Intraplate strike-slip deformation belts

Intraplate Strike-Sli p Deformatio n Belts Geological Societ y Specia l Publication s Society Book Editors R. J . PAN...

Author: Fabrizio Storti | Robert E. Holdsworth | Francesco Salvini | Geological Society of London

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Intraplate Strike-Sli p Deformatio n Belts

Geological Societ y Specia l Publication s Society Book Editors R. J . PANKHURS T (CHIE F EDITOR ) P. DOYL E F. J . GREGOR Y J. S . GRIFFITH S A. J . HARTLE Y R. E . HOLDSWORT H

A. C . MORTO N N. S . ROBIN S M. S . STOKE R J. P . TURNE R

Special Publicatio n reviewin g procedure s The Societ y make s ever y effor t t o ensur e tha t th e scientifi c an d productio n qualit y of it s book s matche s tha t o f it s journals. Sinc e 1997 , all book proposals hav e been refereed b y specialis t reviewers a s well as by the Society's Books Editorial Committee. I f the referees identif y weaknesse s i n the proposal, these must be addressed befor e the proposa l is accepted . Once th e boo k i s accepted , th e Societ y ha s a tea m o f Boo k Editor s (liste d above ) wh o ensur e tha t th e volum e editors follo w strict guidelines o n refereeing an d quality control. We insist that individual papers can only be accepted after satisfactor y revie w b y tw o independen t referees . Th e question s o n th e revie w form s ar e simila r t o thos e fo r Journal o f th e Geological Society. The referees' form s an d comments must be availabl e to the Society's Boo k Editors on request . Although man y o f th e book s resul t fro m meetings , th e editor s ar e expecte d t o commissio n paper s tha t wer e no t presented a t th e meetin g t o ensur e tha t th e boo k provide s a balance d coverag e o f th e subject . Bein g accepte d fo r presentation a t the meetin g doe s no t guarante e inclusio n i n th e book . Geological Societ y Specia l Publication s ar e include d i n th e IS I Index o f Scientifi c Boo k Contents , but the y d o not have a n impac t factor , th e latte r bein g applicabl e onl y t o journals. More informatio n abou t submittin g a proposal an d producing a Special Publicatio n can b e foun d o n th e Society' s web site : www.geolsoc.org.uk. It is recommended tha t reference t o all or part o f this book shoul d be made in one of the following ways: STORTI, F. , HOLDSWORTH , R . E & SALVINI , F . (eds ) 2003 . Intraplate Strike-Slip Deformation Belts. Geological Society , London, Specia l Publications , 210 . UTTAMO, W. , ELDERS , C . & NICHOLS , G . 2003 . Relationship s betwee n Cenozoi c strike-sli p faultin g an d basin openin g i n norther n Thailand . In: STORTI , F. , HOLDSWORTH , R . E & SALVINI , F . (eds ) Intraplate Strike-Slip Deformation Belts. Geologica l Society , London, Specia l Publications , 210 , 89-108 .

GEOLOGICAL SOCIET Y SPECIA L PUBLICATIO N NO. 21 0

Intraplate Strike-Sli p Deformatio n Belt s EDITED BY F. STORT I Universita degl i Stud i "Rom a Tre" , Rome, Ital y

R. E . HOLDSWORT H Durham University , Durham, U K

F. SALVIN I

Universita degl i Stud i "Rom a Tre" , Rome, Ital y

2003 Published b y The Geologica l Societ y London

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Contents

Preface vii Acknowledgements viii STORTI, F. , HOLDSWORTH , R . E . & SALVINI , F . Intraplat e stike-sli p deformatio n belt s 1 VAUCHEZ, A . & TOMMASI , A . Wrenc h fault s dow n t o th e asthenosphere : Geologica l an d geophysical evidenc e an d themo-mechanica l effect s 1

5

SCHREURS, G . Faul t developmen t an d interactio n i n distribute d strike-sli p shea r zones : a n experimental approac h 3

5

BUSLOV, M . M. , KLERKX , J. , ABDRAKHMATOV , K., DELVAUX , D. , BATALEV , V . Yu , KUCHAI, O . A. , DEHANDSCHUTTER , B . & MURALIEV , A . Recen t strike-sli p deformatio n o f the Norther n Tie n Sha n 5

3

CUNNINGHAM, D. , DUKSTRA , A. , HOWARD , J. , QUARLES , A . & BADARCH , G . Activ e intraplate strike-sli p faultin g an d transpressiona l uplif t i n th e Mongolia n Alta i 6

5

UTTAMO, W. , ELDERS , C . & NICHOLS , G . Relationship s betwee n Cenozoi c strike-sli p faultin g and basi n openin g i n norther n Thailan d 8

9

FERRACCIOLI, F . & Bozzo, E . Cenozoi c strike-sli p faultin g fro m th e easter n margi n o f th e Wilkes Subglacia l Basi n t o th e wester n margi n o f th e Ros s Se a Rift : a n aeromagneti c connection 10

9

PERRITT, S . H . & WATKEYS , M . K . Implication s o f lat e Pan-Africa n shearin g i n wester n Dronning Maud Land , Antarctic a 13

5

ROCCHI, S. , STORTI , F. , D i VINCENZO , G . & ROSSETTI , F . Intraplat e strike-sli p tectonic s a s an alternativ e t o mantl e plum e activit y fo r th e Cenozoi c rif t magmatis m i n th e Ros s Se a region, Antartic a 14

5

MARSHAK, S. , NELSON , W . J . & MCBRIDE , J . H . Phanerozoi c strike-sli p faultin g i n th e continental interio r platfor m o f the Unite d States : example s fro m th e Laramid e Orogen , Midcontinent, an d ancestra l Rock y Mountain s 15

9

MURPHY, J . B . Lat e Palaeozoi c formatio n an d developmen t o f th e S t Mary s Basin , mainlan d Nova Scotia , Canada : a prolonged recor d o f intracontinental strike-sli p deformatio n durin g the assembl y o f Pangae a 18

5

REIJS, J . & McCLAY , K . Th e Salin a de l Frail e pull-apart basin , northwest Argentin a 19

7

MOHRIAK, W . U . & ROSENDAHL , B . R . Transfor m zone s i n th e Sout h Atlanti c rifte d continental margin s 21

1

Index 229

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Preface Intraplate strike-sli p deformatio n belts are common tectonic features , particularl y a t convergen t plat e boundaries, wher e the y ar e produce d b y bot h oblique convergenc e an d continenta l indentation . These lithosphere-scal e structures , tha t als o occu r in othe r geodynami c environments such as passive margins, ar e characterise d b y comple x structura l architectures, b y th e occurrenc e o f larg e earth quakes, an d by th e fas t uplif t and/o r subsidenc e of localised crusta l sectors . Intraplat e strike-sli p belt s can als o contro l th e ascen t an d emplacemen t o f deeply-sourced magma . I n som e cases , intraplat e strike-slip belt s lin k wit h oceani c fractur e zone s and transfor m faults , transferrin g transfor m shea r from th e ridge s t o th e interio r o f th e plates . Thi s evidence ha s a n importan t impac t o n th e classica l concept o f transfor m faulting . Thi s volum e con tains a selectio n o f paper s describin g th e tectoni c architecture o f intraplat e strike-sli p deformatio n belts an d relate d structures . Th e volum e contain s 13 papers, includin g experimental an d case studie s from a global se t of contributors. The opening contribution by Storti et al. is an overview of the basic features o f intraplat e strike-sli p deformatio n belt s aimed a t settin g th e scen e fo r th e mor e detaile d papers t o follow . Th e firs t pape r b y Vauche z & Tommasi, discusse s th e geologica l an d geophysi cal evidenc e supportin g an astenospheri c dept h of major intraplate strike-sli p belts. The second paper, by Schreurs , describe s laborator y experiment s o n

the structura l architectur e o f faultin g produce d b y distributed shear . Th e remaining papers ar e organised following a geographic criterion. W e start with three contribution s dealin g wit h intraplat e strike slip belt s locate d i n th e India-Asi a collisiona l region (Buslo v et«/., Cunningha m et al., Uttamo et a/.) ; the n mov e t o Antarctic a (Ferracciol i & Bozzo, Perrit t & Watkeys, Rocch i e t al.), Nort h America (Marsha k e t al., Murphy ) an d Sout h America (Reij s & McClay). Th e las t contributio n (Mohriak & Rosendahl) deal s wit h the evolutio n of riftin g i n th e Sout h Atlantic . The editor s woul d lik e t o than k al l th e parti cipants t o Symposiu m LS04, hel d a t th e BU G X I Congress (Strasbourg , Apri l 2001) , wh o provide d the impetu s fo r thi s volume . Man y thank s t o th e Geological Society , London , for encouraging u s to edit the volume and for their constan t support during th e editoria l process . Th e interes t o f F . Stort i and F . Salvin i o n th e interna l architecture , evol ution an d geodynami c setting s o f intraplat e strike slip deformation belts was driven by their involve ment i n the stud y of th e Cenozoi c geodynamic s a t the northeastern edge of the Antarctic plate, funde d by the Italian National Antarctic Program (PNRA). Fabrizio Storti , Rom a Tre , Ital y Bob Holdsworth , Durham , UK Francesco Salvini , Roma Tre , Ital y

Acknowledgements The editor s than k th e followin g colleague s an d friend s wh o kindl y helpe d wit h th e reviewin g o f th e papers submitte d fo r thi s volume : John C . Behrendt, Boulder , US A Marco Bonini , Florence , Ital y Luigi Burlini , Zurich , Switzerlan d Dickson Cunningham , Leicester, U K Mike Curtis , BAS , Cambridg e Nicola d'Agostino , Roma , Italy Giorgio Vittori o Dal Piaz , Padova , Ital y Tim Dooley, London , U K Ian Fitzsimmons , Curtin , Australi a Mario Grasso , Catania , Ital y Stephane Guillot , Lyon , Franc e Martin Insley , Infoterra , Barwell , UK James Jackson , Cambridge , U K Laurent Jolivet , Paris , Franc e

Robin Lacassin , Paris , Franc e Emanuele Lodolo , Trieste , Ital y Colin Macpherson , Durham , UK Massimo Mattei , Roma , Italy Brendan Murphy , St. Francis Xavier , Canad a Claude Rangin , Paris, Franc e Jean Francoi s Ritz , Montpellier, Franc e Yann Rolland , Grenoble , Franc e Mike Searle , Oxford , UK Robin Strachan , Oxfor d Brookes , U K Christian Teyssier , Minnesota , US A Bruno Vendeville , Austin , Texa s John Waldron , Calgary , Canad a One anonymou s reviewer

Intraplate strike-sli p deformatio n belt s F. STORTI 1, R . E . HOLDSWORTH 2 & F. SALVINI 1 l

Dipartimento di Science Geologiche, Universita "Roma Tre", Largo S. L. Murialdo 1, I00146 Roma, Italy. ^Reactivation Research Group, Department of Geological Sciences, Durham University, Durham DH1 3LE, UK Abstract: Intraplat e strike-sli p deformatio n belt s ar e typicall y steeply-dippin g structure s tha t develop i n bot h oceani c an d continenta l lithospher e wher e the y for m som e o f th e larges t an d most spectacular discontinuities found o n Earth. In both modern and ancient continental settings , intraplate strik e sli p deformatio n belt s ar e o f majo r importanc e i n accommodatin g horizonta l displacements wher e they additionally for m ver y persistent zone s of weakness tha t substantially influence th e rheological behaviou r o f th e lithospher e ove r ver y long tim e period s (u p to 1 Ga or more) . Thes e deformatio n zone s provid e a fundamenta l geometric , kinemati c an d dynami c link between th e more rigi d plate-dominated tectonic s o f the oceans and the non-rigid, comple x behaviour of the continents. During convergence, the y help to transfer major displacement s dee p into the plate interiors . Durin g divergence, the y act as transfer zones tha t segmen t rifts , passiv e continental margin s and, ultimately, oceanic spreadin g ridges. Suc h belts are also of great econ omic importance, controllin g the location o f many destructive earthquakes, offshore an d onshore hydrocarbon deposit s an d metalliferous or e deposits . I n the oceans , intraplat e strike-sli p move ments ar e relativel y mino r alon g transform-relate d fractur e zones , bu t ther e ar e a n increasin g number o f documente d example s tha t ma y reflec t spatia l an d tempora l variation s i n spreadin g rate alon g individua l activ e ridg e segments .

Strike-slip deformatio n belt s ar e region s i n whic h tectonic displacement s occu r predominantl y paral lel t o th e strik e o f th e zon e (fo r a general review , see Woodcock & Schubert 1994) . Th e recognitio n of strike-slip-dominate d plat e boundarie s o r transform faults (Wilso n 1965) , togethe r wit h their geo metric linkag e to , an d kinemati c interactio n with , constructive an d destructiv e plat e margin s wer e central t o th e emergenc e o f plat e tectonic s (e.g . McKenzie & Parke r 1967 ; Morga n 1968 ; Co x 1973). Plat e tectonic theor y assume s that the lithosphere i n th e plat e interior s is , t o a firs t approxi mation, rigi d an d tha t most deformatio n relate d t o plate interaction s wil l be concentrate d int o narrow belts alon g th e plat e margins . Thi s mode l work s reasonably wel l i n region s underlai n b y oceani c lithosphere, wit h th e resul t tha t muc h o f th e seis micity alon g transfor m fault s occur s onl y wher e they for m activ e plat e boundaries . A s the y pas s into th e plat e interior , transfor m fault s becom e relatively tectonicall y quiescen t feature s known as oceanic fractur e zone s whic h for m som e o f th e largest topographi c structure s o n th e Earths ' sur face (e.g . Whit e & William s 198 6 an d reference s therein). Man y o f thes e fractur e zone s segmen t o r even boun d majo r sedimentar y basin s wher e the y impinge upo n th e continenta l margin . Example s

include th e souther n Atlanti c margin s (Francheteau & L e Picho n 1972 ) an d th e wester n margin o f Australi a (Son g e t al. 2001). In continenta l regions , intraplat e structure s an d their relationship t o plate tectonic s ar e complicate d by th e non-rigi d behaviou r o f substantia l region s of continenta l lithospher e (e.g . Molna r 1988) . Thi s behaviour i s well-illustrate d b y th e broad , diffus e zones o f seismicit y observe d i n man y continenta l regions - notabl y Centra l Asi a fro m Tibe t north wards - extendin g dee p int o th e plat e interiors . Geologically, thes e continenta l deformatio n zone s may compris e interlinke d system s o f fault - an d shear zone-bounded blocks an d flakes that partition strains and other geological processes int o comple x regions o f displacement , interna l distortio n an d rotation o n various scale s (e.g . Dewe y e t al. 1986 ; Foster & Gleadow 1992 ; Park & Jaroszewski 1994 ; Tommasi e t al. 1995 ; Butle r et al 1997 ; Salvin i et al. 1997 ; Marshak et al. 2000). It may be useful t o view suc h regions o f non-rigid behaviour as broad, diffuse plat e boundaries (e.g . se e Gordon 199 8 an d references therein) , bu t i n th e presen t pape r w e shall refe r t o al l region s locate d awa y fro m th e major plat e boundarie s a s 'intraplate' . The non-rigi d behaviou r o f continenta l litho sphere probably arise s from the presence of a weak

From: STORTI , F. , HOLDSWORTH , R . E . & SALVINI , F . (eds ) Intraplate Strike-Slip Deformation Belts. Geologica l Society, London , Specia l Publications , 210 , 1-14 , 0305-8719/037 $ 15 © Th e Geologica l Societ y o f Londo n 2003 .

2

F. STORTI, R. E . HOLDSWORTH & F. SALVINI

quartzofeldspathic crusta l laye r an d fro m pre existing mechanica l anisotropies . Suc h aniso tropies, primaril y ol d fault s an d shea r zones , ma y undergo reactivation in preference to the formation of ne w tectoni c structure s durin g regiona l defor mation episode s (e.g . Thatche r 1995 ; Holdswort h et al 1997 , 2001b) . Th e buoyanc y o f continenta l crust mean s tha t i t an d it s underlyin g lithospheri c mantle ar e no t normall y subducted . A s a result , zones o f pre-existin g weaknes s ar e effectivel y 'locked-in' t o the continents and can potentially b e reactivated many times during successive phases of continental deformatio n an d accretion . Thi s long lived architectur e o f inheritanc e i s no t generall y found in oceanic lithosphere (e.g. Sutto n & Watson 1986). Crustal-scale reactivated fault system s in the upper crus t broaden wit h dept h int o ductil e shea r zones o f regiona l extent , i n whic h substantial volumes o f lowe r crusta l an d uppe r mantl e rock s experience episode s o f reworkin g (Holdswort h e t al. 2001a) . In intraplat e region s o f th e continents , deep seated fault s o r shea r zone s ofte n manifes t them selves a t the surfac e by th e developmen t o f linea r zones o f geological , geophysica l o r topographi c features know n as lineaments (e.g . Sutto n & Watson 198 6 an d reference s therein) . Significantly , a majority o f these lineaments seem to coincide with large, steeply-incline d strike-sli p deformatio n zones (e.g . O'Driscol l 1986 ; Dal y e t a l 1989) . Seismological an d geodeti c studie s o f neotectoni c intraplate deformatio n (e.g. Molna r & Tapponnier 1975) sugges t tha t horizonta l movement s ar e pre dominantly accommodate d b y strike-sli p faulting . Thus intraplat e strike-sli p deformatio n belt s ar e particularly importan t i n determinin g th e defor mation response o f continental lithosphere i n plate interiors ove r lon g tim e scales . Interestingly , th e longevity of intraplate continental strike-slip deformation zones and their deeply penetrating natur e is central t o man y o f th e globa l tectoni c hypothese s that hav e bee n propose d prio r t o plat e tectonic s (e.g. rhegmatic tectonics; Vening Meinesz 1947 ) or as alternativ e paradigms . I t i s probabl y significan t that th e mos t prominen t o f thes e - th e Sovie t endogenous regim e mode l (Belousso v 1978 ; Pav lenkova 1995 ) - wa s developed by scientists based in an intraplate region almost entirely underlai n by continental lithosphere . Long-lived strike-sli p deformatio n zone s - o r structures that reactivate them - ar e of considerable economic significance and represent important geological hazards . Although subordinate t o the grea t concentrations o f earthquake s aroun d th e plat e margins, ther e ar e man y region s o f frequen t an d sometimes highl y destructiv e seismicit y focuse d along strike-sli p deformatio n zone s i n intraplat e regions. Example s includ e th e N an d E Anatolian

faults i n Turke y (e.g . Jackso n & McKenzie 1988 ) and th e Ne w Madri d Seismi c Zon e i n th e US A (e.g. Johnsto n & Shedloc k 1992 ; Marsha k e t al . this volume). Intraplate strike-slip fault s ofte n hav e a profoun d influenc e o n th e location , architectur e and subsidenc e histor y o f associate d sedimentar y basins, man y o f whic h ar e ric h i n hydrocarbons, forming importan t tectoni c an d palaeogeographi c boundaries. Goo d example s ar e th e man y long lived intraplat e strike-sli p fault s tha t hav e repeat edly influence d th e evolutio n o f th e ric h hydrocarbon-bearing basin s of S E Asia (e.g . Mor ley 200 2 an d references therein). Finally, there ar e numerous intraplat e strike-sli p deformatio n belt s that have acted a s channels for the flow of magma and hydrotherma l fluids , leadin g t o th e accumu lation of economically significan t or e deposits (e.g. the Easter n Goldfield s Provinc e i n th e Yilgar n Block, wester n Australia; Cox 1999) .

Size an d mechanica l significanc e o f intraplate strike-sli p deformatio n belt s In all plate tectonic settings, strike-slip deformation belts a t th e surfac e ar e characterise d b y steeply dipping anastomosin g arrays o f faults , ofte n wit h many bends an d offset s i n individua l fault strand s (Fig. la ; e.g . Sylveste r 1988 ; Woodcock & Schubert 1994) . Regionall y significan t faul t zone s ar e typically a few tens of kilometres wide and several hundred kilometre s long . B y definition , plate boundary transfor m fault s cu t throug h th e whol e lithosphere i n al l settings . Th e dee p geometr y o f intraplate faults i s less straightforward, particularly as i t i s difficul t t o imag e stee p structure s at depth . However, ou r improve d understandin g o f th e relationships betwee n faul t dimension s an d dis placement (e.g. Walsh & Watterson 1988, Cowie & Scholz 1992 ) suggest s tha t sub-vertica l strike-sli p fault zone s wit h strike-length s an d offset s greate r than 30 0 k m an d 3 0 km , respectively , ar e ver y likely t o b e o f a size sufficien t t o cu t much, if no t all, o f th e lithospher e (Fig . la) . A growin g body of geological , geochemica l an d geophysical obser vations sugges t a direct link between the mantle at depth and regional-scale strike-slip faults an d shear zones i n th e crust . I n summary , thi s evidenc e includes th e followin g (see Vauche z & Tommasi , this volume, and references therein) : i) Geologica l observation s i n ancien t exhumed mid- an d lowe r crusta l rocks preserv e many examples o f verticall y cross-cuttin g strike slip shea r zones , wit h littl e evidenc e o f detachment alon g sub-horizonta l surfaces , even i n partiall y molte n rock s (e.g . Borbor ema Province , Brazil : Vauchez et al . 1995) . ii) Chemica l an d stabl e isotop e studie s o f

INTRODUCTION

3

Fig. 1 . (a ) Cartoo n showin g ho w majo r continenta l intraplat e strike-sli p deformatio n belt s ma y ultimatel y roo t int o the asthenospher e (afte r Teyssie r & Tikof f 199 8 an d Vauche z e t al. 1998) . Strik e sli p fault s i n th e uppe r crus t pas s down int o increasingl y broa d shea r zone s (fabri c trace s show n schematically ) i n th e lowe r crus t an d lithospheri c mantle. Dashe d line s i n expose d faul t & fabri c surfac e neare r t o viewe r ar e transport-paralle l lineations . Schemati c strength v s dept h profil e fo r continenta l lithospher e show n to th e left , (b ) O n th e left , sketc h cross-sectiona l vie w of a load-bearin g laye r (suc h a s th e crus t o r lithosphere ) o f thicknes s t cu t b y a principa l displacemen t zon e (PDZ ) o f length L dippin g a t a n angl e 5° . Th e relativ e siz e o f th e PD Z i s give n b y L/t . Grap h o n th e righ t show s ho w th e relative siz e o f a PD Z rapidl y decrease s a s th e di p increases .

magmas an d hydrotherma l fluid s channelle d along larg e strike-sli p fault s an d shea r zone s suggest CO 2-rich, mantle origin s (e.g . Madagascar: Pil i e t al 1997) . iii) Man y continental-scal e shea r zone s an d reactivated fault s overli e large-scal e aniso tropies suc h a s low-velocit y zone s i n th e upper mantl e (e.g . Housema n & Molna r 2001) and/o r positive gravity anomalies asso ciated wit h localise d uplif t o f th e crust mantle boundar y (e.g . Pil i e t al. 1997) . iv) Shea r wav e splittin g measurement s

v)

(reviewed i n Silve r 1996 ) collecte d alon g several intraplat e strike-sli p fault s an d shea r zones sugges t tha t fabric s relate d t o thes e structures ar e develope d a t al l level s i n th e lithosphere, includin g th e uppe r mantl e (e.g . Fig. 1 ; Tommas i e t al . 199 6 an d reference s therein). Thes e result s ar e broadly supporte d by magnetotelluri c an d electrica l anisotrop y measurements o f dee p mantl e fabric s (e.g . Pous e t al . 1995 ; Senecha l e t al 1996) . Deep seismi c reflectio n profiling studie s have imaged numerou s examples o f regional-scal e

4

F. STORTI , R . E. HOLDSWORT H & F. SALVIN I faults tha t cu t th e entir e crus t an d tha t the y penetrate dee p int o th e mantl e (e.g . Grea t Glen Fault : McGeary 1989) .

The large-scal e mechanica l behaviou r o f the litho sphere ha s bee n investigate d widel y throug h th e application o f experimentall y derive d strengt h v s depth profiles (e.g . Goetze & Evans 1979 ; Brac e & Kohlstedt 1980 ; Kirb y 1983) . Typically , thes e assume a simpl e horizontall y layere d lithosphere , with eac h laye r havin g a unifor m composition , a limited numbe r o f competin g deformatio n mech anisms (usuall y brittl e failur e an d dislocatio n creep) an d specifie d environmenta l (P , T , strai n rate etc ) an d lithologica l (composition , grai n size , crustal thickness ) conditions . Thes e diagram s ar e gross simplification s o f th e likel y rheologica l behaviour (se e fo r exampl e Paterso n 1978 ; Schmid & Handy 1991 ) but a s first order approxi mations the y provid e usefu l informatio n concern ing the vertical distributio n o f strength in the litho sphere. I t i s generall y agree d tha t th e mechanica l properties o f th e stronges t layer(s ) wil l determin e the overal l behaviou r o f th e lithospher e (e.g . England 1983) . I n mos t continenta l settings , th e extrapolations o f experimental dat a suggest that the main load-bearin g regio n i n the lithosphere shoul d lie i n th e uppe r mantle , wit h a secondar y stron g region i n the mid-crust (Fig . la ; Molna r 199 2 and references therein) . Thi s vie w has been questione d recently b y Maggi el al. (2000a, b ) who argue that the distribution of earthquake focal depth s suggest s that th e mai n load-bearin g regio n lie s i n th e crus t and tha t th e aseismi c uppe r mantl e i s weak . Thi s conclusion i s base d o n th e premis e tha t th e pres ence o f seismicit y i s indicativ e o f strength , bu t i t remains a distinct possibility tha t the upper mantle may b e bot h aseismi c an d strong , eve n ove r lon g time scales . Mechanically wea k tectoni c discontinuitie s wil l be most significant whe n they cut through the loadbearing region s o f th e lithospher e and , fro m th e foregoing discussion , i t i s clea r tha t thi s i s parti cularly likel y fo r steeply-incline d t o sub-vertical , regional-scale strike-sli p fault s an d shea r zone s (§.g. Fig ia} , FfSff l a §iffigi § §§§ffl§iri 2 viswgsiiu , the steeper a fault, the smaller it has to be (in terms of length , are a o r displacement ) i n orde r t o cu t through a horizonta l load-bearin g laye r o f thick ness t (Fig . Ib) . Thi s ma y b e on e reaso n wh y strike-slip fault s an d shea r zone s ar e particularl y prone t o reactivatio n i n continenta l settings . Field studie s o f regional-scal e reactivate d fault s suggest that profound weakening can occur following textura l an d retrograd e metamorphi c modifi cation o f faul t rock s unde r mid-crusta l an d uppe r mantle conditions (e.g . Vissers e t al. 1995 ; Stewart et al . 2000 ; Imbe r e t a l 2001 ; Holdswort h e t al .

200Ib). Thes e processe s see m t o b e particularl y effective i n regions where a large influx o f H2O- or CO2-rich flui d ha s occurre d durin g shearing . Sub vertical strike-sli p belt s wil l focu s th e weakenin g effects o f fault-relate d processe s particularl y strongly a s al l faul t strand s ar e verticall y aligned . Thus, thei r persistenc e ove r lon g tim e scale s an d apparent importanc e i n intraplat e deformatio n regimes i s perhaps no t surprising .

Origins of intraplate strike-sli p deformation belt s The generatio n o f intraplat e strike-sli p belt s i s particularly favoure d whe n one or more of the following occurs : (i ) collisio n o f irregularl y shape d continental margin s an d indentors , a proces s tha t often lead s to lateral escape (e.g . India-Eurasia col lision; Tapponnie r e t al. 1986 ; Arabi a - Eurasi a collision generatin g th e Anatolia n faul t block ; McKenzie 1972 ; Dewey 1977) ; (ii ) deformation of lithosphere i n whic h marke d latera l variation s i n rheological strengt h occu r du e t o rift-relate d changes in crustal thickness o r geothermal gradient (e.g. Borborem a Province , Brazil ; Tommas i & Vauchez 1997 ; Vauche z e t al. 1998) ; (iii ) conver gence continue s afte r initia l continenta l collisio n (e.g. India-Eurasia collision ; Molnar & Tapponnier 1975); (iv ) relative motions amon g adjacen t plate s are governed by differen t Euleria n poles (e.g . Australia-East Antarctica-Ne w Zealand; Stoc k & Molnar 1982) ; (v ) differential rotation s occu r within a major plate (e.g. the Cenozoic motion between East and West Antarctica; Cande et al. 2000); (vi ) kinematic strai n partitionin g o f a regiona l intraplat e transpressional o r transtensiona l deformatio n (e.g . The Main Recent Fault, NW Iran; Talebian & Jackson 2002 ; se e als o Jackso n 1992) . In continenta l regions , man y intraplat e strike slip deformatio n belt s ar e reactivate d structure s that formed initially at continental plate boundarie s as transfor m faults , or , a s i n th e cas e o f trench linked an d indent-linke d strike-sli p faults , du e t o the operatio n o f plate-boundar y processe s (e.g . Woodcock 1986) . Other s hav e initiate d a s majo r dig-siig algssfliiimiik s §uy h a s §2§ani 2 §mu£§g , thrusts or rift-bounding faults. Reactivate d oceani c transforms ar e restricte d t o ophiolite s i n ancien t settings an d see m t o b e relativel y uncommon . I n all othe r cases , th e discontinuitie s hav e becom e intraplate feature s followin g continenta l collisio n and ma y hav e undergon e steepenin g int o a sub vertical attitud e tha t i s particularl y favourabl e t o reactivation. Som e intraplat e strike-sli p fault s ma y form a s ne w structure s i f n o favourabl y oriente d zones o f pre-existin g weaknes s ar e present , Once presen t i n th e continenta l lithosphere , strike-slip deformatio n zone s clearl y influenc e th e

INTRODUCTION

segmentation o f rift s an d th e resultin g location o f salient-re-entrant feature s i n passiv e continenta l margins durin g break-u p (e.g . Daly e t al. 1989) . As first recognised b y Wilson (1965) , th e resulting irregularities in the continental margin significantl y determine th e locatio n an d developmen t o f trans form fault s i n th e evolvin g spreadin g ridg e an d their associate d intraplat e fractur e zones . Signifi cantly, man y regions o f enhanced seismicit y occur along pre-existin g strike-sli p deformatio n belt s adjacent t o and continuous with the terminations of transform-related fractur e zone s i n passiv e conti nental margin s (e.g . Sykes 1978) . Thes e obser vations sugges t a direc t lin k betwee n intraplat e faulting i n continental an d oceanic lithosphere an d illustrate tha t th e structura l inheritanc e locked-u p in the continents ultimately plays an important role in controllin g th e geometri c an d kinemati c evol ution o f oceani c plates .

Termination zone s Two mai n classe s o f intraplat e strike-sli p defor mation zones are recognised based on the nature of their termination s (Fig . 2) . Transfer intraplat e strike-slip fault s occu r whe n displacemen t i s accommodated a t a plat e boundary , eithe r b y th e extrusion of a single, rigi d block (rigi d escape), o r by extrudin g a numbe r o f linke d block s wit h a rotational componen t (rotationa l escape). Confined intraplate strike-sli p faultin g occur s whe n th e displacement decreases an d is fully accommodate d by strain withi n th e plat e interior . Deformatio n patterns a t thes e faul t termination s fal l int o fou r end member types : extensional, contractional, strikeslip o r rotational (Fig. 2). In some cases, more than

Fig. 2 . Highl y conceptua l sketc h showin g the tw o main classes of intraplate strike-slip deformation belts and their different mode s o f termination .

5

one type may occur associated wit h individual terminations (see below). These second-orde r accom modation structure s form a t an angle to the maste r strike-slip fault . Th e dominan t type(s) formed will probably depen d o n th e interactio n o f th e strai n fields relate d t o faul t motio n an d shap e (loca l bends, offsets , tips ) with the mechanical propertie s of th e adjacent hos t rocks , particularl y th e orien tation o f pre-existing anisotropie s i n th e basemen t (e.g. se e Sylveste r 1988 ; Woodcock & Schuber t 1994 an d reference s therein) . I n cas e o f bloc k rotation, th e angl e betwee n th e block-boundar y faults an d the master strike-slip belt changes markedly throug h time (Scott i e t al. 1991) . Once formed , secondar y accommodatio n struc tures provid e weaknesse s int o whic h par t o f th e residual strike-sli p displacement ca n be transferred from th e maste r strike-sli p bel t (e.g . Storti e t al . 2001). Repeate d propagatio n o f th e maste r strike slip fault syste m into the plate interior ca n produce a sequence of accommodation structures becoming younger toward s th e faul t ti p (a s see n i n small scale faults: Willemse & Pollard 1998) . Thus, complex an d superimpose d structure s ca n develo p i n the terminatio n regio n o f intraplat e strike-sli p deformation belts . Examples o f terminatio n structure s i n th e ti p region o f intraplat e strike-sli p deformatio n belt s include the strike-slip faults in the northern Aegean Sea, whic h en d i n a regio n o f norma l faultin g i n central Greec e (Tayma z e t al . 1991) , an d th e Priestley Fault , a Cenozoi c intraplat e right-latera l fault syste m i n nort h Victori a Land , Antarctic a which terminate s o n its souther n sid e int o a series of extensiona l an d transtensiona l fault s includin g the Terro r Rif t (Fig . 3, Salvin i e t al . 1997 , 1998 ; Storti e t al . 2001) . Th e norther n sid e o f th e faul t termination is characterised by ESE-WNW striking strike-slip an d transpressional splay faults illustrat ing tha t bot h contractiona l an d extensiona l struc tures can form o n opposite side s of a single termination. Th e geometri c arrangemen t o f thes e termination structure s is a majo r clu e t o th e sens e of faul t movemen t (e.g . see Fig . 3 inset) . A s ye t there ar e n o palaeomagneti c dat a t o constrai n th e amount o f bloc k rotatio n abou t vertica l axe s tha t may hav e occurred , bu t ther e i s n o independen t geological evidenc e t o sugges t tha t thi s i s signifi cant (Stort i e t al. 2001) . A rotationa l an d contractiona l structura l archi tecture is developed a t the termination o f the rightlateral Sa n Gregorio-Sur-Sa n Simeon-Hosgr i faul t system, i n Souther n Californi a (Sorlie n e t al . 1999). Th e souther n Hosgr i Faul t comprise s tw o main strand s bounding compressional fold s tha t lie at hig h angl e t o th e fault s (Fig . 4). Right-latera l shear acros s th e souther n Hosgr i Fault i s absorbe d mainly b y clockwis e vertical-axi s rotatio n o f th e

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F. STORTI , R . E. HOLDSWORT H & F. SALVIN I

Fig. 3 . Structura l architectur e a t the termination o f the Priestley Fault , nort h Victori a Land , Antarctic a (se e Fig . 9 for location). Th e inse t show s ho w extensional , contractional , an d strike-sli p deformatio n accommodate s th e residua l horizontal displacement s a t the ti p o f th e faul t syste m (afte r Stort i e t al. 2001).

Fig. 4 . Cartoo n showin g th e tip of the Hosgri Fault , Cali fornia, wher e contractiona l an d strike-sli p deformation , together wit h bloc k rotatio n abou t vertica l axe s accom modate horizonta l displacement s (afte r Sorlie n e t al . 1999).

elongated block s between the fault strands , as well as b y foldin g an d thrustin g (Sorlie n el al . 1999) . Other example s o f rotationa l termination s o f intraplate strike-slip belt s include the Marlborough

fault syste m o f Ne w Zealan d (Littl e & Robert s 1997), the Sa n Jacinto fault syste m of southeastern California (Armbruste r et al. 1998 ) and the Whittier faul t syste m i n th e Lo s Angele s Basi n (Wright 1991) . Many intraplat e strike-sli p belt s en d i n area s of distributed thrusting , like thos e i n th e easter n an d northern Tibet (Molnar & Lyon-Caen 1989; Meyer et a l 1998) . Bayasgala n e t al . (1999 ) describe d field example s o f contractiona l terminatio n o f intraplate strike-slip belts in Mongolia. At both the eastern end of the Artz Bogd fault syste m (Fig. 5a) and o f fault s i n th e Toromho n regio n (Fig . 5b) , thrust faults develope d a t high angles to the strike slip faul t systems . Thrus t displacemen t decrease s progressively away from th e strike-slip faults , suggesting th e relativ e rotatio n o f th e thrus t footwall and hangin g wall block s abou t vertica l axe s (Fig . 5c; Bayasgalan et al. 1999) .

Bends an d stepover s The fault system s associated with strike-slip deformation zone s ar e rarel y perfectl y straigh t a s th e host rock s ar e invariabl y mechanically anisotropi c

INTRODUCTION

7

Fig. 5 . Simplifie d sketc h ma p o f th e structura l architecture a t th e terminatio n o f th e Art z Bog d Faul t (a ) an d Bog d Fault (b) , Mongolia, showin g th e dominan t role o f contractiona l deformation s tha t accommodat e residua l horizonta l displacement a t faul t tips . Th e progressiv e decreas e o f thrus t displacemen t awa y fro m th e maste r strike-sli p faul t suggests th e occurrenc e o f bloc k rotatio n abou t vertica l axe s (c ) (afte r Bayasgala n e t al. 1999) .

and th e fault s typicall y gro w b y th e linkag e o f second-order non-paralle l faul t segment s (e.g. Wilcox e t al . 1973) . Irregularitie s ca n b e subdivide d into tw o end-membe r types: bends wher e the faul t trace i s continuou s an d stepovers o r jogs wher e a discontinuity occurs in the fault trace (Fig 6a; Sylvester 1988 ; Woodcoc k & Schubert 199 4 and refer ences therein) . I n man y cases , stepove r zone s developed i n sedimentar y cove r sequence s nea r t o the surfac e ma y pas s downward s wit h dept h into bends i n th e faul t wher e i t cut s th e basement . Bends an d stepover s for m loca l zone s o f trans pressional (restraining ) or transtensional (releasing ) deformation dependin g o n the sens e o f overstep o r bending relativ e t o th e overal l sens e o f movemen t along th e principa l displacemen t zon e (PDZ) . A t the surface , restrainin g bends o r offset s produc e localised region s o f uplif t referre d t o a s push-ups whilst releasin g bend s o r offset s ar e associate d with the development o f pull-apart basins. In crosssections derived from seismi c reflectio n dat a across

many strike-sli p deformatio n belts, upward diverging fault patterns are commonly imaged originatin g from a singl e sub-vertica l discontinuit y a t dept h (e.g. Hardin g 1985) . Thes e ar e know n a s flowe r structures an d they ar e particularly commo n i n the region o f faul t bend s an d stopovers . A good example of the effects o f fault bends and offsets - an d what happens when the sense of shear is reverse d durin g successiv e reactivatio n epi sodes - i s provided b y the Late Archaea n t o the Late Proterozoi c Carajas-Cinzent o strike-sli p faul t systems i n th e Amazonia n Crato n o f Brazi l (Fig. 6b; Pinheir o & Holdswort h 1997 a & b ; Hold sworth & Pinheir o 2000) . Lat e Archaea n brittl e dextral movement s alon g E- W trending, sub vertical faul t zone s reactivate d pre-existin g base ment fabric s i n th e underlyin g Itacaiuna s shea r zone, down-faultin g cover sequence s o f lo w grad e and unmetamorphosed rocks into a series of releasing bend s an d offset s (Fig . 6b). Later faul t reacti vation and partial inversio n o f the cover sequence s

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F. STORTI , R . E . HOLDSWORT H & F. SALVIN I

Fig. 6 . (a ) Ma p view s o f stepove r an d ben d geometrie s found alon g strike-sli p fault s (afte r Woodcoc k & Schubert 1994) . (b) Simplified map of the structural architecture along th e Carajas-Cinzent o strike-sli p faul t systems , i n the Amazo n Crato n o f Brazi l (afte r Holdswort h & Pinheiro 2000) . Shadin g highlight s th e locatio n o f th e Archean cover rocks and the Itacaiunas shear zone in the older, underlyin g basement rocks . Lat e Archaea n fault s include the Carajas Fault Zone (CFZ), the Carajas strikeslip syste m (CaSSS ) an d th e Cinzent o strike-sli p system (CzSSS) .

Contractional-, extensional-, and strike-slip-relate d structures characteristicall y alternat e alon g thes e impressively lon g deformatio n belts, i n whic h th e internal architectur e is thought to be influence d b y inherited crusta l fabrics (e.g. Vauche z e t al. 1995; Rossetti e t al. 2002) . There i s a n ongoing debate concernin g the relative importanc e an d role s o f crusta l thickening , extensional collaps e an d strike-sli p faultin g i n bringing abou t lateral extrusion in the Tibetan Plateau an d Asia n regions t o th e N an d th e relation ship between these processes an d the collision and indentation o f Indi a (e.g . Tapponnie r e t al . 1982; Davy & Cobbol d 1988 ; Englan d & Molnar 1990; Shen et al. 2001). Irrespective of the relative merits of the various competing models, there is clear evidence in much of Asia that significant deformation and displacemen t hav e occurre d alon g a serie s o f very larg e intraplat e strike-sli p deformatio n belt s during th e Cenozoic . The simplifie d tectoni c sketc h map o f Asia published b y Jolive t e t a l (1999 ) (Fig . 7) illustrate s the tectoni c architectur e o f majo r intraplat e deformation belts , thei r impressiv e lengt h an d complexity. I n thi s interpretation , th e Pamir-Baikal Okhotsk shea r zon e comprise s interlinke d exten sional rifts , suc h a s th e Baika l basin , an d strike slip faul t segments . Th e deformatio n bel t appear s to exten d fro m th e collisio n zon e t o th e Berin g Strait, separating the stable Eurasian block from the

occurred during a subsequent sinistral shearing episode. This le d to the formatio n of complex assem blages o f folds, thrusts, oblique slip and strike-sli p faults whic h wer e preferentiall y develope d i n th e cover rock s clos e t o th e pre-existin g faul t trace s in bend s an d offset s tha t ha d becom e restrainin g features du e t o th e reversa l i n th e sens e o f shea r (Pinheiro & Holdswort h 1997a ; Holdswort h & Pinheiro 2000) . Th e adjacen t basemen t gneisse s remained comparativel y undeforme d durin g thes e later episodes , undergoing regiona l uplif t an d exhumation that stripped awa y the cover sequences everywhere except where they were initially down= faulted i n bend s an d offset s durin g dextral move ments.

Intraplate strike-sli p belt s an d plate convergence Intraplate strike-sli p belt s hav e bee n extensivel y studied i n convergen t setting s (e.g . Vauche z e t al . 1998). Thes e belt s o f localise d intracontinenta l deformation ar e typically severa l tens o f km wid e and man y hundred s o f k m lon g (e.g . Molnar & Fig. 7 . Highl y simplifie d tectoni c sketc h ma p o f Asi a Tapponnier 1975 ; Pil i e t al. 1997 ; Ludma n 1998) . based o n the interpretatio n of Jolive t e t al . (1999) .

INTRODUCTION

deformed part of the Asian plate (Davy & Cobbold 1988). Thus, th e Pamir-Baikal-Okhotsk shea r zon e represents a possibl e exampl e o f a transfe r intra plate strike-sli p deformatio n belt, sinc e it connects the northwes t corne r o f th e India n indenter , th e western Himalaya n syntaxis , t o th e boundar y region o f th e Pacifi c Plate . Anothe r exampl e o f a transfer intraplat e strike-sli p deformation belt ma y be provided by the roughly N-S envelop e o f rightlateral strike-sli p faul t system s and extensional basins (bot h pull-apar t an d back-arc ) tha t develope d along th e easter n borde r o f Asi a (Fig . 7) . Thi s right-lateral intraplat e deformatio n bel t connect s the northeast corner of the Tibetan Plateau with the Pacific plate boundary region where it abuts a complex arra y o f left-latera l strike-sli p faul t system s (e.g. Jolive t e t al. 1999) . Examples of confined intraplat e strike-slip fault s include th e Re d Rive r Faul t an d th e Alty n Tag h Fault (Molna r & Tapponnie r 1975 ; Lelou p e t al . 2001) (Fig . 7) . The Red River Fault is a left-lateral intraplate strike-sli p deformatio n bel t whic h bounds th e Indonesia n bloc k t o th e north an d ter minates i n th e extensiona l domai n o f th e Sout h China Se a (e.g . Morle y 2002) . Despit e it s interna l complexity, th e Re d Rive r Faul t ca n b e broadl y described a s havin g a n extensiona l termination . The Alty n Tag h Faul t i s a EN E t o E- W strikin g left-lateral strike-sli p deformatio n bel t boundin g the Tibetan Platea u t o the north. At the point where the faul t trajector y start s bendin g clockwise , i t shows a compressiona l componen t (Fig . 7) . Th e Altyn Tagh Fault terminates agains t the NNE-SSW thrust syste m tha t bound s th e Tibeta n Platea u t o the E an d ca n thu s be describe d a s havin g a con tractional termination .

Intraplate strike-sli p belts and plate divergence The occurrenc e o f strike-sli p belt s tha t ar e signifi cantly active in intraplate regions past or present is uncommon i n divergen t plat e boundarie s tha t ar e more generall y dominate d b y se a floo r spreadin g and passiv e margi n development . Substantia l strike-slip movement s d o no t occu r alon g oceani c fracture zone s onc e the y pas s outboar d o f thei r associated ridg e segment s an d away from th e plat e boundary (Fig . 8a) . A good example of intraplate strike-sli p faultin g in a divergent setting comes from the Cenozoic tectonic evolutio n a t the eastern edge o f the Antarcti c Plate, whic h include s th e Souther n Ocea n eas t o f 139°E, nort h Victoria Land, and the Ross Sea (Fig. 9). Interpretatio n o f seismi c reflectio n profile s i n the Ros s Se a an d correlatio n o f th e offshor e tec tonic fabri c wit h th e onshor e majo r structura l lin eaments allow s reconstruction o f a tectoni c archi -

9

Fig. 8 . Conceptua l cartoon showing the possible relation ships betwee n transfor m faultin g an d spreadin g rate s a t mid oceani c ridges , (a ) "conventional " geodynami c framework wit h constan t spreadin g rat e an d transfor m faulting confine d betwee n ridg e segments . Out-of-ridg e transform segment s ar e inactive (fracture zones) , (b ) Differential spreadin g rates at the plate boundary cause plate segmentation by active intraplate strike-sli p faul t system s that includ e bot h "classical " transfor m fault s an d thei r associated fractur e zone s alon g strike .

tecture dominate d b y NW-SE-striking right-latera l strike-slip faul t system s i n nort h Victori a Land , which t o transfe r thei r horizonta l displacemen t i n to the N-S trending basins of the Ross Sea (Salvini et al. 1997) . The continuity o f the NW-SE strikin g right-lateral strike-sli p deformatio n belt s con necting the Ross Sea into the impressive, co-linea r fracture zone s o f th e Souther n Ocea n i s demon strated b y th e developmen t o f prominen t recen t positive flowe r structure s i n reflection seismi c pro files recorded acros s the seismicall y activ e Balleny Fracture Zon e adjacen t t o th e continenta l shel f (Spezie e t al . 1993) . Thi s evidenc e suggest s tha t major fracture zones in the Southern Ocean, east of 139°E, ar e tectonically activ e an d that right-latera l partitioned transtensio n i n th e wester n Ros s Se a (Wilson 1995 ; Rossetti e t al. 2000) accommodate s transform shea r i n th e Souther n Ocea n (Salvin i e t al. 1997) . Suc h a n excess shear appea r to be transmitted fro m th e oceani c ridge s t o th e Ros s Se a through a networ k o f long , intraplat e strike-sli p deformation belt s cuttin g acros s bot h oceani c an d continental lithospher e (Fig . 9) . Simila r processe s might als o explai n wh y som e o f th e larges t intra -

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F. STORTI , R. E . HOLDSWORTH & F. SALVINI

Fig. 9. Cenozoi c geodynamic framework a t the northeastern edge of the Antarctic Plate showing the intraplate termination of transform shear by transtensional faulting the western Ross Sea. (after Salvin i et al. 1997). Earthquake location is fro m th e Harvar d CM T catalog.

plate shock s in the continents ar e located alon g preexisting fault s locate d inlan d fro m th e en d o f oce anic transfor m faul t fractur e zone s (e.g . se e Sykes 1978) . One possibl e explanatio n fo r th e occurrenc e o f intraplate strike-sli p deformatio n belt s i n passiv e margin setting s ma y relat e t o change s i n th e spreading rat e alon g mid-oceani c ridges . Plat e tec -

tonic theor y generall y assume s rigidit y s o that th e rate o f spreadin g is constant and is proportional t o the distanc e fro m th e Euleria n pole . I f th e rigidit y constraint i s relaxe d (Gordo n 1998) , however , intraplate strike-sli p movement s alon g transfor m fracture zone s an d a t thei r termination s ar e poss ible. I n particular, differences in the spreadin g rate at th e mid-oceani c ridg e i n adjacen t transfor m

INTRODUCTION fault-bounded compartment s coul d lea d t o strike slip shea r alon g th e intraplat e fractur e zone s (Fig . 8b). Th e sens e o f shea r i n th e intraplat e segment s is towards the ridge in the low-spreading plate sec tors an d awa y from th e ridg e i n th e fas t spreadin g sectors. Th e exces s shea r alon g th e intraplat e strike-slip belt s ca n terminat e i n th e oceani c plat e interior or in the continental passive margin following on e o r mor e o f th e terminatio n mechanism s described earlier .

Conclusions Intraplate strike-sli p deformatio n belt s for m som e of th e mos t prominen t tectoni c an d topographi c features o n bot h th e Eart h and , possibly , othe r planets (e.g . Grumpie r e t al. 1986) . A majorit y o f these structure s appear t o originate i n plate boundary deformatio n zones an d in the continent s where the lithospher e i s no t subducted , the y becom e incorporated int o the plate interior by the processes of collisio n an d accretion . Onc e establishe d the y actively transfe r displacements fro m plat e margin s into the interior regions, fundamentally influencing the location an d evolution of a broad range of geological features , including sedimentary basins, orogenic belts, active sesimicity , hydrothermal activity and magmatism . I n th e continent s especially , the y form majo r persisten t zone s o f apparen t weakness whose influenc e ma y b e fel t ove r man y hundreds or eve n thousand s o f millio n years . I t therefor e seems likel y tha t intraplat e strike-sli p deformatio n belts for m on e o f th e most significan t source s o f long-term mechanical anisotrop y in the lithosphere. Financial suppor t for this work wa s provided by the Ital ian Programm a Nazional e d i Ricerch e i n Antartid e (PNRA; grant s t o F . Salvini) . Nige l Woodcock , Mar k Allen an d Jonatha n Turne r ar e thanke d fo r detaile d an d thoughtful reviews .

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Wrench fault s dow n to the asthenosphere : geological an d geophysical evidenc e an d thermomechanical effect s A. VAUCHE Z & A. TOMMAS I Laboratoire de Tectonophysique, Universite de Montpellier II et CNRS, PL Eugene Bataillon, F-34095 Montpellier cedex 5, France (e-mail: [email protected]) Abstract: W e revie w a se t of geologica l an d geophysica l observation s tha t strongl y suppor t a coherent deformation o f the entir e lithosphere in major intracontinenta l wrench faults. Tectoni c studies o f wrench fault s erode d dow n t o th e middl e to lowe r crust sho w that , eve n i n case s in which th e lowe r t o middl e crus t i s partiall y melted , strai n remain s localize d (althoug h les s efficiently) i n transcurren t shea r zones . Seismi c profilin g a s wel l a s seismi c tomograph y an d magnetotelluric soundings provide strong argument in favour o f major wrenc h faults crosscuttin g the Moh o an d deformin g th e uppe r mantle . P n velocit y anisotropy , shear-wav e splittin g an d electric conductivit y anisotrop y measurement s ove r majo r wrenc h fault s an d i n transpressiona l domains suppor t that a wrench fault fabri c exist s over most or even the entire lithosphere thickness. Thes e seismi c an d electrica l anisotropie s ar e generate d b y a crystallographi c preferre d orientation o f olivine an d pyroxenes develope d i n the mantl e durin g th e faul t activity , which is frozen i n the lithospheric mantle when th e deformation stops . The preservation of such a 'wrench fault type ' fabri c withi n the upper mantle may have major effect s o n the subsequent tectonothermal behaviour of continents, because olivine is mechanically an d thermally anisotropic . Indeed, the associatio n o f numerical model s an d laboratory dat a on textured mantl e rocks strongl y sug gests tha t th e orogeni c continenta l lithospher e i s a n anisotropi c mediu m wit h regard s t o it s stiffness an d t o hea t diffusion . Thi s anisotrop y ma y explai n th e frequen t reactivation , a t th e continents scale, of ancient lithospheric-scale wrench fault s an d transpressional belts during sub sequent tectoni c events .

Introduction Assumin

g tha t major , i.e . continental-scale , strike-slip fault s observed toda y at the surface conHorizontal displacements in transcurrent faults rep- tinu e dow n to th e bas e o f th e lithospher e implie s resent on e of the fundamental mode s of accommo- a stron g mechanica l couplin g between th e various dation o f deformation i n the crust. It is quite obvi- rheologica l layer s of the lithosphere. Thi s raises the ous tha t transcurren t fault s generate d a t transform questio n o f th e mechanica l propertie s o f th e ho t plate boundaries, like the San Andreas Fault in Cal- middl e t o lowe r crust . Strai n localizatio n shoul d ifornia o r the Alpine Fault in New Zealand, cross - remai n efficien t enoug h t o allo w th e developmen t cut th e entire lithosphere . I t is, however, less clea r o f strike-slip faults zone s at this level. I n addition, whether intracontinenta l strike-sli p faul t system s rheologica l contrast s between th e lowe r crus t and generated i n activ e margin s o r i n collisiona l th e uppe r mantl e shoul d remai n moderate ; other domains are only crustal structures or are rooted in wis e the lower crus t would behave a s a horizontal the uppe r mantle . Th e penetratio n o f a 'wrenc h decouplin g leve l i n whic h uppe r crusta l wrenc h fault type' tectonic fabric (i.e. a vertical flow plane fault s woul d root. Thes e issue s hav e been alread y associated wit h a horizonta l flo w direction ) dee p addresse d in a large number of studies on the rheolinto the upper mantle ma y have major geodynami c ogica l stratificatio n o f th e continenta l lithospher e implications, sinc e it would generate an anisotropy (e.g . Ranall i & Murphy 1987 ; Molna r 1988 ; Vauof th e mechanica l an d therma l propertie s o f th e che z et al. 1998; Meissner et al. 2002), but experilithospheric mantl e and , hence , modif y th e large - menta l dat a o n th e rheolog y o f lowe r crusta l scale rheologica l behaviou r o f continenta l plate s material s ar e s o limite d tha t thes e studie s ar e no t during subsequen t tectonic event s (Tommasi e t al. conclusive . 2001; Tommasi & Vauchez 2001). I n thi s paper , i n orde r t o evaluat e ho w dee p a From: STORTI , F. , HOLDSWORTH , R . E . & SALVINI , F . (eds ) Intraplate Strike-Slip Deformation Belts. Geologica l Society, London , Specia l Publications , 210 , 15-34 , 0305-8719/037 $ 15 © Th e Geologica l Societ y o f Londo n 2003 .

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coherent 'transcurren t fabric ' ma y penetrate , w e analyse direc t observation s fro m surfac e geology , which ar e o f cours e restricte d t o th e crust , an d indirect informatio n fro m geophysic s an d geo chemistry tha t give s a hin t o n th e crust/mantl e coupling. W e conside r evidenc e fro m activ e an d fossil tectoni c domain s an d discus s observation s from bot h individua l shea r zone s an d broa d trans pressive domains . The review of this broad dataset suggests tha t majo r wrenc h fault s d o crosscu t th e entire lithosphere . Thi s lead s u s t o discus s th e effect o f thes e lithospheric-scal e wrenc h fault s o n the thermo-mechanica l evolutio n o f continenta l plates.

Transcurrent shear zone s and strain localization i n a hot middle to lower crust If majo r transcurren t fault s wer e roote d int o th e crust, th e wrenc h deformatio n i n th e uppe r crus t must be decouple d fro m th e mantle flow. Decoupling betwee n crusta l an d mantl e deformation s i s supposed t o b e favoure d i n th e middl e t o lowe r crust (especiall y i n region s displayin g hig h geo thermal gradients ) b y th e lo w stiffnes s o f crusta l material a t hig h homologou s temperatur e (T/Tm , with T m = meltin g temperature) . I t woul d b e marked b y rooting o f the strike-sli p fault s into this low-stiffness layer , and therefore by a listric shape

of the fault i n order t o accommodate th e transitio n from a vertical to a horizontal flow plane. In this section, w e examine a set of continental scale transcurren t faults erode d t o increasingl y deeper level s fro m th e middl e t o th e lowe r crust . In al l these cases , durin g transcurrent deformation, the crustal level s exposed toda y wer e submitte d to high temperature s an d even partia l melting . Thes e levels represen t forme r low-viscosit y layer s int o which crustal-scal e strike-sli p fault s migh t hav e rooted. In northeastern Brazil , the Neoproterozoic province o f Borborema display s a complex networ k of wrench faults (Fig . 1 ) that are several hundred kilometres long and up to 30 km wide (e.g. Vauchez et aL 1995) . Satellit e images highlight a clear textura l contrast betwee n th e shea r zone s an d th e country rock. Thi s contras t i s mostl y du e t o th e transition from a predominan t low-angl e metamorphi c foli ation outsid e the shea r zone s t o a steepl y dippin g mylonitic foliatio n withi n th e shea r zones . A t th e satellite imag e scale , th e boundarie s o f th e faul t zones appea r usuall y rather sharp , althoug h in th e field a continuous transition from th e external flatlying foliatio n t o th e interna l steepl y dippin g foli ation (half 'flower-structure') i s observed where no subsequent reactivatio n conceale d th e origina l relationships. Mylonite s outcroppin g i n th e shea r zones were formed at depths of 16-18 km (P = 500

Fig. 1 . Th e high-temperatur e Borborem a shea r zon e syste m of northeastern Brazil (Vauche z et al. 1995) . (a ) Sketc h map showing the complex patter n of transcurrent faults formed during the Neoproterozoic orogeny : (1 ) Neoproterozoic granitoids, (2 ) Mid - an d Lat e Proterozoi c sedimentar y basins , (3 ) Mesozoi c sedimentar y basins , (4 ) Neoproterozoi c high-temperature shear zones, and (5) Neoproterozoic low-temperatur e shear zones, (b) and (c) are two Landsat images showing segment s o f tw o majo r high-temperatur e wrenc h faults : th e Pato s an d th e Wes t Pernambuc o shea r zones , respectively. Gre y line s i n (b ) mar k th e shea r zon e limits .

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emplaced a s syn - o r late-kinemati c dyke s (Fig . 4 ) and/or elongate d pluton s withi n th e shea r zone s (Vauchez e t al 1995 ; Neve s e t al 2000) ; thi s strongly suggest s tha t the faults were connecte d t o a partiall y melted uppe r mantle .

Fig. 2 . High-temperatur e vertica l foliatio n (S ) an d hori zontal mineral stretchin g lineatio n (L ) in a mylonite fro m the Borborem a shea r zon e system . Deformatio n i n thi s felsic mylonit e occurre d a t T > 600°C . Location o n Fig. la. Scal e ba r i s 0.5 m.

MPa) an d a t hig h temperatur e (>65 0 °C). Unde r these conditions , th e protolith s o f th e mylonite s (metasediments, pre-kinemati c intrusives , felsi c gneisses fro m th e basement) wer e partiall y melte d and th e resultin g roc k i s indee d a migmatiti c mylonite. A t thes e temperatur e conditions , felsi c rocks ar e expected t o displa y lo w viscosity, which will be further decrease d by partial melting. Nevertheless, eve n whe n th e degre e o f melting i s rathe r high, the foliatio n i n the shea r zone s remains con sistently steepl y dippin g an d bear s a shallow dipping stretchin g lineatio n (Fig . 2) . Shear-sens e indicators develope d in the partially melted mylonites consistently suppor t dextral wrenching (Fig. 3) . Evidence o f downwar d decreas e o f th e foliatio n dip, suggesting rooting o f the faults, has never been reported. O n th e contrary , a larg e volum e o f mantle-derived magmas , especiall y diorites , wa s

Fig. 3 . Migmatiti c mylonit e fro m th e Wes t Pernambuc o shear zon e (se e locatio n o n Fig . la) . Downwar d view . Intense shearin g occurred alon g a subvertical foliation in a partially melte d crust . White layers ar e leucocratic neosome. Arrow s indicate dextra l shear .

Fig. 4 . Diorit e dyke s injected in a porphyritic granodior ite emplace d i n th e Pernambuc o shea r zon e (locatio n o n Fig. la) . Dyke s wer e emplace d withi n th e transcurren t shear zon e an d deforme d before complet e solidification . No evidenc e o f solid-stat e deformatio n ha s bee n observed.

The Neoproterozoic Mozambiqu e belt i n Madagascar an d East Afric a is also characterize d by the development o f a larg e networ k o f wrenc h fault s (Fig. 5 ) a t c . 530-50 0 M a (Martela t e t al . 2000) . The present-day level of exposure shows rocks that were 20 to 30 km deep during the deformation [0.5 to 1. 1 GPa; Martelat e t al. 2000; Pili et al. 1997a] . At these depths, deformation took place at temperatures >750°C . Th e majo r shea r zone s i n thi s domain ar e typicall y severa l hundre d kilometre s long an d up to 40 km wide . Numerous minor ductile wrenc h fault s forme d unde r simila r P- T con ditions ar e also documented. The tectonic fabri c in the shea r zone s i s typica l o f ductil e strike-sli p faults: th e foliation is steeply dipping, the mineral stretching lineation i s subhorizonta l an d consistent shear-sense criteria ar e observed. Outside the shear zones, the granulites that form the country rock display a low-angl e foliatio n an d th e fabri c i s meta morphic-migmatitic rather than mylonitic. According t o Martela t e t al . (2000) , th e deformatio n regime in the southern Mozambique belt was transpressional an d th e deformatio n wa s partitioned ; transcurrent shearin g wa s localized within th e ver tical shear zones and large-scale folding accommodated transvers e shortening . Throug h a regional scale investigation of the C- and O-isotope compositions of carbonates from marbles and metabasites, Pili e t al . (1991b) hav e show n tha t CO 2 i n th e major wrenc h fault s o f th e networ k ha s a mantl e origin. Thi s suggest s tha t thes e majo r fault s wer e connected t o th e mantle . O n th e othe r hand , i n

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Fig. 5 . Th e Neoproterozoi c wrenc h faul t syste m o f Madagascar , (a ) Sketc h ma p an d simplifie d cros s sectio n D-D ' (Martelat et al. 2000): (1 ) post-Cambrian sediments, (2) granulites, (3) anorthosites, (4) granitoids, (5 ) and (6) foliation trends outsid e an d withi n shea r zones , respectively 3 an d (7 ) majo r brittl e faults , (fe ) gf| 4 (g ) LSfidBa l IrM f §S 6nSWin £ two majo r shear zonee: the Ampanihy and BOFagot a gHga f g§fi§g , f§Bg§§iiY§ly . Trie WRif e 'inclusiona ' ii i ffio Arnpamhy shear zone (b ) ar e anorthosite massifs aroun d wnic n th e steepl y dippin g myloniti c foliatio n i s deflected .

minor shea r zone s an d i n metamorphi c rock s out side the shear zones, CO 2 has a crustal isotopic sig nature. In the same region, Pili et al. (1991 a) documented a systemati c associatio n o f a short wavelength positiv e gravit y anomal y t o majo r strike-slip shea r zone s tha t als o support s a dee p rooting of th e majo r wrench fault s of the Mozam bique belt . Thi s anomal y wa s interprete d a s du e to a shallowe r crust-mantl e boundar y beneat h th e

faults. Suc h a n upwar d deflectio n o f th e Moh o might result from thinnin g of the crus t in response to th e intens e stretchin g associate d wit h simpl e shear i n th e faul t zone s (Pil i e t al . 1997 a). In northeastern Brazil, as well as in the Madagascar Neoproterozoi c belts , strai n localizatio n i n transcurrent shear zones is observed eve n at crustal levels wher e synkinematic temperature s were high enough to induce partial melting . The width of the


fault zone s i s extremel y larg e (severa l ten s o f kilometres) compare d t o typica l width s o f shea r zones develope d unde r lowe r temperatur e con ditions (centimetres to hundred metres). This points out that , a t these high temperatures , strai n localiz ation wa s les s efficien t an d strai n wa s distribute d over a large r volum e o f rock s tha n i s usuall y observed i n uppe r crusta l shea r zones . Rock s within th e shea r zone s displa y a high-temperature mylonitic fabri c largel y du e t o dislocatio n cree p assisted b y ver y effectiv e diffusiona l processe s (i n particular grai n boundar y migration) , an d consist ent shear criteria. In addition, petrological and geochemical observation s strongl y sugges t tha t fluids percolated fro m th e mantl e int o th e crus t alon g these majo r shea r zones , an d therefor e tha t th e faults wer e continuou s through the uppe r mantle. 4

Moho' fault s versu s lithospheric fault s

The observations presented above strongl y suppor t that majo r transcurren t fault s d o no t roo t i n som e intracrustal decouplin g level , bu t rathe r crosscu t the entire crust an d are, in some way, connected t o the upper mantle. These observations are, however, not sufficien t t o evaluat e whethe r thos e fault s ar e rooted a t th e crust-mantl e interfac e o r penetrat e deeply int o th e uppe r mantle . Clea r evidenc e sup porting that major wrench faults crosscut the Moho and penetrat e deepl y int o th e uppe r mantl e i s nevertheless obtaine d b y combinin g variou s tech niques o f geophysica l exploratio n o f th e litho sphere. Evidence may be subdivided in two groups. Seismic profiling , magnetotelluri c soundings , an d seismic tomograph y hav e image d 'Moh o faults ' (Diaconescu e t al. 1997) , i.e. discontinuities crosscutting th e Moh o beneat h severa l wrenc h fault s observed a t the surface. On the other hand, electric conductivity anisotrop y evidence d i n magnetotel luric soundings , azimutha l anisotrop y o f P n velo cities, an d S-wave s splittin g ar e directly related t o the tectonic fabric o f the upper mantle an d suppor t that the lithospheric mantl e was deformed in majo r wrench faults . Electric conductivit y anisotrop y i n th e uppe r mantle i s interprete d a s du e t o a preferre d orien tation o f graphit e film s elongate d alon g th e foli ation (Marescha l e t al . 1995 ) o r t o a n anisotropi c electrical conductivit y in a 'wet ' mantl e due to the anisotropy o f H + diffusio n i n th e olivin e crysta l (Mackwell & Kohlsted t 1990 ; Simpso n 2001) . I n both cases, a 'wrenc h fault type ' fabri c (i.e . a stee ply dippin g flo w plane , o r foliation , containin g a subhorizontal flo w direction , o r lineation ) withi n the mantl e woul d generat e a highe r conductivity parallel t o th e trac e o f th e wrenc h faul t observe d at th e surface . Seismic anisotrop y i n th e uppe r mantle , whic h

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may b e characterize d b y measuremen t o f a n azi muthal anisotrop y o f Pn velocities or by the split ting of teleseismic S-waves, results from th e lattice preferred orientatio n (LPO ) o f rock-formin g min erals durin g high-temperatur e deformatio n b y dis location creep . Wrenc h faultin g withi n th e lithospheric mantl e woul d generate a LP O o f oli vine, th e dominan t minera l phas e i n mantl e peri dotites, characterize d b y a concentratio n o f [100 ] axes clos e to the lineatio n (i.e . subhorizontal ) an d of [010 ] axe s normal to the foliation plane [Fig . 6; Tommasi e t al 1999] . Olivin e i s elastically aniso tropic. Thu s i f deformatio n produce s coheren t oli vine LPOs a t the scal e o f tens of kilometres i n th e upper mantle , it als o result s i n anisotropi c seismi c properties (Nicola s & Christense n 1987 ; Main price & Silve r 1993 ; Silve r e t al . 1999) . P-wave s that propagat e eithe r paralle l t o th e maximu m of [100] o r [010 ] axe s o f olivin e i n th e mantl e ar e respectively th e fastes t an d th e slowest . O n th e other hand , S-wave s propagatin g throug h a deformed uppe r mantl e spli t int o tw o quasi-Swaves polarize d i n orthogona l planes ; th e fastes t one i s polarize d i n a plan e containin g bot h th e maximum concentratio n o f olivin e [100 ] axi s an d the propagation direction . The delay tim e betwee n the arrival s o f th e tw o spli t wave s i s proportiona l to both the length of wave propagation path within the deforme d laye r an d th e propagatio n directio n relative to the structural fabric; the largest S-waves splitting i s observe d fo r wave s tha t propagat e a t low angle s t o th e maximu m o f [001 ] axis . A wrench faul t fabri c i n th e mantl e woul d therefor e be evidence d (Fig . 6 ) b y a fas t propagatio n o f P waves (i n particular , horizontall y propagatin g Pn waves) paralle l t o th e faul t an d a polarizatio n o f the fast spli t S-wave in a plane containin g bot h the direction o f propagatio n o f th e wav e an d th e lin eation, i.e . paralle l t o the faul t directio n fo r waves having a n almos t vertica l incidenc e (suc h as SKS, SKKS, PKS...) . I t i s als o i n thi s cas e tha t th e birefringence wil l b e th e largest , leadin g t o rela tively larg e tim e lag s betwee n th e arrival s o f th e fast an d slo w spli t S-waves . Indeed, SK S splittin g data abov e transfor m boundaries , suc h a s the Car ibbean or the Alpine fault in New Zealand, systematically displa y fas t shea r wave s polarized paralle l to th e transfor m directio n an d dela y time s signifi cantly large r tha n 1 s, whic h imply tha t th e entir e lithosphere deformed i n a strike-slip regime (Russo et al . 1996 ; Klosk o e t al. 1999) . These techniques 'probe ' the upper mantle fabric with differen t spatia l resolution s an d dept h sensi tivities. Magnetotelluri c (MT ) sounding s usin g a large spectru m o f measuremen t frequencie s allo w an evaluatio n o f th e electrica l conductivit y ani sotropy fro m th e crus t t o th e asthenospheri c mantle. However , M T dat a depen d o n bot h ani -

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Fig. 6. Cartoo n illustratin g th e concept o f lithospheric fault, i n which crustal fault zone s broaden downwar d and tend to coalesc e formin g a broad shea r zon e that cuts acros s th e entir e lithospheri c mantle . It display s the tectoni c fabri c associated wit h th e faul t withi n the crus t an d th e mantle , the crystallographi c fabri c of olivin e expected t o develo p in th e mantl e sectio n o f suc h a faul t zon e (oriente d i n th e structura l framework of th e fault : X = lineatio n an d Z = normal t o th e foliation) , an d th e splittin g o f a polarized incomin g shea r wav e tha t propagates acros s a lithospheri c mantle displayin g a 'wrenc h faul t type ' fabric . A seismi c statio n locate d abov e suc h a lithospheri c shea r zon e will record a fas t shea r wav e polarized paralle l t o th e shea r zon e tren d ( X direction) an d stron g delay time s (> 1 s) .

sotropy an d heterogeneit y o f electrica l conduc tivity, an d reliabl e anisotrop y determination s ma y only b e obtaine d whe n high-quality , long-perio d MT transfer functions ar e available and lateral conductivity gradient s ar e smal l (Simpso n 2001) . Pn waves sampl e th e uppermos t mantl e (3- 5 k m beneath th e Moho) , bu t th e measure d velocitie s depend o n both th e anisotrop y an d th e heterogen eity (i n temperatur e an d composition ) alon g th e wave path. Teleseismic S-wave s splitting provide s reliable evidence o f seismic anisotropy wit h a very good spatia l resolutio n (c. 5 0 km) , bu t thes e measurements integrat e al l anisotropi c contri butions along the wave path (which is roughly vertical from th e core-mantle boundary to the surface for th e mos t commonl y use d SKS-waves) . Th e association o f thes e technique s shoul d therefor e allow u s to better constrai n the structura l fabric of the uppe r mantle . Indeed , compariso n o f electri c conductivity anisotropy determined b y magnetotelluric sounding s an d S-wave s splittin g measure ments show s tha t th e directio n o f larges t conduc -

tivity an d th e fas t spli t S-wav e polarizatio n plan e are often almos t parallel (Wannamake r et al. 1996; Barruol e t al . 1997£ ; Simpso n 2001 ) o r mak e a slight, but consistent angle (Mareschal et al. 1995) . Ji e t al , (1996 ) interprete d thi s sligh t obliquit y a s representing the obliquity between the foliation and the shea r plan e in shea r zones . To investigat e how dee p a 'wrenc h faul t fabric ' may penetrat e int o th e uppe r mantle , w e analys e geophysical dat a fo r severa l ancien t o r activ e wrench faults an d transpressiona l belts . I n eac h case, transcurren t displacement , eithe r i n a singl e fault o r i n a broade r domai n o f transpressiona l deformation, i s supporte d by surfac e geology.

Transcurrent shear zones Recently, Pollit z e t al. (2000 , 2001) , usin g a combination o f GP S an d syntheti c apertur e rada r (InSAR) data , have show n that the deformatio n in the year s followin g th e 1992-Lander s an d 1999 Hector Min e majo r earthquake s i n th e Mojav e

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Fig. 7 . (a ) Structura l sketc h displayin g the activ e fault s i n California , (b ) Shear-wav e splittin g i n wester n Californi a from Harto g & Schwartz (2001) . Anisotropy beneat h th e westernmost stations , i.e . thos e abov e th e San Andreas faul t system, results from th e superposition of two anisotropic layers. The upper layer, which corresponds t o the lithospheric mantle, i s characterize d b y a polarization o f th e fas t shea r wav e (blac k bars ) i n a plane paralle l t o th e Sa n Andrea s fault syste m an d a delay tim e clos e t o o r even highe r thanls . Th e easternmos t station s displa y a simple r anisotrop y pattern (gre y bars ) tha t ma y b e accounte d fo r b y a singl e anisotropi c laye r wit h a roughl y E- W flo w direction . A similar flow direction i s inferred for the lower anisotropic laye r (grey bars) in the westernmost California, (c ) Horizonta l velocity fiel d showin g th e contemporar y interseismi c deformatio n acros s souther n Californi a (relativ e t o a grou p o f GPS and VLBI station s on the stable North American Plate) . Geodeti c dat a include Global Positionin g Syste m (GPS), Very Long Baseline Interferometry (VLBI) , and Electro-optical Distance Measurement (EDM ) obtained by the Crusta l Deformation Working Group of the Southern California Earthquake Center during the past three decades. Error ellipse s are region s o f 95 % confidence . Release 2 , 1998 , availabl e a t http://www.scecdc.scec.org:3128/group_e/release.v2 .

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A. VAUCHE Z & A. TOMMASI

desert (California , USA ) wa s abou t thre e time s greater tha n before th e earthquakes. Thi s interseismic velocity fiel d support s a right-lateral displace ment paralle l t o th e Sa n Andrea s transfor m faul t system (Fig . 7) . Accordin g t o thes e authors , th e visco-elastic relaxatio n o f th e lowe r crus t an d upper mantl e wa s th e dominan t post-seismic pro cess; thi s require s tha t th e lowe r crus t acte d a s a coherent stres s guide coupling the upper crust with the uppe r mantl e (Pollit z e t al 2001) . Thes e con clusions ar e consisten t wit h thos e draw n fro m th e analysis o f the seismi c anisotrop y measured acros s the Sa n Andreas faul t syste m slightly nort h o f th e Landers an d Hecto r Min e earthquake s are a (Silver & Savage 1994 ; Ozalaybey & Savage 1995 ; Hartog & Schwart z 2001) . Th e shea r wave s split ting parameter s retrieve d fro m a larg e numbe r of records consistentl y sugges t tw o layer s o f ani sotropy within the upper mantle (Fig. 7). The upper layer, whic h correspond s t o th e lithospheri c mantle, is characterized by a polarization o f the fas t split shea r wav e parallel t o th e Sa n Andrea s faul t system. Thi s suggest s tha t th e lithospheri c mantl e has a tectonic fabric consistent with the crustal fabric, i.e . a steepl y dippin g foliatio n bearin g a sub horizontal lineation. Both geodetic and seismologi c observations therefor e converg e toward s a coheren t deformation o f th e entir e lithosphere . The Himalayan orogen provides some of the best examples o f activ e wrenc h fault s i n a n intraconti nental setting . Thes e fault s hav e accommodate d large latera l displacement s associate d wit h th e India-Asia collisio n (e.g . Tapponnie r e t al. 1986) . The mai n fault s o f th e syste m hav e bee n mappe d over hundreds o f kilometre s an d ar e commonl y several kilometre s wide . Th e Re d Rive r fault , fo r instance, wa s recognized ove r 100 0 k m from Tibe t to th e Gul f o f Tonkin . Pha m e t al . (1995 ) hav e performed a 70-km-lon g magnetotelluri c profil e across the Red River fault syste m in North Vietnam (Yen Ba i region) . In thi s area , th e Red Rive r sys tem i s formed by thre e paralle l transcurren t faults , a fe w ten s o f kilometre s apar t (Tapponnie r e t al . 1990). Thi s M T surve y (Fig . 8 ) show s that : (1 ) each faul t i s characterize d b y a hig h conductivit y zone dow n t o th e uppermos t mantle , (2 ) th e Sm g Hing fault, th e mai n branch o f the Red Rive r faul t system, separate s tw o lithospheri c domain s presenting contraste d electrica l properties , an d (3 ) a large conductivity anisotrop y i s observed i n both the crus t an d th e uppermos t mantle ; th e directio n of highes t conductivit y i s consistentl y paralle l t o the strike of the faults. This anisotropy is consistent with a steepl y dippin g foliatio n withi n th e upper most mantl e a s wel l a s i n th e entir e crust. In Tibet , seismi c anisotrop y measurement s have been performe d abov e an d i n th e vicinit y o f tw o other well-know n majo r wrenc h faults , th e Alty n

Fig. 8. Magnetotelluri c sounding s fro m Pha m e t aL (1995) acros s th e Re d Rive r faul t system , (a ) M T geoelectrical sectio n obtaine d b y 2 D numerica l modellin g showing marke d resistivit y contrast s betwee n domain s separated b y th e faults . Eac h blo c i s characterize d b y it s longitudinal (i.e . parallel t o th e strik e o f th e faults ) an d transverse (i n brackets ) resistivitie s (i n ftm) . Low resistivity domain s beneat h eac h branc h o f th e faul t ar e displayed i n ligh t grey . Conductiv e zone s i n th e lowe r crust and uppermost mantle are displayed i n medium and dark grey, respectively, (b) MT sounding curves showing a pronounced variation in apparent resistivity between the transverse (norma l t o th e strik e o f th e faults ) an d longi tudinal directio n i n bot h th e crus t an d th e uppermos t mantle. Th e highes t conductivit y is paralle l t o th e strik e of th e faults , a result i n goo d agreemen t wit h a 'wrenc h fault type ' fabri c i n th e uppermos t mantle . Tagh an d th e Kunlu n faults . Thes e faults , severa l hundreds o f kilometre s lon g (180 0 k m fo r th e Altyn Tag h fault) , hav e accommodate d severa l hundred kilometre s o f latera l escap e durin g th e India-Eurasia collisio n (e.g . Tapponnie r e t al . 1986). Wittlinge r e t a l (1998 ) hav e performe d a seismic tomograph y stud y o f a n are a wher e th e Altyn Tagh fault juxtapose s Precambrian basement with th e Qaila m sedimentar y basin. This tomogra phy show s a southeastern domain characterized by low-velocity perturbations i n contrast with a northwestern domai n wher e high-velocity perturbations


dominate. Th e limi t betwee n thes e domain s i s marked b y a low-velocit y anomal y locate d jus t beneath th e Altyn Tagh fault (Fig . 9a). Fro m thes e results Wittlinger e t al. (1998) have suggeste d that the Altyn Tagh fault in the mantle i s c. 40 km wide and i s continuou s down t o a dept h o f 14 0 k m a t least. I n addition , shear-wav e splittin g measure ments abov e th e Alty n Tag h faul t (Herque l e t al . 1999) sho w fas t spli t shea r wave s polarize d i n a plane paralle l t o th e tren d o f th e faul t an d dela y times betwee n th e fas t an d slo w S-wave s arrival s of c . 1 s. Suc h dela y time s requir e a thicknes s o f anisotropic mantle of c. 10 0 km, in agreement wit h the value s o f faul t penetratio n inferre d fro m seis mic tomograph y (Fig . 9). Shear-wav e splittin g measurement abov e an d acros s th e Kunlu n faul t (McNamara et al 1994 ; Herque l e t al. 1999 ) hav e reached simila r results . Approachin g th e Kunlu n fault zon e the orientatio n o f the fas t S-wav e polarization plan e progressivel y rotate s into parallelis m

Fig. 9 . Mantl e structur e beneat h th e Alty n Tag h an d Kunlun active faults i n Tibet, (a) Cross section displaying the mai n geologica l structure s an d th e P-wav e velocit y structure acros s th e Alty n Tag h faul t syste m (Wittlinge r et al. 1996) . Light grey and dark grey colours correspond to th e crus t an d mantle , respectively . Lighte r shade s i n both layer s indicat e domains o f lower P-wav e velocities. (b) Compilatio n o f shear-wav e splittin g measurement s across th e Kunlu n an d Alty n Tag h fault s fro m Herque l et al. (1999) . Both faults ar e characterized b y a fast spli t shear wav e polarize d paralle l t o th e tren d o f th e fault , contrasting significantl y with the anisotropy patter n away from th e faults .

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with the trend of the fault, suggesting a shear strai n gradient an d a n upper mantle fabric similar t o that in the crust. The 2 s of delay time measured above the Kunlun fault requires a thickness o f anisotropic material >20 0 km , assumin g a steepl y dippin g flow plane and a subhorizontal flow direction, thus larger than the lithosphere thickness. This suggests that the asthenosphere fabric also contributes to the recorded anisotrop y an d deforms somewha t coher ently wit h the lithosphere . Similar observation s als o characteriz e ancien t wrench fault s whos e fabri c wa s froze n int o th e lithospheric mantl e a t the end of the orogenic evol ution. Th e Grea t Glen-Wall s Boundar y faul t (GGWBF) is a major wrenc h fault that belongs to a more complex fault array developed i n the northern segment o f th e Caledonia n bel t betwee n 42 8 an d 390 M a (e.g . Stewart e t al . 1999) . Tw o segment s of the initial fault ar e exposed: the Great Glen faul t in Scotlan d an d th e Wall s Boundar y faul t i n th e Shetland Islands . Palaeomagneti c reconstruction s suggest that several hundred kilometres o f sinistral strike-slip displacemen t hav e bee n accommodate d along thi s fault . Shear-wav e splittin g ha s bee n measured (Helffric h 1995 ) a t station s clos e t o th e GGWBF i n Scotlan d (Fig . 10; statio n MCD ) an d in th e Shetlan d Island s (Fig. 10; station LRW) . I n both stations , th e fas t spli t shea r wav e is polarized in a plan e paralle l t o th e trac e o f th e faul t an d a delay tim e of 0.94 an d 0.53 s is observed between

Fig. 10 . Shear-wav e splittin g i n th e norther n Unite d Kingdom fro m Helffric h (1995) . Initial s (e.g . MCD , LRW...) represen t th e nam e o f the stations . AP M i s th e Absolute Plat e Motio n i n th e hot-spo t framewor k calcu lated usin g Morgan an d Morgan's mode l (see Barruol et al. 1997a) . Thick gre y line north of the Shetlan d Islands marks th e locatio n o f th e UNS T dee p seismi c reflection profile displaye d i n Figur e 11 .

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A. VAUCHEZ & A. TOMMASI

the arrival s o f th e tw o SKS-wave s fo r MC D an d LRW, respectively . Th e fas t S-wav e polarizatio n direction clos e t o th e faul t is significantl y obliqu e to the fas t polarizatio n directio n measure d a t other stations i n th e Britis h Caledonide s (Barruo l e t al. 1997'a). Interestingly , severa l seismi c profile s performed acros s th e GGWBF , i n mainlan d Scotlan d as wel l a s i n th e Shetlan d Island s (e.g . McGear y 1989; Klempere r & Hobbs 1991 ; Klempere r e t al . 1991), show a topography and a change in the seismic expression o f the Moho tightly associate d with the trac e o f th e GGWB F a t th e surfac e (Fig . 11) . These feature s have been interprete d a s du e to th e fault crosscuttin g th e Moh o an d boundin g tw o initially remot e domains tha t sho w contrasted seis mic responses. This interpretation i s in good agree ment wit h shear-wav e splittin g measurements . Altogether thes e result s strongl y sugges t tha t th e GGWBF, rathe r tha n being roote d in som e crusta l decoupling level (McBride 1995) , is a lithospheric fault that crosscuts the Moho and penetrates deeply into th e upper mantle . The well-know n Sout h Armorica n Shea r Zon e (SASZ) i n Brittany , France , i s a majo r intraconti nental transcurrent fault forme d during the Hercynian orogeny . Surfac e geolog y evidenc e o f strai n localization an d strike-sli p displacemen t ha s bee n reported i n a large numbe r o f paper s (e.g . Berth e et al . 1979 ; Jegouz o 1980) . Th e faul t i s locate d north o f th e high-pressur e domai n tha t mark s th e trace of the suture between two collided continents. A seismi c velocit y mode l o f th e structur e o f th e lithosphere dow n t o 20 0 k m beneat h Brittan y has been obtained throug h a recent passive seismolog y experiment (Grane t e t al . 2000 ; Judenher c 2000) . P-wave velocit y perturbatio n model s sho w a marked contras t betwee n tw o domain s (Fig . 12a) : the northeastern domain is characterized b y a positive velocit y anomaly , wherea s th e southwester n domain display s negativ e anomalies . Th e limi t between thes e tw o domain s coincide s wit h th e

Fig. 11. Dee p seismic reflection profil e acros s the Shetland platfor m (McGear y 1989) . Ml , M2 , M 3 indicate Moho reflectors. D refer s t o diffractio n hyperbolae .

Fig. 12. Dee p lithospheri c structure beneat h th e Sout h and Nort h Armorica n shea r zone s (SAS Z an d NASZ , respectively) i n Brittany , wes t France , (a ) P-velocit y model fro m Judenher c e t al . (i n press ) showin g tha t th e SASZ separates a northern domain characterized by high seismic velocitie s fro m a souther n domain , wher e lo w velocities dominate . Hig h P-wav e velocitie s below th e lithosphere (below 90 km) are interpreted as representing a fossi l slab , (b ) Shear-wav e splittin g measurements . Approaching the SASZ, the fast spli t shear wave polarization turn s parallel to the trend of the fault , suggestin g a coherent tectonic fabric i n both th e crust an d the mantle . In contrast, shear-wav e splitting measurements above the NASZ do not show fast shea r waves polarized parallel to the fault trend , suggesting that this latter is a crustal fault .

trace of the SASZ and is observed down to the base of th e lithosphere. In addition, th e direction o f fas t propagation o f Pn-wave s an d th e directio n o f th e polarization plan e o f th e fas t spli t shea r wav e ar e consistently parallel t o the trend of the SASZ (Fig . 12b). Th e dela y tim e betwee n th e fas t an d slo w split shea r wave s a t station s clos e t o th e SAS Z i s consistently large r tha n 1 s , als o suggestin g tha t


the entire lithosphere display s a 'wrench fault type' fabric (Judenher c 2000) . Thes e result s ar e ver y consistent an d altogethe r sugges t tha t th e Sout h Armorican Shea r Zon e crosscut s th e entir e litho sphere. Combined M T an d seismi c anisotrop y measure ments (Fig . 13 ) hav e bee n recentl y performe d i n the vicinity o f th e Proterozoi c Grea t Slav e Lak e shear zon e (GSLSZ) , i n northwestern Canada (Wu et al. 2002). Thi s NE-SW-trending dextral wrench fault is 25 km wide and its magnetic expression can be correlate d ove r 130 0 km. Thi s stud y provide d interesting insight s o n th e lithospheri c structure s associated wit h thi s majo r wrenc h fault : (1 ) th e fault i s associate d wit h a crustal-scal e resistiv e zone whic h is coinciden t wit h a magnetic low, (2) the resistivity structur e in the lowe r crus t to uppe r mantle i s approximatel y 2 D wit h a geoelectri c strike N60°E parallel to the large-scale trend of the GSLSZ, an d (3 ) ther e i s a clos e parallelis m between the orientatio n o f the fast spli t shea r wave polarization plan e an d th e geoelectri c strik e retrieved fro m long-perio d M T measurements. This similarity o f seismi c an d electri c conductivit y anisotropies suggest s that they both have a n origin related t o the wrench fault fabri c of the lithospheri c mantle beneat h th e GSLSZ .

Transpressional orogenic domains Often, orogeni c domain s as a whole have been submitted t o a transpressiona l deformatio n charac terized b y the association of thrusting normal to the belt an d lateral escap e accommodate d b y transcur-

Fig. 13. Compariso n o f magneti c fiel d data , M T high conductivity strike s fo r the period ban d o f 20-500 s , and SKS fas t direction s fo r th e Grea t Slav e shea r zon e (W u et al . 2002) . H an d L refer t o magneti c high s an d lows , respectively. Dela y time s betwee n th e arrival s of the two split SKS-wave s ar e o f 1.1-1. 5 s.

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rent faulting parallel t o the belt. Recently, Meissner et al . (2002 ) usin g P n anisotrop y measurement s have show n tha t i n suc h domain s th e uppermos t (sub-Moho) mantl e i s characterize d b y a fas t propagation o f P-waves parallel t o the trend o f the belt, pointin g t o a flo w fabri c i n th e uppermos t mantle dominate d b y the lateral escape of lithosph eric blocks . Shear-wav e splittin g measurement s i n active and fossil orogeni c areas als o record orogen parallel flo w direction s i n th e uppe r mantl e (e.g. Vauchez & Nicola s 1991 ; Savage 1999 ; Silver e t al. 1999) . Fas t shea r wave s ar e polarize d paralle l to th e tren d o f th e transpressiona l belts , eve n i n domains wher e crusta l deformatio n i s essentiall y accommodated b y thrusting , an d dela y time s frequently attai n 1 s indicat e tha t thi s 'wrenc h faul t type' flo w fabri c affect s th e entir e lithospheri c mantle. Taiwan i s currently deformin g in response to the oblique convergenc e betwee n th e Philippine s an d the Eurasia n plates. A s a result, th e crus t display s evidence o f a transpressive deformatio n an d strai n is partitione d betwee n thrustin g and wrenc h fault ing norma l an d paralle l t o th e belt , respectivel y (e.g. Huan g e t al . 2000 ; Lalleman d e t a l 2001) . Shear-wave splittin g measurement s b y Ra u e t al . (2000) display , nevertheless , a coheren t patter n over th e entir e Taiwa n Islan d (Fig . 14). S-waves generated in the Benioff Zon e by local earthquakes that prob e th e mantl e abov e th e subductio n zon e are split . Th e fas t shea r wav e i s polarized paralle l to the tectonic grai n an d delay time s ar e up to 2 s. These observation s sugges t tha t th e uppe r mantl e beneath Taiwa n has a homogeneou s transcurrent/transpression fabri c du e t o northwar d tectonic escape , i.e. a transport directio n parallel to the activ e orogen . The Neoproterozoi c Ribeir a orogeni c bel t o f southeastern Brazil formed durin g the final amalgamation o f Gondwan a betwee n 58 0 an d 54 0 M a (Egydio-Silva e t al 2002) . Th e souther n an d central domain s o f th e bel t wer e subjecte d t o a n oblique convergenc e betwee n th e Sout h America n and Africa n protocontinent s (Fig . 15a) . Thi s resulted i n developmen t o f numerou s dextra l wrench faults , hundred s of kilometres lon g an d u p to 1 0 kilometres wide , oriented paralle l o r slightl y oblique to the belt. In the central domain , th e current leve l o f erosio n (17-2 0 km ) show s mylonite s that forme d a t high temperatur e ( T > 800°C ) an d continued t o defor m durin g a slo w coolin g dow n to c . 740°C . Southward , the erosio n leve l i s mor e superficial an d th e shea r zone s ar e marke d b y mylonites forme d unde r amphibolit e facie s meta morphic condition s (Vauche z e t a l 1994) . Th e wrench fault s reworke d a slightl y olde r low-angl e foliation du e to thrusting towards the South American protocontinent . Durin g th e lat e orogeni c

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Fig. 14. Dee p structur e beneath th e activ e Taiwa n orogen . (a ) Simplifie d ma p showin g the geodynami c situatio n of the Taiwa n oroge n (afte r Lalleman d e t al. 2001) . (b ) Shear-wav e splittin g measurement s (Ra u e t al . 2000 ) usin g S waves fro m loca l earthquake s an d teleseismi c ScS .

stages, bot h orogen-norma l thrustin g an d orogen parallel wrenc h faultin g occurred . A s a whole, th e southern-central Ribeir a bel t represent s a trans pressional orogeni c segmen t abou t 10 0 k m wide and almost 100 0 km long (Trompette 1994) . Shear wave splittin g measurement s performe d ove r th e southern branc h o f th e Ribeir a bel t (Heint z e t al . 2000) have yielded a coherent patter n characterize d by a polarization o f th e fas t S-wav e in a directio n parallel to the orogenic grain (Fig. 15b) , suggestin g that th e bul k volum e o f lithospher e i n th e trans pressional domain ha s a 'wrenc h fault type' fabric . Larger delay times between the fast an d slow shear

waves arrival s (u p t o 2. 5 s ) hav e usuall y bee n retrieved fro m dat a recorded abov e o r close t o th e main shea r zones , suggestin g tha t strai n wa s no t homogeneously accommodate d bu t wa s somewhat localized i n th e mai n shea r zones . The Pyrenee s i n Wester n Europ e (Fig . 16 ) formed durin g th e Mesozoi c du e t o displacemen t o f Iberia relativ e t o Eurasia . Thi s motion , generate d by th e openin g o f th e Atlanti c Ocea n betwee n North Americ a an d Iberia , wa s mainl y accommo dated alon g th e Nort h Pyrenea n faul t (e.g . Chou kroune 1992) . At first, the deformation regim e wa s transtensive an d severa l pull-apar t basin s formed .

WRENCH FAULT S DOW N TO TH E ASTHENOSPHER E

Fig. 15 . Lithospheri c structur e o f th e Neoproterozoi c Ribeira transpressiv e belt , (a ) Cartoo n showin g the geo dynamic situatio n o f th e Ribeira-Aracuai-Wes t Cong o orogen (ligh t grey ) a t th e en d o f Gondwan a assembl y (580-540 Ma): (1) Archean and Mid-Proterozoic cratonic domains, (2) Neoproterozoic belts , (3) main wrench faults in th e Ribeir a belt , an d (4 ) large-scal e kinematic s a t th e end of the Gondwana assembly. Shaded areas mark continental domain s stabilize d before 60 0 Ma. (b ) Cor e shea r waves splittin g measurement s i n th e central-souther n Ribeira bel t an d the souther n Brasili a bel t (Heint z e t al. 2000).

Then, durin g th e fina l stage s o f th e evolutio n i t became transpressiv e an d finall y compressive . Indeed, th e Nort h Pyrenea n fault , i.e . th e ruptur e between Iberi a an d Eurasia , reactivate d a n older , pervasive transpressiv e fabri c forme d durin g th e late stage s o f th e Hercynia n orogen y (e.g . Bou chez & Gleize s 1995 ; Vauche z & Barruo l 1996) . Shear-wave splittin g measurement s performe d across th e Pyrenee s an d adjacen t area s reveale d a

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Fig. 16 . Shear-wav e splittin g i n th e Pyrenee s an d adjac ent areas, (a) Sketch map of the main Hercynian structural directions i n th e Pyrenee s an d adjacen t regions . NP F i s for th e Nort h Pyrenea n Faul t an d SAS Z fo r th e Sout h Armorican Shear Zone (see Fig. 12) . The relative position of Iberi a relativ e t o Europ e i s th e curren t position , (b ) Shear-wave splittin g measurement s i n th e Pyrenee s (Barruol e t al . 1998) . A t eac h location , th e siz e o f th e circle i s proportional t o the delay tim e tha t is usually > 1 s an d th e lin e indicate s th e polarizatio n o f th e fas t spli t shear wave .

very consistent pattern of anisotropy (Barruo l et al. 1998). Th e fas t shea r wav e polarizatio n plan e i s usually oriente d paralle l t o th e belt, an d th e dela y between the fas t an d slow S-wav e arrivals is larger than 1 s, even beyond the Mesozoic Pyrenee s belt . Pn anisotrop y measurement s (Judenher c e t al . 1999) ar e in good agreemen t wit h S-wav e splitting measurements; the fas t propagatio n directio n o f Pn is als o parallel t o the Hercynian/Pyrenea n tectoni c fabric, suggestin g tha t th e entir e lithospher e beneath th e probe d are a ha s a coheren t 'wrenc h fault type ' fabric . The analysi s o f th e seismi c anisotrop y dat a fo r the activ e oroge n o f Taiwan , th e Neoproterozoi c Ribeira bel t an d th e Hercynian/Alpin e Pyrenea n belt lead s t o simila r conclusions . S-wave s splittin g

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results ar e consistent wit h seismic anisotropy models i n whic h th e lithospheri c mantl e deform s b y homogeneous transpression , instea d o f th e par titioned mode displayed by the crust. However, the tectonic fabri c o f th e mantl e doe s no t correspon d to th e classica l transpressio n a s define d b y San derson an d Marchin i (1984 ; i.e . wit h a vertica l stretching), but rather to lengthening-thinning shear (i.e. plan e transpression ; Tikof f & Fosse n 1999 ; Tommasi e t al. 1999) . Thi s deformatio n regim e involves simultaneou s shortenin g norma l an d stretching paralle l t o th e tren d o f th e bel t an d results i n a latera l escap e o f th e lithospheri c mantle. Thi s ma y explai n wh y observatio n o f a seismic anisotrop y coheren t wit h orogen-norma l thrusting at the scal e of the lithosphere is so scarce (e.g. Silve r 1996) .

Lithospheric wrench faults : thermo mechanical effects The variou s example s presente d abov e converg e towards a mode l o f majo r wrenc h fault s deepl y rooted int o the uppe r mantle. Seismi c tomography and shear-wav e splittin g observation s especiall y support that the fault fabri c affects th e entire lithosphere thickness . Th e widt h o f th e domai n presenting a 'wrenc h faul t type ' fabri c probabl y ranges between several tens of kilometres for a single faul t t o severa l hundred s o f kilometre s fo r a transpressional domain involving various transcurrent and thrust faults. Moreover, seismic anisotropy observations usin g long-perio d dat a suc h a s SKSwaves imply that the olivine lattice preferred orientation associated wit h this 'wrenc h faul t type ' fabric, characterized b y horizontal [100] axes and vertical (010 ) planes , bot h parallel t o th e faul t trace , is coheren t a t scale s large r tha n 50 km. On th e othe r hand , th e olivin e crysta l doe s no t only displa y a n anisotropi c elasticity , whic h leads to th e observe d seismi c anisotropy . Th e plasti c deformation an d therma l diffusivitie s o f olivin e also ar e highly anisotropic (Kobayash i 1974 ; Durham & Goetz e 1977 ; Ba i e t al . 1991 ; Cha i e t al . 1996). Thu s i f majo r wrenc h fault s ar e charac terized by a coherent olivine lattice preferred orientation tha t affect s th e entir e lithospher e ove r domains several hundreds (or thousands in the case of a transpressiona l belt ) o f kilometre s lon g an d tens (o r hundreds ) o f kilometre s wide , thes e domains migh t als o b e th e sourc e o f a large-scal e mechanical an d thermal anisotropy within the continental lithosphere tha t may influence th e thermomechanical behaviou r o f th e plat e durin g sub sequent tectonic events .

Strain-induced mechanical anisotropy of the continental lithosphere Experimental deformation of olivine single crystals under different orientations relativ e to its crystallographic lattic e show s tha t olivin e ha s onl y thre e independent sli p system s an d tha t thes e system s display significantl y differen t strengt h o r critica l resolved shea r stres s (CRSS ) value s (Durha m & Goetze 1977 ; Ba i e t al . 1991) . Unde r high temperature conditions , th e (010)[100] slip syste m displays th e lowes t critica l resolve d shea r stress ; this means that, compared t o the other possible slip systems, fo r a give n stres s i t i s abl e to accommodate th e larges t sli p rate , or , conversely , tha t i t requires the lowest built-up resolved shear stress to accommodate a give n strai n rate . I n othe r words , for deformatio n i n th e dislocatio n cree p regime , which i s expecte d t o prevai l i n th e lithospheri c mantle i n activ e areas , olivin e display s a n aniso tropic viscosity . In a lithospheri c wrenc h fault , th e weakes t (010) [100] sli p syste m i s oriente d paralle l t o th e fault, i.e . th e olivine crystals are preferentially oriented with the (010) plane sub vertical and the [100] axis horizontal, parallel t o the shea r direction. The question i s whethe r th e anisotropi c mechanica l behaviour o f th e olivin e singl e crysta l combine d with such an LPO coherent ove r large scale s i n the lithospheric mantl e may result , a t th e scal e o f th e lithospheric mantle , i n a n anisotrop y o f viscosit y large enoug h t o influenc e th e deformatio n o f th e lithosphere durin g subsequen t tectoni c solici tations. Tommasi and Vauchez (2001) used a poly crystal plasticity mode l to investigat e the effec t o f a pervasive 'wrenc h faul t type ' fabri c froze n i n th e lithospheric mantl e o n th e continenta l break-u p process. In this work, the deformation of an anisotropic continental lithosphere in response to an axisymmetric tensiona l stres s fiel d produce d b y a n upwelling mantle plume was evaluated by calculating the deformation of textured olivine polycrystals representative o f th e lithospheri c mantl e a t differ ent position s abov e a plume head (Fig . 17) . These models show that an LPO-induced mechanical anisotropy o f th e lithospheri c mantl e ma y resul t i n directional softening , leadin g t o heterogeneou s deformation. Reactivatio n o f th e inherite d crystal lographic fabric , whic h i s favoure d b y tensiona l stresses obliqu e t o it s trend , i s characterize d b y higher strai n rates than other deformation regimes. The reactivatio n o f th e pre-existin g fabri c als o results i n highe r strai n rates tha n those accommo dated b y an isotropic mantle i n similar conditions . During continenta l rifting , thi s mechanica l ani sotropy ma y thu s induc e strai n localizatio n i n domains wher e extensiona l stres s i s obliqu e (30 -


Fig. 17 . Predicte d deformatio n o f a lithospher e dis playing a wrench fault typ e fabric abov e a mantle plume (Tommasi & Vauchez 2001). (a ) Strai n rate (Vo n Mise s equivalent strain rate, normalized relative t o the isotropi c behaviour) a s a functio n o f th e orientatio n o f th e radia l tensional stres s relativ e t o th e [100 ] axi s maximu m of the pre-existin g LP O fo r point s abov e th e plum e hea d periphery for three models with different initia l LPOs. (b ) Normal an d shea r component s o f th e strai n rat e tenso r (normalized b y th e Von Mise s equivalen t strain rate dis played by an isotropic polycrystal) for the model in which the initia l LP O i s th e mode l aggregate . Th e referenc e frame i s defined relative to the pre-existing mantle fabric: X i s paralle l t o th e [100 ] axi s maximum , i.e . paralle l t o the pre-existin g structura l trend, Y i s norma l t o th e pre existing shea r plane , an d Z i s vertical . Positiv e norma l strain rate s denot e extension an d negative ones, shorten ing. Gre y regio n mark s orientation s tha t ma y trigge r strain localization .

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60°) t o th e pre-existin g mantl e fabric . Th e direc tional softening associated wit h olivine LPO froze n in th e lithospheri c mantl e ma y als o guid e th e propagation of the initial instability that will follo w the pre-existin g structura l trend . Th e inherite d mantle fabric also controls the deformation regime , imposing a stron g strike-sli p shea r componen t t o the deformation. An LPO-induced mechanica l ani sotropy ma y therefor e explai n bot h th e systemati c reactivation o f ancien t collisional belts durin g rift ing (structura l inheritance ) an d th e onse t o f trans tension withi n continental rifts . These results , obtaine d fo r a specifi c geodyn amic case, can be extended to a more genera l situation. I n majo r strike-sli p fault s an d transcurrent/transpressional orogeni c domains , th e inherited fabri c o f th e lithospheri c mantl e shoul d induce a directiona l softening , wit h th e conse quence tha t thi s fabri c shoul d b e preferentiall y reactivated. Development of new structures oblique to th e pre-existin g shea r zone s shoul d onl y b e observed whe n th e ne w tectoni c solicitation s (either distensive or compressive, Fig. 18 ) are normal o r parallel t o the inherite d foliation , i.e. whe n no shea r stresse s ar e applie d paralle l t o th e inherited fabric . I n mos t cases , reactivatio n wil l occur through transtension o r transpression, an d the relative proportio n o f simpl e an d pur e shea r depends o n th e obliquit y o f th e stres s axe s rela tively t o th e inherite d fabric . The crustal fabri c i n lithospheric-scal e shea r zones als o contribute s t o thi s mechanica l ani sotropy. Indeed , localize d deformatio n i n th e middle an d lower crus t gives ris e t o stron g LPOs . Crustal minerals , i n particula r mica s tha t ar e important phase s i n mylonites , displa y a stil l stronger mechanica l anisotrop y tha n olivine ; thei r layered structur e result s i n plasti c deformatio n accommodated by glide on the (001) plane only. In addition, strength variation in polymineralic crustal rocks ofte n give s ris e t o a millimetre - t o centimetre-scale compositiona l layerin g paralle l t o the shea r zon e that , a t a larger scale , als o contrib utes to a directional weakenin g and reactivation of the shea r zone . Finally , grain-siz e reductio n asso ciated wit h shearing i n the upper/middle crus t may result i n a n isotropi c strain-softenin g withi n th e shear zone; at these depths, the shear zone will thus act a s a plana r wea k heterogeneit y localizin g th e subsequent deformation. Repeated reactivation s o f majo r transcurren t shear zones or domains during long periods of time and th e necessit y fo r th e caus e o f thi s persistenc e to b e i n th e lithospheri c mantl e hav e bee n recog nized lon g ag o (e.g . Watterso n 1975) . Man y examples o f suc h reactivatio n i n variou s geodynamic environment s ar e availabl e i n th e literature . Tommasi an d Vauche z (2001 ) hav e alread y dis -

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Fig. 18 . Compressiona l deformatio n o f a lithospher e displayin g a wrenc h faul t typ e fabric . Calculate d strai n rate s (Von Mise s equivalen t strai n rate , normalize d relativ e t o th e isotropi c behaviour ) ar e displaye d a s a functio n o f th e orientation o f th e impose d shortenin g relativ e t o th e (010 ) plan e maximu m o f th e pre-existin g LPO .

cussed thos e relate d t o th e reactivatio n o f lithospheric-scale shea r zon e o r transpressiona l belts durin g continenta l rifting . S o w e wil l focu s on on e o f th e bes t illustration s o f th e reactivatio n of a collisional wrenc h fault a s a transform boundary: th e developmen t o f th e Newfoundland Azores-Gibraltar transfor m plat e boundar y a t th e northern edg e o f the central Atlanti c Ocea n durin g the Early Mesozoi c (Fig . 19) . The Newfoundland Azores-Gibraltar faul t zon e forme d a majo r Her cynian dextral strike-sli p faul t zon e that offset s th e Appalachians orogeni c fron t i n Newfoundlan d (Keppie 1989) . Durin g th e fina l stage s o f th e Appalachian-Variscan convergence , thi s faul t accommodated th e relativ e displacemen t betwee n the Iberia n an d Nort h Africa n blocks . Thi s faul t subsequently playe d a majo r rol e o n th e Centra l Atlantic initia l rifting , limitin g on e of the promontories of the North American stable margin. Indeed, the openin g o f th e centra l Atlanti c Ocea n too k place almos t simultaneousl y fro m Florid a t o th e Newfoundland-Azores-Gibraltar transfor m (th e first Centra l Atlanti c magneti c anomaly , M25 , i s identified alon g thi s entir e segmen t (Owe n 1983)) , but furthe r northwar d propagatio n o f th e Centra l Atlantic leadin g t o separation betwee n Eurasia and North America di d not occur until Late Cretaceou s

time. Fro m Mid-Jurassi c t o Lat e Cretaceou s time , the Newfoundland-Azores-Gibralta r transfor m connected th e Central Atlantic an d the Tethys oce anic basins, accommodating the differential motio n between Afric a an d Europe .

Thermal conductivity anisotropy Heat transfe r i s a ke y proces s controllin g th e Earth's dynamics , sinc e temperatur e i s a majo r parameter controllin g th e rheologica l behaviou r of both crusta l an d mantl e rocks . Therma l conduc tivity i n bot h mantl e an d crus t i s usuall y assume d to b e isotropic . Yet , experimenta l dat a sho w that , at ambient conditions, the dominant mineral phases in th e crus t an d uppe r mantl e displa y a larg e ani sotropy o f therma l diffusivity . I n olivine , fo r instance, heat conduction parallel to the [100] crys tallographic axi s is 1. 5 times faster tha n parallel t o the [010 ] axi s (Chai et al. 1996). Quart z and micas, the main constituents of crustal mylonites, also display a strongl y anisotropi c therma l conductivity , with th e highes t an d lowest conductivitie s paralle l to th e [0001 ] axi s an d withi n th e (001 ) plane , respectively (Clause ? & Huenges 1995) . This therma l anisotrop y i s als o observe d a t th e rock scale . Recent studies combining petrophysica l


Fig. 19. Fi t o f th e Centra l an d Nort h Atlanti c Ocea n showing tha t th e initia l rif t i n th e centra l domai n propa gated paralle l t o th e Hercynia n oroge n an d tha t th e Newfoundland-Azores-Gibraltar Hercynia n wrench faul t was reactivated in the Mesozoic as a transform fault transferring extensio n fro m th e Centra l Atlanti c basi n t o th e Tethys basin .

modelling an d thermal diffusivit y measurement s on upper mantl e rock s (Tommas i e t al. 2001 ) sho w that a deformation-induced olivine LPO may result in a significant thermal diffusivity anisotrop y i n the uppermost mantle: heat transport parallel to the olivine [100] axe s concentration (flo w direction ) is up to 30% faster tha n normal to the flow plane ([010 ] concentration). Moreover , i n th e studie d tempera ture range (30 0 t o 1250°K) , th e thermal diffusivit y anisotropy doe s no t depen d o n temperature , sug gesting i t migh t b e preserve d eve n a t highe r tem peratures correspondin g t o asthenospheri c con ditions. Seismi c anisotrop y data , lik e thos e presented i n th e previou s sections , indicat e tha t major wrench faults ar e characterized by a coherent olivine lattic e preferre d orientatio n tha t affect s th e entire lithospher e ove r domain s severa l hundred s (or thousands in the cas e o f a transpressional belt ) of kilometre s lon g an d tens (o r hundreds ) o f kilometres wide. This 'wrenc h fault type' fabric should therefore induc e a large-scal e therma l diffusivit y anisotropy in the lithospheric mantle, characterized by faster heat conduction within the shear zone parallel t o th e shea r directio n an d slowe r conductio n normal t o th e shea r zone .

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A simila r therma l anisotrop y shoul d b e presen t in th e crusta l sectio n o f a lithospheric shea r zone . Laboratory measurement s o f therma l conductivit y of gneisse s drille d i n th e KT B borehol e sho w u p to 40 % o f anisotrop y (Buntebart h 1991) . I n thes e samples, which display mineralogical composition s (quartz, micas , an d feldspars ) an d microstructure s similar t o thos e o f high-temperatur e mylonite s i n the Borborema , Ribeira , an d Madagasca r shea r zones, hea t conductio n paralle l t o th e foliatio n plane i s o n averag e 1. 2 time s faste r tha n norma l to it . A weake r anisotrop y i s observe d withi n th e foliation plane, with the highest conductivity measured parallel t o the lineation. Compariso n betwee n measured therma l conductivitie s an d thos e pre dicted b y petrophysica l modellin g suggest s that , similarly t o th e mechanica l anisotropy , th e majo r contributions t o th e gneisse s therma l conductivit y anisotropy stems from th e strong LPO of micas and quartz (Siegesmun d 1994) . Existence o f a large-scale , strain-induce d ther mal anisotropy i n the upper mantle implies that the temperature distribution , rheology, and , hence, th e upper mantl e dynamic s depen d o n its deformatio n history. Olivine orientations frozen i n the continental lithospher e ma y modif y plume-lithospher e interactions fo r instance . Enhance d therma l diffu sivity alon g lithospheric-scal e wrenc h zones , i.e . parallel t o th e olivin e [100 ] preferre d orientation , may lea d t o anisotropi c heatin g o f th e lithospher e above a mantl e plume , favourin g th e reactivatio n of thes e structure s durin g continenta l break-u p (Vauchez et al. 1997 ; Tommas i & Vauchez 2001) . Such a contro l o f th e pre-existin g lithospheri c structure o n the propagation o f a thermal anomal y may b e inferred , fo r instance , fro m tomographi c images o f th e Eas t Africa n rif t i n Keny a (Achauer & krisp-group 1994). In these images, the low-velocity seismi c anomalie s displa y tw o mai n trends: a N- S trend , paralle l t o th e surfac e expression o f the Eas t Africa n rift , an d a NW-SE trend followin g Neoproterozoi c structure s tha t were reactivate d durin g the Mesozoi c t o giv e ris e to th e Anz a rift .

Conclusion Geological an d geophysica l observation s i n activ e and fossi l orogeni c belt s converg e t o suppor t tha t major wrenc h fault s ar e roote d int o th e uppe r mantle. Hug e transcurren t shea r zone s (severa l hundreds of kilometres long and a few tens of kilometres wide ) in Brazi l an d Madagascar hav e been eroded dow n t o level s wher e deformatio n wa s accommodated unde r high-temperatur e condition s (650 t o >800°C ) i n partiall y melte d rocks . I t i s remarkable that unde r thes e high-temperatur e and , hence, low-viscosity conditions, which were highly

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favourable t o developmen t o f a decouplin g level , no evidenc e o f rootin g o f thes e shea r zone s ha s been observed ; o n th e contrary , strai n wa s stil l localized i n wide transcurrent shear zones. Seismi c profiling, seismi c tomography , P n azimutha l ani sotropy and magnetotelluric sounding s also support that several major wrenc h faults crosscu t the Mono discontinuity an d penetrate th e uppermos t mantle . In addition, shear-wave splitting measurements and electric conductivity anisotropy above major strike slip fault s ar e i n agreemen t wit h a 'wrenc h faul t type' mantl e fabri c coheren t acros s mos t o r eve n the totalit y o f th e lithospher e thickness . Indeed , transform faul t boundarie s such as the San Andreas Fault, fo r whic h a connectio n wit h th e mantl e i s required, displa y geophysica l characteristic s simi lar t o thos e o f th e mai n intracontinenta l faults , either activ e o r fossil . A simila r conclusio n i s reached fo r transpressiona l orogeni c domain s deforming i n respons et o obliqu e convergence/collision. The existence of a 'wrench fault type ' fabri c into the continenta l mantle , beside s inducin g aniso tropic elasti c an d electrica l properties , ma y resul t in th e developmen t o f a directiona l softenin g an d an anisotropic conduction of heat in the continental mantle. Thes e anisotropi c propertie s probabl y influence th e large-scal e tectoni c behaviou r of th e continents. Reactivatio n o f th e inherite d mantl e fabric represent s i n most cases th e most economi c behaviour in terms of energy. Only in very specifi c situations (solicitation orthogonal o r parallel t o the ancient fabric) , wil l th e pre-existin g fabri c o f th e lithospheric mantl e no t be reactivated. Preferentia l propagation o f continenta l break-u p paralle l t o ancient orogeni c belt s a s wel l a s th e systemati c reactivation of major wrenc h faults probably resul t from bot h a directiona l softenin g an d a n aniso tropic hea t transfe r du e t o wrench-typ e olivine preferred orientation s froze n i n th e continenta l mantle. Finally, th e wor k by Pollit z e t al (2000 , 2001 ) that suggests that the mantle beneath active wrench faults deform s coherentl y wit h th e crus t and , i n some way , determine s th e interseismi c character istics o f th e faul t raise s th e questio n o f th e effec t of th e mechanica l anisotrop y o f th e lithospheri c mantle on the dynamics of active faults. Characteristics of the fault lik e the slip rate, the stress building rate and therefore the magnitude and the recurrence o f earthquake s coul d be affecte d b y a lower stiffness o f th e mantl e in a specifi c direction . J. M. Lardeaux and J. E. Martelat provided th e map and images of the Madagascar shea r zone s and M. Granet th e seismological result s o n the Armorican massif. We thank C. Teyssier an d L . Burlin i fo r constructiv e reviews .

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Fault developmen t an d interaction i n distributed strike-sli p shear zones : a n experimental approac h G. SCHREUR S Institute of Geological Sciences, University of Bern, Baltzerstrasse 1, CH-3012 Bern, Switzerland (e-mail: [email protected]) Abstract: Analogu e model experiments using both brittle and viscous materials were performed to investigate the developmen t an d interaction o f strike-sli p fault s in zone s o f distribute d shea r deformation. A t lo w strain , bul k dextra l shea r deformatio n o f a n initia l rectangula r mode l i s dominantly accommodate d b y left-stepping, en echelon strike-sli p fault s (Riede l shears , R ) that form in response to the regional (bulk ) stress field. Push-up zones form in the area of interactio n between adjacen t left-stepping Riedel shears . I n cros s sections , fault s boundin g push-up zones have a n arcuat e shap e o r merg e a t depth . Adjacen t left-stepping R shear s merg e b y sideway s propagation o r lin k by shor t syntheti c shear s tha t strik e subparalle l t o the bulk shea r direction . Coalescence o f en echelon R shear s results in major, through-goin g faults zone s (master faults) . Several paralle l maste r fault s develo p du e t o th e distribute d natur e o f deformation . Spacin g between master faults i s related t o the thickness o f the brittle layer s overlying the basal viscous layer. Master faults contro l to a large extent the subsequent fault pattern . With increasing strain, relatively shor t antithetic an d synthetic faults develop mostl y between old, but still active maste r faults. Th e orientatio n an d evolutio n o f the ne w fault s indicat e local modification s of th e stres s field. In experiments lacking lateral borders, closely spaced parallel antithetic faults (cros s faults ) define block s that undergo clockwise rotatio n abou t a vertical axi s with continuing deformation . Fault developmen t an d faul t interactio n a t differen t stage s o f shea r strai n i n ou r model s sho w similarities wit h natural examples tha t have undergon e distributed shear .

Introduction 1990)

. Althoug h thes e zone s ar e dominate d b y major syntheti c strike-sli p fault s whic h ar e mutu Deformation o f continental lithospher e i s generally all y subparallel , smalle r antitheti c strike-sli p faults not confine d t o narro w linea r belt s bu t distribute d ofte n strikin g a t larg e angle s t o th e majo r fault s over broa d zone s u p t o severa l hundreds t o thou - occu r a s well. A serie s o f analogu e mode l experi sands kilometre s wid e (Molna r an d Tapponnie r ment s wa s designe d t o better understan d the com1975; McKenzi e an d Jackso n 1983) . Deformatio n ple x fault pattern i n zones o f distributed strike-sli p in th e uppe r continenta l crus t i s predominantl y shear , an d especiall y faul t developmen t an d inter accommodated b y brittl e faultin g an d i s assume d action . to be a t least partly controlle d b y distribute d flow Althoug h quite a number of experimental studie s of th e underlyin g ductil e part s o f th e lithospher e hav e investigated strike-slip faulting , mos t of them (England 1989) . At shallow depths, the presence of use d a singl e basemen t strike-sli p faul t (o r basa l a Theologicall y wea k laye r consistin g o f salt , eva - velocit y continuity ) t o induc e faultin g i n a n over porites, o r overpressure d shale s ma y als o caus e burde n consisting of sand or clay with or without a deformation i n th e overlyin g competen t sedimen - viscou s decollement (e.g. Cloos 1928 ; Riedel 1929 ; tary rock s t o be distributed . Emmon s 1969 ; Tchalenk o 1970 ; Wilco x e t al Major strike-sli p fault s occur in distributed shea r 1973 ; Nay lor e t al . 1986 ; Richard 1991 ; Richard zones, which ar e thousands of kilometres lon g an d e t al . 1995 ; Ueta e t al . 2000 ; Schopfe r & Steyre r up t o severa l hundre d kilometre s wide . Example s 2001) . I n thi s typ e o f experimen t (referre d t o a s a of such zones ar e the Proterozoic Najd fault system Riede l experiment ) fault s i n th e overburde n wer e in Saud i Arabia (Moor e 1979) , th e Dea d Se a faul t i n fac t secondar y structures generally directl y consystem (Quennell 1959) , an d the San Andreas fault necte d t o th e pre-existin g basemen t faul t an d system (e.g . Crowell 1962 ; Atwater 1970 ; Page restricte d t o it s immediat e vicinity . Th e widt h o f From: STORTI , F. , HOLDSWORTH , R . E . & SALVINI , F . (eds ) Intraplate Strike-Slip Deformation Belts. Geologica l Society, London , Specia l Publications, 210 , 35-52, 0305-8719/037 $ 15 © Th e Geologica l Societ y o f Londo n 2003 .

36

G. SCHREUR S

the faul t zon e in map view depended o n the thick ness o f th e overburden . Experimenta l studie s o n zones o f distribute d strike-sli p shea r involve d a large variet y o f differen t experimenta l set-up s an d used mostl y cla y (Cloo s 1955 ; Hoeppene r e t al 1969; Freund 1974 ; An & Sammis 1996; An 1998) , fault goug e (A n & Sammi s 1996 ; A n 1998) , o r sand (Gapai s e t al. 1991 ; Richar d e t al 1995) . In these experiments , first-generatio n fault s generall y included both synthetic and antithetic faults ( R and R' shears , respectively) , bu t wit h increasin g bul k shear on e o f th e tw o set s starte d t o dominate . In contras t t o previou s experiments , w e use d both brittl e an d viscou s analogu e material s t o investigate faultin g i n zone s o f strike-sli p shea r driven b y basa l distribute d flow . I n ou r particula r experimental set-u p th e viscou s materia l impose s deformation to be distributed homogeneously at the base o f the mode l an d als o allow s partia l decoup ling du e t o it s contrastin g rheologica l behaviou r with respec t t o th e overlyin g brittl e materials . A t crustal scale , th e viscou s analogu e materia l represents a detachmen t leve l i n th e middl e t o lower crust , wherea s a t basi n scal e i t simulate s a weak sedimentar y laye r (e.g . evaporites) . Th e brittle analogu e materia l represent s uppe r crusta l

rocks (a t crusta l scale ) o r lithified , non-evaporiti c sediments (a t basin scale) . The aim s o f th e experimenta l programm e wer e to (1 ) stud y th e developmen t an d interactio n o f faults in zones of distributed strike-sli p shea r deformation; (2 ) investigat e th e influenc e o f varyin g boundary conditions; (3) compare results with previous studie s an d wit h natura l examples ; an d (4 ) propose criteria for identifying zone s of distributed strike-slip shea r i n nature.

Experimental apparatu s and procedure The experimental set-u p used to mode l distribute d strike-slip shea r i s show n in Figur e 1 . The experi mental apparatu s i s a slightl y modifie d versio n o f the on e use d b y Schreur s & Collett a (1998 ) an d included tw o basal plates : on e remained fixe d an d the other moved by a geared moto r drive. 50 plexiglass bars , 0. 5 c m wide , 1 cm high, an d 5 0 o r 7 0 cm long, were stacked lik e cards between tw o parallel woode n bar s attache d to th e overlyin g longi tudinal vertical walls. The plexiglass bars were laterally confine d by a thin wooden bar on either side. One end of each bar ( A in Fig. Ic , d) was attached to th e movin g bas e plate , wherea s th e othe r en d

Fig. 1 . Experimenta l set-up , (a ) Perspective view o f experimental apparatu s an d stratified model befor e deformation . Lateral boundarie s consisting o f rubber sheets i n 'confine d experiments ' ar e not shown , (b ) Vertica l sectio n through undeformed model , (c ) Bas e o f model a t initial state , (d ) Bas e o f mode l a t deforme d state .

ANALOGUE MODELLIN G O F STRIKE-SLIP TECTONIC S

was allowe d t o sli p alon g a smal l pi n ( B i n Fig . Ic, d ) attache d t o th e fixe d bas e plate . A s on e of the bas e plate s sli d th e confine d plexiglas s bar s slipped past on e another an d the initial rectangula r configuration change d int o a parallelogram , thu s simulating distribute d strike-sli p shear . Movemen t of the longitudinal sidewall and base plate occurred at a pre-set velocity applied by stepper motors with computer control . As analogu e material s w e use d quart z sand and glass powder having an average grain size of about 100 /im , an d a viscou s polyme r (polydimethyl siloxane, PDMS). San d and glass powder obey th e Mohr-Coulomb criterio n o f failur e an d the y ar e considered t o be good analogue materials for simulating brittl e deformatio n i n th e uppe r crus t (Horsfield 1977) . Thei r cohesio n i s lo w an d thei r angle o f interna l frictio n a s determine d b y shea r tests i s 36 ° fo r san d an d 37 ° fo r glas s powder . These value s ar e approximatel y simila r t o thos e determined experimentall y fo r uppe r crusta l rock s (Byerlee 1978) . PDM S ha s a densit y o f 0.96 5 g cm"3 and an average value of 5 X 10 4 Pa s for th e viscosity i n th e Newtonia n flo w regime , whic h occurs below a strain rate of 3 X 10~ 3 s^ 1 a t 24°C (Weijermars 1986) . I t i s a good analogu e materia l to simulat e viscou s flo w o f evaporite s o r rock s i n the lowe r crus t (Vendevill e 1987) . Models wer e scale d using methods discusse d b y Hubbert (1937 ) an d Ramber g (1981) . Calculate d scale ratio s ar e give n i n Tabl e 1 an d var y depending whether one intends to model (1) evaporites overlai n b y competen t sediment s (Tabl e la ) or (2) ductile lowe r crus t overlai n b y brittle uppe r crust (Tabl e Ib) . Severa l parameter s suc h a s tem -

37

perature increas e wit h depth , por e pressure , faul t zone width , grai n size , an d compactio n wer e no t incorporated i n th e mode l design . Despit e thes e limitations, partiall y scale d model s ca n generat e ideas about the origin and development o f geologi cal structures . O f specia l importanc e i s th e abilit y to monitor the evolution of the model through time, instead o f th e stati c pictur e obtaine d fro m fiel d observations o r seismi c interpretations . A layer of viscous PDMS was placed at the base of the model, directly overlying the plexiglass bars. Sand an d glass powder wer e alternatel y poure d on top t o produc e a stratifie d model . Passiv e squar e grids mad e o f coloure d san d wer e trace d o n th e upper free surfac e o f the model. The widt h of each model wa s 2 5 cm an d displacement o f the moving base plate occurred at 8 cm h-1, resulting in a shear strain rate of 9 X 10~ 5 s" 1. The array of plexiglas s bars wa s initiall y eithe r 5 0 X 25 c m o r 7 0 X 25 cm, wit h th e longes t dimensio n bein g paralle l t o the shea r direction . Th e applie d bulk shea r defor mation wa s arbitraril y chose n a s dextra l i n al l experiments. Severa l analogu e model s wer e ana lysed b y X-ra y computerize d tomograph y (CT) , a non-destructive technique whic h makes i t possibl e to visualiz e th e 3 D geometr y o f a mode l (Mand l 1988; Collett a e t al 1991) .

Experimental result s Two type s o f experiment s wer e performed : (1 ) confined experiments , i n whic h th e latera l bound aries wer e confine d b y rubbe r sheets , an d (2 ) unconfined experiment s havin g n o rubbe r sheet s along the lateral boundaries, thus allowing materia l

Table 1 . Scale ratios a t (a ) basin scale an d fb j crustal scale. Strain rate i n experiments wa s calculated from th e shear strain rate using methods discussed by Ramsay (1967) and Ramsay & Graham (1970) a.

Length

Time

Velocity

Density (g air3)

Viscosity (Pas)

Strain rat e (s-1)

Model

1 cm

1.5 hours

4.8 X 10" 5

500m 2 X 10~ 5

270000 years 6.25 X 10- 10

0.965 (PDMS) 2.5 0.4

5 X 10 4

Nature Ratio: model/nature

8 cm rr1 (basal plate ) 2.2 cm a" 1 3.2 X 10 4

b.

Length

Time

Velocity

Density (g cm- 3)

Viscosity (Pas)

Strain rat e (s-1)

Model

1 cm

1.5 hours

4.8 x icr 5

5000 m 2 X 10- 6

2700000 year s 6.25 X 10" 11

0.965 (PDMS) 2.5 0.4

5 X 10 4

Nature Ratio: model/nature

8 cm hr1 (basal plate ) 2.2 cm a"1 3.2 X 10 4

1021 5 X 10~ 17

3 X 10~ 15 1.6 X 10 10

1019 5 X 10~

15

3 X 10~ 14 1.6 X 10 9

G. SCHREUR S

38

to mov e freel y sideways . Tw o experiment s wer e repeated t o check fo r reproducibility. Faul t pattern and faul t orientation s wer e nearl y identica l i n sur face vie w a t simila r stage s o f deformation , thu s demonstrating tha t results wer e reproducible . Sev eral experimenta l parameter s varie d betwee n experiments i n orde r t o asses s thei r influenc e o n the resultin g structure s (Tabl e 2) . Th e mos t influ ential parameters were thickness of the brittle cover and th e leve l o f confinemen t alon g th e latera l boundaries. We first describe an d illustrate in detail the structura l evolutio n o f tw o confine d experi ments having identical viscous layer thickness, but different brittl e cove r thickness . Subsequently , w e will describ e th e faul t evolutio n i n a n unconfine d experiment.

Confined experiment 1638: 0.5 cm viscous PDMS and 1.5 cm brittle layers During th e initia l stage s o f bul k dextra l shear , deformation i n th e brittle layer s occur s b y distrib uted grai n flow . Wit h increasin g shea r strain , dis crete faulting become s th e dominant mechanism of strain accommodation (Fig . 2) . After a shear strain of abou t y = 0.1 0 dextra l strike-sli p fault s (synthetic Riede l shears ; R i n Fig . 2a ) develop . They ar e e n echelo n an d left-stepping , an d thei r traces strik e between 1 7 and 24° from th e impose d bulk shear direction . Almost simultaneously , sinis tral strike-sli p fault s appea r strikin g a t 7 2 t o 78 ° (antithetic Riede l shears ; R ' i n Fig . 2a , b) . R ' shears ar e restricte d t o th e acut e corner s o f th e model an d ar e considere d t o b e a n edg e effec t related directl y t o th e scisso r effec t o f th e deforming model . Wit h increasin g shear , domain s with a slight vertica l relief (push-u p zones) appea r in the are a comprised betwee n two left-stepping R shears (Fig . 2b , c) . Th e lon g axi s o f th e push-u p zone i s paralle l t o th e strik e o f th e R shears . A s

individual R shear s propagat e alon g strike , the y overlap wit h adjacen t left-steppin g R shears , an d the propagating fault segments acquire gentler dips. The di p directio n o f individua l R shear s change s along strik e and the footwall becomes th e hanging wall wit h a smal l revers e offse t a t eac h faul t tip . Coalescence o f R shear s ma y occu r i n tw o ways : (1) individua l faul t segment s o f closel y adjacen t left-stepping R shear s propagat e alon g strik e and , as the y overlap , thei r surfac e strik e decrease s and they merge with an adjacent R shear, (2) short dextral strike-sli p faults form i n th e overla p are a between two adjacent left-stepping R shears, whose traces strik e a t a n angl e (lowe r angl e syntheti c shear, R L in Fig. 2c, d) with respect to the impose d shear direction that is lower than the angl e of older R shears . Coalescence o f e n echelo n R shear s result s i n the formation of a slightly anastomosing shea r zone that strike s a t a n overall angl e of abou t 15 ° an d t o which w e refe r a s maste r faul t (Fig . 2d) . I n ou r distributed shear experiments, severa l master fault s form subparalle l t o on e another . Thes e long-live d master fault s accommodat e mos t o f th e displace ment. With additional shear two new types of faults form, mostl y confine d betwee n maste r faults : (1 ) sinistral strike-sli p fault s strikin g a t angle s lowe r than R ' shear s (lowe r angl e antitheti c shear s o r cross faults , R' L i n Fig . 2d , e ) an d (2 ) dextra l strike-slip faults (lowe r angl e syntheti c shears , R L in Fig . 2d , e ) tha t strik e a t angle s lowe r tha n th e older R shear s Wit h increasin g shear , secondar y faults generall y strike progressively at lower angles with respect t o the shear direction. During the final stage o f th e experimen t ne w cros s fault s strik e a t angles o f les s tha n 50° , wherea s new lowe r angl e synthetic fault s ar e subparalle l t o th e shea r direc tion o r eve n strik e a t a smal l angl e counter clockwise wit h respec t t o th e impose d shea r direction.

Table 2. Parameters and boundary conditions used i n analogue models. Bold experiments ar e discussed i n detail i n the text; Experiment 1959A was not analysed by X-ray computerized tomography

Experiment number

Nature of transverse borders

Initial dimensions of stratifie d model (cm)

Initial thickness of viscous laye r (cm)

Initial thickness of granular material s (cm)

Maximum shear strai n

1553 1625 1638 1666 1959 1959A

Unconfined Confined Confined Confined

50X25 50X25 50X25 70 X 25 70 x 25 70 X 25

0.5 0.5 0.5 0.5 0.5 0.5

3.0 3.0 1.5 3.0 3.0 3.0

0.57 0.57 0.62 0.56 0.37 0.33

Unconfined

Unconfined

ANALOGUE MODELLIN G O F STRIKE-SLI P TECTONIC S

Experiment 1638

39

Fault geometr y i n vertica l section s i s visualized using transvers e computerize d tomograph y (CT) scans, which show vertical or slightly arcuate faults that exten d dow n t o th e bas e o f th e brittl e layer s (Fig. 3 a, b) . Usin g closel y space d sequentia l C T scans of the final stage of the experiment, compute r visualization softwar e allowed u s to generat e hori zontal slice s (Fig . 3d) an d 3 D perspectiv e view s (Fig. 3e , f) . Th e horizonta l slic e nea r th e bas e o f the mode l clearl y show s anastomosin g syntheti c master fault s (R ) an d bot h secondar y cros s fault s (R'L) an d lowe r angl e syntheti c fault s (R L) confined in betwee n maste r fault s (Fig. 3d). The perspective view s illustrat e push-u p zone s i n area s where e n echelon R shear s overla p (arrow s in Fig. 3e). Th e faul t plane s boundin g th e push-u p zones typically steepe n downwar d an d hav e a smal l reverse componen t o f slip . (Fig . 3f). Th e push-u p zone indicate d b y the righ t whit e arro w i n Fig. 3e has late r bee n transecte d b y a younger R L shear .

Confined experiment 1666: 0.5 cm viscous PDMS and 3 cm brittle layers Left-stepping e n echelo n R shear s for m initially , striking a t 17-23 ° wit h respect t o th e shea r direc tion, wherea s a fe w antitheti c strike-sli p fault s striking a t 71-80° develo p nea r th e acut e border s of the model (Fig. 4a). In vertical cros s section s en echelon an d overlappin g R shear s creat e push-u p zones (labelle d '+ ' i n Fig. 4a). R shea r plane s ar e vertical o r slightly arcuat e an d may merge a t depth (Fig. 4a) . With increasin g shear , R shea r plane s coalesc e to for m majo r through-goin g maste r fault s (Fig . 4b). Th e spacin g betwee n maste r fault s i s large r than i n th e previou s experimen t havin g a thinne r brittle layer . Onc e th e maste r fault s form , second ary cros s fault s (R' L ) an d lowe r angl e syntheti c faults (R L) develo p in between . Th e surfac e strik e of thes e newl y forme d fault s decrease s wit h additional shear . Durin g th e fina l stage s o f defor mation (Fig. 4c), a new generation of synthetic and antithetic fault s form s locally , clos e t o th e are a where maste r fault s an d cros s fault s intersec t (e.g. faults A , B an d C i n Fig. 4c). The coalescenc e o f master faults an d younger cross faults an d RL faults

Fig. 2 . Sequentia l development of faulting in experimen t 1638. Overhea d photograph s wit h superpose d lin e drawings fo r fiv e successiv e stage s o f distribute d strike-sli p shear. Thi n line s represen t passiv e marker s o n th e san d layer's uppe r surfac e (initiall y squar e grid) ; thic k line s represent trace s o f visibl e faults . R = syntheti c Riede l shear, R' = antithetic Riedel shear , R L = lower angle synthetic fault , R/ L= lower angle antithetic fault (cros s fault) .

40

G. SCHREUR S

Fill ?• Flai l IrcBMon i n eK5crimcrtf 1538 : la} Vortica l acction a a t 7 = 0.19; orientatio n o f section! indicated i n nig. 2rL (ft ) Vertical section s a t 7 - 0.37 ; oncntatio n o f section s indicate d i n Fig . 2d . (c ) Line drawin g fro m overhea d photograph showin g faul t patter n a t y = 0.60. (d ) Horizonta l slic e 7 mm above base o f model a t y = 0.60. Locatio n of sectio n i s show n in Fig . 3c . (e ) 3 D perspectiv e vie w o f mode l a t y = 0.60. Not e th e push-u p zones indicate d b y white arrows , (f ) 3 D perspective cut-ou t a t y = 0. 60 . Fo r notatio n se e Figur e 2 .

at the surfac e an d a t depth i s illustrated b y th e 3 D block diagrams in Figure 5. The evolutio n o f faul t geometr y i n vertica l sec tion is shown in Figure 6 . For a low amount of bulk strain, subvertica l fault s correspon d t o R shear s in

surface view . Closel y adjacen t R shear s (left stepping i n surfac e view ) converg e downwar d and delineate a push-up zone (Fig. 6), marked by slight vertical relief. Lower angle synthetic faults, linkin g overlapping e n echelo n R shear s i n surfac e view ,

ANALOGUE MODELLIN G OF STRIKE-SLI P TECTONIC S

41

Fig. 4. Sequentia l developmen t of faulting i n map view and cross section for experiment 1666 . Overhead photographs with superpose d lin e drawing s of visibl e fault s an d vertica l section s (C T images) ar e show n for successiv e stage s of distributed strike-sli p shear , (a ) y = 0.19; push-up zones labelled b y '+' . (b ) y = 0.37. (c) y = 0.56. The area covere d by th e C T scanne r wa s slightl y smalle r tha n th e widt h o f model , an d therefor e a smal l par t o f th e left-han d sid e o f the sectio n wa s no t considere d i n imag e computing . Notations a s i n Figur e 2 .

extend dow n t o th e bas e o f th e brittl e layer s o r merge a t dept h wit h R shears . Th e mor e diffus e fault zone s correspond t o cros s fault s tha t intersec t the CT acquisition plane a t a low angle thus reduc-

ing th e resolution . Th e sligh t shif t i n positio n o f faults i n successiv e section s i s due to th e displace ment o f fault s wit h respec t t o th e fixe d sectio n orientation durin g progressive bul k shear .

42

G. SCHREUR S

Fig. 5 . Bloc k diagram s o f faul t patter n a t fina l stag e o f distributed shear deformation i n experiment 1666 . (a ) 3D perspective view , (b) 3D perspective cut-out. For notation see Figure 2 .

Unconfined experiment 1553: 0.5 cm viscous PDMS and 3 cm brittle layers The absence of transverse rubber sheets in laterally unconfined experiment s allow s materia l t o mov e sideways durin g shea r an d result s i n a faul t evol ution (Fig . 7 ) markedl y differen t fro m tha t i n lat erally confine d experiments . Dextra l strike-sli p faults for m a t a shea r strai n o f abou t 0.09 . Thes e synthetic fault s ( R shears ) nucleat e a t th e uncon fined lateral boundarie s and propagate toward s the central par t o f th e model . Thei r trace s strik e between 2 8 an d 35 ° fro m th e directio n o f applie d bulk shear. Sinistra l strike-sli p fault s appear almos t at the sam e time an d strike a t about 70° (antithetic Riedel shears; R') . With increasin g bul k shear , ol d R shear s remai n active , an d som e o f the m propa gate alon g th e entir e lengt h o f th e model . A t th e same tim e ne w fault s form . The y ar e mostl y restricted t o area s locate d betwee n subparallel oriented major R shears (maste r faults) an d include evenly space d R' L shear s (cros s faults ) strikin g a t 60-65° an d R L shears . Cros s fault s rotat e wit h increasing strain , propagat e sideways , an d acquir e a sigmoida l Z shap e i n pla n view . The y hav e a small dip-slip component an d the sense o f fault di p changes alon g strike . Strike-sli p displacemen t along cros s fault s i s mino r compare d wit h tha t along maste r faults . Cros s fault s usuall y merg e

Fig. 6 . Vertica l section s fo r successiv e stage s o f distrib uted strike-sli p shea r i n experimen t 1666 . Heigh t o f brittle-viscous mode l i s abou t 3. 5 cm . Fo r notatio n se e Figure 2 .

with o r terminat e agains t maste r faults . Transten sional graben s develop mostl y nea r the unconfined lateral borders . A s shea r increases , th e array s o f cross fault s an d intervenin g unfaulte d domain s undergo significant clockwis e rotatio n about a vertical axis. At the end of deformation ( y = 0.57) th e central segment s of sigmoida l cross fault s strik e a t right angle s an d rotation amount s t o about 30°. 3 D views an d horizonta l slice s illustrat e ho w sig moidal cros s fault s coalesc e wit h maste r fault s (Fig. 8) , th e latte r one s strikin g a t abou t 25° wit h respect t o the bul k shea r direction .

Discussion Model results The presenc e o f a thi n basa l laye r o f viscou s material i s sufficien t t o allo w for distribute d shea r in th e brittl e cove r ove r th e entir e mode l width . After initia l distribute d grai n flow , th e san d an d glass powder layers defor m according t o the MohrCoulomb sli p criteri a an d distribute d shea r i s

ANALOGUE MODELLIN G O F STRIKE-SLI P TECTONIC S

43

Fig. 7 . Faul t evolutio n o f experimen t 155 3 fo r successiv e stage s o f distribute d strike-sli p shear . Lin e drawing s o f visible faults ar e superpose d o n photographs. Ticked line s in (e) and (f ) indicat e fault s wit h important dip-slip compo nent. Note how cross faults between R shears rotate with time and acquire a sigmoidal shape. For notation see Figure 2.

Fig. 8 . Detai l o f sigmoidal cros s fault s betwee n maste r faults , (a ) Surfac e view at y = 0.39, (b ) 3 D perspective vie w at y = 0.57 . Fo r notatio n se e Figure 2 .

44

G. SCHREURS

accommodated b y subvertica l strike-slip fault s tha t extend acros s th e entir e thicknes s o f th e brittl e cover. Accordin g t o th e Mohr-Coulom b sli p criteria, failur e i n a materia l tha t ha s no t ye t bee n faulted occur s at angles of ± (45 ° - 6 earthquake s i n th e norther n Tie n Sha n ar e spatially associate d wit h activ e tectoni c zone s around th e Issyk-Ku l microcontinen t (Fig . 6). As a result of recent crusta l movement s a t variable direction s an d rates , th e souther n shor e o f Lake Issyk-Ku l i s subjec t t o uplift , an d ther e ar e indications fo r subsidenc e i n the eastern an d western shores , wit h a ris k o f collapse . Activit y of th e mountains aroun d th e Issyk-Ku l microcontinen t and reactivatio n o f faul t border s o f th e Aktyuz Boordin microcontinen t i s expecte d t o continu e in the future . W e sugges t tha t reactivatio n o f fault s and th e relate d seismi c an d geologica l hazar d can

Fig. 8 . Activ e strike-sli p movement s in northern Tie n Sha n (afte r geodeti c surveys) .

62

M. M. BUSLOV ETAL.

be predicted fro m change s i n directio n an d rate of block movements .

faulting ma y be responsible fo r th e formatio n of a pull-apart structur e i n the central par t o f the basin .

Conclusions

We ar e gratefu l fo r th e constructiv e criticism s an d suggestions from th e reviewers. Especially we would like to expres s our cordia l thanks t o F . Stort i fo r hi s critical reading and valuable comments. Our thanks are extended to I . Safonov a an d T . Perepelov a fro m th e Institut e o f Geology for their help with the preparation of the English version o f th e manuscrip t an d t o L . Smirnov a fro m th e same institute for her assistance with figure drawing . The work wa s supporte d by grant s INCO-COPERNIKUS N ° PL 96-321 2 and fro m th e Russia n Foundatio n fo r Basic Research N ° 02-05-64627.

We investigate d th e relationship s betwee n th e present-day structure , reactivation o f ancient faults, and interactio n o f old granite-metamorphic block s (microcontinents) withi n relativel y mobil e oro genic belts in the region o f Tien Shan , on the basis of geological information , detailed interpretation of satellite imagery , analysi s o f seismicit y an d faul t plane solutions , an d geodeti c measurements . The tectonic s o f th e Tien-Sha n evolve s i n response t o th e convergenc e betwee n Indi a an d Eurasia since their collisio n i n the Eocene (Molna r and Tapponnie r 1975 ; Tapponnie r an d Molna r 1979, etc.), a s India continues its northward motion at 5 0 m m a" 1 (Avoua c an d Tapponnie r 1993 ; Avouac e t aL 1993) . Th e propagatio n an d distri bution o f strai n induce d b y th e collisio n i s con trolled by the complicated structur e of the crust and lithosphere. Geophysica l dat a indicate tectonic layering o f th e lithospher e beneat h th e norther n Tie n Shan. Th e presenc e o f horizonta l viscoelasti c lay ers ma y influence the rotatio n an d underplating of the Tarim plate and indentation of its basement into the middle crust of the Tien Shan . The thrusting of the Tari m plat e unde r th e norther n Tie n Sha n has caused th e shortenin g o f th e uppe r crus t a t a rat e of 5wt% . Source s o f data : MIG-NVL : mafi c dykes fro m Rocch i e t a l (2002) ; MMVG-NVL : lavas from th e Melbourn e an d Hallet t Volcani c Province s (Worner et al. 1989 ; Rocholl et al 1995) ; MMVG-EVP: Erebus Volcani c Provinc e (Kyl e e t a l 1992) ; MBL : Marie Byrd Land (Hol e & LeMasurier 1994 ; Har t et a l 1997); Pete r I 0y (Prestvi k et al 1990 ; Har t et al 1995) ; Balleny Island s (Green 1992) .

Relations between faulting an d dyk e injection The orientatio n o f dyke s alon g th e wester n shoulder o f the Ross Se a is almos t bimodal i n th e area north of the Reeves Glacier and is unimodal in the southern secto r (Fig . 3) . In the northern sector, dykes strike NW-SE and almost N-S , i.e . paralle l to th e majo r right-latera l strike-sli p faul t system s and t o th e basi n boundar y faults , respectively . A t Terra Nova Bay Station, dyke arrangement in left stepping e n echelo n tensio n gas h array s wit h a

Fig. 5 . Multipl e plot summarizing the isotopic variations of mafi c product s acros s th e WARS, the adjoining , contemporaneously activ e volcani c provinces , an d the main OIB reservoirs . The arro w in th e middl e diagra m points to die high U3 Nd/1IulNd ratio for DMM-A: Soufg§ 8f ^SIS! MIG-NVL: mafi c dyke s fro m Rocch i ct a l 2002 ; MMVG-NVL: lavas from the Melbourne and Hallett Volcanic Provinces (Worner et al 1989 ; Rocholl et al 1995) ; MMVG-EVP: Erebu s Volcani c Provinc e (Kyl e e t a l 1992); MBL : Mari e Byr d Lan d (Hol e & LeMasurie r 1994; Hart et al 1997) ; Peter I 0y (Prestvi k et al 1990 ; Hart e t a l 1995) ; Ballen y Island s (Har t 1988 ; Gree n 1992); JRIVG : Jame s Ros s Islan d Volcani c Grou p (Antarctic Peninsula ; Hol e e t a l 1995 ; Lawve r e t a l 1995); SNVG : Seal Nunataks Volcanic Group (Antarcti c Peninsula; Hol e 1990 ; Hol e et al 1993) ; BSVG: Bellingshausen Se a (i.e . Alexande r Island ) Volcani c Grou p (Antarctic Peninsula ; Hol e 1988 ; Hol e e t a l 1993) ;

STRIKE-SLIP TECTONIC S VS . MANTL E PLUM E

151

Fig. 7 . Outcrop-scal e relation s betwee n right-latera l strike-slip faultin g an d dyk e injection , (a ) Left-steppin g en echelo n tensio n gas h array s of basalti c dyke s a t Terr a Nova Ba y Statio n afte r Stort i e t al . (2001) . (b ) Cartoo n showing th e tension gash geometr y o f dykes a t Starr Nunatak (afte r Rossetti e t al. 2000). For locations, se e Fig. 3.

genetic relationship s betwee n tectoni c an d mag matic activitie s constrain the ag e o f onshore fault ing, which had t o be activ e since Eocene times , at least nort h of Priestle y Faul t (Fig . 6).

Fig. 6 . Ag e dat a o f igneou s activit y i n Victori a Lan d north o f Campbel l Fault . 40 Ar-39Ar age s afte r Rocch i e t al. (2002) ; Rb-S r age s afte r Tonarin i e t al. (1997) , K-A r ages after Mtiller et al. (1991). Acquisition o f geochronological dat a fo r dyke s fro m sout h o f Campbel l Faul t i s in progress .

NW-SE envelop e tren d (Fig . 7a), support s thei r emplacement i n a NW-S E right-latera l strike-sli p shear zon e whic h constitute s a spla y faul t o f th e Priestley Faul t (Stort i e t al. 2001). I n the southern sector, dykes are arranged in left-stepping arrays at an angl e o f abou t 30 ° t o th e N- S transtensiona l master fault s an d in places sho w tension-gash-lik e relationships wit h these faults (Fig . 7b), indicatin g syntectonic dyk e emplacement i n a dextral regim e (Rossetti e t al. 2000). This field evidence indicates that dyke emplacement an d geometr y wer e initiate d an d drive n b y the ongoin g tectonics. In th e norther n sector , bot h the NW-S E right-latera l strike-sli p faul t system s and th e basi n boundar y faults alon g th e Ros s Se a shoulder induce d magm a emplacement . I n th e southern sector , th e emplacemen t o f dyke s wa s linked t o th e activit y o f transtensiona l faults . Th e PAVF: Pal i Aik e Volcani c Fiel d (souther n Patagonia ; D'Orazio et al. 2000). BSE (Bulk Silicat e Earth), DMM A (Deplete d MOR E Mantle-typ e A) , an d OIB-HIM U (Ocean Islan d Basalt s wit h hig h ^U/^P b ratio ; Zindler & Hart 1986) .

Discussion: a role fo r a mantle plume on WARS development ? Evidence from the regional tectonic framework Typical features o f a mantle plume dominated tec tonic scenario are the development of a low-amplitude, broad-wavelengt h uplifte d regio n wit h a roughly circula r symmetr y an d a n almos t radia l pattern o f extensiona l faul t system s (Olse n 1995) , particularly for the Antarctic plate, whic h has been almost stationar y sinc e Cretaceou s time . Th e present-day tectoni c an d morphologica l architec ture o f Mari e Byr d Lan d ha s bee n interprete d a s fitting these features (LeMasurier & Landis 1996) . Conversely, Victori a Lan d i s characterize d b y a n elevated linea r rif t shoulde r (th e Transantarcti c Mountains) developed by N-S extensiona l to transtensional faultin g an d transvers e faultin g (e.g . Cooper e t al . 1991 ; Behrend t et al . 1996 ; Wilso n 1999; Rossetti et al 2000 ) that, in the northern sector, abu t NW-SE-strikin g intraplat e right-latera l fault system s (Salvini et al. 1997 ) with no evidence for eithe r domin g or radial structures. The relativ e chronolog y o f uplif t an d extension also counter s th e traditiona l concept s o f litho spheric evolutio n above a mantle plume. The main extension episod e occurre d i n th e lat e Cretaceou s (e.g. Lawve r & Gahaga n 1994) , whil e th e mai n uplift episod e occurre d durin g th e Eocen e (Stump & Fitzgeral d 1992 ; Fitzgeral d & Stump 1997). A thermal source for the uplift o f the Transantarctic Mountain s has bee n suggeste d (Smit h &

152

S. ROCCHI£TAL.

Drewry 1984 ; Berg et al 1989 ; Ster n & Ten Brink 1989), bu t alternativ e mechanism s have been pro posed suc h a s isostati c uplif t o f th e hangin g wal l of a major faul t cutting the lithosphere (Ster n et aL 1992) o r uplift linke d t o a shallow-dipping detach ment faul t i n a n asymmetri c passiv e rif t settin g (Fitzgerald e t al . 1986 ; Fitzgeral d & Baldwi n 1997). Finally , th e activit y o f a plum e carryin g higher-than-normal mantle temperatures is difficul t to reconcile with the prolonged subsidenc e o f Ross Sea basins durin g Cretaceou s an d Cenozoic time .

Evidence from geochemical data The pluton s an d dyk e swarm s o f NV L (Meande r Intrusive Group ) displa y th e typica l OI B geo chemical features . The geochemical characteristic s of th e whol e igneou s associatio n sho w restricte d variations ove r 5 0 myr an d acros s th e whol e are a (Rocchi e t al . 2002) . Additionally , a clos e simi larity i s observe d wit h the neighbourin g Cenozoi c magmatic provinces o f the Antarctic plate an d with magmatic province s fro m oceani c an d continenta l setting classicall y definin g th e HIM U affinity , i.e . characterized b y ^Pb/^P b i n exces s o f 20. 5 (Fig. 5; Hofman n 1997) . Thes e geochemica l fea tures represent on e of the classical argument s used to infe r th e activit y o f a dee p mantl e plum e fo r many volcanic provinces in both oceanic an d continental settings , base d o n th e ambiguou s relatio n between OIB-HIM U chemistr y ( a chemica l reservoir) an d mantle plume (a physical entity). On this ground, the wide diffusion o f Cenozoic volcanism wit h simila r geochemica l affinit y throughou t Antarctica, Souther n Ocea n islands , Tasmania , New Zealand, an d Campbell Plateau led Hart et al. (1997) t o hypothesiz e th e origi n o f magma s fro m a fossi l plum e hea d source , whic h impacte d th e Gondwana lithosphere befor e break-up in this area, i.e. befor e th e lat e Cretaceous . Th e momen t o f plume impingemen t i s controversia l an d coul d b e related t o eithe r th e middl e Jurassi c emplacemen t of th e Ferra r Larg e Igneou s Province (LIP ) o r th e mid-late Cretaceou s break-u p o f Ne w Zealan d from Wes t Antarctic a (Weave r e t al. 1994) . However, th e occurrenc e o f Ferra r basalt s exclusivel y along th e Transantarcti c Mountain s couple d wit h their absenc e i n Mari e Byr d Lan d an d th e othe r sites o f Cenozoi c OI B magmatis m i s evidenc e against a role for widespread sublithospheri c mas s contribution t o the sourc e of Cenozoic magmatis m by a Jurassi c plume . O n th e othe r hand , th e ide a of a late Cretaceous plume activity in West Antarctica is overruled by the lack of Cretaceous magmatism acros s th e whol e Victoria Lan d couple d with evidence fo r subsidenc e instea d o f buoyan t uplif t (LeMasurier & Landi s 1996) . Actually , th e geo chemically grounded clai m for a fossil plume hea d

source i s se t u p t o satisf y th e nee d fo r a shallow , weak, enriched layer, common to wide areas below the Souther n Ocea n an d th e adjoinin g continents . One o f th e mos t use d isotopi c issu e t o clai m fo r mantle plumes is the high 206pb/204Pb ratio, thought to b e derive d fro m dee p mantl e plume s tha t entrained sla b materia l recycle d int o th e dee p mantle ove r a lon g tim e perio d (10 9 years) . However, Halliday et al. (1995) showed that such a high 206 Pb/204Pb ratio can be also attained by the magma source a t rathe r shallo w depth , i n shorte r tim e interval (108 years, provided the source has a rather high U/P b ratio ) an d propose d a mode l fo r U/P b fractionation clos e t o mid-ocea n ridge s an d late r sampling o f suc h hig h U/P b sourc e b y oceani c islands magmatism . I t i s wort h notin g tha t Ceno zoic mafi c dyke s an d lava s fro m NV L hav e a n average U/Pb ratio of 0.44 ±0.1 1 and 0.66 ± 0.17 . This implie s a hig h U/P b rati o i n th e magm a source, whic h therefor e ha s bee n abl e t o produc e high 206 Pb/204Pb ratios i n a time spa n o f the orde r of 10 8 years . Therefore , w e propos e a model (se e further on ) i n whic h th e sourc e enrichmen t occurred i n th e lat e Cretaceou s som e ten s o f million year s before the magmatism.

Evidence from the thermal and magmatic regional framework Two classical piece s o f evidence for the activit y of mantle plume s ar e th e preservatio n o f hot-spo t tracks an d th e hig h volum e o f magma s produced. In th e WARS , chronological-area l progressio n o f magmatism i s lackin g an d the volum e o f magma s produced i s low . However , thes e fact s canno t b e unequivocally use d t o counte r th e plum e hypoth esis owing to the very low mobility of the Antarctic plate sinc e th e lat e Cretaceou s an d th e peculia r 'stationary' settin g o f th e Antarcti c plate , almos t completely encircle d b y mid-ocean ridge s (Hol e & LeMasurier 1994) . The presenc e o f seismicall y slo w (hot ) mantl e in th e WARS has been imaged fro m surfac e wave tomography (Danes i & Morell i 2000 , 2001) . Th e depth to which hot mantle extends cannot be safely modelled belo w 20 0 km, no t dee p enoug h t o sup port o r discar d th e occurrenc e o f a n active plume . Nevertheless, i t i s wort h notin g th e slo w mantl e does not have a circular symmetry, as expected for a plume : th e grou p velocit y map s o f Rayleig h waves (Danes i & Morell i 2000 ) sho w minimu m values arrange d o n a line corresponding t o the belt of transformatio n of the ridg e betwee n the Antarctic an d Australia n plate s (Fig . 8). Thi s indicate s that shallow hot mantle is related to a linear geodynamic featur e >400 0 km long , suc h a s th e bel t o f Southern Ocea n fractur e zones . Thes e large-scal e tectonic lineament s cros s th e continenta l litho -

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Fig. 8 . Souther n Ocea n Fracture Zones. Redrawn afte r Salvin i e t al. (1997), wit h mantle low-velocit y anomalie s afte r Danesi & Morelli (2000) .

sphere through NV L t o th e Ros s Embaymen t (Salvini e t al . 1997 ) an d coul d b e responsibl e fo r the transtensiona l Cenozoi c riftin g phas e wit h a rise o f mantl e geotherm s t o generat e mel t i n th e mantle an d strike-sli p t o transtensiona l tectoni c activity controllin g th e emplacemen t o f magma s within th e crus t o r a t the surface .

Intraplate strike-slip faulting: an alternative geodynamic scenario for magma genesis and emplacement The equivoca l geochemica l data , coupled wit h the lack o f diagnosti c tectonomagmati c an d geo morphic evidence , d o no t suppor t th e activit y o f a mantl e plum e a s th e drivin g mechanis m fo r th e generation an d emplacemen t o f Cenozoi c magma s in NVL . O n th e othe r hand , spatia l an d tempora l links betwee n th e Cenozoi c strike-sli p tectoni c regime an d th e igneou s activit y suppor t intraplat e right-lateral shea r a s a n effectiv e an d alternativ e geodynamic scenari o fo r magm a emplacement . According t o Salvin i e t al . (1997 ) th e magmati c activity i s focuse d i n a bel t alon g th e wester n shoulder o f th e Ros s Se a owin g to th e Theologica l zoning of the brittle crus t induced by th e eastward shallowing o f the Moho moving from the Transantarctic Mountain s to th e Easter n Basi n i n th e Ros s Sea (Coope r e t al . 1991) : crusta l thicknes s belo w the wester n Ross Se a shoulde r would be appropri ate fo r th e fracturing/permeabilit y condition s

required fo r magm a ascent . Additionally , thi s belt corresponds a t dept h t o a topographi c gradien t a t the base of the lithosphere, that could enhance convection-driven meltin g (Anderso n 1995) . The recen t dat a reviewe d i n thi s pape r o n (1 ) the structura l architecture of som e of the intraplate right-lateral faul t system s an d thei r relation s wit h dyke emplacement , (2 ) th e attitud e an d chemica l composition o f Cenozoic dyke s alon g a significan t segment o f th e wester n shoulde r o f th e Ros s Sea , and (3 ) th e geochronologica l constraint s suppor t the strike-slip-relate d mode l fo r magm a emplace ment. Th e space-tim e distributio n o f pluton s an d dykes i n NV L (Fig . 9), sugges t tha t igneou s activity ha s been activ e in different crustal sector s and/or alon g differen t boundar y faults in differen t times. The boundaries between these sectors are the major right-latera l faul t system s identifie d b y Sal vini e t al . (1997) . I n particular , th e crusta l secto r affected b y Cenozoi c pluto n emplacemen t i s bounded t o th e nort h b y th e Lea p Yea r Faul t an d to th e sout h by th e Campbel l Faul t (Fig . 9a). Th e three adjacen t sector s wit h differen t dyk e geo metries an d frequency ar e bounded, from th e north to th e south , b y th e Lea p Yea r Fault , th e Aviato r Fault, an d th e Priestle y Fault , respectivel y (Fig. 9b); the two adjacent sectors characterized by different timin g o f magm a emplacemen t ar e bounded by the Leap Year Fault, the Aviator Fault, and th e Priestle y Fault , respectivel y (Fig . 9c) . During th e las t 5 0 myr NV L ha s bee n affecte d

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Fig. 9. Space-tim e distributio n o f Cenozoic plutoni c an d subvolcani c igneou s product s i n northern Victori a Land .

by lithospheri c shea r processe s whic h divide d th e area into sectors characterized by different tectono magmatic history . Suc h a compartmentalizatio n may relat e t o th e crusta l fabri c inherite d fro m th e previous tectonic histor y o f the region, namel y the early Palaeozoi c Ros s Orogen y an d th e lat e Cre taceous Ros s Se a initia l opening . Differen t sli p rates alon g th e majo r intraplat e right-latera l faul t systems ma y hav e induce d a tempora l zonin g t o magma genesi s an d emplacement .

A general model for the tectonomagmatic history of the western Ross Embayment The alternativ e contex t propose d fo r magm a gen esis an d emplacemen t o n th e wester n shoulde r of the WAR S lead s t o a mode l fo r th e Mesozoic Cenozoic tectonomagmati c histor y o f th e wester n Ross Embaymen t (Fig . 10) . Durin g th e lat e Cre taceous, a n earl y rif t phas e occurre d wit h orthog onal extensio n tha t stretche d th e crus t an d th e underlying strong lithospheric mantle . Lithospheri c attenuation probabl y le d t o the productio n o f ver y small degre e partia l melts . Thes e wer e no t suf ficient t o giv e wa y t o surfac e magmatis m (amagmatic rift phase), but were essential i n distributing fertile , enriched , low-meltin g poin t veins/domains i n a wide zone of the Antarctic plate mantle. A t th e middl e Eocene , th e increas e o f differential velocit y alon g th e Souther n Ocea n Fracture Zones reactivate d th e Palaeozoi c tectoni c discontinuities in northern Victoria Land as intraplate dextral strike-slip fault systems . The activit y of

these lithospheric deformation belts promoted local decompression meltin g o f th e enriche d mantl e domains created during the late Cretaceous and isotopically mature d sinc e then (Fig . 11) . The magm a rose an d wa s emplace d alon g th e mai n NW-S E discontinuities an d alon g th e N- S transtensiona l faults array s departin g fro m th e maste r NW-S E systems (Fig . 10) . Thi s mode l relate s th e drivin g forces of events such as uplift, activ e faulting, magmatism, and seismicity to the dynamics of the Antarctic plat e rathe r tha n t o deep-sourc e force s suc h as mantl e plumes.

Conclusions The occurrenc e o f Cenozoi c magmatis m i n th e Ross Embayment has long been related to the presence of a mantle plume, associate d with the origi n and developmen t o f the whole West Antarctic Rif t System. Th e plum e hypothesi s wa s propose d o n the basi s o f geochemica l constraint s an d morpho logical evidenc e i n Mari e Byr d Land . Ou r revie w of the tectonomagmatic framewor k along the western shoulde r o f th e Ros s Se a cast s doub t o n th e mantle plume-related sourc e for magma generation and ascen t an d favour s intraplat e right-latera l strike-slip faultin g a s an alternative mechanism for magma genesis and emplacement. In particular, the tight lin k betwee n tectoni c activit y an d magm a emplacement suggest s tha t th e inherite d litho spheric fabri c o f northern Victoria Lan d led t o th e tectonomagmatic compartmentalizatio n o f th e whole lithosphere, wit h the boundaries between the

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Fig. 10 . Genera l mode l fo r th e Meso-Cenozoi c tectonomagmati c histor y o f th e wester n Ros s Embayment .

sectors playin g a n activ e rol e i n bot h mel t gener ation an d emplacement . We are gratefu l t o P. Armienti, M. D'Orazio, F . Mazzarini, an d F . Salvin i fo r th e stimulatin g discussion s o n th e

geology o f th e Ros s Se a region . Thank s ar e du e t o B . Murphy an d C . Macpherso n fo r th e constructiv e an d sti mulating review that helped u s to improve and clarify th e paper. Th e whol e wor k i s par t o f th e Italia n Antarcti c Research Progra m (PNRA) .

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Fig. 11. P- T diagram , modifie d afte r Smit h & Lewi s (1999) . Geotherm s an d adiabati c decompressio n path s afte r McKenzie & Bickle (1988) .

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Cenozoic Cap e Crossfire , Cap e Kin g an d N o Ridg e igneous complexe s (norther n Victori a Land , Antarctica). In : GAMBLE , J . A . (ed. ) 8t h International Symposium on Antarctic Earth Sciences Special Volume, Wellington, Ne w Zealand . ROCHOLL, A. , STEIN , M., MOLZAHN , M. , HART , S . R . & WORNER, G . 1995 . Geochemica l evolutio n o f rif t magmas b y progressiv e tappin g o f a stratifie d mantle source beneat h th e Ros s Se a Rift , Norther n Victori a Land, Antarctica. Earth and Planetary Science Letters, 131, 207-224 . ROSSETTI, F. , STORTI , F . & LAUFER , A . 2002 . Brittl e architecture o f th e Lanterma n Faul t an d it s impac t o n the final terrane assembly i n north Victoria Land , Antarctica. Journal o f th e Geological Society, London, 159, 159-173 . ROSSETTI, F. , STORTI , F . & SALVINI , F . 2000 . Cenozoi c noncoaxial transtensio n alon g th e wester n shoulde r o f the Ros s Sea , Antarctica , an d th e emplacemen t o f McMurdo dyk e arrays . Terra Nova, 12 , 60-66. SALVINI, F . & STORTI , F . 1999 . Cenozoi c tectoni c lin eaments o f th e Terr a Nov a Ba y region , Ros s Embay ment, Antarctica . Global an d Planetary Change, 23 , 129-144. SALVINI, F. , BRANCOLINI , G. , BUSETTI , M. , STORTI , F. , MAZZARINI, F . & COREN , F . 1997 . Cenozoi c geodyn amics of the Ross Sea region, Antarctica: crustal extension, intraplate strike-sli p faulting, and tectonic inheritance. Journal o f Geophysical Research, 102(B11) , 24669-24696. SALVINI, F., STORTI, F. , BRANCOLINI, G., BUSETTI , M . & DE CILLIA , C . 1998 . Strike-sli p induce d Cenozoi c basin inversio n i n the Ross Sea , Antarctica : kinemati c versus dynami c deformation . Terra Antartica, 5 , 209-215. SMITH, A . D . & LEWIS , C . 1999 . Th e plane t beyon d th e plume hypothesis . Earth Science Reviews, 48 , 135 182. SMITH, A . G . & DREWRY , D . J . 1984 . Delaye d phas e change du e to hot asthenospher e cause s Transantarcti c uplift? Nature, 309 , 536-538 . STERN, R . J., TEN BRINK, U . S . & BOTT , M . H . P. 1992 . Numerical modellin g o f uplift an d subsidence adjacent to the Transantarctic Mountains . In: YOSHIDA, Y. (ed,) Recent Progress i n Antarctic Earth Science. Terr a Scientific Publishin g Company , 515-521. STERN, T . A. & TEN BRINK, U. S. 1989 . Flexura l uplift of the Transantarcti c Mountains . Journal o f Geophysical Research, 94(B8) , 10315-10330 . STOREY, B. , LEAT , P . T. , WEAVER , S . D. , PANKHURST , R. J. , BRADSHAW , J . D . & KELLEY , S . 1999 . Mantl e plumes an d Antarctica-New Zealan d rifting : evidenc e from mid-Cretaceou s mafi c dykes . Journal of th e Geological Society, London, 156 , 659-671 .

STOREY, B. C. & ALABASTER, T. 1991 . Tectonomagmatic controls on Gondwana break-up models: evidence fro m the proto-Pacifi c margi n o f Antarctica . Tectonics, 10, 1274-1288. STORTI, F. , ROSSETTI , F . & SALVINI , F . 2001 . Structura l architecture an d displacemen t accommodatio n mech anisms a t the termination o f the Priestle y Fault , northern Victori a Land , Antarctica . Tectonophysics, 341 , 141-161. STUMP, E . 1995 . Th e Ross Orogen of th e Transantarctic Mountains. Cambridg e Universit y Press . STUMP, E . & FITZGERALD, P . G. 1992 . Episodi c uplift of the Transantarcti c Mountains . Geology, 20, 161-164 . SUN, S . S . & McDoNOUGH , W . F . 1989 . Chemica l an d isotopic systematic s of oceanic basalts: implications for mantle compositio n an d processes . In: SAUNDERS , A . D. & NORRY, M. J. (eds) Magmatism i n the Ocean Basins. Geological Society o f London Specia l Publication , 313-345. TESSENSOHN, F . & WORNER , G . 1991 . Th e Ros s Se a rif t system, Antarctica : structure , evolutio n an d analogues . In: THOMSON , M. R . A. , CRAME , J. A . & THOMSON , J . W. (eds ) Geological Evolution of Antarctica. Proceedings of the Fifth International Symposium on Antarctic Earth Sciences. Cambridge University Press, 273-277 . TONARINI, S. , ROCCHI , S. , ARMIENTI , P . & INNOCENTI , F. 1997 . Constraint s o n timin g o f Ros s Se a riftin g inferred fro m Cainozoi c intrusion s fro m norther n Vic toria Land , Antarctica . In : RICCI , C . A . (ed. ) Th e Antarctic Region: Geological Evolution and Processes. Terra Antartic a Publication , Siena , 511-521 . TREY, H., COOPER, A.K., PELLIS, G. , DELLA VEDOVA , B., COCHRANE, G. , BRANCOLINI , G. & MAKRIS , J . 1999 . Transect acros s th e Wes t Antarcti c rif t syste m i n th e Ross sea , Antarctica . Tectonophysics, 301, 61-74 . WEAVER, S . D. , STOREY , B . C. , PANKHURST , R . J. , MUKASA, S . B. , DlVENERE , V . J . & BRADSHAW , J . D .

1994. Antarctica-Ne w Zealan d riftin g an d Marie Byrd Land lithospheri c magmatis m linke d t o ridge subduc tion an d mantle plume activity. Geology, 22, 811-814. WILSON, T . J . 1995 . Cenozoi c transtensio n alon g th e Transantarctic Mountains-Wes t Antarcti c Rif t bound ary, souther n Victoria Land, Antarctica. Tectonics, 14, 531-545. WILSON, T . J. 1999 . Cenozoic structura l segmentation o f the Transantarctic Mountain s rift flank in southern Victoria Land . Global an d Planetary Change, 23 , 105 127. WORNER, G. , VIEREK , L. , HERTOGHEN , J . & NIEPHAUS , H. 1989 . Th e Mt . Melbourn e Volcani c fiel d (Victori a Land, Antarctica) . II . Geochemistr y an d magm a gen esis. Geologisches Jahrbuch, E38 , 395-^-33 . ZINDLER, A . & HART , S . 1986 . Chemica l geodynamics . Annual Revue o f Earth an d Planetary Sciences, 14 , 493-571.

Phanerozoic strike-sli p faulting i n the continental interio r platfor m o f the Unite d States: example s fro m the Laramid e Orogen , Midcontinent, an d Ancestral Rock y Mountain s S. MARSHAK 1, W. J . NELSON 2 & J. H . McBRIDE 3 1

Department of Geology, University of Illinois, 1301 W Green Street, Urbana, Illinois 61801, USA ^Illinois State Geological Survey, 615 E Peabody Drive, Champaign, Illinois 61820, USA ^Department of Geology, Brigham Young University, P. O. Box 24606, Provo, Utah 84602, USA Abstract: Th e continental interior platform of the United States i s that part of the North Amer ican crato n wher e a thin venee r of Phanerozoic strat a cover s Precambrian crystallin e basement . N- to NE-trending and W- to NW-trending fault zones , formed initially by Proterozoic/Cambrian rifting, brea k th e crus t o f th e platfor m int o rectilinea r blocks . Thes e zone s wer e reactivate d during th e Phanerozoic , mos t notabl y i n th e lat e Palaeozoi c Ancestra l Rockie s even t an d th e Mesozoic-Cenozoic Laramid e orogen y - som e remai n activ e today . Dip-sli p reactivatio n can be readil y recognize d i n cros s sectio n b y offse t stratigraphi c horizon s an d monoclina l fault propagation folds . Strike-sli p displacemen t i s har d t o documen t becaus e o f poo r exposure . Though offset palaeochannels , horizontal slip lineations, and strain at fault bends locally demon strate strike-sli p offset , mos t report s o f strike-sli p movement s fo r interior-platfor m fault s ar e based o n occurrenc e o f map-vie w belt s o f e n echelo n fault s an d anticlines . Eac h bel t overlie s a basement-penetratin g maste r fault , whic h typically splay s upwards into a flowe r structure . In general, bot h strike-sli p an d dip-slip component s o f displacemen t occu r i n th e sam e faul t zone , so some belts o f en echelon structure s occur o n the flank s o f monoclinal folds . Thus, strike-sli p displacement represent s th e latera l componen t o f oblique faul t reactivation ; dip-sli p an d strike slip component s ar e th e sam e orde r o f magnitude (tens of metres t o ten s o f kilometres). Effec tively, faults with strike-slip components of displacement act as transfers accommodating jostling of rectilinea r crusta l blocks . I n thi s context , th e sens e o f sli p o n a n individua l strike-sli p faul t depends o n block geometry , not necessaril y o n the trajectory of regional or l. Strike-slip faultin g in the North American interior differ s markedl y from tha t of southern and central Eurasia , poss ibly because o f a contrast in lithosphere strength . Weak Eurasia strained significantl y during the Alpine-Himalayan collision , forcin g crusta l block s t o underg o significan t latera l escape . Th e strong North American craton straine d relatively littl e durin g collisional-convergent orogeny , so crustal block s underwen t relatively smal l displacements .

Introduction ('basement'

) overlai n b y a relatively thi n (0-7 km thick) venee r o f unmetamorphose d Phanerozoi c The continenta l interio r platfor m o f th e Unite d sedimentar y strat a ('cover') . Locally , Neoprotero States, broadl y defined , consist s o f a zoi c t o Cambria n faile d rifts , o r aulacogens , fille d 2000 X 150 0 km regio n bounde d b y th e Canadia n wit h many kilometres o f sedimentar y an d volcanic Shield o n th e north , th e Appalachia n thrus t fron t rocks , cu t the crust. The interio r platform, together on th e east , th e Ouachit a thrus t front o n th e south , wit h th e Canadia n Shield , compris e th e Nort h and th e Cordillera n thrus t fron t o n th e wes t America n craton , tha t portion o f th e continen t no t (Fig. 1) . I n thi s region , th e crus t consist s o f affecte d b y penetrativ e deformatio n an d regiona l Archean through Mesoproterozoic crystallin e rocks metamorphis m during the past 1 billion years. The From: STORTI , F. , HOLDSWORTH , R . E . & SALVINI , F . (eds ) Intraplate Strike-Slip Deformation Belts. Geologica l Society, London , Specia l Publications , 210, 159-184, 0305-8719/037$ 15 © Th e Geologica l Societ y o f Londo n 2003 .

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interior platfor m o f Nort h Americ a include s thre e physiographic provinces : (1 ) Th e Midcontinent , a region o f broad, temperate plains in which bedrock exposures ar e generall y poo r t o non-existent ; (2 ) The Colorado Plateau, a semi-arid region now at an average elevatio n o f about 1. 5 km, i n whic h cove r bedrock is well exposed; an d (3) The Rocky Mountains, a semi-ari d regio n o f tal l basement-core d uplifts, separate d fro m on e another by deep basins, in whic h bedrock is locall y wel l exposed . The interio r platfor m o f Nort h Americ a lie s i n the forelan d o f Phanerozoi c convergen t an d colli sional orogens . Thus , i n term s o f location , th e region resemble s th e continenta l interio r o f southern an d centra l Eurasia , a regio n tha t lie s i n th e foreland o f th e Cenozoi c Alpine-Himalaya n orogen. Collisiona l tectonis m along the margins of central/southern Eurasi a generate d significan t (o f the order of tens to perhaps hundreds of kilometres ) strike-slip displacement s o n majo r regional-scal e faults i n orde r t o accommodat e latera l escap e o f crustal block s (e.g . Molna r & Tapponie r 1975 ; Tapponier & Molnar 1976) . I t i s fai r t o ask : Have significant strike-sli p displacements occurre d in the interior platform of the United States in associatio n with Phanerozoi c continental-margi n tectonis m of North America ? In thi s paper , w e presen t a revie w o f evidenc e for Phanerozoi c strike-sli p displacement s o n interior-platform fault s o f th e Unite d States . Afte r briefly reviewin g th e geologica l settin g o f th e interior platform, we addres s th e challenge of how to identif y strike-sli p displacemen t o n faul t zone s of th e region . W e the n revie w cas e studie s o f strike-slip displacements , firs t fo r th e Palaeozoi c (primarily, th e Carboniferous-Permia n Ancestra l Rockies event) , the n th e Mesozoic-Cenozoi c (th e Laramide event), and finally for the Holocene. W e Fig. 1 . (a ) Map of the USA continental interior, illustrating the distributio n of Midcontinent fault-and-fold zones . Modified fro m Marsha k e t al. (2000) . Thi s ma p show s the limit s o f Nort h America's interio r cratoni c platform, the portio n o f th e platfor m tha t ha s develope d int o th e Cenozoic Rocky Mountains, and the portion that has been uplifted t o for m th e Colorad o Plateau . The Midcontinent proper lie s betwee n th e Rock y Mountai n fron t an d th e Appalachian front. Abbreviations : BE = Belt embayment; UT = Uinta trough; WB = Williston basin ; O A = Oklah oma aulacogen; NU = Nemaha uplift; MC R = Midcontinent rift ; O D - Ozark dome ; R R = Reelfoo t rift ; I B = Illinois basin; N M — New Madri d seismi c zone ; L D = LaSalle belt ; N D = Nashvill e dome ; M B = Michiga n basin; CA = Cambridge arch; BG = Bowling Green fault ; M-S = Mojave-Sonor a megashear . Th e darke r shade d area i s th e Rock y Mountain s province , an d th e lighte r shaded are a i s th e Colorad o Plateau , (b ) Locatio n ma p showing the locatio n o f other area s discusse d in the text. The number s refe r t o figur e numbers .

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conclude that , though evidenc e for strike-sli p on a given faul t zon e ca n b e circumstantial , sufficien t data exis t overal l t o demonstrat e tha t strike-sli p deformation doe s occu r i n th e interio r platform of the Unite d States . But , in contras t wit h the strike slip deformation of south/central Eurasia, the magnitude o f thi s strike-sli p deformatio n i s small . Further, the pattern o f faulting i n the United States is very different fro m tha t of south/central Eurasia. We suggest tha t this contrast reflect s differences in lithosphere strength .

Geological backgroun d Structural style of the interior platform As note d above , th e continenta l interio r platfor m of th e Unite d State s ca n b e divide d int o thre e physiographic provinces . Becaus e th e Colorad o Plateau and Rocky Mountains provinces ar e so dramatically affecte d b y th e Cenozoi c uplif t an d deformation, whil e th e Midcontinen t provinc e i s not, th e forme r two ma y als o b e considere d t o b e part o f th e Nort h America n Cordillera . However , we emphasize tha t all three provinces shar e simila r crustal structur e - namely , Precambria n basemen t overlain b y a venee r o f Phanerozoic cove r - an d all have responded t o deformation by the formation of a similar style of faults an d folds, s o we consider all thre e province s i n thi s paper . Specifically , deformation o f th e interio r platfor m cause s dis placement o n basement-penetratin g fault s tha t splay up-dip in cross sectio n forming a fan o f subsidiary faults that resembles a flower structure (e.g . Marshak & Paulsen 1997 ; cf. Lowell 1985 ; Wood cock & Fischer 1986 ; Sylvester 1988) . While som e faults hav e bee n exhume d an d reac h th e contem porary land surface, man y are blind and die out updip in monoclinal folds before reaching the surface. In cross section , normal-sens e offse t remain s a t the level of the basement/cover contact on some faults , even i f reverse-sens e offse t occur s neare r th e ground surfac e (Fig . 2). Thi s configuratio n sug gests tha t thes e fault s initiate d wit h a norma l component o f slip , bu t wer e late r reactivate d wit h a revers e component . Base d o n trend , faul t zone s in Nort h America' s interio r platfor m fal l into tw o sets, on e nort h t o northeast , an d th e othe r wes t to northwest (e.g . Marshak & Paulsen 1997 ; Marsha k et al. 2000; Timmons et al. 2001). Thus, faults divide th e crus t o f th e interio r platfor m int o roughly rectilinear blocks (e.g . Chamberli n 1945) . A variety o f names have been use d for th e styl e of faultin g an d folding characteristic o f the interio r platform. Commonly , suc h structure s ar e calle d 'Laramide-style structures' , becaus e structure s o f this style formed in the Rocky Mountains and Colorado Platea u durin g th e 80-4 0 Ma Laramid e

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Fig. 2. A schemati c cros s sectio n o f the Rough Cree k Grabe n i n wester n Kentuck y an d adjacen t Illinoi s (lin e YY'). Note how the border faults hav e reverse-sense slip near the surface , bu t residual normal sens e sli p at depth. Note the flower structure at shallowe r depth. Source : Nelson & Lumm 1987 , as simplifie d b y Marshak & Paulsen (1997) .

orogeny. However , sinc e th e sam e styl e o f struc tures als o forme d i n th e interio r platfor m durin g pre-Laramide events, and in regions not affected b y the Laramide orogeny , th e ter m ca n be confusing . Alternative adjective s use d fo r thes e structure s include 'thick-skinned' , t o contras t the m wit h 'thin-skinned' structure s tha t li e abov e a shallo w subhorizontal detachment , an d 'basement-cored' , to emphasiz e deformatio n involve s basement penetrating faults . Example s fro m th e Midcontin ent region have also been called 'plains-typ e structures' t o emphasiz e tha t the y occu r i n th e Grea t Plains regio n (Power s 1928) , o r 'Midcontinen t fault-and-fold zones ' (Marsha k & Paulse n 1996 ; Marshak e t al. 2000). Monoclina l fold s forme d i n response t o faultin g i n th e interio r platfor m hav e also been referre d to by a variety of names. Earlier literature refers t o them a s 'force d folds ' o r 'drap e folds', t o emphasiz e tha t the y forme d i n respons e to a pus h fro m below . I n mor e recen t literature , these fold s ar e calle d fault-propagatio n folds , fol lowing th e terminolog y o f Supp e an d Medwedeff (1990), an d the process o f forming these folds can be referre d t o a s 'trishea r fault-propagatio n fold ing', because the strained region above the fault tip can be viewed as a triangular zone of shear (Erslev 1991; Allmendinge r 1998) . So far , w e hav e emphasize d th e similarit y o f structural style s i n th e thre e physiographi c prov inces o f th e interio r platform . But , though lat e Palaeozoic Midcontinen t fault-and-fol d zone s ar e identical i n styl e t o Mesozoic-Cenozoic structure s formed durin g the Laramide orogen y in the Rocky Mountains an d Colorad o Plateau , th e thro w o n Midcontinent zone s i s generall y muc h less tha n is typical fo r Rock y Mountain s or Colorad o Platea u examples (Marsha k & va n de r Pluij m 1997 ; McBride 1997 ; McBrid e & Nelson 1999 ; Marshak et al. 2000) . Specifically , maximum throw reache s about 1 5 km i n th e Rock y Mountain s province. , about 1. 5 km i n th e Colorad o Plateau , an d gener ally n o mor e tha n 15 0 m i n th e Midcontinen t

(though a fe w Midcontinen t fault s hav e displace ments o f u p t o 1. 5 km). Marshak an d Paulsen (1996) and Marshak et al . (2000), amon g others , argu e tha t majo r fault s o f the continenta l interio r platfor m initiate d durin g unsuccessful riftin g event s i n th e Proterozoi c an d Early Palaeozoic . Onc e formed , th e fault s hav e remained a s long-live d weaknesse s i n th e crust , because the y hav e neve r bee n anneale d b y meta morphism, o r stitche d togethe r b y plutons . Whe n boundary loads on the continent change orientation or magnitude , th e fault s ar e susceptibl e t o reacti vation. Thus , Phanerozoi c movement s o n thes e faults represent s sli p o n pre-existin g fault s — an individual faul t ma y b e reactivate d severa l time s (Holdsworth e t al. 2001) . Fault zone s an d relate d fold s ar e no t th e onl y consequences o f tectonis m i n Nort h America' s interior platform. The region has undergone epeirogenic movement s (gradua l vertica l displacement s of broa d areas ) t o form regional-scal e basin s an d domes (Fig . 1) , whos e presenc e ha s profoundl y affected th e distributio n of sedimentar y facies an d the thickness o f formations (e.g. McBride 199 8 and references therein) . Also, strat a of the interior plat form record regionally consistent strain in the for m of twinning in carbonates and deformation bands in sandstones. Strain magnitude recorded by twinning decreases markedl y from orogeni c front s toward s the interior . Bu t eve n i n th e centr e o f th e Unite d States, twinnin g strain s o f 1-3 % ca n b e docu mented (Craddoc k e t al . 1993 ; va n de r Pluij m e t al 1997) .

Deformation events in the interior platform Faults o f th e interio r platfor m hav e bee n affecte d by severa l episode s o f deformation. Contemporary seismicity indicate s tha t movemen t happen s today in a few locations , mos t notabl y th e Ne w Madri d seismic zon e i n th e centra l Mississipp i Valle y (Fig. 1) , and t o a lesse r degre e alon g a portio n of

STRIKE-SLIP FAULTIN G I N TH E US A CRATO N

the Nemah a uplif t i n northeaster n Kansa s an d th e southern Oklahom a aulacogen . Structure s o f th e Rocky Mountain s an d th e Colorad o Platea u wer e active durin g th e Laramid e orogeny , a shortenin g event tha t occurre d betwee n 8 0 and 40 Ma. Coney and Reynolds (1977) , amon g others, argu e that this event happene d i n response t o shallo w subductio n along the west coast, though, more recently, Maxon and Tikoff (1996) suggest that it is due to the colli sion o f a n exoti c terran e wit h Nort h America . Reactivation in Jurassic-Cretaceous time , in associ ation wit h Nort h Atlanti c rifting , ma y hav e trig gered normal-sens e reactivatio n o f fault s i n th e eastern par t o f th e interio r platform . Evidence fo r th e timin g o f Palaeozoi c tectoni c activity i n th e interio r platfor m come s primaril y from stratigraphi c studies . Fo r example , localize d unconformities an d shoals , bordere d b y clasti c wedges, indicat e formatio n o f uplifts , wherea s anomalously thick sections o f clastic strata indicate formation o f basins. Stratigraphic data indicate that movement o n fault-and-fol d zone s occurre d i n pulses durin g th e Ordovician , Devonian , an d Carboniferous-Permian (e.g . Klut h & Coney 1981 ; Nelson & Marsha k 1996 ; McBrid e & Nelso n 1999). Th e Carboniferous-Permia n even t wa s th e most significan t Palaeozoi c event , in that its conse quences are more widespread an d of greater magnitude that those of other events. Melton (1925 ) used the phrase 'Ancestra l Rocky Mountains' t o identif y uplifts tha t formed during this event, because many of th e uplift s ar e i n th e sam e o r simila r position s to th e present-da y Rock y Mountain s (Fig . 3). Th e uplifts o f th e Ancestra l Rockie s ar e fault-bounded

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blocks, man y o f whic h borde r dee p basin s fille d with thousands of metres o f sediments. Ve r Wieb e (1930) recognize d tha t th e Ancestra l Rockie s tec tonic even t als o affecte d a large are a o f the south ern Grea t Plains , producin g a series o f basins and uplifts (e.g . Nemaha, Matador), som e of which host major petroleu m fields . McBrid e an d Nelso n (1999) extende d th e Ancestra l Rockie s origi n far ther east , recognizing tha t many structure s in Missouri and Illinois share d structura l timing and style with th e classi c Ancestra l Rockie s structure s further west . Here , w e poin t ou t tha t timin g o f movements o n fault s i n Ohi o i s roughl y the same , emphasizing tha t th e Ancestra l Rockie s even t affected th e entir e continenta l interio r platfor m (Fig. 4). The caus e o f th e Ancestra l Rockie s even t ha s been debate d fo r decades. Klut h and Coney (1981 ) and Kluth (1986) sugges t that it represents a continental interior response to loads applied to the eastern an d souther n margi n o f Nort h Americ a durin g collision with Gondwanaland during the AlleghanianOuachita orogen y (Fig . 5A). Thi s mode l implie s that th e faultin g i s analogou s t o faultin g i n th e interior o f Asi a resultin g fro m collisio n o f Indi a with Asi a during the Himalayan orogeny . Alterna tively, Ye et al. (1996) compar e th e classic Ances tral Rockie s an d th e Cenozoi c Laramid e orogeny , and suggest that the former, like the latter, resulte d from compressio n i n th e forelan d o f a n Andeantype convergen t boundar y tha t existe d alon g th e southwest margi n o f Nort h Americ a (Fig . 5B). Thus, Ye et al. (1996) imply that the loading which triggered faul t movemen t wa s du e t o shallo w sub duction. Y e e t a/.' s mechanis m canno t explai n structures i n th e Grea t Plain s an d easter n USA , leading u s t o conclud e tha t bot h Alleghanian Ouachita collisio n an d Cordillera n convergenc e may hav e contribute d t o generatin g Ancestra l Rockies structures .

Tools for identifyin g strike-slip displacements o n faults of the continenta l interior platform

Fig. 3 . Ma p showin g th e distributio n o f th e 'classic ' Ancestral Rockie s uplift s i n th e wester n Unite d States . Source: Klut h 1986 .

Subsurface dat a (seismic-reflectio n profiles ; wel l logs) allo w geologist s t o characteriz e vertica l movements o n continental interio r fault s relativel y easily - offset s o f stratigraphi c horizon s an d the shapes o f layer s giv e a clea r imag e o f thi s move ment. Bu t ho w ca n w e determin e i f ther e i s a strike-slip componen t o n thes e faults ? Geologist s analyse th e kinematic s o f well-expose d strike-sli p faults b y studyin g sli p lineations , offse t markers , mesoscopic folds , e n echelo n veins , th e geometr y of Riede l shear s (and , i f myloniti c rock s ar e present, C- S fabrics , rotate d porphyroclasts , mic a

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Fig. 4. Ma p of the conterminous United States, illustrating the continent-wide distribution of structures resulting fro m the Ancestra l Rockie s event , an d th e maximu m principa l stres s directio n calculated fro m calcit e twinnin g i n th e Midcontinent (van der Pluij m e t al. 1997) . (Modified fro m McBrid e & Nelson 1999 ; Marshak e t al. 2000). Abbreviations: U = Uncompahgre, M = Matador-Red River , A B = Arbuckle Mts ; OA = Oklahoma aulacogen, N = Nemaha, L = LaSalle belt, BG = Bowling Green fault, CS = Cambridge-Burning Springs fault, W = faults of southern Wisconsin, C = Commerce fault zone , RC = Rough Creek, CG = Cottage Grove. The thick dashed line is the edge of the AtlanticGulf Coas t coastal plain.

fish, and porphyroclast tails). While suc h structures can be found locall y i n fault zone s of the Colorad o Plateau an d Rock y Mountains , i t i s rar e fo r the m to be visible in association wit h continental interior fault zone s o f th e Midcontinent . Indeed , man y interior-platform fault-and-fold s ar e blin d and , even wher e emergen t o r exhumed , ar e weathere d or poorl y exposed . I n essence , definin g strike-sli p kinematics o n these faul t zone s is a bit lik e recon structing th e skeleto n o f a Palaeolithi c homini d from thre e teet h an d a to e bone . W e emphasiz e from th e outse t that 7 i n studie s o f continenta l interior faul t zones , i t i s simpl y no t possibl e t o obtain th e qualit y o f dat a tha t i s usuall y expecte d for kinemati c studies . Indeed , par t o f ou r purpos e in this paper i s to illustrate the challenges involve d in studyin g thes e structures . Features tha t hav e bee n use d t o indicat e th e occurrence o f strike-sli p displacemen t o n interior platform faul t zone s includ e th e following : (1) Displacement o f markers: I t i s locall y poss ible t o estimat e th e sens e an d amoun t o f strike-slip offse t o n continental interio r fault s

by examinin g offse t isopachs , facie s bound aries, offse t palaeochannels , an d potential field anomalies . Offse t anomalies , however , are no t particularl y reliabl e indicators , because it is hard to demonstrate that anomalies on opposite sides of a fault were once continuous. (2) Mesoscopic structural analysis'. Standar d methods of mesoscopic structura l analysis can be use d to identif y strike-sli p component s of displacement o n continenta l interio r faul t zones, where the zones ar e exposed. Feature s that constrai n kinematic s includ e sli p lin eations, mesoscopi c fold s withi n th e faul t zone, rip-ou t clasts , an d mesoscopi c e n ech elon extension-gas h arrays . (3) E n echelon faults an d folds i n surface an d subsurface ma p view: It i s wel l know n fro m model studie s an d studie s o f well-expose d examples o f strike-sli p fault s tha t en echelo n faults an d fold s develo p i n strike-sli p shea r zones (e.g . Na y lor e t al . 1986 ; Sylveste r 1988). Thes e ca n eithe r b e a consequence o f

STRIKE-SLIP FAULTIN G I N TH E US A CRATON

Fig. 5 . Tw o competin g hypothese s fo r th e origi n o f Ancestral Rockie s structures , (a ) Klut h (1986 ) mode l relating th e Ancestra l Rockie s t o th e Alleghanian Ouachita collision . Th e cross-hatche d are a i s th e Trans continental Arch, a relatively high region of the continent during the Palaeozoic, (b) Ye et al. (1996) mode l relating the Ancestral Rockie s to a subduction zone on the southwestern margi n o f Nort h America . Th e patterne d area s are Ancestra l Rockie s uplifts .

accommodation fo r shortenin g an d extensio n oblique t o th e strik e o f th e fault , assumin g a model i n which the zone accommodates sim ple shea r in ma p view , or can consis t o f Riedel shear s forme d earl y durin g the rupturing of th e strat a a s displacemen t o n th e underlying basement-penetratin g faul t progresse s (Fig. 6; Smit h 1965 ; Mand l 1988) . (4) Flower structures'. Flowe r structure s ar e defined b y a n upward fan o f faul t splay s that merge a t dept h wit h a steepl y dippin g faul t (Sylvester 1988 ; Hardin g 1990) . I f ther e i s a thrust componen t o n th e faul t splays , a positive flowe r results, whil e if ther e i s a normal component o n th e faul t splays , a negativ e flower results. Flowe r structur e can b e ident ified i n seismic-reflectio n profiles , an d the y have been documented along many strike-slip faults. Th e presenc e o f flowe r structure s alone, however , i s no t sufficien t evidenc e t o confirm strike-sli p displacement , becaus e similar fault geometrie s ca n also develop simply b y inversio n o f antitheti c an d syntheti c fault splay s formed in the hanging-wall block above a n originall y norma l fault . (5) Vertical displacement components a t fault bends: A s describe d b y Sylveste r (1988) ,

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Fig. 6 . Cla y mode l illustratin g th e geometr y o f Riede l shears an d norma l fault s (extensio n gashes ) forme d i n a weak cla y laye r ove r tw o stron g woode n blocks , (e.g . Mandl 1988) .

among other s (e.g . Hold s worth & Pinheir o 2000), restrainin g bend s an d releasin g bend s may develo p alon g a strike-sli p faul t system . Reverse faultin g an d uplif t occu r acros s th e former, yieldin g push-up ridges, whereas normal faultin g an d subsidenc e occu r acros s th e latter, commonl y formin g a pull-apar t basin . The presenc e o f suc h bends , an d th e strai n that occur s a t them , suggest s th e occurrenc e and sens e o f strike-slip . In terms of reliability, offset linea r markers and slip lineations provid e th e bes t indicato r o f strike-sli p components o f movement . Displacement s o n restraining bends and releasing bends, may also be definitive. Th e occurrenc e of e n echelon structures may b e reliable , i f th e natur e o f th e structure s (Riedel shear s vs . extensio n gashes ) ca n b e specified. Claim s o f strike-sli p offset s tha t rely o n apparent offse t o f magneti c anomalies , o r o n th e occurrence o f flowe r structures , ar e les s reliable , but, nevertheless , suc h feature s ma y provid e th e only hin t o f strike-sli p movements .

Case studie s o f Palaeozoic strike-sli p In thi s sectio n w e discus s representativ e example s of structure s forme d durin g th e lat e Palaeozoic . Most o f thes e represen t Carboniferous-Permia n deformation - th e Ancestral Rockies even t - in the

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continental interio r platfor m of North America. Fo r some o f th e case s provide d below , ther e i s evi dence fo r earlie r movement .

North-central New Mexico Based o n a stud y o f offse t magneti c anomalies , Woodward et al. (1999) summarized evidenc e indicating that four nearly N-S-trending dextral strike slip fault s cu t acros s centra l an d norther n Ne w Mexico. Specifically , the y argue d tha t a serie s o f distinct magneti c low s define s th e northwester n boundary o f th e Mazatza l provinc e ( a NE-SW trending belt of post-1.7 Ga Proterozoic basement) , and tha t thes e low s ar e offse t t o th e sout h o n th e east sid e o f eac h faul t (Fig . 7). The y suppor t th e strike-slip hypothesi s b y notin g tha t seismic reflection profilin g indicate s tha t fault s spla y up dip into flower structures. Woodward et al., following Ye et al. (1996) , sugges t tha t the faulting happened i n respons e t o subductio n alon g th e south western margi n o f Nort h America . Th e map s provided by Woodward e t al indicat e that the combined displacemen t acros s thes e faults is 14 5 km in a right-latera l sense . W e poin t ou t tha t thi s dis -

placement i s muc h larger tha n strike-sli p displace ment o n other Ancestra l Rockie s faults . Thus, part of th e displacemen t ma y reflect Laramid e reacti vation, a s describe d b y Karlstro m an d Danie l (1993).

Paradox basin and Uncompahgre uplift The NW-SE-trendin g Uncompahgr e uplift , whic h cuts diagonall y acros s southwester n Colorado , i s one o f th e larges t uplift s o f th e classi c Ancestra l Rockies. I t lies alon g strike o f the Oklahoma aulac ogen, though there i s a gap in faulting between th e two. Formatio n o f th e uplif t brough t Precambria n metamorphic rocks up , relative t o adjacent Palaeo zoic strata . Th e metamorphi c rock s serve d a s a source fo r coars e sediment s tha t collecte d i n th e adjacent Parado x basin, which subsided until about 7.6 km o f structura l relief ha d forme d betwee n th e top o f the Uncompahgr e uplif t an d th e floo r o f th e Paradox basin . Baars & Stevenson (1982 , 1984 ) an d Stevenso n and Baar s (1986 ) argu e that th e faults forming th e boundary between th e Uncompahgre uplif t an d the Paradox basi n ha d a right-latera l componen t o f

Fig. 7. (a ) Ancestral Rockies in the New Mexico/Colorado region (adapted from Pazzaglia e t al. 199 9 and references therein), showin g the locatio n of tw o o f the strike-sli p faults propose d by Woodward et al. (1999) . The dar k shaded areas ar e Ancestra l Rockies uplifts , whil e th e ligh t stipple d areas ar e basin s formed durin g th e Ancestra l Rockies event, (b ) Woodwar d et a/'s . interpretatio n - offse t o f th e shea r zone definin g th e norther n edg e o f the Mazatza l province. Th e shade d area i s underlai n by pre-1. 7 G a basement , while the whit e area i s underlai n by th e younge r Mazatzal province.


strike-slip. Base d o n th e shap e o f th e Parado x basin, the y conclud e tha t th e Parado x basi n itsel f is a pull-apart basi n forme d i n respons e t o dextra l strike-slip. The principal evidence that they present for thi s mode l i s th e occurrenc e o f a n e n echelo n set of anticlines involvin g Pennsylvanian-age strata in th e centr e o f th e Parado x basin , alon g th e Colorado/Utah border (Fig. 8) . Here, isopach maps demonstrate tha t a n arra y o f NNW-trendin g anti clines occu r between NW-trending enveloping surfaces.

Matador uplift/Red River Arch (Texas) The Matador-Red River uplift consists of a narrow zone of fault-bounded uplifts an d troughs that trend

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E-W, fro m easter n Ne w Mexic o acros s th e Texa s Panhandle, t o th e Oklahom a border , a distanc e of more than 450 km (Fig. 4). Offset o f the basementcover contac t acros s th e zon e indicate s tha t ther e is a maximu m o f 1. 4 km o f dip-sli p offse t acros s the zone . Becaus e n o well s cu t acros s fault s eve n in densel y drille d areas , Rega n & Murphy (1986 ) concluded tha t fault s ar e essentiall y vertical . Notably, th e mai n fault s o f th e zon e chang e di p along strik e (i.e . the y ar e 'propelle r faults') , so , as a result , th e upthrow n sid e i s no t alway s o n th e same sid e o f a give n fault . Radica l difference s i n the thicknes s o f Pennsylvania n rock s occu r acros s the fault , indicatin g tha t displacemen t occurre d during the Ancestral Rockies event. Most faults die out up-di p within th e Earl y Permia n (Rega n & Murphy 1986) . Two source s o f evidenc e sugges t tha t a compo nent of strike-slip displacemen t accompanie d verti cal component s o f movemen t i n th e faul t zon e of the Matador-Re d Rive r uplift . First , NW-trendin g faults i n th e zon e defin e e n echelo n array s i n E W-trending envelopin g surface s - thi s geometr y hints a t a dextra l sens e o f shear . Second , pro prietary seismic-reflectio n profile s acros s a n uplifted segmen t revea l tha t i t i s underlai n b y a positive flowe r structure . A ne w stud y b y Briste r et al. (2002 ) reveal s a pull-apar t basi n alon g th e uplift. Th e geometr y o f thi s basi n suggest s a component o f left-latera l sli p alon g th e uplift .

Southern Oklahoma aulacogen

Fig. 8 . (a ) Locatio n ma p showin g th e Uncompahgr e uplift an d th e adjacen t Parado x basin , (b ) E n echelo n folds withi n th e Parado x basin , suggestiv e o f strike-sli p displacement accordin g t o Baar s & Stevenso n (1982) . Baars and Stevenson stat e that the fault i s dextral, though the arrangemen t o f anticline s look s lik e the y woul d b e associated wit h sinistral movement . Locatio n o f this ma p is shown in 'A'. (Adapted from Baar s & Stevenson 1982) .

The souther n Oklahom a aulacoge n originate d i n Cambrian tim e o r earlie r a s a failed rif t tha t filled with several kilometres of igneous rocks an d strata. Palaeozoic inversio n of the rif t bega n in Late Mis sissippian tim e an d continue d int o Earl y Permia n (i.e. durin g th e Ancestra l Rockie s event) . Thi s inversion yielde d a bel t o f WNW-trendin g uplift s and fault s tha t cu t acros s souther n Oklahom a an d the Texa s Panhandl e (Fig . 9). Locally , erosio n stripped th e cover t o expose underlyin g Precambrian crystalline rocks (Ham et al. 1964) . At the same time, flankin g basin s san k so , a s a result , vertica l relief between basin floors and crest of the adjacent uplifts i s a s grea t a s 1 4 km (Ha m 1978 ; Donova n 1986, 1995 ; Johnso n 1989 ; Perr y 19890) . Inversion o f th e aulacoge n clearl y involve d shortening oriente d roughly perpendicula r t o th e rift axis . Som e o f th e majo r rift-boundar y faults , originally basement-penetratin g norma l faults , became revers e faults , and , a s the y moved , fold s formed (Fig . 10A) . Bu t strike-sli p displacement s unquestionably als o occurre d durin g thi s event . Direct field evidence includes observations of horizontal an d obliqu e slickensides , e n echelo n fold s and shear zones, pull-apart grabens, and lateral off-

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Fig. 9 . Ma p o f the souther n Oklahoma aulacoge n an d adjacen t Ancestra l Rockies uplifts . Inse t shows the locatio n of this ma p area . (Adapte d fro m Budni k 1986) .

Fig. 10. (a ) Ma p o f a portion o f the souther n Oklahom a aulacogen, includin g th e Arbuckl e Mountain s an d Ard more basin (Hardin g 1985) . (b ) Classi c interpretatio n o f a seismi c lin e acros s nea r th e Ardmor e basi n (approximately lin e X—X' i n 'A' ) a s portrayed b y Hard ing (1985) .

sets o f fol d axes , formatio n contacts , an d isopac h lines (cf . Hardin g 1990) . Flowe r structure s ar e locally wel l develope d acros s thes e fault s (Fig. 10B) . Faults of the southern Oklahoma aulacogen stan d ou t amon g fault s o f th e interio r plat form becaus e their strike-slip displacements can be more clearly documented, and are an order of magnitude large r tha n those o f othe r examples . Both right-latera l an d left-latera l displacement s have been reported for the southern Oklahoma aulacogen, i n som e case s o n side-by-sid e fault s (Dunham 1955) . Estimate s o f th e left-latera l dis placement o n individua l fault s rang e fro m 4. 8 km (Perry 1989£ ) t o 6 4 km (Tanne r 1967) . Unfortu nately, a s Deniso n (1995 ) pointe d out , reliabl e piercing point s ar e difficul t t o com e by , an d esti mates of slip based on offset isopac h lines or facies trends of Palaeozoic units are subject to large errors due t o lac k o f wel l control , an d du e t o structura l complications. Nevertheless , withi n th e las t tw o decades, a consensu s favour s overal l left-latera l transpressive displacement, with strike-slip components o f offse t o n individual fault s in th e rang e of a few kilometres. Allowin g for the width and complexity of the fault zone , the overall lateral compo nent acros s th e whole zon e coul d be substantial up to tens of kilometres (McConnel l 1989; Denison 1995). Som e fault s i n th e Oklahom a aulacoge n appear to exhibit Holocene left-latera l obliqu e dis placements o f th e orde r o f 1 2 to 2 0 m (Cron e &

STRIKE-SLIP FAULTING IN THE US A CRATO N

Luza 1986 ; Madol e 1986 ; Ramell i & Slemmon s 1986).

Nemaha uplift (Kansas) and faults of northcentral Oklahoma The Nemah a uplif t trend s NN E fo r abou t 65 0 k m from Oklahom a Cit y t o Omaha , Nebrask a (Fig. 11) . I t i s a 10-8 0 k m wid e fault-bounde d uplift, containin g abundan t smal l horst s an d grab ens tha t overlie s th e souther n exten t o f th e 1. 1 Ga Midcontinent Rif t System . Thus, the Nemaha uplift formed b y inversio n o f rif t faults . Th e Nemah a structure was a high during most of the Palaeozoic , though occasionall y i t wa s submerge d (Berendsen & Blai r 1995) . Faul t movemen t occurred durin g the Early to Middle Pennsylvanian (i.e. durin g th e Ancestra l Rockie s event) . The main fracture zone that borders the east sid e of th e Nemah a uplif t i n Kansas i s called th e Hum boldt faul t zone . Seismic-reflectio n profile s indi cate tha t th e zon e include s high-angl e revers e faults. A s muc h a s 79 0 m o f cumulativ e dip-sli p displacement, dow n to the east, occurred across th e zone. Man y NW-trendin g transfe r fault s wit h

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throws a s grea t a s 450 m cros s th e Humbold t an d subdivide th e Nemah a uplif t (Berendse n & Blai r 1995). Based on the occurrenc e o f en echelon faul t patterns an d apparen t pull-apar t grabens , Berendsen an d Blai r inferre d tha t a left-latera l component o f strike-sli p displacemen t occurre d along th e Humbold t faul t zone , bu t th e tota l amount o f displacemen t remain s unknown . Seg ments of the Humboldt fault zone appear to be seismically activ e today , particularl y nea r Manhattan , Kansas (Burchet t e t al 1985) . Several belts of en echelon faults occur southeas t of th e Nemah a uplif t i n Oklahom a (Fig . 11) . Th e belts tren d N t o NNE , paralle l t o th e Nemah a uplift, an d ar e compose d o f fault s tha t strik e NW . The fault s tha t make u p the e n echelon belt s strik e N45-70°W an d di p 5 0 t o 65 ° eithe r northeas t o r southwest. Al l ar e norma l faults . Th e longes t i s about 5 km and the greatest throw about 40 m. The fault belt s paralle l th e strik e o f Upper Pennsylvan ian strat a i n thi s par t o f Oklahoma . Mappin g o f these fault s le d t o wha t may b e th e earlies t recog nition o f strike-sli p faultin g in th e America n Mid continent, by Path (1920 ) an d Foley (1926) . Usin g simple cla y model s fo r analogues , Pat h an d Fole y

Fig. 11. Faul t traces and en echelon fracture trace s from th e Nemaha uplift an d nearby fault zones . The shade d areas are th e area s represent the interior s of rifts . (Compile d from Fole y 1926 ; Berendsen & Blair 1995).

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proposed tha t the e n echelon zone s ar e the surface expression o f left-latera l movemen t o n fault s i n Precambrian basement .

Grays Point fault (Missouri) The Grays Point fault is part of the Commerce faul t system i n southeaster n Missouri . Thi s faul t paral lels th e northwester n margi n o f th e Reelfoo t rif t (Fig. 1) , whic h underlie s th e Mississipp i embay ment. Clendenin an d Diehl (1999) have interpreted the movemen t histor y o n th e Gray s Poin t fault , where it cuts exposures o f Ordovician an d Silurian strata expose d i n a quarry. By examinin g sidewal l rip-out clast s i n faul t veins , a s viewed i n thin sec tion, an d b y lookin g a t the geometr y o f subsidiar y faults whic h the y interpre t t o b e Riede l shears , Clendenin an d Dieh l argu e fo r a phas e o f lat e Palaeozoic dextra l strike-sli p o n th e Gray s Poin t fault. Th e detail s o f faul t geometr y ar e complex , and thus the interpretation o f this fault remain con troversial.

Plum River fault zone The Plu m Rive r faul t zone , whic h trend s N85° E across easter n Iow a an d northwester n Illinois , i s approximately 18 0 km lon g an d it s maximu m width i s abou t 1.2k m (Fig . 12) . Th e ne t vertica l

throw resultin g fro m movemen t i n th e zon e i s down t o th e north , an d range s fro m 3 0 t o 12 0 m. In bot h Illinoi s (Kolat a & Buschbac h 1976 ) an d Iowa (Bunke r e t al. 1985) , th e Plu m Rive r zon e consists o f nearl y vertica l fault s tha t border horst s and graben s i n Ordovicia n throug h Pennsylvanian sedimentary rocks . Fault s ar e marke d b y wid e zones o f silicifie d breccia , whic h contai n rotate d blocks o f dolomite . Th e zon e probabl y root s i n a Proterozoic basement-penetratin g faul t (Anderso n 1988). Stratigraphi c relationships suggest that most of th e movemen t o n it too k plac e betwee n Middle Devonian an d Middle Pennsylvania n - thu s the structure may have been activ e prior to the Ancestral Rockie s event . A strike-sli p componen t o f movemen t i n th e Plum Rive r faul t zon e i s indicate d b y th e occur rence o f a band o f N45° W t o N67° W e n echelo n faults borderin g the zone (Templeton 1951) . Thes e faults ar e vertica l t o steepl y dipping , displa y mostly normal offsets , an d contain vein-fille d breccias. Dee p graben s o f thi s orientatio n occu r i n Iowa. Th e occurrenc e o f horizontal sli p lineation s on smal l fault s tha t strik e N69° W t o N90° W i n a quarry just nort h o f the Plu m Rive r zon e support s this proposal . Mor e recen t mappin g (Fig . 13A) , indicates tha t th e zon e als o contain s NE-trendin g low-angle thrust faults. Because of the geometry of en echelo n fault s i n the Plu m River zone , we suggest tha t the zon e experienced a n episode o f right lateral oblique , down-to-the-north displacement. In this context , th e dee p graben s o f Iow a ar e smal l pull-apart basins . Ou r proposa l concur s wit h th e speculations o f Trap p an d Fenste r (1982 ) an d Heyl (1983) .

Sandwich fault zone

Fig. 12 . Sketc h ma p of the Illinois basin region, showing the locatio n o f principa l structura l features referred t o in the text . R R -Reelfoot rift; RC -Rough Cree k graben ; CG = Cottage Grov e fault ; C L = portion o f Commerc e geophysical lineament ;F A = Fluorspar Area ; LDB^LaSalle belt ; S F = Sandwich faul t = PR = Plum River fault ; W V = Wabash Valle y faul t zone .

The Sandwic h faul t zon e run s NW-S E approxi mately 13 5 km acros s norther n Illinoi s (Fig . 12) . As mappe d an d describe d b y Kolat a e t al. (1978) , the fault zon e is 1 to 3 km wide and contains vertical to steeply dipping normal and reverse fault seg ments tha t outlin e horst s an d grabens . Th e ne t throw alon g th e middl e portio n o f th e zon e i s a s much as 250 m down-to-the-northeast, but near the southeastern terminus of the zone the southwestern block i s downdropped . Stratigraphi c constraint s require onl y tha t movemen t occurre d betwee n th e Silurian an d the Pleistocene. Base d on exposures in quarries, Kolat a et al . (1978 ) an d Nelso n (19956 ) described a n arra y o f subparallel , NW-strikin g high-angle norma l fault s alon g wit h a fe w nearl y vertical revers e faults . I n a n unpublishe d manu script, Templeton (1951 ) presented details on structural feature s nea r the town o f Orego n (Fig . 13B) . Here, a narrow , NW-trending hors t o f th e Cambr ian Franconi a Sandstone , upthrow n b y 75-9 0 m

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Fig. 13. (a ) Sketc h ma p o f a portion o f th e Plu m River faul t zon e showin g en echelon grabens , indicativ e o f dextral displacement, (b ) Sketc h ma p o f a portion o f the Sandwic h fault zon e nea r th e tow n of Oregon , illustratin g graben s and horst s incline d t o th e mai n faul t zone .

and juxtapose d wit h th e Middl e Ordovicia n S t Peter Sandston e o n bot h sides , i s expose d alon g the eas t sid e o f th e Roc k River . Templeto n als o describes a se t o f WNW-strikin g fault s tha t for m an e n echelo n array . The structura l features described b y Templeton , Kolata e t al., an d Nelso n ar e evocativ e o f strike slip faultin g - e n echelon border-faul t arrays , and both horst s (th e slic e o f Franconia ) an d graben s occur alon g map-vie w bend s i n th e system . Notably, no t al l th e feature s indicat e th e sam e sense o f shear . Specifically , the orientatio n o f th e Franconia slic e suggests tha t it lies at a restrainin g bend, ye t thi s upthrow n slice ha s th e sam e orien tation a s th e releasin g ben d o r extensiona l duple x northwest o f Oregon . Perhap s th e Sandwic h faul t zone has ha d tw o (o r more) episode s o f tectonism with strike-sli p components , on e dextra l an d on e sinistral movement .

Southwestern Wisconsin Several strike-sli p fault s hav e been reporte d i n th e Upper Mississipp i Valle y Zinc-Lea d District . I n a highly detaile d treatis e o n th e district , Hey l e t al . (1959) cite d severa l example s o f strike-sli p fault s encountered withi n undergroun d mines . On e

WNW-trending fault, in the Liberty Mine of Lafayette County , Wisconsin , ha s horizonta l striation s and display s left-latera l offse t o f 7. 6 m. A nearb y fault strikin g N- S produce d a n apparen t right lateral offse t o f 60 m on an ore body between tw o adjacent mines . A NW-strikin g fault , th e Miffli n Fault o f Iow a County , Wisconsin , produce d approximately 30 0 m of right-lateral displacemen t on a n or e bod y an d tw o NE-trendin g fol d axes .

Cottage Grove fault system (Illinois) The Cottag e Grov e faul t zone , a n E-W-trendin g structure tha t ca n b e trace d fo r 115k m acros s southern Illinoi s (Fig . 12) , i s arguabl y th e best documented exampl e o f strike-sli p faultin g i n th e Midcontinent (Clark & Royds 1948 ; Heyl & Brock 1961; Hey l e t al . 1965 ; Wilco x e t al . 1973 ; Nel son & Krauss e 1981) . Althoug h thi s structur e ha s little surfac e expression, seismic-reflectio n profiles, mineral-exploration boreholes , an d exposure s o f the zone in coal mines provide abundant kinematic information. Seismic-reflectio n section s (Fig . 14) show tha t faul t displacement s affec t th e entir e Palaeozoic sectio n and disrupt the top of Precambrian basement 3. 0 km below th e surface (Duchek e t al. 2001) . Vertica l fault s i n th e lowe r Palaeozoi c

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S. MARSHAKCTAL .

Fig. 14 . Migrate d seismic-reflectio n profil e acros s th e Cottage Grov e faul t syste m illustratin g th e flowe r struc ture at shallow depths. K x = Knox Group; pC = top of the Precambrian. Line i s located ove r the easter n par t of th e fault (Fig . 15) .

section propagat e upward s int o flowe r structures . The timin g o f faultin g ca n b e constraine d i n par t by stratigraph y (th e younges t rock s displace d ar e early Lat e Pennsylvanian) , an d i n par t b y radio metric datin g o f ultramafi c dykes , whic h cu t th e faults (Earl y Permian). These observations indicat e that th e Cottag e Grov e faul t syste m i s Lat e Pennsylvanian to Early Permian i n origin, and thus moved durin g th e tim e o f th e Ancestra l Rockie s event (Nelso n & Lumm 1987) . Near the ground surface, the Cottage Grov e fault zone include s a sinuou s maste r faul t tha t locall y bifurcates, formin g tw o paralle l strands . A s exposed i n coa l mines , th e maste r faul t include s both high-angl e norma l an d revers e faults , wit h vertical throw s o f a s muc h a s 6 0 m. Notably , th e side o f th e faul t tha t ha s move d relativel y dow n reverses severa l time s alon g the length o f the faul t (i.e. th e faul t i s a scissor fault) . Hundred s o f sub sidiary NW-trendin g norma l an d oblique-sli p e n echelon faults , wit h maximu m vertica l separatio n of 18m , border the maste r faul t (Fig . 15) . Kinem atic indicator s o n these faults includ e horizontal o r obliquely plungin g sli p lineations , an d latera l off sets o f vertica l contact s expose d undergroun d mines (Nelso n & Krausse 1981) . Several anticline s have bee n mappe d alon g th e Cottag e Grov e faul t system, al l of which lie close to the master fault , or directly in line with the fault's westwar d extension

(Fig. 15) . Som e o f th e anticline s tren d ENE , for ming a n e n echelo n syste m (Nelso n & Krauss e 1981). Evidence fo r a strike-sli p componen t o f dis placement o n th e faul t zon e come s fro m a variety of sources . First , a s noted above , e n echelo n fold s and subsidiar y fault s occu r withi n th e zone . Th e geometry of these suggests that they formed during dextral shea r acros s th e zone . Second , i n under ground coa l mines , Nelso n an d Krauss e (1981 ) mapped many mesoscopic faults wit h horizontal or gently plungin g slickenside s an d mullion , an d lat eral offset s (u p t o a fe w metres ) o f stratigraphi c contacts o r o f othe r fault s provid e evidenc e fo r lateral motion . Third , sli p o n th e maste r faul t produced 0.6-1. 6 km o f dextra l offse t o f th e boundaries o f a Pennsylvania n palaeochanne l (Nelson & Krauss e 1981) . Finally , bot h seismic reflection profiles, an d cross sections prepared fro m coal min e data , sho w positive an d negative flower structures along the master fault, wit h the appropri ate orientations to be associated with dextral strikeslip displacement .

Bowling Green Fault (Ohio) The Bowling Gree n faul t zon e ha s been trace d fo r about 10 0 km, along a NNW trend in northwestern Ohio an d adjacen t Michiga n (Fig . 4). I t lie s ove r the Grenville Front , a major Lat e Proterozoic crus tal boundar y tha t separate s high-grad e metamor phic rock s o f th e 1. 1 Ga Grenvill e oroge n o n th e east fro m a n unmetamorphose d 1. 3 Ga granite rhyolite terrane on the west (Onasch & Kahle 1991; Wickstrom et al 1992 ; Root 1996). Though largely buried, th e zon e i s expose d i n a numbe r o f lime stone quarries , an d ha s als o bee n studie d throug h data fro m oi l an d ga s exploratio n hole s an d seis mic-reflection profiles . These dat a indicate tha t the zone consist s o f high-angl e revers e an d norma l faults tha t exten d fro m th e bedroc k surfac e down ward i n Precambria n basemen t (Wickstro m e t al . 1992), as well as low-angle thrust faults (Onasc h & Kahle 1991) . Overall displacement across the faul t dropped th e easter n sid e dow n by 15 0 m. Two recen t publication s presen t divergen t interpretations o f th e Bowlin g Gree n faul t zone , but bot h conclud e tha t a t times , left-latera l offse t developed acros s th e zone . Onasc h an d Kahl e (1991) inferre d si x episode s o f movement , begin ning in the Late Ordovician. Fiv e of these episode s involved dip-slip but the third produced left-lateral motion a s show n by nearly horizontal slickenside s on Siluria n dolomite . In contrast, Wickstro m e t al . (1992) suggest the zone had three major period s of activity; Precambrian , Lat e Ordovician , an d post Silurian. Thes e author s inferre d a significan t left lateral componen t o f displacemen t durin g th e

STRIKE-SLIP FAULTIN G I N THE US A CRATO N

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Fig. 15 . Geologica l ma p o f th e Cottag e Grov e faul t syste m an d nearb y structure s o f souther n Illinois . Not e th e e n echelon fault s that border the master fault. Modified from Nelso n and Krausse (1981) an d Nelson (1995^) . Inset shows the strai n ellips e fo r dextra l strike-sli p faultin g (Nelso n & Krauss e 1981) . Se e Figur e 1 2 for location .

Ordovician - rock s eas t o f th e faul t movement northward. The y bas e thi s conclusio n o n th e geometry o f a restraining bend .

Burning Springs-Cambridge fault zone (Ohio and West Virginia) This zone can be traced for 350 km, trending NNW across Ohi o an d int o West Virginia , thoug h it ca n barely b e recognize d a t th e groun d surfac e (Roo t 1996). Th e zon e i s severa l kilometre s wide , an d contains severa l faul t strand s wit h a ne t displace ment measure d i n ten s o f metre s (Roo t & Onasc h 1999). A seismi c sectio n acros s th e zon e (Deylin g 1993, reproduced in Root 1996) , portrays a vertical fault i n Precambria n basemen t tha t branche s upward into a positive flower structur e that dies out up-dip i n a bo x anticline . Roo t (1996 ) propose d that th e zon e underwen t right-latera l movemen t during th e Alleghania n orogeny , base d o n recog nition o f flowe r structur e alon g th e fault , an d th e occurrence o f a left-steppin g restraining bend .

Rough Creek fault system (Kentucky and Illinois) The Roug h Cree k faul t syste m extend s E- W approximately 21 0 km acros s wester n Kentuck y and souther n Illinoi s (Fig s 2 & 12 ; Nelso n 1991 ; Nelson 1995a). I t i s on e o f th e larges t faul t zone s of th e Midcontinent , an d ha s undergon e severa l episodes o f displacemen t datin g bac k a t leas t t o Cambrian time . Th e zon e marks th e northern mar gin o f a Lat e Proterozoi c o r Cambria n faile d rift , the Roug h Cree k Grabe n (Soderber g & Kelle r 1981). A number of geologists , includin g Clar k &

Royds (1948) , Hey l & Broc k (1961) , Hey l e t al (1965), an d Heyl (1972 ) postulated tha t the Rough Creek ha s a left-latera l strike-sli p componen t o f displacement, becaus e th e faul t i s bordere d b y a belt o f NE-trending en echelo n faults , an d becaus e the master fault splay s up dip into a positive flower structures. Nelson and Lumm (1987) an d Lumm et al. (1991), however, examine d th e fault syste m and concluded tha t th e flowe r structur e o f th e bel t i s dominantly a consequenc e o f faul t inversio n o f a rift margin , no t o f strike-slip . I f a strike-sli p component o f displacemen t occurre d o n th e fault , then th e displacemen t wa s minimal , fo r no signifi cant pull-apart basin has developed a t the west end of th e faul t zone , wher e th e zon e makes a n abrupt 60° ben d t o th e south . Further , Pennsylvania n palaeochannels tha t cros s th e faul t syste m in Ken tucky ar e no t offse t laterall y (Davi s e t al . 1974) . In one place where the fault syste m exhibits 450 m of dip-sli p displacement , palaeochanne l mappin g limits possibl e strike-sli p t o les s tha n 30 0 m.

Case studie s o f Mesozoic-Cenozoic strike slip Structures that formed in the portion o f the Unite d States continenta l interio r platfor m tha t la y i n th e foreland o f the Cordilleran oroge n wer e active during th e Laramid e orogen y o f lat e Mesozoi c an d early Cenozoic time . Their developmen t resulte d in both th e monocline s o f th e Colorad o Platea u an d the towering basement-core d uplifts o f the present day Rock y Mountains . Numerou s author s hav e pointed out that strike-slip component s of displacement occur on some faults o f the region (e.g. Sale s 1968; Ston e 1969) .

174

S. MARSHAK£TAL.

East Kaibab Monocline (Utah-Arizona) The Eas t Kaiba b Monoclin e i s on e o f th e large r Laramide monoclines of the Colorado Plateau, with vertical structura l relie f o f 1. 6 km. Th e monoclin e has a sinuou s trace , 18 0 km long , bu t generall y trends N- S wit h a n E-facing stee p lim b (Fig . 16) . Exposures i n th e Gran d Canyo n demonstrat e tha t the monoclin e i s a fault-propagatio n fol d forme d due to reverse-sense displacemen t o n a W-dipping basement-penetrating faul t (Huntoo n 1993 ; Hun toon et al. 1996). Exposures within the canyon also show that the fault originated as a normal fault during Proterozoi c tim e (boundin g a half-grabe n which fille d with strat a o f th e Gran d Canyo n Ser ies, the n wa s reactivate d a s a reverse faul t durin g the Laramid e orogen y (Walcot t 1890 ; Maxso n 1968; Beu s & Morale s 1990 ; Tindal l & Davi s 1999). Tindall an d Davi s (1999 ) provide d a detaile d structural analysi s o f th e norther n 50k m o f th e East Kaiba b Monocline , showin g that , i n additio n

to reverse-sens e componen t displacement , ther e i s a componen t o f right-latera l displacement . Thei r mapping reveal s tha t a multitud e o f e n echelo n faults, whic h bear shallowl y raking sli p lineations , cut th e stee p lim b o f th e monoclin e i n souther n Utah. I n detail , tw o e n echelo n faul t set s ar e present, on e trendin g N W an d th e othe r trendin g NE. Tindal l an d Davis (1999) conclud e that development o f th e monoclin e involve d approximatel y 1.6 k m o f revers e displacemen t an d a s muc h a s 8.0 k m o f dextra l strike-sli p displacement . Thus , movement o n th e faul t underlyin g th e monoclin e was oblique-slip . Th e sens e o f sli p i s compatibl e with regiona l NE-S W Laramid e shortening .

Owl Creek Mountains (Wyoming) Laramide structure s o f th e Rock y Mountain s i n Wyoming rang e i n tren d fro m NN W t o WN W (Fig. 17A) . Some author s have argued that the different trend s forme d i n respons e t o tw o separat e shortening event s wit h radicall y differen t orien -

Fig. 16. (a ) Sketc h map o f the Eas t Kaibab monoclin e i n Uta h an d Arizona , (b ) Detai l of e n echelon faulting alon g the trac e of th e Eas t Kaibab monocline, (c ) Cross sectio n of the Eas t Kaibab monocline. (Modified fro m Tindal l & Davis 1999 ; Huntoon e t al. 1996) .


175

Fig. 17 . (a ) Regional map showing th e Laramide uplift s and structures o f the Colorado Plateau and the Rocky Moun tains, (b ) Ma p fro m Ston e (1969 ) showin g trace s o f fold s adjacen t t o fault s i n Wyoming , suggestiv e o f strike-slip . (c) Ma p fro m Paylo r an d Yi n (1993 ) illustratin g th e e n echelo n fold s alon g th e Nort h Ow l Cree k fault . Boxe s show locations .

tations (e.g . Grie s 1983) . However , a growin g consensus favour s a singl e NE-S W shortenin g direction, wit h a relativel y mino r amoun t o f rotation (e.g . Varg a 1993 ; Ersle v & Wiechelma n 1997). If there is a uniform regional NE-SW short ening direction, the n one might predict oblique-sli p to strike-sli p displacemen t o n fault s tha t ar e no t NW-trending. Stone (1969 ) applie d thi s concept t o the entire Rocky Mountai n region , an d interprete d numerous array s o f e n echelo n anticline s an d thrusts t o be indicative of strike-slip displacement s (Fig. 17B) . Brow n (1993 ) reviewe d th e evidenc e for strike-sli p displacements an d suggested severa l locations wher e it probably occurred . On e of thes e locations occur s alon g th e flan k o f th e Ow l Cree k Mountains. Paylor an d Yin (1993) investigated th e kinematics o f the North Ow l Creek faul t syste m in detail, an d demonstrate d tha t E-W-trendin g faul t segments d o displa y strike-sli p lineations , an d ar e bordered b y e n echelo n fold s an d thrust s tha t locally defin e a transpressiona l duple x (Fig . 17C) . They conclud e that the fault is , effectively, a sinis-

tral latera l ram p tha t act s t o transfe r displacemen t between tw o E-dippin g fronta l ramps .

Cat Creek anticline and Lake Basin fault zones (Montana) The Ca t Creek anticlin e i s a 10 0 km long by 8-1 9 km wid e fol d tha t trend s WN W acros s th e plain s of central Montan a (Nelso n 19930 , b, 1994, 1995Z? ; Fig. 18A , B) . It s northeas t lim b dip s steepl y (30 90°), wherea s it s southwes t lim b ha s a gentl e di p (1-6°). Borehol e penetration s an d seismi c dat a indicate tha t the stee p flan k o f the Ca t Cree k anti cline (Fig . 18B ) i s underlai n b y a SW-dippin g reverse fault , th e Ca t Cree k fault , whic h dip s 55 70° i n Mesozoic strata , flattenin g slightl y a t dept h to 45-60° in Palaeozoic strata . The Cat Creek faul t bifurcates upwar d an d die s ou t withi n th e Uppe r Cretaceous shal e sectio n befor e reachin g th e sur face, an d ha s undergon e a t leas t fiv e episode s o f displacement. I t began a s a normal fault durin g the Proterozoic (Sonnenber g 1956 ; Shepar d 1987) ,

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S. MARSHAKETAL.

Fig. 18. (a ) Sketc h o f Montana , showin g th e locatio n o f Laramid e strike-sli p faults , includin g the Ca t Cree k fault . (b) Cros s sectio n o f th e Ca t Cree k fault , showin g th e associate d fold .

underwent revers e movemen t i n th e Cambria n (Nelson 1993a , Z>) , and again in the Devonian. Normal displacemen t occurre d betwee n Middl e Pennsylvania an d Middl e Jurassic , an d fina l reverse/left-lateral obliqu e displacemen t occurre d during th e Laramid e orogen y (Nelso n 1993a , b, 1994, 1995/7) . The left-latera l componen t o f displacemen t o n the Ca t Cree k faul t durin g th e Laramid e i s indi cated b y e n echelo n fold s an d faults . Specifically , nine domes are arrayed along and south of the crest Of th e Ca t Cree k anticlin e (Fig . 18C) . Thei r axe s are slightl y obliqu e t o tha t o f th e mai n anticline , so the y for m a left-hande d en echelo n set . A bel t of NE-trendin g e n echelo n norma l fault s follow s the cres t o f th e anticline . N o piercin g point s hav e been identified , s o th e exac t magnitud e o f left lateral displacemen t canno t b e measure d directly , However, th e sigmoida l bendin g o f NE-striking en echelon fault s adjacen t t o th e mai n faul t suggest s a maximu m latera l displacemen t o f abou t 1.4km , a valu e tha t i s slightl y greate r tha n th e maximum dip-slip componen t (Nelso n 1995&) . The Ca t Creek anticlin e i s not a n isolated struc ture. I n centra l an d easter n Montana , Laramid e faults strik e east ENE and ESE, an d are either left lateral strike-slip or are oblique transpressive , wit h reverse-left-lateral motio n (Fig . ISA) . Th e left lateral componen t o f thes e structure s i s indicate d by belts o f NE strik e en echelon fractures , a s illus-

trated b y the Lake Basi n fault, whic h trends abou t N80°W an d contain s a bel t o f e n echelo n norma l faults tha t strik e northeas t an d di p a t 3 0 60° (Chamberlin 1919 ; Hancoc k 1919 , 1920 ; Robin son & Barnum 1986 ; Lope z 2000) . Judging by the small magnitud e of surfac e displacements , and th e absence of a through-going master fault , th e latera l component o n th e Lak e Basi n faul t i s probabl y small ( 1 km or less).

Holocene examples Commerce fault zone Outcrops and borehole studie s indicate that Palaeo zoic strat a o f souther n Illinoi s an d Missour i ar e extensively cu t by NE-trending faults . This regio n of faultin g include s the Fluorspa r Are a faul t com plex, know n fo r economi c deposit s o f fluorspa r precipitated fro m fluid s passin g alon g th e faults , and th e Commerc e faul t zone , whic h lie s t o th e west of the Fluorspa r Area faul t comple x (Fig . 12 ; Nelson 1991 ; Nelso n e t al 1997 , 1999) . Th e Commerce fault zone has displaced Holocene sediment s (Harrison e t al . 1999) . I n southeaster n Missouri , the fault-zon e correspond s t o th e regionall y extensive Commerc e geophysica l lineament , tha t parallels th e Reelfoot rift and the trace of the New Madrid seismi c zon e (Harriso n & Schult z 1994 ; Hildenbrand & Rava t 1997 ; Langenhei m & Hild -


177

enbrand 1997) . Langenhei m an d Hildenbran d argue tha t ther e ha s bee n left-latera l sli p o n th e fault, base d o n thei r interpretatio n o f offse t mag netic anomalies . However , exposure s o f th e faul t are decorated wit h slip lineations that indicate dextral strike-slip displacement (Harriso n e t al. 1999).

New Madrid seismic zone By fa r th e mos t activ e seismi c regio n i n th e Mid continent i s th e Ne w Madri d seismi c zon e (Johnston & Shedlock 1992) . The zone was the site of thre e o r fou r earthquake s tha t ha d body-wav e magnitude o f 7. 0 o r greate r i n 1811-181 2 (Fig. 19A) . Th e fault s o n whic h th e earthquake s occurred li e burie d beneat h gravel s o f th e Mississippi Valley , s o they cannot be examined in outcrop, bu t geophysica l studie s sho w tha t th e earthquakes occu r i n and along th e Reelfoot rift , a NNE-trending troug h that forme d in Lat e Protero zoic t o Earl y Cambria n time , an d wa s reactivate d in pulse s throug h th e Phanerozoi c (Ervi n & McGinnis 1975 ; Brail e e t al . 1986) . Seismi c activity concentrate s alon g tw o NE-trendin g belt s linked b y a shor t NW-trendin g belt . Foca l mech anisms indicat e tha t earthquake s o n th e northeas t segments o f Ne w Madri d seismi c zon e ar e du e t o right-lateral strike-slip , whil e movemen t o n th e NW-trending segment is due to thrust displacement (with th e hangin g wal l movin g northeast ; Staude r 1982; Prat t 1994 ; Va n Arsdale e t al 1998) . Thus, the northwes t segmen t behave s lik e a restrainin g bend linkin g tw o non-coplana r fault s (Fig . 19B). Notably, a smal l uplif t ha s develope d ove r th e thrust segment . Th e movemen t i s compatibl e wit h the contemporar y regiona l stres s fiel d o f easter n North America , whic h indicate s tha t maximu m compressive stres s trajectorie s tren d NE-S W (Zoback & Zoback 1980) .

Discussion an d conclusions The continenta l interio r platfor m o f th e Unite d States is the portion of the craton where a veneer of Phanerozoic sedimentar y strata covers Precambrian crystalline basement . I t ca n b e divide d int o thre e physiographic provinces: Rocky Mountains, Colorado Plateau , an d Midcontinent . Regional-scal e faults occu r i n al l thre e provinces , thoug h i n th e Midcontinent most fault s ar e not well exposed and thus ar e known primarily fro m subsurfac e studies . Overall, faults fal l int o two sets , based on trend: N to NE trending, an d W to NW trending. The fault s probably initiated in the Proterozoic, i n response to crustal extension , an d thu s thei r orientatio n doe s not reflec t Phanerozoi c stres s fields . Rather , Phanerozoic episode s o f slip on the faults represen t reactivation in response t o boundary loads applie d

Fig. 19. (a ) Ma p showin g th e schemati c locatio n o f earthquake epicentres i n the New Madrid seismic zone of southern Missouri, (b ) Interpretive sketch , illustrating the sense o f sli p o n faults , base d o n fault-plan e solution s (Chiu e t al . 1992) .

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S. MARSHAKEJAL.

to the continent by marginal convergent and/o r collisional orogeny . Becaus e th e faults ar e typicall y not paralle l t o th e tren d o f a principa l stress , sli p during their reactivation mus t be transpressional o r transtensional (i.e . oblique-slip ; Bot t 1959) . Vertical displacemen t component s o n interior platform fault s ca n b e identifie d relativel y easil y on seismic-reflectio n line s o r drillhol e controlle d cross sections , becaus e the y caus e distinc t strati graphic offsets . Bu t strike-sli p displacements , a s we hav e shown , ar e generall y har d t o identify , because the y ar e not obviou s in cros s section , an d rarely caus e obviou s latera l offset s a t th e surface . Specific clue s tha t hav e bee n use d t o strike-sli p include belts o f en echelon faults , e n echelon anti clines, offse t palaeochannels , occurrenc e o f sub horizontal lineation s o n faul t surfaces , an d displacements a t restrainin g an d releasin g bends . Our survey of studies claiming strike-sli p displace ments o n interior-platfor m fault s indicate s tha t most of these interpretations are based primarily on the occurrenc e o f e n echelo n structures . Researchers studyin g interior-platfor m faultin g cannot, in general, obtai n the qualit y of data available t o thos e studyin g fault s i n Phanerozoi c ero genic belts . Based o n th e example s describe d i n thi s paper , we conclude tha t 'typical ' strike-sli p displacemen t on fault zones of the interior platform of the United States i s expresse d a t the surfac e b y a belts o f e n echelon second-orde r faults . Suc h belt s ar e u p t o 100-600 km long, an d 2-20 k m wide. Most of the en echelo n fault s ar e norma l o r normal-obliqu e faults and , in cross section , hav e listric geometrie s so they cu t onl y th e uppe r part o f the sedimentar y rock column . In som e cases , however , th e e n ech elon fault s ar e strike-sli p o r oblique-slip . E n ech elon dome s o r anticline s als o for m alon g som e strike-slip faults. Where subsurfac e data ar e avail able, the y demonstrat e tha t through-goin g maste r faults underli e e n echelo n system . Suc h maste r faults ar e vertica l o r nearl y s o an d bifurcat e upward, producin g flowe r structures . Dip-sli p components o f motion s o n thes e fault s lea d t o development o f monoclina l uplift s (fault-propa gation folds) . Information o n the magnitud e o f offse t i s avail able for relatively fe w interior-platform fault zones. In th e cas e o f documente d examples , thes e zone s have bot h strike-sli p an d dip-slip component s tha t range from a few tens of metres to more than 2 km. Generally, th e overal l strike-sli p componen t i s comparable t o or less tha n the dip-sli p component , so mos t interior-platfor m faul t zone s ar e bes t described a s oblique slip-fault s - pur e strike-sli p faults ar e rare . Th e observatio n tha t continenta l interior-platform strike-sli p faul t zone s ar e mani fested nea r th e surfac e b y e n echelo n faul t belts ,

Fig. 20. Sketc h map showin g the sense-of-sli p o n selected strike-slip faults, and regional palaeostress trajectories (from va n der Pluij m e t al. 1997) . As can be seen , some known strike-slip senses on mapped faults match the 'predicted' sens e of strike-slip while others do not. The inset shows a 'conjugat e shear ' interpretatio n o f faulting .

and that basement-penetrating maste r faults d o not reach th e surfac e confirm s tha t interio r platfor m strike-slip fault s ar e small displacement faults. This style of deformation is typical of laboratory model

Fig. 21. Th e jostlin g bloc k mode l o f interior-platform faulting, (a ) I n thi s model , strike-slip is a component of oblique-slip faulting tha t occurs along the lateral edge of a block . (Modifie d fro m Ston e (1969). (b ) Th e sens e of slip o n a strike-sli p faul t depend s o n th e geometr y o f faulting, relativ e to regional strain.


studies i n whic h a clay cak e ha s bee n place d ove r two woode n block s tha t begi n t o shea r pas t eac h other i n a strike-slip sens e b y only a slight amount (Mandl 1988) . I n thi s context , e n echelo n strike slip fault s ca n b e considere d t o b e Riede l shears , while e n echelo n norma l fault s ar e effectivel y extension gashe s resultin g fro m th e sligh t stretch ing that accompanie s simpl e shea r acros s a belt of finite widt h (Fig . 6) . Many i f no t mos t o f th e faul t zone s o f th e interior platfor m date to th e Proterozoic, an d have undergone multiple episodes of displacement under a variety of stres s fields . Strike-sli p components of displacement wer e imparte d durin g severa l event s that coincid e wit h margina l orogenie s o f Nort h America. Fo r example , som e displacemen t occurred durin g th e Ordovicia n (th e Taconi c event), the Devonian (th e Acadian event) , and dur-

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ing th e lat e Palaeozoi c (th e Alleghenian-Ouachit a event). Th e las t o f thes e wa s th e mos t significant , causing faul t reactivatio n acros s th e entir e interio r platform. Thi s faul t reactivatio n i s th e Ancestra l Rockies event . Convergenc e alon g th e southwestern margi n o f th e continen t ma y hav e contributed to the Ancestral Rockies event. Faults in the Rocky Mountains and Colorado Plateau were also reactivated durin g th e Mesozoic-Cenozoi c Laramid e event. Som e fault s (e.g . fault s o f th e Ne w Madri d zone) remai n activ e today . I n mos t cases , th e strike-slip episod e wa s no t th e earlies t displace ment fo r th e faul t i n question . Developing a regiona l interpretatio n o f shea r sense o n th e interior-platfor m strike-sli p fault s o f the Unite d State s remain s problematic , fo r shea r sense dat a ar e incomplete . Severa l author s hav e assumed tha t the shear sens e o n a given faul t mus t

Fig. 22. Compariso n map s o f th e interio r platfor m o f th e Unite d States , a t th e tim e o f th e Alleghanian-Ouachit a orogeny, an d eastern Eurasia today, (a ) Th e interior platfor m o f the United States is a rigid craton , whose upper crust has bee n broke n int o a mosaic o f blocks b y faults . I n easter n Eurasia , the souther n margin o f th e continen t is a sof t Phanerozoic orogen. (b) During the Alleghanian-Ouachita collision, the craton strain s onl y slightly , s o crustal blocks move only slightly . In eastern Eurasia , crustal blocks undergo lateral escap e when the lithosphere strain s significantly . (c) Th e mosai c o f crusta l block s i n th e interio r platfor m o f th e Unite d State s contrast s wit h majo r regiona l fault s i n eastern Eurasia . (Eurasi a figure s modifie d fro m Tapponie r & Molnar 1976) .

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be compatibl e wit h th e prediction s o f a conjugate shear model, i n which the predicted shea r sense on a given faul t i s taken to be the shea r sens e that the fault would have if it were a member of a conjugate shear se t whos e acut e bisectri x i s th e maximu m principal compressiv e stress . As an example of this model, conside r th e stres s fiel d resultin g fro m th e late Palaeozoic Alleghanian-Ouachita orogeny (th e collision o f Sout h Americ a an d Afric a wit h North America). Studie s o f calcite twinning i n limestone units o f th e US A continenta l interio r sugges t tha t crl during this event trended roughly NW (Figs 4 & 20A). In the conjugate shear model, faults trendin g approximately N-S shoul d be sinistral, while those trending approximatel y E- W shoul d b e dextral . Taken a t fac e value , th e strike-sli p shea r sens e reported for faults activ e during the late Palaeozoi c in th e interio r platfor m d o no t al l fi t thi s mode l (Fig. 20B) . I n som e cases , paralle l fault s hav e opposite shea r senses . We propos e tha t a n alternativ e approac h t o understanding th e regiona l patter n o f shea r sens e on continenta l interio r fault s come s fro m examin ing ho w regiona l strai n ca n b e accommodate d i n the contex t o f th e 'jostlin g block ' mode l (e.g . Davis 1978 ; Tikoff & Wojtal 1999) . A s noted earlier, the two sets of faults o n the continental interio r platform divid e th e upper crus t into roughl y recti linear blocks. Movement on the faults occur s when these block s jostl e wit h respec t t o on e anothe r i n response t o a regiona l strai n o f th e interio r plat form - suc h strains result primarily fro m collisional and/or convergen t orogen y alon g th e continenta l margin. I n thi s model , whic h ha s bee n applie d t o individual examples previousl y (e.g . Paylo r & Yin 1993), strike-sli p o r oblique-sli p fault s ar e effec tively transfe r fault s accommodatin g th e dip-sli p displacement o n a frontal faul t alon g anothe r edge of a block. Thus , the sens e o f sli p simpl y depends on th e di p o f th e fronta l faul t t o whic h th e strike slip faul t link s (Fig . 21). If, for example, the transfer faul t link s t o a NW-dipping revers e fault , the n it will have a dextral sens e o f slip, whil e i f it link s to a SE-dipping revers e fault , the n it has a sinistral sense o f slip , Becaus e o f th e complexit y o f th e regional patter n o f faults , an d th e fac t tha t som e faults hav e a propeller shape , th e regiona l patter n of strike-sli p shea r sens e woul d be expecte d t o b e quite complex . The souther n Oklahom a aulacoge n stand s ou t among th e faul t zone s o f th e interio r platfor m i n hosting a n order of magnitude more sli p than othe r faults (Fig s 1 & 9) . Bot h it s vertica l an d latera l components o f displacement ar e significantly large r than o n othe r faults . Thi s contras t ma y reflec t th e fact that the fault zones of the aulacogen ar e longer, and i f linke d t o thos e o f th e Uncompahgr e uplift , effectively extende d to the late Palaeozoi c wester n

continental margin. Thus, during Ancestral Rockies strain, the continent north of this block wa s fre e t o translate westward s by ten s o f kilometres . I n thi s regard, th e faul t behave d lik e intracontinenta l transform, muc h like its neighbour to the south, the similarly trendin g Mojave-Sonor a megashea r (Fig. 1) , did durin g the Mesozoic . We conclude b y comparing th e nature of strikeslip faultin g i n th e interio r platfor m o f th e Unite d States t o th e strike-sli p faultin g o f centra l an d southern Eurasia (Fig. 22). Kluth and Coney (1981) suggested tha t ther e i s a n analog y betwee n deformation o f th e Nort h America n continenta l interio r during the Alleghanian orogeny-Ancestral Rockie s event, an d deformation of central Asi a in respons e to the collision o f India. While there is some merit to thi s concept , i n tha t far-fiel d erogeni c stresse s are drivin g intracontinenta l deformatio n i n bot h regions, w e emphasize tha t the interior platfor m of North Americ a contrast s significantl y wit h central/southern Eurasia , i n that i t strike-sli p faulting magnitudes are one to two orders of magnitude less tha n they ar e i n Eurasia. We sugges t that difference reflect s th e differenc e i n th e relativ e strengths o f th e lithospher e o f th e tw o continents . In Nort h America , th e interior-platfor m fault s cu t the crust of an essentially strong craton. The upper crust o f thi s crato n containe d a numbe r o f pre existing faults, breakin g it into a mosaic of blocks, and thes e blocks jostled wit h respect t o each othe r when th e regio n underwen t strain, bu t becaus e of the strength o f the lithosphere, eve n the great collision of the Alleghanian orogeny did not cause large strains. I n Asia, however, th e collision of souther n continents deforme d lithospher e tha t ha d bee n weakened during the heating that accompanied pre-

Fig. 23. Bloc k diagrams indicatin g ho w stronge r conti nental lithosphere strains by a smaller amount while weak lithosphere strains by a larger amount. Crusta l blocks of soft lithospher e undergo greate r displacemen t tha n thos e of stron g lithosphere .


vious Phanerozoi c orogenie s (Sengo r & Nata l 1996). I n thi s wea k lithosphere , larg e fault s developed an d accommodate d significan t latera l escape (Fig . 23) . This research wa s supported, in part, by the US Geologi cal Surve y (USGS ) unde r USG S awar d numbe r 99HQGR0075 (Universit y o f Illinois) . Th e view s an d conclusions containe d i n thi s documen t ar e thos e o f th e authors an d shoul d no t b e interprete d a s necessarily representing the official policies , either expressed or implied , of th e U S Government . Thi s wor k wa s als o supporte d in par t b y th e Earthquak e Engineerin g Researc h Center s Program o f th e Nationa l Scienc e Foundatio n unde r Award Number EEC-9701785. We also acknowledge the support o f thi s researc h b y Landmar k Graphic s vi a th e Landmark University Grant Progra m a t the Universit y of Illinois a t Urbana-Champaign . Dat a processin g fo r thi s study wa s performed usin g Landmark's ProMA X 2-D™ . Finally, w e wish t o than k R . Hold s worth an d B . Tikoff , for ver y helpfu l reviews , an d th e editor s o f thi s volum e for thei r patience .

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