, N-7034 Trondheim, Norway * Department of Geology, University of Oslo, P.O. Box 1047 Blindern, N-0316 Oslo, Norway Abstract: Th e Cenozoi c depositiona l histor y alon g a regiona l E- W profil e acros s th e northern North Sea has been simulated using a forward process-based simulatio n program of dynamic-slope type . I t involve s a depth-dependent , dual-litholog y diffusio n equatio n tha t handles transport, erosion and deposition of sediments. The data used in the simulation were derived fro m a seismi c lin e calibrate d agains t wells , an d fro m th e regiona l literatur e concerning the norther n North Sea . Th e mos t importan t o f the factor s used are : th e initia l basin form (Paleocene bathymetry) , tectoni c subsidence , isostati c variables , sediment suppl y (sand-shale), sedimen t compactio n (porosity-dept h relationship s fo r sand-shale ) an d eustatic sea-leve l changes . Th e interactio n betwee n th e dat a value s extracte d fro m th e literature coul d no t reproduc e a cross-sectio n simila r t o th e observe d cross-sectio n fro m seismic data. Therefore, th e subsidence pattern and th e initial basin form were reconsidered. The resultin g model gav e a n anomalou s Cenozoi c subsidenc e pattern, differen t fro m th e expected post-rif t therma l subsidence, with deviations corresponding t o Paleocen e and Lat e Miocene-Pliocene times . Th e model-derive d Paleocen e subsidenc e migh t hav e bee n overestimated b y usin g an over-shallo w palaeobathymetric value, although a deepenin g of the basi n i s also indicate d b y biostratigraphi c data . Th e pronounce d Neogen e subsidenc e created accommodatio n spac e fo r a thic k Pliocen e sequence , derive d fro m th e uplifte d eastern sourc e area .

Computer simulation s ar e increasingl y impor tant fo r th e improve d understandin g o f basi n development an d infilling , an d severa l forwar d models tha t simulat e sedimentar y processe s i n basins ar e available . Kendal l e t al. (1991 ) summarized variou s approache s i n forwar d model ling an d conclude d tha t th e significanc e an d utility o f an y particula r mode l i s a matte r o f need, compute r hardwar e an d programmin g resources. However , th e qualit y o f th e simu lation depend s o n th e exactnes s o f th e inpu t values (Aigne r e t al . 1990) . DEMOSTRAT (Rivenae s 1993) , whic h i s a for ward computer-simulatio n mode l fo r siliciclastic basin-fill i n tw o dimensions , ha s bee n use d t o simulate th e Cenozoi c seismi c stratigraphi c framework alon g a regiona l E- W transec t i n the norther n Nort h Se a region . Th e progra m DEMOSTRAT i s a process-base d dynamic-slop e type mode l fo r clasti c settings , wit h a dual lithology, depth-dependen t diffusio n algorithm , which ha s successfull y demonstrate d realisti c erosion an d depositio n (Syvitsk i e t al . 1988 ; Flemings & Jordan 1989 ; Sinclair etal. 1991; Riv-

enaes 1992) . The process-elements in the progra m are performed as logically linked events in a timestep loop (Helland-Hanse n e t al. 1988; Lawrence et al . 1990 ; Rivenae s 1992) . The mos t importan t elements i n th e time-ste p loo p tha t wil l b e dis cussed her e are : tectoni c subsidenc e histor y (user-specified subsidenc e rates) ; eustati c sea level variation s (user-define d sea-leve l curve) ; erosion an d depositio n (user-define d transpor t coefficients tha t describ e ho w easil y erosion an d transport occur , an d a user-define d rat e o f sedi ment san d an d shal e supply) ; compactio n o f sediment (user-define d empirical porosity-dept h relations); flexural isostatic subsidence caused b y loading (o r unloading ) o f sedimen t an d wate r (user-defined siz e of the flexura l rigidit y describing th e lithospheri c strength) . Fo r detaile d des criptions o f th e element s i n th e program , th e reader i s referred t o Rivenae s (1993). The presen t stud y i s base d o n th e seismi c lines SG8043-10 1 an d NSDP84- 2 (transec t 2 ) (Jordt e t al . 1995 , 2000 ; Christiansso n e t al . 2000; Odinse n e t al . 2000), adjacen t well s and a documented regiona l Cenozoic seismi c sequenc e

From: NOTTVEDT , A . e t al . (eds ) Dynamics o f th e Norwegian Margin. Geologica l Society , London , Specia l Publications, 167 , 273-294 . 1-86239-056-8/OO/ S 15.00 © Th e Geologica l Societ y o f Londo n 2000 .

274

R. KYRKJEB 0 E T AL .

analysis (Jord t e t al. 1995 , 2000) . I n addition , information ha s bee n extracte d fro m th e litera ture concernin g th e Nort h Sea . Th e mai n objec tives o f th e presen t simulatio n hav e bee n t o reproduce th e Cenozoi c successio n a s see n i n transect 2 b y applyin g a se t o f realisti c inpu t values an d t o discus s th e tectoni c subsidenc e predicted b y th e simulation .

Geological setting The Nort h Se a Basi n (Fig. 1 ) is an intracratoni c basin situate d a t th e northwester n margi n o f

the Europea n Platform . Sinc e th e crusta l accre tion complete d durin g earl y Devonia n time , the Nort h Se a are a ha s experience d severa l episodes o f lithospheri c extensio n (Devonian Carboniferous, Permo-Triassic , lat e Jurassic early Cretaceous) , eac h followe d b y a stag e o f thermal subsidenc e (Ziegle r 1982# ; Glenni e 1984; Giltne r 1987 ; Badley e t al . 1988 ; Gabriel sen e t al . 1990) . Th e Devonian-Carboniferou s part o f th e structura l histor y i s no t ver y wel l understood i n th e stud y area , mainl y becaus e of lac k o f dat a (Gabrielse n e t al . 1990) . Th e timing o f th e Permo-Triassi c rif t phas e i s stil l a matter o f debate , an d bot h Permia n (Eyno n

Fig. 1 . Regiona l map o f th e norther n Nort h Se a wit h mai n structura l elements and th e locatio n o f transec t 2. Transect 2 a combination o f the conventional SG8043-10 1 lin e and th e deep seismi c line NSDP84-02. crosses the Horda Platfor m (HP), Viking Graben (VG), Eas t Shetlan d Basi n (ESB) and th e Shetland Platform (SP) . and ha s a tota l lengt h o f 240k m i n thi s stud y (fro m Christiansso n e t al . 1999) .

CENOZOIC TECTONI C SUBSIDENC E 1981; Badle y et al 1988 ; Gabrielsen e t al. 1990 ; Faerseth e t al , 1995 ) an d Triassi c (Beac h e t al . 1987; Giltne r 1987 ; Robert s e t a l 1995 ) age s have bee n suggested . Mos t o f th e Permo Triassic stretchin g occurre d betwee n th e 0ygarden Faul t Zon e t o th e eas t an d th e Shetlan d Platform an d th e Hutto n alignmen t t o th e west (Odinsen e t a l 20006) . Modellin g result s (te r Voorde e t a l 2000 ) sho w tha t n o hea t acces s from th e broa d extende d Permo-Triassi c rif t

275

event wa s lef t a t th e onse t o f th e mor e focuse d late Jurassic-earl y Cretaceou s riftin g (Odinse n et a l 20006) , whic h le d i n tur n t o th e develop ment o f th e mai n structura l element s o f th e Viking Grabe n area . In earl y Cretaceou s time , therma l subsidenc e commenced; thi s was marked b y the cessation of fault bloc k tilting . Th e followin g post-rif t sedi mentation (Cretaceous-Cenozoic ) fille d i n an d buried th e rif t topography . I n th e norther n

Fig. 2. Correlation chart showing the seismic seuence stratigraphic framework. connected to chronostratigraphy and lithostratigraphy (from Jordt et al. 1995. 2000).

276


North Sea , sout h o f 61 : N, earl y Cretaceou s deposits ar e mainl y restricte d t o th e Vikin g Graben an d mostl y la p ont o th e basi n margins , but lowe r Cretaceou s sediment s ar e als o foun d in th e Stor d Basi n an d a t th e Hord a Platform . Late Cretaceou s sedimentatio n burie d th e rif t topography an d Cenozoi c sedimentatio n over stepped th e grabe n boundaries . Period s o f in creased subsidenc e associate d wit h faultin g ar e detected i n the post-rift stage , particularly in late Cretaceous-early Tertiar y tim e (Thorn e & Watts 1989 ; White & Lati n 1993 ; Hal l & Whit e 1994; Leper q & Gaulie r 1996 ; Nadin & Kusznir 1996). bu t thi s cannot b e considered a rif t phas e (Badley e t al. 1988 ) The Cenozoi c basin-fil l wa s derive d fro m tw o main sources : th e Eas t Shetlan d Platfor m t o th e west an d th e Fennoscandia n Shiel d t o th e east . Deposition i n Tertiar y tim e wa s controlle d b y several phase s o f uplif t affectin g th e sourc e areas, combine d wit h relativ e sea-leve l change s and therma l subsidenc e o f the basi n (Jord t e t al. 1995. 2000) . Th e wester n sourc e area s domi nated th e sedimen t influ x fro m lat e Paleocen e until earl y Miocen e time s (Stewar t 1987 ; Gallo way e t a l 1993 ; Jord t e t al . 1995 , 2000) . Th e basin shallowe d fro m relativ e deep-marin e conditions durin g th e lat e Paleocen e time , t o shallow-marine an d locall y fluvia l condition s i n late Earl y Miocen e tim e (Rundber g e t al . 1995 ; Jordt e t al . 1995 , 2000) . Th e sedimen t influ x from th e wes t cease d i n Earl y Miocen e tim e and mos t o f Mid - an d Lat e Miocen e time , wa s characterized b y non-depositiona l conditions . In Pliocen e time , a majo r uplif t o f th e Fennos candian landmas s cause d a majo r sedimen t influx fro m th e east . I n lat e Pliocen e an d Pleistocene time , th e glaciation s o f Fennoscan dia caused th e removal o f proximal Tertiar y an d older deposit s fro m th e basin' s easter n margin .

CSS-1 t o CSS-1 0 (Cenozoi c Seismi c Sequences) . The correlatio n char t i n Fig . 2 connect s th e sequence stratigraphi c interpretatio n wit h th e standard chrono - an d lithostratigraphi c subdivisions o f Deega n & Scul l (1977), revise d b y Isak sen & Tonstad (1989 ) fo r th e Norwegia n secto r and b y Kno x & Hollowa y (1992 ) fo r th e U K sector. A depocentr e ma p wit h indicate d out building direction s i s show n i n Fig . 3 . Figur e 4 shows th e ful l cross-section , wherea s th e dis tribution an d thicknes s (depth-converted) of th e Cenozoic sequence s ar e give n i n Fig . 5 . CSS-1 (o f Lat e Paleocene-earlies t Eocen e age), whic h correlate s wit h th e Rogalan d Group (Fig . 2) . ha s it s mai n outbuildin g direction fro m th e west , wher e i t wa s source d b y th e uplifted Shetlan d Platform . As can b e see n fro m Fig. 5 . westerly derived sediment s reache d fa r t o the east . Th e sediment s wer e deposite d a s submarine fa n complexe s i n a relativel y dee p basin, wit h wate r depth s suggeste d t o b e middle bathyal (500-100 0 m) i n wel l 3/25- 1 (locate d close t o transec t 2 i n th e U K sector ) (Gradstei n et al . 1994) . I n th e east , sedimentar y wedge s prograded westward s toward s a depocentr e probably situate d nort h o f th e transect , wes t o f Sognefjorden (Fig . 3) . I t i s probable tha t CSS- 1 covered a large r area , an d tha t part s o f western Norway wer e transgressed i n late Paleocene tim e (Jordt e t al . 1995) . However, thes e deposit s wer e removed b y pos t Paleocen e erosio n durin g th e Late Pliocen e an d Quarternar y glaciations .

Cenozoic seismi c stratigraphy The seismic-stratigraphi c framework use d i n this study wa s establishe d b y Jord t e t al . (1995) , on th e basi s o f a regiona l analysi s o f th e Ceno zoic i n the centra l an d norther n Nort h Sea . Th e sequence description s ar e no t repeate d here : instead, th e importan t constraint s fo r th e com puter modellin g ar e examined . From eas t t o west , transec t 2 (Fig . 1 ) crosse s the mai n structura l feature s o f th e norther n North Se a Basin , namely , th e 0ygarde n Faul t Zone, th e Hord a Platform , th e Vikin g Grabe n and th e easter n par t o f th e Eas t Shetlan d Plat form. Th e Cenozoi c succession in the North Sea has bee n subdivide d int o te n sequences , labelle d

Fig. 3. Depocentr e location s an d outbuildin g directions indicated by arrows for each of the Cenozoic seismic sequence s give n b y number s (fro m Jord t et al . 1999) .

Fig. 4. Transec t 2 in full basi n scale and locatio n o f key well 30/11-2 (from Christiansson e t al. 2000). The transec t show s the Paleogen e an d Neogen e succession s relativ e to deepe r structure s and th e mai n structural elements. Lin e locatio n i s indicated i n Fig . 1 .

278


Fig. 5. Cenozoi c basin fill (CSS-1 to CSS-10) along transect 2. Arrows show main outbuilding directions, and th e interpreted borde r betwee n easterl y an d westerl y derive d sediment s i s marked fo r sequence s CSS-1 and CSS-2 . CSS-2 (o f Eocen e age ) correlate s wit h th e lower par t o f th e Hordalan d Grou p (Fig . 2) . This sequenc e ha s bee n subdivide d int o tw o units: a lowe r Eocen e sequence , CSS-2.1 , foun d mainly t o th e east , an d a middle-uppe r Eocen e sequence, CSS-2.2 , wit h it s depocentr e i n th e Viking Grabe n (Fig . 3) . CSS-2. 1 i s predomi nantly eas t derived , an d it s depositio n wa s ter minated wit h the correspondin g submergenc e o f Fennoscandia (Rundber g 1989 ; Sterbau t e t al. 1991; Jord t e t al . 1996) . Th e depositiona l styl e of CSS- 2 seem s t o b e simila r t o tha t o f CSS-1 , both fo r westerly and easterl y derived sediments. CSS-3 (of Early Oligocene age ) correlates with the middle part o f the Hordaland Grou p (Fig. 2). At th e Eocene-Oligocen e transition , uplif t o f Fennoscandia cause d a relativ e fal l o f se a level and a n outbuildin g o f a sand y wedg e fro m th e east, sout h o f transec t 2 . The depocentr e o f thi s sandy unit is located north of the transect (Fig. 3). Another depocentre i s located i n the Viking Graben, an d wa s source d fro m th e wester n hinter land (Jord t e t al . 1995) . CSS-4 (o f lat e Early-Lat e Oligocen e age ) correlates with the middle part o f the Hordalan d Group (Fig . 2) . I t prograde d eastwards , an d has a depocentr e alon g transec t 2 (Fig . 3) . Another depocentr e i s locate d sout h o f th e transect, wher e CSS- 4 prograde d t o th e north west. A n irregula r surfac e o n th e to p o f CSS- 4 (Fig. 5 ) was probabl y cause d b y th e remobilization o f clay s (Jordt e t al . 199 5 and 1996) . CSS-5 (o f Lates t Oligocene-Earl y Miocen e age) correlate s wit h th e uppe r par t o f th e Hordaland Grou p (Fig. 2) , and is the uppermost preserved strat a o f th e grou p i n th e stud y are a (see CSS-6). CSS-5 prograde d fro m th e west, but formed a north-trendin g shee t i n th e middl e o f the basin (Figs 3 and 5) . No sedimen t influ x fro m the east has been recognize d i n this sequence. Th e irregular top o f CSS-5 is interpreted a s an incised valley surface , probabl y forme d b y subaeria l erosion. This latter feature is more distinct in the

seismic sections taken further nort h i n the North Sea. Rundber g e t al . (1995 ) an d Jord t e t al . (1996) suggeste d tha t th e subaeria l exposur e i n this are a wa s primaril y controlle d b y tectoni c uplift o f the northernmos t Nort h Sea . CSS-6 (o f lates t Early-earl y Mid-Miocen e age) i s th e uppermos t uni t correlativ e wit h th e Hordaland Grou p (Fig. 2). It is only found south of th e Vikin g Grabe n an d o n th e Hord a Plat form. I n th e presen t stud y area i t i s represented by a hiatu s (o r belo w seismi c resolution) . CSS-7 (o f Mid-Lat e Miocen e age ) correlates with th e lowe r par t o f th e Nordlan d Grou p (Fig. 2). This sequence prograded fro m the east as a resul t o f th e uplif t o f Fennoscandi a relativ e to th e subsidin g basi n i n Mid-Miocen e time . The depositio n o f thi s sequenc e marke d a shif t of th e mai n sourc e are a fro m a westerl y t o an easterl y domain . I n th e transec t area , th e sequence is represented b y a relatively thin depo sitional wedg e (Fig. 5) , with th e majo r depocen tres locate d nort h an d sout h o f th e transec t (Fig. 3) . Thi s sequenc e als o correlate s wit h th e Utsira Formatio n (Fig . 2) . However , correla tions o f seismi c an d wel l dat a show ? tha t th e sandy unit s of bot h CSS- 5 an d CSS- 8 hav e als o been interprete d as th e Utsir a Formatio n (Jord t et al . 1995) . CSS-8 (o f Pliocen e age ) correlate s wit h th e Nordland Grou p (Fig . 2 ) an d wa s deposite d i n response t o erosion of a strongly uplifted Fenno scandian hinterland , togethe r wit h Nort h Se a basinal subsidenc e providin g accommodatio n space (Jord t e t al. 1995 , 2000). The CSS- 8 clastic wedge prograded to the west, forming two depocentres (Fig . 3) : a smalle r wedge wa s locate d o n the transec t i n th e Stor d Basin , and a large r on e in th e Tampe n Spu r area . CSS-9 (o f earl y Quaternar y age ) correlate s with th e uppe r Nordlan d Grou p (Fig . 2) . but i s represented b y a hiatu s i n transec t 2 . CSS-10 (o f lat e Quaternar y age ) correlate s with the uppermost par t o f the Nordland Grou p

CENOZOIC TECTONI C SUBSIDENC E (Fig. 2) . This sequence show s an angular unconformity a t th e lowe r boundar y (Fig . 5) , espe cially in the eastern part o f the study area, where the olde r sequence s ar e truncate d b y CSS-10. Input factor s The modellin g was performe d b y systematically changing inpu t value s t o reproduc e th e geom etry observe d i n th e seismi c reflectio n data . We starte d wit h a fixe d se t o f value s fo r initia l basin form , amoun t o f tectoni c subsidenc e (uplift), rate s o f sedimen t supply , sea-leve l curve an d value s fo r isostati c calculations , whereas th e value s o f th e transpor t coefficient s were varied. The fixed input values are summarized i n Tabl e 1 an d th e choic e o f value s i s discussed below.

Initial basin form Water depth is a critical factor in subsidence and burial analysis, because it represents the amoun t of th e basin' s 'underfill' . Larg e palaeobathy Table 1 . Initial input factors summarized for th e postrift modelling Variable

Source

Initial basi n form

Water depth s alon g transec t 2 based o n wor k b y Rocho w (1981), Barto n & Wood (1984) , Ziegler (1990 ) an d Gradstei n et al. (1994) Haq e t al . (1987) long-term curve; age based o n Harlan d e t al . (1990) tim e scal e Derived fro m backstripping ; supply fro m bot h side s According t o th e typ e curves shown i n Fig . 7

Sea-level function Sediment suppl y Transport coefficient function Surface laye r thickness Compaction

Isostasy Time span , number o f time step s Basin length , number o f columns Tectonic movements

30cm Sclater & Christie (1980) curves ; the compactio n o f th e bas e is measured an d give n togethe r with tectoni c subsidenc e Flexural rigidit y D = 1. 0 x 10 22 Nm; n o axia l stress 60.5 Ma, 20 0 time steps , whic h gives each tim e ste p a duratio n of 30 2 500 a 240km, 20 0 column s Thome & Watts (1989 )

279

metric value s a t th e en d o f Cretaceou s tim e provided muc h o f th e accommodatio n spac e for Tertiar y sedimentation . O n th e basi s o f a n analysis o f fauna l trend s alon g palaeoslop e transects i n th e norther n Nort h Sea , Gradstei n et al . (1994) and Gradstei n & Backstrom (1996 ) estimated conservativel y that th e earl y Palaeo cene wate r dept h wa s betwee n 25 0 an d 500m , and tha t th e Lat e Paleocen e wate r dept h wa s between 50 0 and 1000 m i n th e UK-wel l 3/25-1 , which i s clos e t o th e modelle d transect . Othe r wells (Fig . 6 ) analyse d i n th e are a sho w tha t water depth s i n earl y Paleocen e tim e wer e be tween 50 0 an d 750 m i n UK-wel l 9/23- 1 an d between 20 0 and 500 m i n UK-well 9/13-1. Lat e Paleocene wate r depth s wer e betwee n 50 0 an d 1000m i n UK-wel l 9/13-1 , an d betwee n 75 0 and 1500 m i n UK-wel l 9/23-1 (Gradstein e t al . 1994, Gradstei n & Backstrom , 1996) . Palaeo bathymetric maps (Barto n & Wood 1984 ) based on informatio n obtaine d fro m wel l samples , combined wit h th e palaeogeographica l map s of Ziegler (1981 , 19820) , show a 500 m contour fo r the Paleocen e wate r depths , indicatin g tha t water depth s i n th e deepes t par t o f th e basi n must hav e exeede d 500m . Accordingly , a max imum initia l basi n dept h i n thi s stud y wa s estimated t o b e c. 550m. The initial basin configuration along the transect at the beginning of Late Paleocene tim e was constructed b y combining the work of Barton & Wood (1984 ) (500 m contours) , palaeogeogra phical map s o f Rocho w (1981 ) an d Ziegle r (1990) (Fig . 6) , an d th e presen t bas e Tertiar y relief. Th e initia l for m o f th e basi n (Fig . 7 ) i s believed t o hav e bee n nearl y symmetrical , with the wester n flan k reachin g se a leve l o n th e Eas t Shetland Platfor m (Rochow 1981 ; Ziegler 1990), whereas parts of the eastern flank on th e Hord a Platform wer e submerged (Ziegle r 1990) .

Sediment input The depth-converte d sejsmi c profil e wa s back stopped t o determin e th e decompacte d cross sectional are a o f eac h sequenc e afte r removin g the overburden . Th e backstrippin g proces s wa s carried out using the Balancing Section Progra m (XBSP; Midlan d Valle y Exploratio n Ltd) . Th e exponential porosity-dept h relationship s o f Sclater & Christi e (1980 ) fo r shale s an d sand s were use d i n th e backstrippin g procedure . Th e lithological compositio n o f eac h sequenc e wa s established b y investigatin g the completio n lo g from wel l 30/11- 2 (Fig . 4) ; th e shale-to-sand ratio wa s estimate d fro m th e gamma-ra y log . The volumetri c sand fractio n (Tabl e 2 ) appear s

280


Fig. 6 . Integratio n o f Lat e Paleocen e palaeogeographica l map s b y Rocho w (1981) . Barto n & Wood (1984 ) an d Ziegler (1990 ) wit h interpreted fauna l trends fro m well s location s b y Gradstei n e t al . (1994 ) an d Gradstei n & Backstrom (1996) .

CENOZOIC TECTONI C SUBSIDENC E

281

Fig. 7 . Th e initia l form o f the basi n i n the beginnin g Lat e Paleocen e tim e based o n Fig . 6 . The maximu m wate r depth i s suggested t o b e 550m . Th e basi n for m i s symmetrical wit h shallo w wate r depth s a t th e flanks .

to b e relativel y high fo r mos t o f th e sequences . The estimates ar e admittedly uncertain , becaus e of limitation s o f th e metho d an d th e uncer tainty o f the log-derive d values . Because th e sectio n i s 2D , th e decompacte d sediment-flux rate s ( C2D volumes') fo r san d an d shale are given in square metres per year (m2 a"1 ) (see Fig . 8) . Changes i n sediment-flu x rate s ar e closely relate d t o tectoni c movement s an d t o changes i n the sedimen t suppl y system ; they ar e synchronous wit h th e developmen t o f deposi tional sequenc e boundarie s (Jord t e t al. 1995) . To tes t th e sensitivit y of compactio n t o th e lithological variations, a backstripping was done for 100 % shal e an d 100 % san d content . Th e difference betwee n the shale and san d volumes in the basina l cross-sectio n wa s foun d t o rang e from 0 to 20%, with the greatest difference i n the oldest (mos t compacted ) units , an d decreasin g upwards. Hence , th e sensitivit y analysi s indi cates tha t th e error s i n bul k volum e cause d b y departures i n lithological composition ar e small. The lack o f information o n the distribution of lithologies along th e profile mad e i t necessary to simplify thi s par t o f th e problem . A s th e litho facies architectur e o f th e basina l cross-sectio n Table 2 . Interpreted sand fraction fo r each sequence applied in the simulation Sequence

Sand fractio n

CSS-1 CSS-2 CSS-3 CSS-4 CSS-5 CSS-7 CSS-8 CSS-10

0.76 0.62 0.47 0.35 0.80 0.99 0.49 0.72

was not th e main objective of the simulation, the compaction-related error s caused b y lateral lithological variation s hav e bee n neglected . Instead , the modellin g focuse d o n th e bul k geometrica l relationships o f eac h seismi c sequence. Some sequences , especiall y those derived fro m the easter n source , hav e bee n erode d fro m th e flank. This erosional truncation is shown in Fig. 5. As th e backstrippin g take s onl y th e volum e o f sediment present in the profile (2D ) into accoun t and n o out-of-plan e transpor t o f sediment s i s possible in the forwar d simulation , a redistribution patter n fo r th e erode d sedimen t ha d t o b e suggested. The volum e of the redistribute d sediment wa s calibrated t o matc h th e overal l thickness framework , an d th e tota l sedimen t bul k volume. Th e patter n o f redistributio n woul d obviously have to reflec t th e palaeogeographica l situation. Sequence s CSS-1 , CSS- 2 an d CSS- 4 therefore ha d t o includ e additiona l sedimen t i n the easter n par t t o fi t th e pre-Miocen e palaeo geography. Thi s wa s accomplishe d b y extrapo lating th e observe d sequence s eastwards ; th e amount o f volum e adde d wa s calibrate d t o th e related volume s o f sequence s CSS- 7 an d CSS-8 . While CSS- 7 an d CSS- 8 wer e bein g deposited , the additiona l sedimen t volum e i n th e eas t i s assumed t o hav e bee n erode d an d redeposited . These depositiona l event s correspon d t o th e uplift o f th e easter n sourc e area . The sediment s are assume d t o ente r th e basi n at poin t source s o n eac h sid e o f th e cross section. O n th e seismi c reflectio n dat a i t i s pos sible to distinguish between easterly and westerly derived sediments (see location o f depocentres i n Fig. 3 ) for som e of the sequence s (CSS-1, CSS-2 , CSS-4 and CSS-10) (Jordt e t al 1995) , and therefore calculat e th e decompacte d cross-sectiona l area t o ente r fro m eac h sid e o f th e section . Because o f restricte d knowledg e o f th e sourc e

282


Fig. 8 . Sedimen t suppl y (irra ' ) o f san d (continuou s line ) an d shal e (dashe d line ) fro m wes t (a ) an d eas t (b). Volumes ar e calculate d fro m backstripping . Thes e volume s hav e 0 % porosity , whic h i s the inpu t for m i n DEMOSTRAT. Th e progra m wil l conver t inpu t volume s to maximu m porosities. 0.49 fo r san d an d 0.6 3 fo r shale , according t o th e empirica l porosity-dept h relation s o f Sclate r & Christi e (1980 ) fo r Nort h Se a settings .

area, erosio n o f sourc e are a topograph y i s no t specified i n detai l i n th e simulation .

Compaction The function s of Sclate r & Christi e (1980 ) hav e commonly bee n use d t o describ e th e porosity depth relationshi p i n th e Nort h Se a region . DEMOSTRAT use s on e curv e fo r eac h lithology , whether san d o r shale , wherea s XBS P (Midlan d Valley Exploratio n Ltd ) uses a singl e lithology dependent curve . An importan t aspec t i s th e volum e contro l on th e sediment s belongin g t o eac h sequence . By using the same empirical porosity-depth relationship an d san d fractio n value s bot h i n th e

inverse an d forwar d modelling , th e volum e i s expected t o b e th e sam e i n th e simulate d an d in th e observe d section . Th e porosit y value s in the simulate d cross-sectio n ar e no t necessar ily th e sam e a s i n th e observe d section . Thes e deviations ar e du e t o uncertaint y relate d t o the usag e o f empirical porosity-depth functions . However, a s th e erro r woul d b e th e sam e fo r the invers e an d forwar d simulations , th e un certainty o f th e porosity-dept h relationshi p i s not relevant . The underlyin g Cretaceous an d olde r deposit s have a non-unifor m thickness distribution along the cross-sectio n (Fig . 4) , bein g thickes t i n th e Viking Grabe n an d thinnin g greatl y toward s the basi n flanks . Thi s probabl y cause d differ ential compactio n an d base-Tertiar y subsidence .

CENOZOIC TECTONI C SUBSIDENC E which i t i s no t possibl e t o represen t i n th e DEMOSTRAT simulation . I n th e decompactio n process based o n XBS P (Midland Valle y Exploration Ltd) , th e Cretaceou s strat a were include d and th e resultin g value s wer e adde d t o th e tectonic subsidenc e i n th e DEMOSTRA T simula tion. I n al l presentations o f tectoni c subsidenc e curves, thes e compactio n value s ar e subtracted . These are , how - ever, minimu m value s fo r th e compaction o f th e underlyin g sediments , a s n o sediments olde r tha n Cretaceou s hav e bee n taken int o account in the decompaction proces s and i n the depositiona l modelling .

Transport coefficients In DEMOSTRA T (Rivenaes 1992, 1993) , the depth dependent diffusio n dual-litholog y algorith m handles erosio n an d deposition . Diffusio n ca n be characterized a s a time-dependent smoothin g process. I n geologica l term s thi s ca n b e erosio n of topographic high s an d depositio n o f the ero sional product in sedimentary basins . Th e model has severa l limitations . I n th e presen t case , th e most importan t ar e lac k o f transvers e sedimen t transport (ou t o f th e plan e o f th e section ) an d inability t o mode l turbidites . Input data are values for transport coefficients , termed K fo r san d an d shale . Th e transpor t coefficients giv e a measure of the transport effici ency o f sediments , tellin g u s ho w easil y erosio n and depositio n occu r (Rivenaes 1992). Sediments are supplie d fro m point source s o n eac h sid e of the cross-section . I n general , dept h variation s of K value s i n th e simulatio n ar e adapte d a s

283

follows (Fig . 9) : a highe r transpor t efficienc y above se a level , a n exponentia l decreas e nea r and belo w se a leve l whe n th e loa d enter s th e basins, an d belo w a certai n leve l th e transpor t efficiency i s constant an d 'low' . Rivenaes (1993) discussed the quantification of the transpor t coefficient s an d summarize d som e measured an d som e synthetic K values propose d by variou s workers , showing tha t K value s ar e a functio n of the scal e of investigation an d tha t K value s ar e dependen t o n environment . Rive naes (1992) suggested syntheti c lvalues rangin g from 300m 2 a"1 i n mountai n environment s t o 1.2 x 10 4 m2 a"1 fo r nonmarin e mud. Kenyo n & Turcotte (1985 ) have measured ^ values for the Mississippi delt a fron t o f 5. 3 x 10 5 m2 a"1. Unfortunately, n o values for the transport coefficients fo r the Cenozoic basin-fil l in the North Sea ar e available , an d i n th e presen t stud y i t has bee n necessar y t o assum e th e transpor t coefficients representin g th e contributin g pro cesses invoke d i n sedimentatio n an d erosion . The applie d K values , therefore , ar e take n a s those tha t reproduc e th e latera l distributio n o f sediments. Th e K curv e patter n use d i n thi s simulation i s complex , becaus e o f change s i n depositional patter n bot h i n tim e (difference s in sediment supply for eac h sequence) and spac e (differences i n sedimen t suppl y fo r th e wester n and easter n part , an d differenc e i n depositiona l regime with depth) . The sequence s CSS- 1 t o CSS- 5 ar e mainl y derived from a western source, with limited sediment supply from the east. This suggests a higher transport efficienc y fo r th e westerl y source d sediments. CSS-1 , whic h wa s source d fro m th e

Fig. 9. Th e transport coefficient function s used in the simulation. The values decrease to a depth of 40m an d ar e constant a t deepe r levels . Difference s i n transport efficiency ar e neede d t o reproduc e the observe d geometrie s in Fig . 5. A hig h valu e above sea level, decreasin g t o a hig h valu e a t 20 m belo w se a level, an d constan t high further dow n (lin e a ) ar e adapte d for th e wester n derive d CSS-1 . A lo w value both above and belo w se a level, describing a period of non-deposition, and tryin g to avoi d erosion, is used for CSS- 6 to CSS- 9 from th e west. Intermediate value s to thos e of line a an d lin e c are applie d fo r th e res t o f the sedimentar y record.

284


west, ha s th e larges t sedimen t suppl y an d a higher A T value i s assumed . Furthermore , CSS- 1 consists partl y o f submarin e fa n deposit s an d therefore a hig h K valu e a t grea t depth s ha s been applie d t o matc h th e observe d geometrie s (Fig. 9 , lin e a) . I n period s o f erosio n o r non deposition (CSS- 6 t o CSS- 9 fro m th e wester n source), K values are assume d t o hav e bee n very low (c . 1.0) (Fig . 9 , lin e c) . Thi s migh t no t b e correct considerin g th e erosio n observe d i n un derlying sequence s an d th e northward-directe d transport o f erosiona l product s (Jord t e t al. 1995). bu t becaus e o f th e 2 D limitation s w e ar e not abl e t o transport sediment s ou t o f the plane. The CSS- 8 sequence , source d fro m th e east , ha s high K value s t o simulat e erosio n o f previousl y deposited CSS- 1 to CSS- 4 on the easter n flank (Fig. 9 , lin e b) . We mak e n o attemp t t o predic t a detaile d lithological facie s distributio n alon g th e cross section. Therefore , n o particula r sedimen t sort ing (sand-shale ) i s applie d t o differentiat e between th e K values . Thi s i s du e t o restricte d knowledge o f sedimen t compositio n alon g th e profile, time-ste p resolutio n (302.50 0 a), an d t o the necessaril y limite d scop e o f thi s study. CSS-1 an d CSS- 2 ar e partl y submarin e fa n deposits (turbidites ) an d eac h turbidit e even t cannot b e regarde d t o hav e a diffusiv e nature , but th e su m o f turbidit e event s withi n eac h sequence ca n b e assume d t o b e diffusive .

Tectonic movements The genera l hypothesi s o f Cenozoic basin devel opment i n th e norther n Nort h Se a i s tha t th e basin wa s dominated b y thermal subsidenc e tha t followed th e Lat e Jurassic-Earl y Cretaceou s rifting (e.g . Sclate r & Christi e 1980 ; Badle y et al . 1988 ; Gabrielse n e t al . 1990) . Severa l workers hav e quantified and explaine d th e caus e of thi s subsidence . Thorn e & Watt s (1989) , White & Lati n (1993) . Hal l & Whit e (1994) , Lepercq & Gaultier (1996 ) and Nadin & Kusznir (1996); hav e inferre d a highe r subsidenc e rat e in Earl y Tertiar y tim e an d a lowe r rat e i n th e rest o f th e Tertiar y time . Th e subsidenc e dat a obtained b y Thorn e & Watt s (1989 ) (Fig . 10 ) were applied i n the initial simulation run . This is considered t o be a minimum subsidenc e pattern , because i t wa s a resul t o f backstrippin g usin g Airy isostasy, which favours isostatic compensa tion. However , Thorn e & Watt s (1989 ) appar ently di d no t conside r th e effect s o f uplif t o r subsidence o f basi n flanks . Therefore , tectoni c movements o n th e flank s wer e connecte d

(linked) t o th e qualitativ e interpretatio n o f Rundberg (1989 ) an d Jord t e t al . (1995). Only on e faul t i n th e basina l cross-sectio n i s thought t o hav e substantiall y influence d th e basin geometry . Th e faul t i s 4 6 k m fro m the wester n end o f the section an d cut s sequenc e CSS-1. wherea s sequenc e CSS- 2 i s unaffected . As thi s faul t ha s affecte d th e depositiona l pattern, i t i s include d i n th e simulation . Th e relative tectoni c dro p o f th e easter n par t o f the sectio n i s taken t o b e about 350m . an d th e faulting i s assume d t o hav e occurre d i n Lat e Paleocene time .

Eustasy The mode l require s th e inpu t o f sea-leve l variations through the geological time . The long-term curve of Haq e t al. (1987), therefore, was chosen to reflec t se a level . Th e curv e wa s sample d an d converted t o the Harland e t al. (1990) time scale . However, eustati c sea-leve l change s ar e no t within th e mai n scop e o f th e presen t study , a s the rat e o f sea-level chang e i n terms o f the long term curv e (Ha q e t al . 1987 ) i s to o lo w fo r it s effects t o be represented i n the depositional style . An exceptio n i s th e larg e fal l i n se a leve l fro m 15 Ma t o th e present, whic h affects th e sequence s CSS-7 t o CSS-10 . T o b e comparabl e wit h th e possible effect s o f th e short-ter m eustati c curv e of Haq e t al. (1987), the simulation ha s been ru n with 20 0 tim e steps . Th e result s confir m tha t there i s n o significan t difference wit h respec t t o the mai n simulatio n base d o n th e long-ter m eustatic curve , probabl y becaus e o f th e combi nation o f the depositional condition s in the dee p basin an d th e larg e scal e o f th e stratigraphi c approach. A n increas e i n th e numbe r o f tim e steps an d horizonta l resolution (numbe r of columns) migh t giv e greate r detai l o f change s i n a shallow-marine depositional system with respec t to th e short-ter m curv e of Ha q e t al . (1987) .

Isostasy The lithospher e i n th e Nort h Se a regio n wa s more stabl e i n Cenozoi c tim e tha n durin g Mid Jurassic t o Cretaceou s times . Th e regiona l tec tonic stres s i n Cenozoi c time s wa s apparentl y low, with the strain n o greater tha n th e develop ment of minor features. Therefore, n o axial stress has been applied i n the simulation, and a flexural rigidity o f th e lithospher e o f 1 . 0 x l 0 2 2 N m was used (Fjeldskaa r 1994) . The matri x densities for th e isostati c compensatio n hav e been take n as follows : 2.65gcm" 3 fo r sand , 2.72gem" 3 for shale , an d l.OSgcm" 3 fo r wate r (th e sam e

CENOZOIC TECTONI C SUBSIDENCE

285

Fig. 10 . Cenozoi c tectonic subsidence pattern give n by Thorne & Watts (1989). The subsidenc e analysis is based on backstrippin g wit h Airy isostasy (flexura l rigidit y zero) o f wells in the norther n Vikin g Graben area , an d suggests a n accelerate d subsidenc e i n Paleocene tim e followe d b y gentle subsidenc e rate s i n the res t o f th e Cenozoic period , reflectin g post-rif t thermal subsidence (McKenzi e 1978). In th e simulation , the subsidence data obtaine d b y Thorne & Watts (1989 ) were applied fo r modellin g case 1 . A flexural rigidity of 1. 0 x 10 22 N m was used .

286


as used b y Sclate r & Christi e (1980)) . Fo r the crus t an d th e uppe r mantle , densitie s of 2.8gcm~ 3 an d 3.4gcm~ 3 , respectively , hav e been assumed .

Modelling results, case 1 The us e o f th e value s specifie d i n Tabl e 1 resulted i n a n overfil l o f th e basi n b y Earl y Oligocene time , an d subaeria l depositio n o f sequences younge r tha n Earl y Oliogocen e (se e Fig. 1 1 a). Therefore , i t i s no t possibl e t o com pare th e modelle d sequenc e geometrie s wit h th e observed geometries . Furthermore , th e model ling result s place d th e bas e Tertiar y to o shallo w at presen t time . Th e effec t o f introducin g Airy isostasy (flexura l rigidit y zero ) wa s tested , bu t was no t sufficien t t o avoi d earl y overfillin g (see Fig lib) , whic h occurre d i n Mid-Oligocen e times i n thi s model , o r t o improv e th e positio n of th e presen t dept h leve l o f th e bas e Tertiary . It i s eviden t fro m th e simulate d cross-sectio n

that th e basi n mode l underestimate s eithe r th e total subsidenc e o r the initial water depths alon g the cross-section . Accordingly , a ne w initia l form o f th e basi n (Fig . 12) , with a n asymmetri cal form , enhance d initia l wate r depth s i n it s eastern part , an d a sligh t increas e i n th e maxi mum initia l wate r dept h wa s suggested . The overall objective in this study was to carry out a simulatio n tha t coul d matc h th e observe d cross-section wit h respec t t o th e geo-histor y related t o th e basin-fillin g processe s an d th e observed geometrica l relationships . The eustatic sea-level, isostas y an d sedimen t inpu t wer e considered t o b e documente d value s an d i t was assume d tha t th e softwar e handle s erosio n and depositio n properly . Th e factor s tha t ar e associated wit h th e larges t uncertaintie s wer e therefore tectoni c subsidenc e an d wate r depths . We could, by an increas e in the maximu m initial water dept h o f th e basi n t o a n orde r o f 1000m , still use the subsidence rate s proposed by Thorne & Watt s (1989) . Thi s i s als o i n agreemen t wit h the wate r depth s fo r earl y Paleocene tim e in th e

Fig. 11 . (a ) The interactio n betwee n th e chose n value s faile d t o reproduc e the cross-section. The basi n wa s overfilled i n Early Oligocene time, (b) Use of D = 0, Airy isostasy, could no t improv e the results, as it also led to an overfille d basi n i n Lat e Oligocen e time .

CENOZOIC TECTONI C SUBSIDENCE

287

Fig. 12 . O n th e basi s o f the assumptio n tha t th e overfille d basi n i n Earl y Oligocene time in the mode l was mainly caused b y a n underestimate d wate r depth o f the initia l basin, a revise d initia l for m o f the basi n i n Lat e Paleocene time with increased wate r dept h along the cross-section was required. Also, the tectonic subsidence pattern was reconsidered t o obtai n agreement o f the modelled an d observe d geometrica l relationships .

northern Nort h Se a propose d b y Bertra m & Milton (1989 ) and Nadi n & Kusznir (1995) , bu t differs fro m wate r depths proposed b y Barton & Wood (1984 ) an d Gradstei n e t al (1994 ) fo r early Paleocen e time . However , Gradstei n e t al. (1994) an d Gradstei n & Backstro m (1996 ) suggested a rapi d deepenin g o f th e basi n fro m Early (200-50 0 m) to Lat e Paleocen e tim e (500 1000m). I t wa s therefor e decide d t o kee p th e maximum initia l water dept h a t a maximu m of c. 700m at the beginning of Late Paleocene time . Paleocene uplif t i n northwes t Britai n include s the shel f area s northwestwar d t o Shetland , which wer e associate d wit h th e earl y Tertiar y opening o f th e Nort h Atlanti c (Bot t 1975 ; Rochow 1981) . Delta-to p lignit e sequence s i n the Mora y Firt h Basi n suggest marginal marine to subaeria l sedimentatio n i n this area (Rocho w 1981; Ziegler 1990 ; Nadin & Kusznir 1995) . N o corresponding Paleocen e uplif t even t ha s bee n detected fo r th e Fennoscandia n hinterland , which probabl y wa s partl y submerge d durin g this perio d (Ziegle r 1990 ; Jordt e t al 1995) . I n principle, th e sedimentatio n patter n shoul d reflect th e basi n topograph y (Jord t e t al . 1995) . The depositiona l patter n o f th e firs t fou r sequences, CSS- 1 t o CSS-4 , i s characterize d by sediment suppl y predominantl y fro m th e wes tern source , wit h th e sediment s reachin g fa r t o the east . Thi s give s reaso n t o assum e tha t th e basin floo r wa s dippin g slightl y t o th e eas t during th e deposition o f the first four sequences . Therefore, a n asymmetrica l basi n form , wher e the westernmos t par t o f th e cross-sectio n i s characterized b y shallow water depths as a result of the uplifted sourc e area to the west, and water depths ar e abou t 7.00 m i n th e deepes t par t o f

the norther n Vikin g Graben, i s assumed fo r th e beginning o f Lat e Paleocen e time . Th e easter n part o f th e basi n i s anticipate d t o hav e ha d relatively dee p wate r depth s a t thi s time , indi cating submergin g o f th e westernmos t par t o f the Norwegian mainland . Several simulatio n run s usin g th e revise d initial for m o f th e basi n showe d tha t a n earl y overfill o f th e basi n persisted . Th e rapi d deepening o f th e basi n fro m Earl y (200-500 m) to Late Paleocen e tim e (500-1000m) recognized on biostratigraphi c dat a b y Gradstei n e t al . (1994) an d Gradstei n & Backstro m (1996) , i s assumed t o hav e ha d a tectoni c origin . There fore, i t was decided t o keep the maximum initial water dept h a t 700m , an d instea d increas e th e tectonic subsidenc e rate s compare d wit h thos e given b y Thorne & Watts (1989) . The tectoni c subsidenc e suggeste d b y Thorn e & Watts (1989) and Hal l & White (1994) is based on backstrippin g wit h th e respons e t o sedimen tary loading assumed t o be of local Airy-isostasy type (flexural rigidit y D = 0), thus giving a minimum value . T o improv e th e model , a litho spheric flexura l rigidit y o f D = 1. 0 x 10 22 Nm was applied . Therefore , subsidence rates should be increase d compare d wit h thos e give n b y Thorne & Watts (1989) . Geological studie s (Jord t e t al . 1995 , 1996 , 2000) suggest that th e basin was filled at th e end of sequenc e CSS- 5 time and locall y subjecte d t o episodic subaeria l erosion i n Mid-Late Miocen e time. Thi s scenari o ha s le d to a modification of the tectoni c subsidenc e (Fig . 13 ) patter n sug gested b y Thorn e & Watt s (1989) , wit h th e purpose o f avoiding complete basin filling before the depositio n o f the CSS- 5 sequence.

Fig. 13 . Tectoni c subsidenc e pattern s wit h a flexura l rigidit y of 1. 0 x 10 2 2 Nm a t selecte d position s alon g th e transect a s predicted b y the simulation . An accelerate d tectoni c subsidenc e pattern is detected fo r Paleocen e tim e along th e entir e cross-section. I n Lat e Miocene-Pliocen e time , tectonic subsidenc e o f th e centra l an d wester n part o f the basi n is detected, corresponding t o th e uplif t o f the eastern sourc e area (Fennoscandia) . Bot h o f these subsidence events are anomalou s relativ e to post-rif t therma l subsidenc e (McKenzie 1978) .

CENOZOIC TECTONI C SUBSIDENC E

Fig. 14 . Simulate d basi n geometr y an d stratigraph y a t selecte d tim e step s i n th e Cenozoi c basi n evolution .

289

290


Modelling results, cas e 2 Figure 1 3 show s simulated tempora l subsidence for selecte d localitie s alon g th e cross-sectio n resulting fro m th e simulation . Accelerate d sub sidence is seen for the Paleocene CSS-1 sequenc e at al l positions alon g th e cross-section , an d th e subsidence rates are highest in the central part of the basi n (a t 90km , Fig . 13) . I n th e wester n (Fig. 13 , Okm an d 45km ) an d centra l part s o f the basi n (Fi g 13 , 90km) subsidence rate s i n the Eocene-Oligocene CSS- 2 t o CSS- 4 tim e sho w reduced, bu t stil l hig h rates . I t i s note d tha t sedimentation rate s outpace d subsidenc e i n th e western par t o f the basi n and th e Eocen e CSS- 2 sequence buil t u p t o se a leve l (se e Fig . 14) . For th e centra l par t o f th e basin , Oligocen e sedimentation rate s als o outpace d th e subsi dence rate s an d durin g Lates t Oligocene-Mid Miocene time, CSS-5 built up t o se a level at th e same tim e tha t subsidenc e ceased (se e Fig . 14) . During CSS-6 time, parts o f the basin were subject t o subaeria l erosion . I n th e easter n centra l part o f th e basi n (Fig . 13 , 130km) , subsidenc e rates wer e low i n Eocen e time . I n Lat e Eocen e time, subsidence ceased. In the eastern part of the basin (Fig . 13 , 180k m an d 240km) , subsidence ceased i n Eocen e CSS- 2 time . I n th e wester n (Fig. 13 , Okm an d 45km ) an d centra l part s o f the basin , a ne w phase o f subsidenc e started i n Mid-Miocene tim e (CSS-7 ) and laste d through out Pliocen e tim e (CSS-8) . Subsidenc e rate s fo r

Fig. 15. Modelle d (a) and observe d (b) cross-section.

the CSS- 7 tim e ar e relativel y gentle . Durin g deposition o f CSS-8, a considerable acceleratio n in subsidenc e occurred an d provide d accommo dation fo r th e Pliocen e CSS- 8 (Fig . 14 ) wedge, especially i n th e centra l par t o f th e basi n (Fig. 13 , 90 km and 13 0 km). The eastern part of the cross-sectio n (Fig . 13 , 180k m an d 240km ) shows uplif t throug h Oligocene-Miocen e tim e (CSS-3 t o CSS-7) , an d a prominen t uplif t i n Pliocene tim e (CSS-8). The uplift cause d erosio n of previousl y deposite d sequence s CSS- 1 t o CSS-4 from th e flank and deposition in a rapidly subsiding basi n i n th e west . Th e fina l cross section wit h time-lines is shown i n Fig . 14 . A compariso n o f th e simulate d an d th e observed basina l cross-section s (Fig . 15 ) shows some geometrica l discrepancies , mainl y cause d by smoothin g processe s resultin g fro m th e diffusive algorith m use d t o handl e erosio n an d deposition. Otherwis e ther e i s a strikin g similarity betwee n the two . The similarit y is sufficientl y high to render the simulation realistic and justif y the mai n conclusion s derive d fro m thi s study . This pertain s particularl y t o th e patter n o f tectonic subsidenc e o r uplif t (Fig . 13 ) implied by th e simulatio n an d th e patter n o f source area activity. Discussion The se t of factors used in the simulatio n is not a unique solution . Othe r combination s o f factors

CENOZOIC TECTONI C SUBSIDENC E could undoubtedl y giv e th e sam e results . Fo r instance, woul d inclusio n o f th e sourc e are a reveal a totall y differen t patter n o f transpor t coefficients? A differen t sea-leve l fluctuatio n curve would affec t th e depositiona l patter n an d the tectoni c subsidenc e pattern , bu t amon g th e published sea-leve l curves there are not curve s of such a radica l patter n t o contradic t ou r mai n results. An increase d time-ste p resolution would make i t possibl e t o simulat e a highe r orde r o f sea-level variation s an d t o predic t lithofacie s distributions. W e assum e tha t ou r documen tation an d choic e o f factor s i s reasonable , an d that th e trend s o f th e mai n results , therefore , are adequate . The subsidenc e curve s fo r differen t localitie s along th e transec t (Fig . 13 ) sho w a n anoma lous subsidenc e pattern , differen t fro m th e expected curv e o f post-rif t therma l subsidenc e (McKenzie 1978) , both fo r Lat e Paleocen e an d Late Miocene-Pliocen e times . Thorn e & Watts (1989), Whit e & Lati n (1993 ) Hal l & Whit e (1994) and Leperc q & Gaultier (1996 ) have als o pointed t o th e anomalou s subsidenc e pattern i n Paleocene time , wit h apparent subsidenc e accel eration. Th e Lat e Miocene-Pliocen e subsidenc e increase, u p to 1 km in magnitude, i s anomalous , for i t i s no t predicte d b y th e existin g mode l of lithospheric stretching , unles s a lat e phas e o f Tertiary stretchin g is assumed t o hav e occurred . However, ther e is little regional evidence of an y significant norma l faultin g in late Tertiary time. In thi s model , i t wa s assume d tha t som e o f the Paleocen e subsidenc e i s incorporated i n th e initial basi n form , becaus e th e simulatio n com mences wit h th e Lat e Paleocen e development . In Fig. 16 , the Cenozoic subsidence pattern for a pseudo-well, a t 110. 7 k m fro m th e section' s lef t

291

margin (west ) (30/11-2) , ha s bee n plotte d i n combination wit h th e subsidenc e analysi s b y Hall & Whit e (1994 ) fro m wel l 30/9-1 , locate d relatively close t o the pseudo-well site . The plot, showing th e subsidenc e patter n fro m Jurassi c time t o th e present , indicate s anomalou s sub sidence rates in Paleocen e (1 ) (CSS-1 sequence) , and Lat e Miocene-Pliocen e times (2 ) (CSS-7 t o CSS-8 sequences) . The Paleocene subsidence rate may not be real, but rathe r a n artefac t o f the backstrippin g pro cedure, which may have underestimated palaeo bathymetries. Accordin g t o Nadi n & Kuszni r (1995, 1996) , n o mechanis m i s require d fo r the formation of Tertiary accommodation spac e other tha n post-Jurassi c therma l subsidenc e 'buffered' b y a n even t o f transien t Paleocen e uplift. Bertra m & Milto n (1989 ) came t o th e same conclusion : i f a wate r dept h o f 1 km i s assumed fo r th e Cretaceou s an d Tertiar y development o f th e norther n Nort h Se a basin, the n no post-Jurassi c riftin g need s t o b e invoke d i n the basi n t o explai n it s subsidenc e pattern . Th e latter workers also suggested an episode of uplif t in th e Paleocen e whic h affecte d muc h o f th e basin's northwes t margin . Thi s uplif t wa s sub sequently eliminate d b y th e margi n collapse , except fo r th e Inne r Mora y Firt h area . If th e Paleocen e subsidenc e rat e fro m ou r simulation is considered, and w e let it follow th e curve of post-rift thermal subsidence (McKenzi e 1978) (se e uppe r dashe d lin e i n Fig . 16) , th e reduced subsidenc e create s th e nee d fo r deepe r water depth s i n th e basin . I n thi s case , a maximum basin depth o f c. 1000m would account fo r the anomalous subsidence pattern o f Late Paleocene time. Water depths of that orde r hav e been suggested b y Jone s (1988 ) an d Gradstei n e t al.

Fig. 16 . Subsidenc e pattern for Earl y Jurassic time to th e presen t fro m a pseudo-well on transec t 2 . Anomalous subsidence events , which d o no t follo w th e expecte d post-rif t therma l subsidenc e pattern, ar e detecte d i n Paleocene (1 ) and Lat e Miocene-Pliocene (2) times.

292


(1994), albeit thos e workers have also postulate d a rapid deepenin g of the basin in Paleocene time . The presen t simulatio n show s tha t th e basin , after th e Miocen e episod e o f subaeria l sedimentation, require s a n anomalou s amoun t o f sub sidence t o provid e sufficien t accommodatio n space fo r Lat e Miocene-Pliocene sedimentation. To ou r knowledge , such a n accelerate d Pliocen e subsidence ha s no t bee n recognize d i n th e northern Nort h Sea . A possibl e coeva l even t has bee n suggeste d b y Kooi e t al (1989 ) fo r th e North Sea' s Centra l Grabe n t o th e south, where the increase d subsidenc e i s attribute d t o a late stage puls e o f compression . The presen t stud y doe s no t revea l th e causa l mechanism t o explai n th e Paleocen e an d Lat e Miocene-Pliocene anomalou s subsidence , bu t indicates that suc h a subsidenc e pattern (Fig s 1 3 and 16 ) is required accordin g t o the input values, which are considered t o be realistic. The need fo r the anomalou s Paleocen e subsidenc e ca n b e avoided onl y i f th e initia l dept h o f th e basi n i s taken t o hav e bee n greater .

Conclusions The mai n conclusion s o f thi s stud y ar e a s follows. The numerica l simulatio n ha s produce d a lithostratigraphic cross-sectio n tha t matche s fairly wel l th e observe d basina l cross-sectio n (see Fig s 1 4 and 15) . The geometri c departure s are mino r an d d o no t significantl y affec t th e similarity o f th e simulate d cross-sectio n t o the observe d one . Th e visua l tes t render s th e simulation resul t satisfactory. The simulatio n indicate s anomalou s tectoni c subsidence, differen t fro m th e patter n o f post rift therma l subsidenc e predicte d b y genera l models, fo r Lat e Paleocen e an d Lat e Miocene Pliocene time . Thi s forme r episod e o f acceler ated subsidenc e ma y partl y b e a n artefac t o f the basin' s underestimate d initia l wate r depth , but i t i s mor e difficul t t o attribut e th e latte r episode t o a mistake n inpu t value . A n applica tion o f a modifie d sea-leve l curve wil l no t caus e any majo r chang e o f th e simulatio n result . A decrease d flexura l rigidit y will caus e a lowe r degree o f tectoni c subsidenc e durin g th e whol e time interva l considered . In short , th e resul t o f the simulatio n seems t o be vali d an d require s furthe r lithostratigraphi c analysis o f th e basin . We than k O . Hanse n o f IK U fo r hi s valuabl e contribution t o the computer simulation. We gratefully acknowledge th e constructiv e comment s mad e b y A. Ryseth. J. C. Rivenaes, W. Nemec and R. Gabrielsen .

References AIGNER, T. , BRANDENBURG . A. , VA N VLIET . A. . DOYLE, M. , LAWRENCE , D . & WESTRICH . J . 1990. Stratigraphi c modellin g o f epicontinenta l basins: Tw o applications : Sedimentarv Geologv. 69, 167-190. BADLEY, M . E. . PRICE . J . D. , RAMBECH-DAHL . C . & AGDESTEIN . T . 1988 . The structura l evolution of th e norther n Vikin g Grabe n an d it s bear ing upo n extensiona l model s o f basi n formation . Journal o f th e Geological Society, London. 145 . 455-472." BARTON, P . & WOOD . R . 1984 . Tectonic evolutio n o f the Nort h Se a basin : crusta l stretchin g an d subsidence. Geophysical Journal o f th e Royal Astronomical Society. 79 . 987-1022. BEACH, A. , BIRD , T . & GIBBS . A . 1987 . Extensional tectonics an d crusta l structure: deep seismic reflection dat a fro m th e norther n Nort h Se a Vikin g Graben. In : COWARD . M . P. , DEWEY . J . F . & HANCOCK. P . L . (eds ) Continental Extensional Tectonics. Geologica l Society , London . Specia l Publications, 28 , 467-476. BERTRAM, G . T . & MILTON , N . J . 1989 . Reconstructing basi n evolutio n fro m sedimentar y thickness : the importanc e o f palaeobathymetri c control , with referenc e t o th e Nort h Sea . Basin Research. 1, 247-257 . BOTT, M . H . P . 1975. The structur e an d evolutio n o f the Nort h Se a Basin , th e Faero e Bloc k an d th e intervening region . In : WOODLAND , A . W . (ed.) Petroleum and the Continental Shelf of North West Europe, Vol. 1: Geology. Applie d Science . London, 105-116 . CHRISTIANSSON, P. . FALEIDE . J . I . & BERGE . A . M . 1999. Crusta l structur e in the northern Nort h Sea: an integrate d geophysica l study . This volume. DEEGAN, C . E . & SCULL , B . J . *1977 . A Proposed Lithostratigraphic Nomenclature for the Central and Northern North Sea. Norwegia n Petroleu m Directorate Bulleti n 1. EYNON, G . 1981 . Basi n developmen t an d sedimenta tion i n the Middl e Jurassic of the norther n Nort h Sea. In : ILLING , L . V . & HOBSON . G . D . (eds) Petroleum Geology of the Continental Shelf of NW Europe. Heyden , London , 196-204 . FJELDSKAAR, W . 1994 . Viscosity an d thicknes s o f th e asthenosphere detecte d fro m th e Fennoscandia n uplift. Earth an d Planetary Science Letters. 126. 399-410. FLEMINGS. P . B . & JORDAN . T. E . 1989 . A syntheti c Stratigraphic model of foreland basin development. Journal o f Geophysical Research. 94 , 430-434. FÊRSETH, R . B. , GABRIELSEN , R . H . & HURICH . C. A . 1995. Influenc e of basemen t i n structurin g o f th e North Se a Basi n offshor e southwes t Norway . Norsk Geologisk Tidsskrift. 75 , 105-119 . GABRIELSEN, R . H. . F/ERSETH . R . B. . STEEL . R . J. . IDIL, S . & KLOVJAN . O . S . 1990 . Architectura l styles of basi n fill in th e norther n Viking Graben . In: BLUNDELL . D. J. & GIBBS, A. D . (eds) Tectonic Evolution o f th e North Se a Rifts. Oxfor d University Press , Oxford . 158-179 .

CENOZOIC TECTONI C SUBSIDENC E GALLOWAY, W . E. , GARBER , J . L. , Liu , X . & SLOAN , B. J . 1993 . Sequenc e stratigraphi c an d deposi tional framewor k o f th e Cenozoi c fill , Centra l and norther n Nort h Sea . In : PARKER , J . R . (ed. ) Petroleum Geology of Northwest Europe: Proceedings o f th e 4t h Conference. Geologica l Society , London, 33-43 . GILTNER, J . P . 1987 . Applicatio n o f extensiona l models t o th e norther n Vikin g Graben . Norsk Geologisk Tidsskrift, 67 , 339-352. GLENNIE, K . W . (ed. ) 1984 . Introduction t o th e Petroleum Geology o f th e North Sea. Blackwell , Oxford, 1-236 . GRADSTEIN, F . M . & BACKSTROM , S . 1996 . Cainozoic biostratigraphy an d palaeobathymetry , norther n North Se a an d Haltenbanken . Norsk Geologisk Tidsskrift, 76 , 3-32 . , KAMINSKI , M. A. , BERGGREN , W . A. , KRISTIAN SEN, I . L . & D'lORio , M . A . 1994 . Cenozoi c biostratigraphy o f th e Nort h Se a an d Labrado r Shelf. Micropalaeontology, 40(Suppl.) , 1-152 . HALL, B . D. , & WHITE , N . 1994 . Origi n o f anom alous Tertiar y subsidenc e adjacen t t o Nort h Atlantic continenta l margins . Marine an d Petroleum Geology, 11 , 702 - 714 . HAQ, B . U. , HARDENBOL , J . & VAIL , P . R . 1987 . Chronology o f fluctuatin g se a level s sinc e Tri assic (25 0 Myr ag e t o present) . Science, 235 , 1156-1167. HARLAND, W . B. , ARMSTRONG , R . L. , Cox , A . V. , CRAIG, L . E., SMITH , A . G. & SMITH, D . G . 1990 . A Geologic Time Scale 1989. Cambridg e Uni versity Press, Cambridge . HELLAND-HANSEN, W. , KENDALL , C . G . S T C. , LERCHE, I . & NAKAYAMA , K . 1988 . A simulatio n of continenta l basi n margi n sedimentatio n i n response t o crusta l movements , eustati c sea leve l change, an d sedimen t accumulatio n rates . Mathematics an d Geology, 20 , 777-802. ISAKSEN, D . & TONSTAD , K . (eds ) 1989 . A Revised Cretaceous and Tertiary Lithostratigraphic Nomenclature for the Norwegian North Sea. Norwegian Petroleu m Directorat e Bulleti n 5. JONES, G . D 1988 . A palaeoecologica l mode l o f Lat e Palaeocene 'Flysch-type ' agglutinate d foramini fera usin g th e palaeoslop e transec t approach , Viking Graben , Nort h Sea . Abhandlungen de r Geologischen Bundesanstalt, 41 , 143-153 . JORDT, H . 1996 . Th e Cenozoic geological evolution o f the Central and Northern North Sea based on seismic sequence stratigraphy. Dr . Scient . thesis , University o f Oslo . ,, & IBRAHIM , M. T . 1995 . Cenozoi c sequence stratigraph y o f th e Centra l an d North ern Nort h Se a basin : tectoni c development , sediment distributio n and provenanc e areas . Marine Petroleum Geoleum, 12 , 845-879. , THYBERG , B . I. & NOTTVEDT, A. 2000 . Cenozoi c evolution o f th e centra l an d norther n Nort h Se a with focu s o n differentia l vertica l movements o f the basi n floo r an d surroundin g clasti c sourc e areas. This volume. KENDALL, C . G . S T C. , STROBEL , J. , CANNON , R. , BEZDEK, J. & BISWAS , G. 1991 . The simulatio n of

293

the sedimentar y fill of basins . Journal o f Geophysical Research, 96, 6911-6929. KENYON, P . M. & TURCOTTE, D. L. 1985 . Morpholog y of a progradin g delt a b y bul k sedimen t trans port. Geological Society o f American Bulletin, 96 , 1457-1465. KNOX, R. W. & HOLLOWAY, S . 1992. Palaeogene of th e Central and Northern North Sea . In: KNOX , R . W. & CORDEY, W. G. (eds ) Lithostratigraphic Nomenclature of the UK North Sea. Britis h Geologica l Survey, Keyworth . Kooi, H. , CLOETINGH , S . & REMMELTS , G . 1989 . Intraplate stresses and th e stratigraphic evolution of th e Nort h Se a Centra l Graben . Geologic e n Mijnbouw, 68 , 49-72. LAWRENCE, D . T. , DOYLE , M . & AIGNER , T . 1990 . Stratigraphic simulatio n o f sedimentar y basin : Concepts an d calibration . AAPG Bulletin, 74 , 273-295. LEPERCQ, J . Y & GAULIER , J . M . 1996 . Two-stag e rifting i n th e Nort h Vikin g Graben are a (Nort h Sea): inference s fro m a ne w three-dimensiona l subsidence analysis . Marine an d Petroleum Geology, 13 , 129-148 . MCKENZIE, D . P . 1978 . Some remarks on the develop ment o f sedimentar y basins . Earth an d Planetary Science Letters, 40, 25-32. NADIN, P. A. & KUSZNIR, N. J . 1995 . Palaeocene uplif t and Eocen e subsidenc e i n th e norther n Nort h Sea Basi n fro m 2 D forwar d an d revers e strati graphic modelling . Journal of th e Geological Society, London , 152 , 833-848. & 1996 . Forwar d an d revers e stratigraphi c modelling o f Cretaceous-Tertiar y post-rif t sub sidence and Palaeogen e uplif t i n the Outer Mora y Firth Basin , central North Sea . In : KNOX, R . W. , CORFIELD, R . M . & DUNAY , R . E . (eds ) Correlation of the Early Palaeogene in Northwest Europe. Geological Society , London , Specia l Publication , 101, 43-6 2 ODINSEN, T. , CHRISTIANSSON , P., GABRIELSEN , R. H. , FALEIDE, J . I . & BERGE , A . M . 2000« . The geom etries an d dee p structur e o f th e norther n Nort h Sea rif t system . This volume. , REEMST , P. , VA N DER BEEK, P. , FALEIDE , J . L , GABRIELSEN, R . H . 20006 . Permo-Triassi c an d Jurassic extensio n i n th e norther n Nort h Sea : results fro m tectonostratigraphi c forwar d model ling. This volume. RIVEN^S, J . C . 1992 . Application o f a dual-lithology , depth-dependent diffusio n equatio n i n strati graphic simulation . Basin Research, 4 , 133-146 . 1993. A computer simulation model for siliciclastic basin stratigraphy. Dr.Ing . thesis , Universit y o f Trondheim. ROBERTS, A. M. , KUSZNIR, N. J., WALKER, I. & DORN LOPEZ, D . 1995 . Quantitative analysis of Triassi c extension i n th e norther n Nort h Sea . Journal o f the Geological Society, London, 152 , 15-26 . ROCHOW, K . A . 1981 . Seismi c stratigraph y o f th e North Se a Talaeocene' deposits. In: ILLING , L . V. & HOBSON , G . D . (eds ) Petroleum Geology o f the Continental Shelf o f NW Europe. Heyden , London, 255-266 .

294


RUNDBERG, Y . 1989 . Tertiary sedimentary history and basin evolution of the Norwegian North Sea between 60 Q-62ZN - An integrated approach. Dr.Ing. thesis , Universit y of Trondheim . , OLAUSSEN , S . & GRADSTEIN , F . 1995 . Incisio n of Oligocen e strata ; evidenc e for norther n Nort h Sea Miocen e uplif t an d ke y t o th e formatio n o f the Utsir a Sands . Geonytt, 22 , 62. SCLATER, J. G. & CHRISTIE, P. A. F . 1980 . Continental stretching: An explanation of the post-Mid Creta ceous subsidenc e o f the Central North Se a Basin . Journal o f Geophysical Research, 85, 3711-3739 . SINCLAIR, H . D. , COAKLEY , B . J. , ALLEN , P . A . & WATTS, A . B . 1991 . Simulatio n o f forelan d basi n stratigraphy using a diffusio n mode l fo r mountain belt uplif t an d erosion : a n exampl e for m th e Cen tral Alps , Switzerland . Tectonics, 10 , 599-620. STERBAUT, E., SPIEGLER , D., WEINELT, M. & THIEDE, J . 1991. Cenozoic Erosion an d Sedimentation o n th e Northwest European Continental Margin. Geo mar Researc h Centr e fo r Marin e Geosciences , Christian-Albrechts Universita t Kiel . STEWART, I . J . 1987 . A revisite d stratigraphi c inter pretation o f th e Earl y Palaeogen e o f th e Centra l North Sea . In : BROOKS , J . & GLENNIE , K . (eds ) Petroleum Geology of North West Europe. Graham an d Trotman , London , 557-576 .

SYVITSKI, J . P . M. , SMITH , J . N. , CALABRESE , E . A . & BOUDREAU, B . P . 1988 . Basi n sedimentatio n an d the growt h o f progradin g deltas . Journal o f Geophysical Research, 93, 6895-6908. TER VOORDE , M. , FÊRSETH, R. B. , GABRIELSEN, R . H . & CLOETINGH , S . A., P . L . 1997 . Repeate d litho sphere extension in the northern Viking Graben: a coupled o r a decoupled rheology ? This volume. THORNE, J . A . & WATTS , A . B . 1989 . Quantitativ e analysis of the Nort h Se a subsidence. AAPG Bulletin, 73, 88-116. WHITE, N . & LATIN , D . 1993 . Subsidenc e analysi s from th e North Se a 'triple junction'. Journal of the Geological Society, London, 150 , 473-488 . ZIEGLER, P . A . 1981 . Evolution of sedimentar y basin s in Northwes t Europe . In : ILLING , L . V . & HOBSON, G . D . (eds ) Petroleum Geology o f th e Continental Shelf o f NW Europe. Heyden , London . 3-39. 19820. Faultin g an d grabe n formatio n i n western and centra l Europe . Philosophical Transactions of th e Roval Society o f London, Series A, 305 . 113-143. ' 1982/7. Geological Atlas o f Western an d Central Europe, Elsevier , Amsterdam . 1990. Geological Atlas o f Western an d Central Europe. Shel l International , Th e Hague .

NE Atlanti c continenta l riftin g an d volcanic margi n formatio n JAKOB SKOGSEID, 1'3 SVERR E PLANKE, 1 JA N ING E FALEIDE, 1 TOM PEDERSEN, 2 OLA V ELDHOLM 1 & FLEMMIN G NEYERDAL 1 1

Department of Geology, University of Oslo, P.O. Box 1047 Blindern, N-0316 Oslo, Norway 2 Institute for Energy Technology, P.O. Box 40, N-2007 Kjeller, Norway ^Present address: Saga Petroleum ASA, P.O. Box 490, N-1302 Sandvika, Norway (e-mail: [email protected]) Abstract: Dee p seismic data fro m th e Hatton-Rockall region, the mid-Norway margin an d the SW Barents Sea provide images o f the crustal structure that mak e it possible to estimat e the relative amount s o f crustal thinnin g for the Late Jurassic-Cretaceous and MaastrichtianPaleocene N E Atlanti c rif t episodes . I n addition, plat e reconstructions illustrate the relativ e movements between Eurasia an d Greenland bac k to Mid-Jurassic time. The NE Atlantic rif t system developed a s a result of a series of rift episode s fro m the Caledonian orogeny t o early Tertiary time . The Late Palaeozoi c riftin g i s poorly constrained, particularly with respect t o timing. However, rifte d basi n geometries, inferre d t o b e of this age, are observed a t depth in seismic dat a o n th e flank s o f th e younge r rif t structures . Intra-continenta l riftin g i n Lat e Jurassic-Cretaceous time s cause d c . 50-70 km o f crusta l extensio n an d subsequen t Cretaceous basi n subsidenc e fro m th e Rockal l Trough-Nort h Se a area s i n th e south , t o the S W Barents Se a in the north . I n lat e Earl y t o earl y Lat e Cretaceous times, ne w riftin g occurred i n th e Rockal l Troug h an d Labrado r Se a associate d wit h th e northwar d propagation o f Nort h Atlanti c sea-floo r spreading . Whe n sea-floo r spreadin g wa s approached i n the Labrado r Se a the Rockal l rif t apparentl y becam e extinct . Th e fina l N E Atlantic rift episod e was initiated near the Campanian-Maastrichtian boundary , lasted until continental separatio n nea r th e Paleocene-Eocen e transition , an d cause d c . 140km extension. Th e lat e syn-rif t an d th e earlies t sea-floo r spreadin g period s wer e affecte d b y widespread igneous activity across a c. 300 km wide zone along the rifted plat e boundary. Th e deep seismi c dat a provid e lower-crusta l structura l geometrie s tha t represen t boundar y conditions fo r a better mappin g an d understandin g o f the extensional thinnin g o f the crust. The crustal geometrie s question extensio n estimates previously made fro m basi n subsidenc e analysis, an d ai d i n th e definitio n o f bodie s o f magmati c underplatin g beneat h th e oute r volcanic margins .

The openin g o f th e N E Atlanti c Ocea n a t th e base d o n interpretatio n o f al l availabl e magne Paleocene-Eocene transitio n marke d th e culmi - ti c dat a fo r th e N E Atlanti c (Fig . 1) . Finally, nation o f a c . 340 Ma histor y o f extensiona l de- a presentatio n o f differen t aspect s o f plume formation an d sedimen t basi n formatio n sinc e lithospher e interactio n an d associate d vertica l the en d o f th e Caledonia n orogen y (Fig . 1) . motio n durin g th e margi n formatio n i s linke d In this paper we utilize previously published cru- wit h a discussio n o f exploratio n challenge s a t stal transects betwee n th e Hatton-Rockal l mar- volcani c margins i n general, gin an d th e wester n Barent s Se a to evaluat e th e Dee p seismi c reflectio n profile s provid e degree o f crustal thinnin g in the various regions image s o f bot h Moh o an d intra-crusta l geome with respect t o th e rifting history of the area . tries , whic h i n particula r allo w u s t o bette r The evaluatio n include s firs t a discussio n o f constrai n th e dimension s o f th e Maastrichtian the observed structure s an d associated extensio n Paleocen e break-up-relate d rift , an d th e Lat e estimates mad e fo r separat e rif t episode s wit h Jurassic-Cretaceou s intra-continenta l rift . Inte respect t o a relativel y simpl e tectoni c model , gratio n o f deep seismi c reflection dat a wit h deep We the n attemp t t o illustrat e th e spatia l an d seismi c refractio n transect s provide s als o th e temporal geologica l evolutio n b y usin g exten - velocit y distribution o f th e lowe r crus t (Mutte r sion estimate s i n plat e reconstruction s an d e t al. 1984 , 1988 ; Gudlaugsson e t al. 1987 ; Hinz transect restorations . Th e reconstructio n t o e t al. 1987; White et al. 1987; Jackson e t al. 1990; magnetic anomal y 24 b tim e (5 3 Ma), th e oldes t Faleid e e t al . 1991 , 19936 ; Makri s e t al . 1991 ; magnetic spreadin g anomal y i n th e region , i s Olafsso n et al. 1991; Planke et al. 1991; Larsen & From: N0TTVEDT , A . e t al . (eds ) Dynamics o f th e Norwegian Margin. Geologica l Society , London , Specia l Publications, 167 , 295-326 . 1-86239-056-8/00/S15.0 0 © Th e Geologica l Societ y o f London 2000 .

296

J. SKOGSEI D E T AL .

Fig. 1 . Magneti c sea-floo r spreadin g anomalie s (numbere d 5-7 , 13 , 20, 24) and structura l setting in th e N E Atlantic region. Transects 1- 7 discussed i n text are located b y bold lines . The outline of the flood basalt province in the regions off Britain and i n vicinity of the Faeroe Islands is based o n Hitche n & Ritchie (1993) and Boldree l & Andersen (1994) . BS, SW Barents Sea ; GB, Grea t Britain ; MM, Mor e margin ; NS. Nort h Sea ; RT. Rockal l Trough; VM , Vorin g margin ; HM , Hatton-Rockal l margin . Marcussen 1992 ; Shanno n e t al. 1993 ; Eldhol m & Grue 1994 ; Skogsei d & Eldholm 1995 ; Mjeld e et al . 1997 ; Christiansson e t al . 1999) . Such dat a reveal th e basi n geometries , an d allo w quantita tive evaluation an d modellin g o f the variou s rift episodes. Non e th e less , th e dee p seismi c data base i s limited . Moreover , multipl e rifting , extensive floo d basalt s an d sil l intrusion s alon g the oute r margi n mak e th e interpretatio n o f th e crustal structur e ambiguou s i n larg e part s o f the NE Atlanti c stud y area .

Regional geologica l framewor k Late Palaeozoic-Early Mesozoic phase The locatio n an d structura l expressio n o f th e late Palaeozoi c N E Atlanti c rif t syste m withi n the Caledonia n orogeni c domai n wa s influence d by Caledonian , an d possibl y pre-Caledonian , structures. Betwee n Norwa y an d Greenlan d

the rif t syste m followe d th e NE-oriente d Caledonides int o th e S W Barent s Sea , wher e north-trending structure s sugges t a structura l connection t o th e Arcti c rif t system . Betwee n Britain an d Greenlan d basi n trend s indicat e a possible reactivatio n o f th e Appalachia n defor mation system . A phas e o f lat e orogeni c extensiona l collaps e during lates t Silurian-Earl y Devonia n tim e apparently initiate d the formatio n o f a serie s of large half-grabe n basins , whic h subsequentl y were fille d wit h thic k succession s o f mainl y intra-continental deposit s (e.g . Ziegle r 1988 ; Coward 1993 ; Hart z & Andrese n 1995) . A n orogenic collaps e o f th e Barent s Se a Caledo nides southeas t o f Bj0rn0y a ha s bee n suggested , on th e basi s o f structure s beneat h th e flank s o f the S W Barent s Se a Permo-Carboniferou s rif t system (Gudlaugsso n e t al . 1994) . I f we conside r orogenic collaps e mainl y a syn-orogeni c phe nomenon, w e assum e tha t riftin g i n respons e

NE ATLANTI C CONTINENTA L RIFTIN G to lithospheri c extensio n betwee n Eurasi a an d Greenland wa s initiated a t th e end o f Devonia n time. Th e mai n Lat e Palaeozoi c rif t episode s took plac e i n Mid-Carboniferous , Carbonifer ous-Permian an d Permian-Earl y Triassi c time s (e.g. Ziegle r 1988).

Late Jurassic-Cretaceous phase During Mid-Jurassi c tim e th e developmen t o f oceanic crus t i n the central Atlanti c marke d th e onset o f a ne w kinematic regime in the Atlanti c domain (e.g . Larse n 1987 ; Ziegle r 1988) . Fol lowing the separation between Africa an d Nort h America, sea-floor spreading was confined t o the region sout h o f th e Azore s Fractur e Zon e fo r c. 50 Ma. Riftin g persisted , however , int o th e North Atlanti c domain , a s documente d b y rift associated sediment s o f Lat e Jurassi c ag e commonly foun d i n Nort h Atlanti c basins . Late Jurassic-Cretaceou s riftin g ha s i n thi s context bee n considere d a precurso r t o th e progressive continenta l separation i n th e south ern par t o f th e Nort h Atlantic , wher e th e final break-up betwee n Iberi a an d th e Gran d Bank s has bee n give n a Hauterivia n ag e (c . 134 Ma) (Srivastava & Tapscott 1986 ; Keen & de Voog d 1988; Whitmarsh et al. 1993). Thus, at the end of the Jurassi c perio d dee p rif t basin s probabl y existed fro m th e Rockal l Troug h t o th e S W Barents Sea, with a separate branch in the North Sea, wit h a possibl e linkag e t o spreadin g i n th e Tethys Ocea n (Lundi n & Dor e 1997 ) (Fig . 1) . Sea-floor spreadin g progresse d northward , an d the opening betwee n Newfoundland an d Goban Spur-Porcupine Ban k bega n i n lat e Aptia n to earl y Albia n (c . 112 Ma) time s accordin g to D e Graciansk y e t al . (1985 ) and Srivastav a et al . (1988). Further north , th e Labrado r Se a an d th e Rockall Troug h develope d contemporaneousl y with riftin g an d driftin g t o th e south . I n th e Labrador Se a gabbroi c dyke s o f Berriasian Valanginian (133-13 8 Ma) ag e i n southwes t Greenland ar e relate d t o incipien t riftin g (Wat t 1969; Larse n e t al . 1999) . O n th e basi s o f dat a from commercia l well s of f Canada , th e oldes t known rif t deposit s here are coarse-grained nonmarine sandstone s o f Barremia n age . Thes e sediments lie , however , partl y o n Neocomia n basalts (Berrasian-Barremian, Alexi s formation) (Balkwill et al. 1990) . The basalts are thus of the same ag e a s th e Greenlan d dyke s an d ma y suggest tha t eve n Uppe r Jurassi c sedimentar y rocks ma y occup y th e deep , undrille d part s o f some rif t basins . Th e younges t rif t deposit s sampled in wells are Coniacian in age. Extension

297

in th e Labrado r Se a continue d t o continenta l separation betwee n Nort h Americ a an d Green land i n Lat e Cretaceou s (9 2 Ma, Roes t & Sri vastava 1989 ) to earlies t Tertiar y time s (62 Ma, Chalmers e t al . 1993) . The developmen t o f th e Rockal l Troug h i s more dispute d (Robert s 1974 , 1915a,b; Russe l 1976; KristofTersen 1978 ; Russel & Smythe 1978; Srivastava 1978 ; Robert s e t al . 1981 ; Smyth e 1989). Irrespectiv e o f th e natur e o f th e crus t flooring the troug h (oceani c o r continental ) i t is clear tha t significan t amounts o f extension hav e been take n u p alon g th e structur e (Joppe n & White 1990 ; Makri s e t al . 1991 ; Dor e 1992 ; Keser Neis h 1993 ; Knot t e t al . 1993 ; O'Reilly et al. 1996). There seems to be consensus about a Cretaceous ag e fo r th e las t majo r extensiona l phase i n th e area . Fro m plat e reconstructions , Srivastava & Verhoef (1992 ) assumed tha t 65 % of th e trough' s presen t widt h can b e accounte d for b y Lat e Jurassi c an d Cretaceou s extension . Further nort h the Late Jurassic boundary fault s along th e Wes t Shetlan d Platfor m wer e mildly reactivated i n Aptia n time , an d i n th e Faero e Basin, whic h ma y b e considere d th e norther n extremity o f th e Rockal l Trough , th e Earl y Cretaceous perio d wa s characterize d b y in creased subsidenc e rates (Duinda m & Van Hor n 1987; Hitchen & Ritchie 1987 ; Mudge & Rashid 1987; Nelson & Lamy 1987). Off Norwa y the M0re and V0rin g basins were formed primaril y a s a resul t o f th e Lat e Jurassic-Early Cretaceou s rifting , wherea s i t may b e questione d whethe r o r no t large-scal e extension too k plac e i n this region contempora neous wit h riftin g t o break-u p i n th e Labrado r Sea. Accordin g t o Surly k e t al . (1981) , coars e clastic submarin e fan s associate d wit h deep water shale s sugges t activit y alon g th e majo r boundary fault s i n centra l Eas t Greenlan d per sisting int o Aptian-Albia n time , an d coeva l activity ha s bee n reporte d alon g th e flank s o f main faul t zone s in the More and V0rin g basin s off Norwa y (Blysta d e t al . 1995 ; Dor e e t al . 19970). Furthermore , accordin g t o Dallan d (1981) Albian-Aptia n tectonis m i s reporte d from Andoya , norther n Norway , an d thre e Early Cretaceou s tectoni c phases , Berriasian Valanginian, Hauterivian-earl y Barremia n an d Aptian-Albian, hav e bee n reporte d fro m th e SW Barents Se a (Faleide e t al . \993a,b).

Late Cretaceous-Tertiary phase The fina l Maastrichtian-Paleocen e rif t episod e lasted fo r c . 20 Ma, leadin g int o continenta l separation an d onse t o f sea-floo r spreadin g a t

298


the Paleocene-Eocen e transition . Thi s rif t epi sode forme d a mor e tha n 300k m wid e zon e associated wit h lithospheri c thinnin g an d post break-up subsidenc e (Skogsei d 1994) . In Paleo cene tim e (63-6 2 Ma) th e rif t wa s affecte d b y the impingemen t o f th e Icelan d mantl e plum e beneath th e thinne d lithosphere , whic h induce d considerable regiona l uplift. Subsequen t igneou s activity characterize d th e rif t history . The mag matism wa s governe d b y th e lithospher e relief , which provide d pressur e condition s (thin-spots ) for voluminou s mel t generatio n (Thompso n & Gibson 1991 ; Skogsei d e t al . 19920) . Th e con tinental separatio n an d initia l sea-floo r spread ing wer e associate d wit h a 2- 3 Ma perio d o f massive volcani c activit y alon g th e mor e tha n 2600km lon g ne w plat e boundar y (Eldhol m & Grue 1994 ) (Fig. 1) . Stretching estimate s In thi s stud y th e amoun t o f crusta l thinnin g is estimated fro m th e presen t thicknes s o f th e continental crus t alon g th e transect s i n Fig . 1 .

The thinnin g i s compare d wit h result s fro m independent estimate s base d o n standar d basi n subsidence analyses , i.e . unifor m extensio n models (McKenzi e 1978 ; Steckle r & Watt s 1978), applie d t o th e N E Atlanti c margin s (Skogseid e t al . 19926 ; Skogsei d 1994 ; Robert s et al . 1997 ; Walke r e t al . 1997) . Al l method s have limitations , and ideall y requir e inpu t dat a that ar e difficul t t o obtai n alon g mos t transects. Nevertheless, w e clai m tha t th e integratio n o f subsidence modellin g an d crusta l structur e evaluation provides the best platform for analysing crusta l extensio n an d fo r furtherin g ou r understanding of the extensional developmen t of the region . With referenc e t o th e riftin g histor y w e subdivide th e sedimentar y successio n i n th e crustal transect s int o thre e mega-sequence s (Figs 2 , 3 an d 4) . Th e pre-Cretaceou s strat a represent a n unspecifie d tectoni c an d basi n subsidence histor y fo r whic h littl e informatio n exists i n th e offshor e regions . Th e Cretaceou s sequence outline s th e basin s forme d afte r th e Late Jurassic-Cretaceou s rifting ; wherea s the Tertiar y sequenc e an d th e presen t wate r

Fig. 2 . Crusta l transect s 2 and 3 across th e norther n an d souther n V0rin g margi n (modifie d fro m Skogsei d & Eldholm 1995 ) with a n alternativ e interpretatio n o f the bas e Cretaceou s horizo n fro m Blysta d e t al. (1995) . Crustal velocitie s beneat h th e Voring Marginal Hig h (VMH ) ar e annotate d i n kms" 1 . COB , Continent-ocea n boundary; FG-GR , Fenris Graben-Gjallar Ridge ; HG , He l Graben; HHA, Helland-Hanse n Arch ; HT . Halte n Terrace; NH, Ny k High ; NR , Nordlan d Ridge ; NS , Nagrind Syncline ; TB , Troena Basin; VS , Vigrid Syncline . Location show n i n Fig . 1 .

NE ATLANTI C CONTINENTA L RIFTIN G depth approximat e th e amoun t o f subsidenc e since th e Maastrichtian-Paleocen e rif t episode . It shoul d b e noted , however , tha t a larg e Plio Pleistocene sedimentar y wedg e i s par t o f th e Tertiary mega-sequenc e i n th e easter n par t o f the transects . Thes e deposit s ar e relate d t o continental uplif t an d glacia l erosio n o f Fen noscandia (Rii s & Fjeldskaa r 1992 ; Stuevol d et al 1992) . The basi n analysi s depend s o n th e definition of th e bas e Cretaceou s an d th e bas e Tertiar y levels, separatin g th e thre e mega-sequences . These horizons , whic h correspon d t o majo r hiatuses o r condense d sequence s o n th e basi n flanks, ma y b e boundin g o r foun d withi n thic k sedimentary unit s reflectin g rapi d differentia l subsidence i n th e deepes t basins . I n fact , th e base Cretaceou s unconformit y doe s no t mar k the transition fro m syn-rif t t o post-rift sediments in th e Nort h Atlanti c rift , wher e riftin g con tinued well into Earl y Cretaceous times . Ideally, tectono-structural consideration s shoul d hav e

299

been mad e wit h reference t o the top o f the Mid dle Jurassi c shallo w marin e sandstones , whic h marks th e onse t o f Lat e Jurassi c riftin g an d rapid subsidence . I t is , however , difficul t t o ascertain thi s leve l becaus e of the lac k of stratigraphic contro l i n th e dee p rif t basins . Similarly, th e Campanian-Maastrichtia n hori zon shoul d b e used t o boun d th e las t rif t episode. Non e th e less , her e th e near-bas e Tertiary rif t unconformit y is at presen t th e bes t approximation i n th e seismi c data . Erosio n across th e to p o f rotate d faul t block s beneat h this horizo n i s furthe r use d a s a n indicato r o f near sea-leve l conditions i n Paleocene time . The uncertainties in the seismic interpretation, which ar e subsequentl y implemente d i n th e modelling, ca n b e illustrate d bot h i n th e S W Barents Se a an d o n th e V0rin g margin . I n th e SW Barent s Se a th e bas e Cretaceou s horizo n represents a mai n rif t unconformit y o n th e flanks o f th e dee p Troms 0 Basin , wherea s i t i s located 1- 2 s tw t abov e th e to p Middl e Juras -

Fig. 3 . Crusta l transect s 1 and 4 across th e southwester n Barents Sea and M0r e margins, respectively (modifie d from Olafsso n et al . 1991 ; Faleide e t al . 19936 ; Breivi k e t al . 1998) . Crustal velocities are annotate d i n kms" 1 . COB, Continent-ocea n boundary ; HB , Hammerfes t Basin ; SB, S0rvestnaget Basin ; SFZ, Senj a Fractur e Zone ; SR, Senj a Ridge ; TB , Troms 0 Basin . Location show n in Fig . 1 .

300


sic leve l i n th e basi n itsel f (Faleid e e t al. \993b). O n the V0ring margi n th e structural an d stratigraphical continuit y associate d wit h th e base Cretaceou s horizo n fro m th e Ra s an d Traena basin s westwar d int o th e Vigri d an d Nagrind synclines , respectively , i s poorl y con strained. Th e mai n reaso n i s tha t th e seismi c data beneat h th e Tertiary Helland-Hanse n Arc h are generall y poor , an d tha t igneou s intrusion s make th e structura l settin g difficul t t o interpre t with confidence . T o illustrat e th e variatio n i n interpretation w e hav e implemente d th e sug gested bas e Cretaceou s horizo n fro m Blysta d et al . (1995 ) (Fig . 2) . However , ou r interpreta tion i s aide d b y th e observe d lower-crusta l geometries. Th e dee p seismi c reflectio n dat a show a c . 45-70 km wid e zon e o f pronounce d crustal thinnin g beneath th e Ra s Basin , wherea s the Moh o interface lies significantly deepe r bot h eastward an d westwar d o f thi s zone . Thi s con figuration resemble s th e structura l settin g i n the Rockal l Trough , th e Mor e an d Troms o basins, an d th e norther n Nort h Sea , wher e a

similar spatia l correlatio n exist s betwee n th e upper-crustal rif t an d area s o f pronounce d crustal thinnin g (Fig s 2-4) . I n ou r opinio n w e find tha t a relativel y narro w centra l rif t basi n therefore i s more plausibl e tha n th e ver y broa d deep basi n suggeste d b y Blysta d e t al . (1995) . Furthermore, ou r interpretatio n allow s for thick units o f pre-Cretaceou s strat a beneat h th e Voring Basin , whic h i s consisten t wit h ob served stratigraphica l succession s bot h beneat h the adjacen t Trondela g Platfor m (Fig . 2 ) an d in Greenland . Voring margin transects We us e th e Vorin g margi n transect s (Fig s 2 . 5 and 6 ) t o illustrat e th e evaluatio n procedur e for th e crusta l thinnin g derive d extension . We interpre t th e c . 45 km lon g reflecto r segment at 8.5- 9 stwt (c . 20km) a s Moh o beneat h th e Ras Basin . O n eithe r sid e Moh o deepen s t o 10-1 Is twt . i.e . c.25k m (Skogsei d & Eldhol m

Fig. 4 . Crusta l transect s 5 and 6 and associate d degre e o f crustal thinnin g (3) acros s th e Rockal l Troug h (modified fro m Makri s e t al . 1991 ; Keser Neis h 1993 ; Shannon e t al . 1993) . Crustal velocitie s are annotate d i n kms -1 . (Note the Felativel y rapid deca y i n width o f the Rockal l Troug h betwee n th e two transects , whic h ar e located c . 200km apart. ) Locatio n show n i n Fig . 1 .

Fig. 5 . (a ) Transect 2 across th e norther n V0rin g margin , wher e th e thre e mai n tectono-stratigraphi c units , the base Cretaceou s interpretatio n fro m Blysta d e t al. (1995) and th e lowe r crusta l geometrie s ar e outlined . Abbreviations a s in Fig. 2 . (b) Transect backstrippe d t o th e base Cretaceou s leve l and isostaticall y balanced. Th e stretching factor , /3tot , illustrate s the tota l pos t mid-Jurassi c (bas e Cretaceous ) thinnin g calculated a s th e rati o between depth s t o isostati c an d backstrippe d Moho . (c ) Same a s i n (b) , but wit h the bas e Cretaceou s interpretation fro m Blysta d et al (1995). (Note th e extreme thinning beneath th e western portions o f the transect associated wit h this interpretation. ) (d ) A summar y o f extension estimate s made alon g th e transect . /9tot , tota l post mid-Jurassi c thinning ; (32 and /?3 , previously made subsidenc e derive d stretchin g estimate s fo r th e Lat e Jurassic-Cretaceous an d th e Maastrichtian-Paleocen e rif t episode s (Skogsei d e t al . 19926) ; /32_resulting , ne w estimates of Late Jurassic-Cretaceous thinnin g derived a s the ratio betwee n /?to t and /33. Where (3 3 is larger tha n /?tot, /32_resultin g i s se t t o 1.0 .

Fig. 6 . (a ) Transect 3 across th e souther n V0ring margin, where the thre e mai n tectono-stratigraphic units, the base Cretaceou s interpretatio n fro m Blysta d e t al. (1995) and th e lower-crusta l geometrie s ar e outlined . Abbreviations a s i n Fig . 2 and labe l as i n Fig . 5 . (b) Transect backstrippe d t o th e bas e Cretaceou s leve l an d isostatically balanced . Th e stretchin g factor , j3, illustrates the tota l pos t mid-Jurassi c (base Cretaceous ) thinning calculated a s the rati o betwee n depths t o isostati c and backstrippe d Moho . (c ) Same as in (b), bu t wit h the bas e Cretaceous interpretatio n fro m Blysta d et al . (1995). (Note th e broa d (c . 250km) area affecte d b y high thinning factors i n contrast t o the two more distinc t zone s o f thinning in (b).) (d) A summary of extension estimate s mad e along th e transect . /?tot , tota l pos t mid-Jurassi c thinning; (32 and /33 , previousl y made subsidenc e derived stretching estimates fo r th e Lat e Jurassic-Cretaceou s an d th e Maastrichtian-Paleocen e rif t episode s (Skogsei d et al . 1992/7) ; /32_resulting, ne w estimates o f Lat e Jurassic-Cretaceou s thinning derived a s describe d i n Fig . 5 . (Note th e larg e differences tha t exis t between /32_resulting an d (3 2 beneath th e dee p Ra s Basin. )

NE ATLANTI C CONTINENTA L RIFTIN G 1995). Th e overlyin g dee p Cretaceou s rif t trough i s betwee n 5 0 an d 70k m wide , where as th e Cretaceou s Vorin g Basi n i s c . 300 km wide. To the north, transect 2 reveals two Cretaceous depocentres , th e Traen a Basi n an d th e Hel Graben . Betwee n thes e depocentres , th e Nagrind Synclin e contain s a 3- 7 km thicknes s of Cretaceous strata above a very thick pre-Late Jurassic section . Skogsei d & Eldhol m (1995 ) suggested a shallower base Cretaceous horizon in this region, resulting in an even thicker pre-Late Jurassic succession . Recen t commercia l drilling on the Nyk High , well 6707/10-1, demonstrated , however, tha t Lat e Cretaceou s sediment s stil l exist at 503 9 m depth below sea level. The bas e o f th e crus t i s determine d b y integrating dee p seismi c refractio n an d wide angle reflectio n dat a (Plank e et al. 1991 ; Mjelde el a l 1996 , 1997) . Eas t o f th e Nagrin d an d Vigrid synclines refraction Moho (Skms" 1 ) corresponds i n dept h t o a dee p crusta l reflecto r (Fig. 2 ) (Skogsei d & Eldhol m 1995) . Towards the west , however , refractio n Moh o i s locate d deeper than this observed reflector. Her e the two levels boun d a 7 + kms"1 velocit y bod y inter preted t o represen t magmati c underplatin g emplaced durin g riftin g an d break-up . Thus , the reflecto r probabl y reflect s th e origina l base of th e crust , a ke y crusta l thinnin g factor . Th e underplating explains why the thinned crust ha s not subside d t o for m a dee p earl y Tertiar y rift basin , lik e th e dee p Cretaceou s Ra s Basin , which forme d a s a resul t o f th e Lat e Jurassic Cretaceous extensio n episode . Th e crus t again thicken s beneat h th e V0rin g Margina l High, presumabl y reflecting emplacemen t o f a n expanded igneou s sequenc e approachin g th e continent-ocean boundar y (COB) . The tota l amoun t o f crusta l thinnin g ca n b e derived fro m th e observe d crusta l thicknes s i f we ar e abl e t o determin e a reasonabl e esti mate o f th e initial , or pre-rift , crusta l thickness. In standar d basi n analysi s i t i s commonl y assumed tha t a 3 2-3 5 km thic k crystallin e crust of densit y 2.8gem" 3 , an d a 125-130k m thic k lithosphere i s balance d a t se a leve l (McKenzi e 1978). Hence, sediments will be deposited belo w sea level associated with lithospheric thinning or flexural bending. I n th e N E Atlantic , however , eclogites in Greenland an d Norway indicate synorogenic an d mayb e earl y post-orogenic crusta l thicknesses o f 80-9 0 km (Anderse n & Jamtveit 1990; Brueckne r e t al . 1999) . Furthermore , between th e Caledonia n front s i n Norwa y an d Greenland th e Lat e Palaeozoic-earlies t Meso zoic sediments are generall y of intra-continental and/or lacustrin e facies . Marin e deposit s wer e regionally firs t introduce d i n Permian-earl y

303

Triassic times . Th e easter n Barent s Se a and th e southern Nort h Se a have a somewha t differen t palaeo-environmental history , bot h area s probably receiving large amounts of detritus from th e high-standing Caledonia n mountai n bel t (e.g . Coward 1993) . Our preferre d margi n stratigraph y include s very thic k sequence s o f uppe r Palaeozoi c an d lowest Mesozoi c sediments . I n term s o f th e erogenic an d earl y post-orogeni c histor y th e question o f reference crustal thicknes s i s important, an d standar d assumption s applied in basin modelling ar e considere d no t applicabl e t o th e pre-Late Jurassi c basi n history . Mor e impor tantly, th e so-calle d pre-rif t sediment s d o no t reflect amount s o f rift-relate d tectoni c subsi dence, a ke y facto r i n calculatio n o f extensio n factors fro m subsidenc e analysi s also fo r th e later rif t episode s (L e Pichon & Sibuet 1981) . None th e less , crustal thinnin g estimates can be mad e fo r th e Lat e Jurassic-Cretaceou s an d Maastrichtian-Paleocene rif t episodes , assum ing: (1 ) a regiona l marin e depositiona l environ ment i n Mid-Jurassi c time ; (2 ) a correctl y identified bas e Cretaceou s horizon ; (3 ) wate r depths wer e small an d ma y b e ignored ; (4 ) th e lithosphere ha d coole d ove r sufficientl y lon g time sinc e precedin g rif t episodes ; (5 ) isostati c conditions. B y backstrippin g th e crus t t o th e base Cretaceou s (Middl e Jurassic ) horizon , a n image o f th e pre-Lat e Jurassi c basi n an d to p basement configuratio n i s achieve d (Fig . 5b) . Isostatic balancin g o f thi s sectio n (assumin g an averag e -porosity o f 20 % i n sediment s with matrix densit y o f 2.7gm~ 3 , an d crystallin e crustal an d mantl e densitie s o f 2.7 5 an d 3.3gcm~3, respectively ) result s i n a n 'isostati c Moho', whic h i s use d a s ou r referenc e crustal thickness. Thus , th e tota l post-Mid-Jurassi c crustal thinnin g i s image d b y th e differenc e between th e isostati c Moh o an d th e backstrip ped presen t Moho . Thi s approach , lik e th e traditional subsidenc e analysis , account s fo r only compactio n o f th e sediments , wherea s i t does no t assum e extensiona l thinnin g o f th e sedimentary column . Th e latte r facto r ma y no t be importan t fo r smal l stretchin g factor s (/?
304

J. SKOGSEI D E T AL.

et al. (1995). Th e averag e stretchin g factor s an d the integrate d amoun t o f stretchin g (ove r th e 325km portio n o f th e transec t wit h stretchin g factors >1.0 ) fo r th e tw o alternativ e interpreta tions ar e 1. 7 and 127k m fo r ou r interpretation , v. 2. 8 an d 200k m fo r th e interpretatio n o f Blystad e t al . (1995) . Similarly , th e souther n V0ring margi n transec t (390k m portio n wit h stretching factors >1.0 ) yields values of 1.7 5 and 170km and 2. 1 and 194km , respectively (Fig. 6). These result s d o no t alon e exclud e an y o f th e interpretations. W e shoul d kee p i n mind , how ever, tha t als o th e conjugat e eas t Greenland margin exhibits rifted basi n geometries reflecting extension tha t wil l ad d t o thes e estimates if they are related t o the same rif t episodes . In any case, the ver y hig h value s o f integrate d stretchin g indicate that th e width of the present margi n was increased almos t b y a facto r o f tw o durin g th e 170-55 Ma period . I n additio n t o ou r previou s comments o n th e upper - an d lower-crus t struc tural correlation , w e believ e th e extrem e max -

imum thinnin g ( 0 betwee n seve n an d eight ) suggested b y the Blysta d et al. (1995) interpreta tion argue s agains t th e possibilit y tha t Cretac eous sediment s ar e restin g directly on basement . To distinguis h betwee n th e Maastrichtian Paleocene an d Late Jurassic-Cretaceous compo nents of thinning we prefer to us e the previously subsidence define d stretchin g estimate s fo r th e Maastrichtian-Paleocene rif t episode . Ther e i s general consensus that th e Maastrichtian-Paleocene stretchin g affected a 150-20 0 km wid e zone landward of the continent-ocean boundary, with factors approachin g 2.0-2. 5 acros s th e Fenri s and He l graben s o n th e oute r margi n (Skogsei d et al . 19920 ; Skogsei d 1994 ; Roberts e t al . 1997 ; Walker e t al. 1997). The estimates fro m Skogsei d et al . (19920 ) an d Skogsei d (1994 ) wer e derive d from subsidenc e analyse s correcte d fo r reduce d subsidence becaus e o f magmati c underplating . The averag e stretchin g factor s an d amoun t o f extension fro m the stretching distribution show n in Figs 5d and 6 d (j33) ar e 1. 6 and 10 7 km an d 1. 6

Fig. 7 . Crusta l transects across th e V0rin g (a) an d Hatton-Rockal l (b ) margins illustrating crustal geometries, velocity structur e (annotation in kms" 1 ) an d modelle d thicknes s of'magmatic underplating' . COB , Continent ocean boundary . Location a s transect s 3 and 6 in Fig . 1 . Subsidenc e (continuous curve) an d crusta l thinning (stippled curve ) derived stretchin g distribution for th e Maastrichtian-Paleocen e riftin g episod e ar e shown . The modelled thickness of underplating is derived from th e difference betwee n the thinning derived and th e subsidence derived stretchin g factors as described b y Skogsei d (1994) . The Hatton-Rockal l transec t is modified fro m White et al . (1987 ) and Makri s e t al . (1991) .

NE ATLANTI C CONTINENTA L RIFTIN G and 104k m fo r th e souther n an d norther n transects, respectively . I t shoul d b e note d tha t the thicknes s o f underplatin g i s th e measure d 7 + km s-1 velocity body on the northern V0ring margin transect , whereas the thickness is derived from modellin g o n th e souther n Vorin g margi n transect (Skogsei d 1994 ) (Fig . 7) . Althoug h i t may b e questione d whethe r th e 7 + kms-1 velocity bod y ma y contai n som e intrude d lowe r continental crust , we consider thes e estimates as well constrained base d o n the present knowledge of the crustal structure , stratigraphic record an d the subsidence history. In fact, they are compar able wit h stretchin g estimate s o n typica l non volcanic margins (Ginzburg et al. 1985), but ten d to be high compared wit h estimates derived fro m amounts o f extensio n fro m faul t analysi s (Roberts et ai 1997 ; Ren et al. 1999). On bot h V0rin g margi n transect s th e Maas trichtian-Paleocene stretchin g estimate s corre late wit h th e tota l pos t Mid-Jurassi c crusta l thinning factor s acros s th e wester n province s (Figs 5 an d 6) . Alon g th e souther n V0rin g margin transec t thi s compariso n ha s n o signifi cance, a s the thickness o f modelled underplatin g (Fig. 7 ) wa s define d b y th e differenc e betwee n

305

observed an d theoretica l tectoni c subsidenc e based o n th e crusta l thicknes s (Skogsei d 1994) . On th e northern transect , an d th e More margin transect (Fig . 8) , however , th e correlation s indicate tha t mos t o f th e observe d crusta l thin ning beneat h th e Ny k High-He l Grabe n an d western Mor e Basi n relate s t o th e las t rif t episode only . The Lat e Jurassic-Cretaceou s componen t o f the thinning , /?2_resulting , ca n b e compare d with th e subsidence-derive d estimate s o f Skog seid e t al . (19920,6) - W e not e tha t significantl y greater thinnin g seem s t o hav e take n plac e beneath th e Ra s an d Mor e basin s tha n pre viously suggeste d (Fig s 6 d and 8) . In these dee p basins, n o notabl e differenc e i s introduce d b y various interpretation s o f th e bas e Cretaceou s level, an d i t appear s tha t les s subsidenc e ha s taken plac e tha n woul d b e expecte d fro m th e crustal structure . Our interpretatio n o f onl y tw o mai n rif t epi sodes, th e Lat e Jurassic-Cretaceou s an d Maas trichtian-Paleocene phases, contrasts wit h those of Blysta d e t al . (1995 ) an d Lundi n & Dor e (1997). Thes e worker s argue d tha t additiona l extensional deformation affecte d th e regio n als o

Fig. 8 . Transect s 1 an d 4 acros s th e Wester n Barent s Se a an d th e M0r e margin s (modifie d fro m Faleid e et al . (I993b) an d Olafsso n e t al . (1991), respectively ) backstrippe d t o th e mid-Jurassic-bas e Cretaceou s level . Extension estimate s are mad e wit h sam e procedure a s described fo r Fig s 5 and 6 . /3tot, tota l post mid-Jurassi c thinning; (3 2 and /?3 , previousl y mad e subsidenc e derive d stretchin g estimate s fo r th e Lat e Jurassic-Cretaceous and th e Maastrichtian-Paleocen e rif t episode s (Skogsei d e t al . 1992/7) ; /?2_resulting , ne w estimate s o f Lat e Jurassic-Cretaceous thinning derive d a s described i n Fig . 5 . (Note large difference s between /?2_resultin g and (32 beneath th e Mor e Basin , wherea s /fto t an d (3 3 show ver y consisten t value s toward s th e wes t (continent-ocea n boundary).)

306


in Lat e Cretaceous time , either as a separat e rif t episode o r a s continuous stretching . N o distinc t rift structure s or basin s of this age are identified, however. O n th e othe r hand , wit h referenc e t o the Troms 0 Basi n w e shoul d us e a broade r definition o f th e Cretaceou s rif t basins , indicat ing tha t substantia l amount s o f Lat e Jurassi c sediments ma y constitut e th e deepes t part s o f the rif t structures , an d tha t riftin g ma y hav e continued als o toward s lat e Cretaceou s time . Late lithospheri c thinnin g can , i n fact , partl y explain bot h th e relativel y larg e discrepanc y between subsidence and crusta l thinning derived stretching factor s beneat h th e deepes t Cretac eous basins (i.e. the apparen t lac k o f subsidence with referenc e to crusta l thickness ; /52-resultin g -32), a s wel l a s som e o f th e discrepanc y ob -

served betwee n th e extensio n derived fro m nor mal fault s an d fro m subsidenc e fo r th e las t rift episode .

Hatton-Rockall, More and SW Barents Sea transects The tota l crusta l thinnin g has als o bee n calcu lated fo r th e othe r transect s i n Fig . 1 , and i t i s shown tha t significantl y mor e pos t Mid-Jurassic extension ha s affecte d th e Hatton-Rockal l margin, includin g th e Rockal l Trough , tha n i s possible t o envisag e of f Norwa y an d i n th e western Barent s Se a (Fig . 9). The norther n Nort h Se a rif t ha s bee n eval uated b y Christiansso n e t aL (2000) , usin g a

Fig. 9. Crusta l thinning estimates for all transects stacked along the axis of the deep Late Jurassic-Cretaceous rif t zones. Th e fairl y consistent rif t dimension s fo r th e Maastrichtian-Paleocen e rif t episod e (/?3 ) on al l transect s should b e noted, an d likewis e for th e Lat e Jurassic-Cretaceou s rif t episode , /32_resulting , fo r al l transects fro m the M0r e margi n an d northwards . The Rockal l Troug h transects , o n th e othe r hand , sho w totally different rif t dimensions. ,3tot i s the tota l pos t mid-Jurassi c thinnin g derived a s describe d i n Fig . 5 .

NE ATLANTI C CONTINENTA L RIFTIN G comparable approach . The y showe d tha t th e basin wa s mainl y affecte d b y Lat e Jurassi c extension, an d tha t i t i s characterize d b y a narro w centra l rif t zone , 30-5 0 km, withi n a 250 km wide region affecte d b y Cretaceous post rift subsidence . Th e rif t follow s th e Lat e Palaeozoic-Early Mesozoi c basi n trend , an d i s surrounded b y rift flan k terraces . Th e estimate d stretching ove r th e tw o profile s discusse d b y Christiansson e t al. (2000 ) show s average s o f 1.2 an d 1.25 , respectively , resultin g i n c . 4050 km extension . West o f Britain , th e wid e Rockal l Troug h appears t o follo w th e SW-oriented Appalachia n trend. With reference to the above procedure, n o backstripping i s needed o f the Rockall transects , as th e existenc e o f pre-Cretaceou s sediment s i s speculative an d accordingl y no t include d i n this analysis. I n Fig . 4 a rapi d deca y i n th e mag nitude o f extensio n i s observe d fro m sout h t o north i n th e trough . Ove r th e distanc e o f c. 200km betwee n th e tw o transect s th e widt h of th e Rockal l Troug h i s reduced b y c . 100 km. Extension estimate s base d o n crusta l thicknes s show tha t th e averag e stretchin g facto r an d magnitude o f extension ar e 2. 9 and 315k m an d 2.9 an d 203k m fo r th e souther n an d norther n transects, respectively , assuming a 30 km pre-rif t crustal thickness . Farthe r wes t th e c . 300 km wide Hatton-Rockall Basin shows average thinning factor s o f c . 1.7, interprete d a s 126k m o f Maastrichtian-Paleocene extension according t o Skogseid (1994 ) (Fig . 7) . The dee p M0r e an d V0rin g basin s represen t the northwar d continuatio n o f th e Rockal l Trough an d th e Faeroe-Shetlan d Basi n trend . On th e M0r e margi n th e averag e stretchin g factors an d tota l amoun t o f extensio n ar e 1. 9 and 150km , o f whic h 87k m relate s t o th e Maastrichtian-Paleocene extensio n (Fig . 8) . Roberts e t al . (1990 ) mad e simila r extensio n estimates fo r th e Vikin g Graben , M0r e an d Faeroe-Shetland basins . Althoug h the y onl y estimated Lat e Jurassi c stretchin g (assumin g i t to b e c . 25% o f th e tota l Lat e Jurassic-Earl y Cretaceous episode) , the y conclude d tha t mos t of th e extensio n in th e M0re Basi n had t o hav e occurred wes t of Shetland. In th e S W Barent s Se a th e Lat e Jurassic Cretaceous crusta l thinnin g wa s severe , wit h maximum crusta l thinnin g value s approachin g four i n th e Troms 0 Basi n (Fig . 8) . Her e th e depth t o the Middle Jurassic horizon i s c. 13 km, and i n th e westernmos t basin, th e S0rvestnage t Basin (Fig . 3) , th e dept h i s comparabl e o r greater. Th e extensiona l development o f thes e two basin s appear s t o b e similar , givin g rise t o thick Uppe r Jurassic-Lowe r Cretaceou s syn-

307

rift sequence s (Breivi k et al. 1998) . The fina l rif t phase i n the Late Jurassic-Cretaceous episode is well constraine d t o b e o f Aptia n ag e i n thes e western Barent s Se a basins , wherea s th e sub sequent perio d wa s characterized b y rapid post rift subsidenc e an d sedimentation . Mos t o f th e structural relie f wa s covere d b y Cenomania n time (Faleid e e t al . 19932400k m lon g easter n margi n ha s a volcanic signature. Hinz el al. (1995) showed tha t the conjugat e margi n of f Sout h America , fro m the Sa o Paul o Platea u t o th e Falklan d Escarp ment, ha s a simila r character. I n particular , th e Uruguay an d Argentin e margin s (Fig . 3 ) have a tectono-magmatic zonatio n simila r t o tha t o f Namibia, an d th e cross-sectiona l dimension s of th e dippin g wedge s ar e als o similar. Assuming tha t th e Nort h Namibi a margi n transect i n Fig . 3 is representative, th e extrusive volume i s c . 0.2 x 10 6 km 3 fo r th e margi n seg ment i n Fig . 5 . Volum e estimate s farthe r sout h are uncertain , bu t appea r smalle r pe r lengt h

Fig. 5 . Namibi a margi n tectono-magmati c zonation . MC S profile s fro m Intera-ECL8 9 9 1 and PG S Nope c surveys. Magneti c anomalie s fro m Rabinowit z & LaBrecqu e (1979) . Bathymetr y in metre s (GEBC O 1994) . BR. break-u p relate d rif t zone ; COB . continent-ocean boundary.

ATLANTIC VOLCANI C MARGIN S unit. W e estimat e a volum e o f c . 0.58 x 10 6 km3 for th e entir e margin , an d c.O.S x 10 6 km 3 fo r its conjugate . Th e entir e Sout h Atlanti c LIP , including th e Parana-Etendek a CFB s (Milne r el al 1992 ; Peate e t al . 1992) , has a n extrusive volume o f a t leas t 2.3 5 x 10 6 km3 (Tabl e 3) . The onse t o f riftin g leadin g t o break-u p i s proposed a t c . 160 Ma (Ulian a et al. 1989; Niirnberg & Muller 1991) , an d dynami c modelling o f lithospheric extensio n i n th e Parana-Etendek a region suggests a rift duratio n o f c. 25Ma (Harr y & Sawye r 1992) . Th e latte r perio d i s consisten t with seismi c dat a o n th e Namibia n shel f (Ligh t et al . 1993) . We estimat e rif t width s o f 12 0 and

419

150km of T Namibia an d Argentina , respectively; and tha t th e u p t o 300k m an d 2400k m lon g rift underwen t extensio n fo r c . 25 Ma befor e break-up.

US Eas t Coas t margi n The US East Coas t margin (Fig . 6 ) was initiated by break-u p o f Nort h Americ a an d Afric a following a Lat e Triassic-Earl y Jurassi c rif t episode (Klitgor d e t al . 1988) . I t i s covere d b y very thic k sediment s limitin g seismi c resolutio n in the deep basins an d th e underlying crust . Th e

Fig. 6 . Distibutio n of seaward-dippin g wedges on th e th e U S Eas t Coas t margi n (O h e t al . 1995 ; Talwani et al . 1995) wit h selected seismi c profiles use d fo r volum e estimates in Tabl e 3 . GEBCO (1994 ) bathmetry i n metres, East Coas t Magneti c Anomaly (ECMA) fro m Talwan i e t al . (1995), an d fractur e zone s and sea-floo r spreading anomalies fro m Klitgor d e t al . (1988). SMV, submarin e volcanic rocks interprete d b y Austi n et al . (1990).

420

O. ELDHOL M E T AL .

on- and offshor e rift basin s exten d ove r a 200 km wide zone acros s Chesapeake Ba y into th e Baltimore Troug h (Benso n & Doyl e 1988) . Distinc t magnetic an d gravit y anomal y belt s delineat e crustal feature s o n th e margi n (e.g . Rabinowit z 1974; Also p & Talwani 1984) . Seaward-dipping reflector s wer e image d b y Klitgord e t al. (1988 ) an d Austi n e t al (1990) , and a 7.2-7. 5 km s"1 LC B was mapped b y wideangle profile s i n th e Baltimor e Canyo n (LAS E Study Group 1986 ) and Carolina trough s (Trehu et al. 1989) . Recent survey s have led to improve d mapping o f geometries an d distributio n of these rock complexe s (Holbroo k & Keleme n 1993 ; Sheridan e t al . 1993 ; Holbroo k e t al . 1994

Dynamics of the Norwegian Margin

The Phonology of Norwegian

The Use Of The Margin

Margin of Error

Margin of Error

Norwegian Wood

Margin of Error

Norwegian Wood

Norwegian Wood

Murder at the Margin

Onshore-Offshore Relationships on the North Atlantic Margin, Volume 12 (Norwegian Petroleum Society Special Publications)

Norwegian Wood

Norwegian wood

The Rise of the Norwegian Parliament: Studies in Norwegian Parliamentary Government (Library of Legislative Studies,)

Norwegian Wood

Norwegian wood

Norwegian Wood

Kress, Nancy - Margin of Error

Norwegian Wood

Norwegian Wood

Norwegian Wood (Tokyo blues)

Tokyo blues (Norwegian Wood)

The Phonology of Norwegian (The Phonology of the World's Languages)

The dynamics of patterns

The Dynamics of Heat

True Norwegian Black Metal

The dynamics of persuasion

The discovery of dynamics

Dynamics of the Atmosphere

Norwegian - An Essential Grammar

Dynamics of the Norwegian Margin

The Phonology of Norwegian

The Use Of The Margin

Margin of Error

Margin of Error

Norwegian Wood

Margin of Error

Norwegian Wood

Norwegian Wood

Murder at the Margin

Onshore-Offshore Relationships on the North Atlantic Margin, Volume 12 (Norwegian Petroleum Society Special Publications)

Norwegian Wood

Norwegian wood

The Rise of the Norwegian Parliament: Studies in Norwegian Parliamentary Government (Library of Legislative Studies,)

Norwegian Wood

Norwegian wood

Norwegian Wood

Kress, Nancy - Margin of Error

Norwegian Wood

Norwegian Wood

Norwegian Wood (Tokyo blues)

Tokyo blues (Norwegian Wood)

The Phonology of Norwegian (The Phonology of the World's Languages)

The dynamics of patterns

The Dynamics of Heat

True Norwegian Black Metal

The dynamics of persuasion

The discovery of dynamics

Dynamics of the Atmosphere

Norwegian - An Essential Grammar

Recommend Documents