Permian And Triassic Rifting in Northwest Europe (Geological Society Special Publication)

Permian and Triassic Rifting in Northwest Europe

Geological Society Special Publications Series Editor A. J. FLEET

GEOLOGICAL

SOCIETY

SPECIAL PUBLICATION

N O . 91

Permian and Triassic Rifting in Northwest Europe EDITED BY

S. A. R. B O L D Y Amerada Hess Ltd, London, UK

1995 Published by The Geological Society London

THE GEOLOGICAL SOCIETY The Society was founded in 1807 as The Geological Society of London and is the oldest geological society in the world. It received its Royal Charter in 1825 for the purpose of 'investigating the mineral structure of the Earth'. The Society is Britain's national society for geology with a membership of around 7500. It has countrywide coverage and approximately 1000 members reside overseas. The Society is responsible for all aspects of the geological sciences including professional matters. The Society has its own publishing house, which produces the Society's international journals, books and maps, and which acts as the European distributor for publications of the American Association of Petroleum Geologists, SEPM and the Geological Society of America. Fellowship is open to those holding a recognized honours degree in geology or cognate subject and who have at least two years' relevant postgraduate experience, or who have not less than six years' relevant experience in geology or a cognate subject. A Fellow who has not less than five years' relevant postgraduate experience in the practice of geology may apply for validation and, subject to approval, may be able to use the designatory letters C. Geol. (Chartered Geologist). Further information about the Society is available from the Membership Manager, the Geological Society, Burlington House, Piccadilly, London W1V 0JU, UK. The Society is a Registered Charity, No. 210161. Published by The Geological Society from: The Geological Society Publishing House Unit 7 Brassmill Enterprise Centre Brassmill Lane Bath BA1 3JN UK (Orders: Tel 01225 445046; Fax 01225 442836)

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Contents GLENNIE, K. W. Permian and Triassic rifting in northwest Europe

1

COWARD, M. P. Structural and tectonic setting of the Permo-Triassic basins of northwest Europe

7

CARTER, m., YELLAND, A., BRISTOW, C. & HURFORD, A. J. Thermal histories of Permian and Triassic basins in Britain derived from fission track analysis

41

SWIECICKI, T., WlLCOCKSON, P., CANHAM, A., WHELAN, G. & HOMANN, H. Dating, correlation and stratigraphy of the Triassic sediments in the West Shetlands area

57

HITCHEN, K., STOKER, M. S., EVANS, D. & BEDDOE-STEPHENS, B. Permo-Triassic sedimentary and volcanic rocks in basins to the north and west of Scotland

87

ANDERSON, T. B., PARNELL, J. & RUFFELL, A. H. Influence of basement on the geometry of Permo-Triassic basins in the northwest British Isles

103

GOLDSMITH, P. J., RICH, B. & STANDRING, J. Triassic correlation and stratigraphy in the South Central Graben, U K North Sea

123

GRIFFITHS, P. A., ALLEN, M. R., CRAIG, J., FITCHES, W. R. & WHITTINGTON, R. J. Distinction between fault and salt control of Mesozoic sedimentation on the southern margin of the Mid-North Sea High

145

CHADWICK, R. A. & EVANS, D. J. The timing and direction of Permo-Triassic extension in southern Britain

161

RUFFELL, A., COWARD, M. P & HARVEY, M. Geometry and tectonic evolution of megasequences in the Plymouth Bay Basin, English Channel

193

SHANNON, P. M. Permo-Triassic development of the Celtic Sea region, offshore Ireland

215

KEELEY, M. L. New evidence of Permo-Triassic rifting, onshore southern Ireland, and its implications for Variscan structural inheritance

239

Index

255

Contents GLENNIE, K. W. Permian and Triassic rifting in northwest Europe

1

COWARD, M. P. Structural and tectonic setting of the Permo-Triassic basins of northwest Europe

7

CARTER, m., YELLAND, A., BRISTOW, C. & HURFORD, A. J. Thermal histories of Permian and Triassic basins in Britain derived from fission track analysis

41

SWIECICKI, T., WlLCOCKSON, P., CANHAM, A., WHELAN, G. & HOMANN, H. Dating, correlation and stratigraphy of the Triassic sediments in the West Shetlands area

57

HITCHEN, K., STOKER, M. S., EVANS, D. & BEDDOE-STEPHENS, B. Permo-Triassic sedimentary and volcanic rocks in basins to the north and west of Scotland

87

ANDERSON, T. B., PARNELL, J. & RUFFELL, A. H. Influence of basement on the geometry of Permo-Triassic basins in the northwest British Isles

103

GOLDSMITH, P. J., RICH, B. & STANDRING, J. Triassic correlation and stratigraphy in the South Central Graben, U K North Sea

123

GRIFFITHS, P. A., ALLEN, M. R., CRAIG, J., FITCHES, W. R. & WHITTINGTON, R. J. Distinction between fault and salt control of Mesozoic sedimentation on the southern margin of the Mid-North Sea High

145

CHADWICK, R. A. & EVANS, D. J. The timing and direction of Permo-Triassic extension in southern Britain

161

RUFFELL, A., COWARD, M. P & HARVEY, M. Geometry and tectonic evolution of megasequences in the Plymouth Bay Basin, English Channel

193

SHANNON, P. M. Permo-Triassic development of the Celtic Sea region, offshore Ireland

215

KEELEY, M. L. New evidence of Permo-Triassic rifting, onshore southern Ireland, and its implications for Variscan structural inheritance

239

Index

255

From Boldy, S. A. R. (ed.), 1995, Permian and Triassic Rifting in Northwest Europe,

Geological Society Special Publication No. 91,' 1-5

Permian and Triassic rifting in northwest Europe K. W . G L E N N I E

University of Aberdeen, UK The Permo-Triassic roughly coincided with the life span of Pangaea. So far as Europe is concerned, Pangaea's creation began in the early Carboniferous (Visean) when the northward-drifting megacontinent Gondwana began to collide with the Iberian portion of the slower moving Laurussia, while Proto-Tethys was subducted beneath the southern margin of central Europe. Creation was complete by the end of the Carboniferous or early in the Permian with the final development of the Variscan orogenic belt, which trends from Brittany eastward through central Europe, and with the addition of western Siberia along the line of the Ural orogen (Ziegler 1989). Despite, or perhaps because of, its bulk, Pangaea was not a stable megacontinent. No sooner had it formed than it tried to break apart again. The disintegration of Pangaea had already started before the end of the Triassic with the westerly extension of Tethys between Iberia and Africa, though not yet underlain by oceanic crust, and, by early in the Jurassic, rifting was taking place between Africa and the Americas in the newly forming Central Atlantic Ocean (Ziegler 1988). Indeed, E-W extensional movements within a Proto-Atlantic Ocean possibly began as early as the Late Carboniferous (Haszeldine & Russell 1987), whilst mid-Permian extension is well documented in East Greenland (Surlyk et al. 1984). These extensional movements may have propagated southward to initiate fracturing in the Viking and Central Grabens of the North Sea, along which the Late Permian Zechstein Sea was to break into the subsiding Rotliegend basins, and towards the Central Atlantic, where a shallow seaway eventually developed early in the Jurassic. Thus in the northern half of Pangaea, the continued existence of the former Laurussia was already at risk in the Permian, although crustal separation in the North Atlantic, which possibly started in the Rockall Trough in the Early Cretaceous, was finally achieved along the line of the Reykjanes Ridge only in the Paleocene. These major events on the periphery of what is now Europe, separately and jointly, were factors that probably controlled a whole sequence of tectonic events within the continent, which, in turn, controlled its patterns of mostly terrestrial sedimentation. Climatically, the northward drift of Laurussia had carried NW Europe from a region of equatorial rain forest during the later Carboniferous to the latitudes of a trade wind desert, like the modern Sahara, in the Permian. The Late Permian basins of Upper Rotliegend and Zechstein deposition were arid. Rotliegend sediments are characterized by dune sand and the saline mudstones of a semipermanent desert lake, and the Zechstein by shallow-water carbonates and anhydrite and deeper-water halite. During the Triassic, however, brackish-water fluvial and lacustrine sediments occupied the basinal areas, although even here, halite horizons (e.g. Rrt, Muschelkalk and Keuper in the North Sea area) associated with local transgressions from Tethys, testify that arid conditions were never far away.

2

K.W. GLENNIE

The Variscan Orogeny seems to have been doomed to failure; it was to become a range of highlands but not a major mountain range. No sooner had it formed than it began to collapse, with the coeval development of a very widespread N W - S E and conjugate N E - S W system of fractures through it and across its northern foreland. This may have been the outcome of a right-lateral reorientation of the relative movement between the former Laurussia and Gondwana (Ziegler 1990). Some of these fractures were obviously extensional as many were associated with igneous activity concentrated around 290-295 Ma BP; this comprised dyke swarms and sills as well as tufts and basaltic lavas of the Lower Rotliegend volcanics (Dixon et al. 1981; Sorensen and Martinsen 1987). Thermal subsidence of the Permian basins of the North Sea area seems to have begun about 20 Ma after the end of the main volcanic activity and was most marked over North Germany, which was the site of the strongest Lower Rotliegend volcanism and the development of a system of associated horsts and grabens (e.g. Gast 1988). The timing and amount of rifting associated with the North Sea graben system is still a matter of some dispute. Some workers (e.g. Ziegler 1990, and others), believe that rifting of the North Sea grabens was not initiated until the Triassic. They base much of their interpretation on seismic data (e.g. failure to recognize Zechstein halite in the middle portion of the Central Graben, even though there is good evidence elsewhere of the local removal of halite by solution; Johnson et al. 1986). In the deeper parts of structurally and stratigraphically complex areas such as the Central Graben, however, it is very difficult to recognize on seismic lines all lithologies of various ages, let alone decide on that basis just when rifting began, especially if initially it had gone through both transtensional and transpressional phases of movement. Other workers, including the author (e.g. Glennie 1990a,b), consider that rifting probably began during the Early Permian. Such rifting was possibly coeval with rotation of the north-trending series of en echelon half grabens (Worcester, Cheshire Basin, etc.) as well as intra-Variscan basins such as the Western Approaches and Celtic Sea Basins. This interpretation would seem to be supported in the North Sea area by the occurrence of Lower Rotliegend volcanism in the Central, Horn and Oslo Grabens, and by the preservation of Zechstein halite within the South Viking Graben together with Rotliegend dune sands as far north as the Beryl Embayment. The Zechstein Sea is believed to have flooded the Rotliegend basins with water of boreal origin via this route (Glennie and Buller 1983) rather than through the Bakevellia Sea and around the southern edge of the Pennine uplift, for which there is no supporting evidence. Debate is still generated concerning the style and amount of North Sea extension (e.g. Gibbs 1987; Latin et al. 1990). There is general agreement that the most active phase of crustal extension took place during the Late Jurassic to Early Cretaceous time span, but there is no concensus on the relative contributions of the Triassic and earlier Jurassic Periods. Despite associated volcanic activity, a possible Permian component is usually ignored, whereas apart from the mid-Jurassic volcanics at the Moray Firth-Viking-Central Graben trilete junction, Late Jurassic extension across the Central Graben was not associated with volcanic activity. B-factors vary from worker to worker depending on the style of extension assumed and the time spans during which crustal stretching is considered to have been operative (e.g. Sclater and Celerier 1988, and associated articles).

PERMIAN AND TRIASSIC RIFTING IN NW EUROPE

3

The cross-sectional geometry of Triassic sequences within the East Shetland Basin clearly indicates that in that area extension was related to fault-block rotation, which was accentuated in the Late Jurassic. A controlling factor must have been the proximity of the North Viking Graben, which limits the eastern margin of the East Shetland Basin, but little is known about the timing or amount of extension within that graben, where Permian strata may be as much as 10 km below present sea level (Ziegler 1990). Apart from the marine Zechstein and Muschelkalk sequences, the PermoTriassic of NW Europe consists largely of arid or semiarid terrestrial sediments that are very poorly dated. Fossils generally are either absent or non-diagnostic for age. Because of a lack of faunal or floral control, the apparent age of some sedimentary sequences has been changed in recent years from Triassic to Permian on the basis of regional correlations. For instance, following interpretations in vogue during the 1930s (Sherlock 1948), no sediments of Permian age are shown in the West Midlands of England on the 1948 edition of the Geological Survey Map of Great Britain. North Sea exploration has now made it likely that at least part of this sequence is Permian in age (e.g. the Bridgnorth Sandstone: Smith et al. 1974; Karpeta 1990; Warrington et al. 1980); the 1979 edition of the same map has advanced only by designating much of the sequence as undifferentiated Permian and Triassic. Other than the radiometric ages of igneous rocks, which are still few and far between, there is an almost complete lack of dating in many of the smaller red-bed basins of presumed Permo-Triassic age in NW Europe. Germany seems to be better off in this respect, and is able to use Russian faunal stages for the Permian, controlled to a limited extent by magnetostratigraphy (Gebhardt et al. 1991). Further west, rare palynofloras are beginning to provide a little control, but in their absence, as is the case northwest of the Scottish mainland, even seismic correlation from one isolated half graben to the next is, at best, conjectural, and New Red Sandstone cannot be separated from its Devonian Old Red counterpart with any confidence. The Permo-Triassic had a time span of some 90 Ma. On the basis of radiometric dating of Westphalian lavas in Germany, it now seems likely that the PermoCarboniferous transition occurred about 300 Ma ago (Lippolt et al. 1984; Leeder 1988). Following Lower Rotliegend volcanism, much of the greater North Sea area seems to have been the site of erosion or non-deposition for up to 20 Ma or more (Saalian Unconformity: see Table 1 in Brown 1991) before Upper Rotliegend deposition began. Some areas of Late Carboniferous inversion (e.g. axis of Sole Pit Basin) were subjected to erosion down to Namurian horizons. Post-Saalian subsidence was greatest over the area of former volcanic activity in northern Germany and Poland. During a very short time span, estimated to be no more than 20 Ma, straddling the Permo-Triassic transition (early Tatarian to Scythian), rates of subsidence reached 220 m per million years (Menning 1991). Some of this subsidence may be related to the rapid crustal loading of Rotliegend basins whose surfaces were already below global sea level, first by the influx of the Zechstein Sea (250 to 300 m of water: Glennie 1990a), and then by the dense evaporites that were deposited in thicknesses of up to 3000m or more (Taylor 1990). Menning (1991), indeed, estimates the duration of Zechstein deposition at around 5 Ma, which implies that deposition proceeded locally at the rate of over 600m per million years. The

4

K.W. GLENNIE

succeeding terrestrial sedimentation slowed duration the early Triassic (Bunter), and in the areas of former maximum subsidence may have adjusted to isostatic equilibrium at about the time of the Hardegsen Unconformity. This brief preface not only indicates some of my own interests in the Permo-Triassic of N W Europe, but hopefully also highlights some of the important facts and difficulties in interpreting the structure and sedimentation patterns associated with Permo-Triassic rifting in NW Europe. It provides only armchair explanations of some important events and processes, and ignores others: more will be discussed in the succeeding pages. And perhaps a few comments will stimulate one or two readers to put their knowledge and ideas on paper. The relatively extensive reference list for such a short contribution may include some useful papers that are not mentioned elsewhere.

References BROWN, S. 1991. Stratigraphy of the oil and gas reservoirs: UK Continental Shelf. In: ABBOTTS, I. L. (ed.) United Kingdom Oil and Gas Fields, 25 years Commemorative Volume. Geological Society Memoir, 14, 9-18. DIXON, J. E., FITTON, J. G. & FROST, R. T. C. 1981. The tectonic significance of postCarboniferous igneous activity in the North Sea Basin. In: ILLING, L. V. & HOBSON. G. D. (eds) Petroleum Geology of the Continental Shelf of North-West Europe. Heyden, London, 121-137. GAST, R. E. 1988. Rifting im Rotliegenden Niedersachsens. Geowissenschaften, 6, 115-122. GEBHARDT, U., SCHNEIDER, J. & HOFFMANN, N. 1991. Modelle zur Stratigraphie und Beckenentwicklung im Rotliegenden der Norddeutschen Senke. Geologisches Jahrbuch A, 127, 405-427. GEOLOGICAL SURVEYOF GREAT BRITAIN, 1948. Ten-Mile Map, Sheet 2. 3rd edition, 1979. GIBBS, A. D. 1987. Deep seismic profiles in the northern North Sea. In: BROOKS, J. & GLENNIE, K. (eds) Petroleum Geology of North West Europe. Graham and Trotman, London, 1025-1028. GLENNIE, K. W. 1990a. Outline of North Sea history and structural framework. In: GLENNIE, K. W. (ed.) Introduction to the Petroleum Geology of the North Sea. 3rd edition, Blackwell, Oxford, 34-77. --, 1990b. Lower Permian- Rotliegend. In: GLENNIE, K. W. (ed.) Introduction to the Petroleum Geology of the North Sea. 3rd edition, Blackwell, Oxford, 120-152. & BULLER, m. T. 1983. The Permian Weissliegend of N.W. Europe: the partial deformation of aeolian dune sands caused by the Zechstein transgression. Sedimentary Geology, 35, 43-81. HASZELDINE, R. S. & RUSSELL, M. J. 1987. The late Carboniferous northern Atlantic Ocean: implications for hydrocarbon exploration from Britain to the Arctic. In: BROOKS, J. & GLENNIE, K. (eds) Petroleum Geology of North West Europe. Graham & Trotman, London, 1163-1175. JOHNSON, H. D., MACKAY, T. A. & STEWART, D. J. 1986. The Fulmar Oil Field (Central North Sea): geological aspects of its discovery, appraisal and devleopment. Marine and Petroleum Geology, 3, 99-125. KARPETA, W. P. 1990. The morphology of Permian palaeodunes - a reinterpretation of the Bridgnorth Sandstone around Bridgnorth, England, in the light of modern dune studies. Sedimentary Geology, 69(1/2), 59-75. LATIN, D. M., DIXON, J. E,. FITTON, J. D. & WHITE, N. 1990. Mesozoic magmatic activity in the North Sea Basin: implications for stretching history. In: HARDMAN, R. F. P. & BROOKS, J. (eds) Tectonic Events Responsible for Britain's Oil and Gas Reserves. Geological Society Special Publication, 55, 207-227. LEEDER, M. R. 1988. Recent developments in Carboniferous geology: a critical review with implications for the British Isles and NW Europe. Proceedings Geologist's Association, 99(2), 74-100.

PERMIAN AND TRIASSIC RIFTING IN NW EUROPE

5

LIPPOLT, H. J., HESS, J. C. & BURGER, K. 1984. Isotopische Alter von pyroclastischen Sanidinen aus Kaolin-kohlensteine als Korrelationsmarken fiir das mitteleuropaische Oberkarbon. Fortschr. Geol. Rheinid. u. Westf., 32, 119-150. MENNING, M. 1991. Rapid subsidence in the Central European Basin during the initial development (Permian-Triassic boundary sequences, 258-240Ma). Zentrablatt ffir Geologie und Pala6ntologie, Stuttgart. 1, 809-824. SCLATER, J. G. & CELERIER, B. 1988. Errors in extension measurements from planar faults observed on seismic reflection lines. Basin Research, 1(4), 217-221. SHERLOCK, R. L. 1948. The Permo-Triassic Formations. Hutchinsons, London. SMITH, D. B., BRUNSTROM, R. G. W., MANNING, P. I. SIMPSON, S. & SHOTTON, F. W. 1974. P e r m i a n - a Correlation of Permian Rocks in the British Isles. Geological Society, London, Special Report, 5. SORENSEN, S. & MARTINSEN, B. B. 1987. A palaeogeographic reconstruction of the Rotliegendes deposits of the Northeastern Permian Basin. In: BROOKS, J. & GLENNIE, K. (eds) Petroleum Geology of North West Europe. Graham and Trotman, London, 497508. SURLYK, F., PIASEKI, S., ROLLE, F., STEMMERIK, L., THOMSEN, E. & WRANG, P. 1984. The Permian Basin of East Greenland. In: SPENCER, m. M. et al. (eds) Petroleum Geology of the North European Margin. Norwegian Petroleum Society, Graham and Trotman, London, 303-315. TAYLOR, J. C. M. 1990. Upper Permian - Zechstein. In: GLENNIE, K. W. (ed.) Introduction to the Petroleum Geology of the North Sea. 3rd edition, Blackwell, Oxford, 153-190. WARRINGTON, G., AUDLEY-CHARLES, M. G., ELLIOTT, R. E. et al. 1980. Triassic- a Correlation of Triassic Rocks in the British Isles. Geological Society, London, Special Report, 13. ZIEGLER, P. A. 1988. Evolution of the Arctic-North Atlantic and Western Tethys. American Association Petroleum Geologists Memoir 43. 1989. Evolution of Laurussia. Kluwer, Dordrecht. -1990. Geological Atlas of Western and Central Europe. 2nd edition, Shell, The Hague.

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From Boldy, S. A. R. (ed.), 1995, Permian and Triassic Rifting in Northwest Europe, Geological Society Special Publication No. 91, 7-39

Structural and tectonic setting of the Permo-Triassic basins of northwest Europe M. P. C O W A R D

Geology Department, Imperial College, London SW7 2BP, UK Abstract: The Permo-Triassic basins of NW Europe formed by (i) late-orogenic

spreading of the Variscan mountain belt, eventually leading to ocean floor formation in the Mid-Atlantic, (ii) continental rifting propagating south from the Arctic and (iii) thermal subsidence following Carboniferous volcanic activity in the North Sea. Spreading values in the northernmost North Sea, west of Shetland and west and southwest of Britain, are relatively high but the sediments are dominantly continental; the Permo-Triassic rifting affected lithosphere thickened during Caledonian and Variscan orogenesis. However, subsidence associated with the Arctic rifting, together with global sea level changes, allowed marine incursions into the North Sea, producing the large Zechstein salt basins. Interference of the different basin formation mechanisms led to a complex three dimensional fault pattern and multidirectional extension across Britain and the North Sea. This fault pattern formed the basic framework for subsequent Jurassic and Cretaceous basin formation.

The Permo-Triassic marks a change in tectonic regime in N W Europe, from Palaeozoic plate accretion, producing the Caledonian and Vafiscan orogenic belts, to continental extension, generating rift basins in the Central Atlantic extending from the Caribbean to Gibraltar and the North Atlantic from north of Ireland to the Boreal Sea. The Permo-Triassic marked the time of rift initiation in many of Britain's offshore basins, including the North Sea. It was also the time of extensive sand accumulation and deposition, generating many of the reservoir rocks for Britain's oil and gas. This paper aims to give a short overview of Permo-Triassic tectonics and basin development in NW Europe, including the Permo-Triassic plate configuration and its influence on the palaeo-environment and the stratigraphic record. In particular, the paper aims to discuss the origin of the various basins, the mechanisms for rifting and subsidence and the variations in rift opening direction in time and space. Unfortunately there is a lack of regional structural and stratigraphic data dealing with this time period. This paper aims to redress this, and compiles and summarizes data and ideas developed in the keynote stratigraphic publications dealing with the Permo-Triassic of NW Europe. The paper also adds some new and possibly controversial conclusions about local and regional structures.

Pre-Permian tectonic framework Figure 1 shows a simplified map of the pre-Permian tectonic zones of N W Europe, based on Ziegler (1982) and Coward (1990).

8

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PERMO-TRIASSIC BASINS OF NW EUROPE

9

Precambrian crystalline basement The Precambrian basement of NW Europe consists of crystalline material derived from three tectonic domains, accreted together during Caledonian collision. The North Atlantic Block, which lies northwest of the Hebrides, comprises Archaean gneisses (~2900Ma) reworked extensively in the Middle Proterozoic (18001600Ma). Mid-crustal tectonic fabrics, which are mostly flat or dip gently to the SE, were generated by large-scale NW-directed thrust tectonics in the Middle Proterozoic followed by NW/SE extension. These fabrics are modified by large NW/SE trending shear zones which acted as large-scale tear faults-transfer zones during the phases of crustal thickening and subsequent extension. The crystalline basement is covered by a locally deep Late Proterozoic basin (the Torridonian) and thin Lower Palaeozoic sediments. The Scandinavian Block, or Baltica, is formed of crystalline crust generated by magmatic arc accretion during the Middle Proterozoic (Svecofennian terrain) followed by Late Proterozoic reworking and possibly plate accretion in southern Scandinavia (Sveconorwegian terrain). This latter phase of collision tectonics was associated with SE-directed overthrusting on N/S-trending mylonite belts. Lower Palaeozoic sediments occupy rift basins along the southern edge of Baltica, possibly related to continental break-up and rifting of Baltica away from some unknown continent. The Welsh-Brabant Massif forms the crystalline basement to Central England and the southern North Sea. It is a Late Precambrian magmatic arc and was essentially undeformed until the Caledonian tectonic events. However, in NW Wales there are Late Precambrian mylonites and blueschists and along the northern margin of the Brabant Massif, north of the English Channel, there is evidence of middle-lower crustal fabric development (Blundell et al. 1991), suggesting that the Massif may have formed from the accretion of several magmatic arcs. During the Early Palaeozoic the western edge of the Brabant Massif was subjected to rifting associated with arc-related magmatism and volcanicity, possibly developed above a SE-dipping subduction zone.

Caledonian plate coll&ion The Caledonian orogenic belt extends from northern Norway to the Gulf of Mexico and formed as a result of the closure of a system of oceans, generally grouped together under the name Iapetus. During the Palaeozoic, thin strips of a southern continent (Gondwana) were broken off to generate at least three elongate ocean basins. The strips moved northwest relative to Gondwana to close, sequentially, the Proto-Iapetus Ocean during the Ordovican, the Neo-Iapetus Ocean during the Silurian-Devonian and the Rheic Ocean during the Late Carboniferous. The accretion direction was generally towards the northwest, perpendicular to the strike of the belt, and the thrusts verge towards the northwest or southeast. The tectonic pattern was complicated by large-scale lateral escape structures, whereby much of NW Europe was expelled towards the northeast away from the zone of most intense collision in the central Appalachians (Coward 1990, 1993). Therefore in Britain several domains can be recognized bounded by major strike-slip faults (Fig. 2). Northwest of the Great Glen Fault, Proterozoic metasediments (Moines) were intensely foliated and metamorphosed to upper greenschist and amphibolite facies in

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Fig. 2. Caledonian domains of Britain. The structures are dismembered by large NE/SWtrending strike-slip faults. From Coward (1990).

Silurian times and thrust to the WNW over the North Atlantic Craton. Crustal scale shear zones, related to these thrust tectonics, can be recognized on deep seismic data from the middle to lower crust and upper mantle (Brewer & Smythe 1984; Cheadle et al. 1987). During this period of thrust tectonics the region northwest of the Great Glen lay 200-1000 km further northeast relative to the rest of Britain, to be displaced in a left-lateral sense during the Devonian (Coward 1990, 1993). Hence, the Moine structures appear to be part of a major mountain belt formed by continent-continent collision between the North Atlantic and Scandinavia Cratons during the Late Ordovician to Silurian, later dismembered by Devonian strike-slip tectonics. Southeast of the Great Glen Fault, a Proterozoic basement was rifted during Late Precambrian times to form several deep basins, infilled with > 10 km of sediments and volcanics. The extensional faults were reactivated in a major compressional episode of Late Cambrian-Ordovician age, resulting in large-scale positive inversion. This compression was associated with the accretion of one or more magmatic arcs, closing the Proto-Iapetus Ocean to the northwest and opening the Neo-Iapetus Ocean to the southeast.

PERMO-TRIASSIC BASINS OF NW EUROPE

11

Closure of the Neo-Iapetus Ocean was accompanied by NW-directed subduction generating a thick Ordovician-Silurian accretionary prism in the Southern Uplands. Closure continued until the Late Silurian with the final docking of the Brabant Massif, with the accretionary prism and the production of large NW-dipping shear zones through the crust, but no large-scale obduction. The simple pattern of ocean closure between the Brabant Massif and the terrain to the northwest is not mimicked in Scandinavia, where Silurian collision was associated with SE-directed thin-skinned overthrusting and crustal shortening in the order of 500 km (Hossack & Cooper 1986).

Devonian-Carboniferous collapse of the Caledonian mountain belt and regional rifting The main crustal fabrics formed by Caledonian tectonics were modified and offset by Late Caledonian displacement on NE/SW-trending shear systems. These shears are interpreted to have resulted from the lateral ~pulsion of an England-North Sea Block to the northeast away from an Acadian indentor in the Early Devonian (Coward 1990). The block was approximately triangular in shape, and bounded by (i) the Ural Ocean to the east, (ii) the left-lateral Great Glen-Midland Valley-North Atlantic shear systems to the northwest, and (iii) the right-lateral English ChannelSouth Polish Trough shear systems (in Devonian times) and the South WalesSouthern North Sea-Polish Trough shear system (in Early Carboniferous times) to the south. Figure 3 shows a suggested plate reconstruction for Dinantian times.

Fig. 3. Simplified plate tectonic reconstruction for the Dinantian.

12

M. COWARD

Pull-apart basins developed along the major shear systems. In the northern North Sea, the West Orkney Basin-East Shetland Platform-Viking Graben formed as a large pull-apart basin in the left-lateral shear system during Devonian-Early Carboniferous times (Coward 1990, 1993). This basin stretched crust previously thickened during the Caledonian Orogeny so that although stretching values were large, the basins were non-marine and filled with terrestial or lacustrine sediments. In the English Channel and SW England, deep basins and locally oceanic crust developed in the right-lateral shear system. These basins were partly closed during early Variscan tectonics in the Devonian and Early Carboniferous. However, in the southern North Sea and East Midlands, NW-trending Carboniferous basins developed in the right-lateral shear system, associated with clockwise rotation of Caledonian crustal fragments (Coward 1993). Early Carboniferous continental escape was synchronous with regional back-arc extension associated with northward subduction of Variscan oceanic crust. NW/SEdirected extension was characteristic of the northern England and Irish basins. The S England and North Sea rift basins affected crust of normal thickness, generating marine conditions. However, the northern pull-apart basins acted as the loci of sediment transport from the Caledonian mountains into the southern basins; deltaic facies prograded southwards during the Early-Middle Carboniferous from the southern end of the proto-Viking Graben.

Variscan plate collision and the inversion o f Palaeozoic basins Variscan structures of NW Europe also formed as a result of NW-accretion of crustal blocks and magmatic arcs onto the Laurentian foreland. Closure of the NeoIapetus Ocean was associated with opening of Rheic Ocean or system of oceans to the southeast. The Variscan structures were associated with the closure of these oceans as indicated by the presence of ophiolites in the internal Variscan belt. The Trans-European Fault Zone was active at this time, forming the lateral boundary of the Variscan Bohemian Massif (Ziegler 1990). In SW Britain Variscan tectonics involved a NW-verging thin-skinned fold and thrust belt (Shackleton et al. 1982). NW/SE-trending tear faults were developed at this time; some of the larger faults, such as the Bray Fault, may have bounded blocks of different crustal age (Coward 1993). The northwest edge of the Variscan thin-skinned belt can be traced from the South Celtic Sea Basin, through north Devon and to southern England. It bounds the edge of the Carboniferous Culm Basin. However, to the north, Devonian and Carboniferous basins were locally strongly inverted, with thick-skinned basement uplifts in South Wales/Bristol Channel and the Mendips. Minor inversion occurred throughout the Carboniferous, particularly at the beginning of the Westphalian, when Variscan folding affected parts of Britain from South Wales to the Midland Valley. However, the main phase of inversion occurred during the Late Carboniferous-Early Permian, when all the Carboniferous basins were inverted and the shear sense along all the strike-slip faults was reversed.

Permo-Triassic basins By Early Permian times, the Caledonian and Variscan Oceans had fully closed and mountain ranges stretched from the northwest edge of South America, through the

PERMO-TRIASSIC BASINS OF NW EUROPE

13

Fig. 4. Simplified plate tectonic reconstruction for the Early Permian.

Fig. 5. Permian basins of NW Europe, showing the relationship between rift basins west and north of Britain and simple subsidence basins in the North Sea.

14

M. COWARD

northern Caribbean, along the Appalachian and European Variscan mountain belts, around the Black Sea and SE Caspian and along the Urals to the Arctic (Fig. 4). These mountain belts formed a topographic and climatic barrier similar to the present day Alpine-Himalayan belt, influencing structure and stratigraphy during Late Palaeozoic and Early Mesozoic times. Southeast of this mountain belt in Asia lay the Tethyan Ocean. Accretionary tectonics continued in southern Asia as a new slice of Gondwana, termed Cimmeria, broke off and moved northwards about a pole of rotation in the present day Eastern Mediterranean. The northward movement of Cimmeria closed Palaeo-Tethys and opened Neo-Tethys. Two rift systems dominated NW Europe (Figs 4 & 5). The Arctic rift system propagated towards western Ireland along the eastern edge of Greenland, reworking many of the Caledonian and Devonian structures. Rift systems also developed along the Appalachian and Variscan mountain belts. Both the Arctic and Appalachian/ Variscan rifts had approximately NW/SE extension directions, with slight interference in the western part of Britain. Permo-Carboniferous volcanic rocks occur in the North Sea and northern Germany (Fig. 6) and Triassic volcanic rocks occur in what

NW EUROPE IN EARLY PERMIAN TIMES

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eventually became the Central Atlantic and also in the western Tethyan-Dauphinois Basin. Salt-filled rift or thermal sag basins formed along the northern edge of the Appalachian-Variscan mountain belt, in the present-day Texas Gulf, the southern North Sea and the Peri-Caspian Basins.

Bas&s formed by collapse of the Appalachian-Variscan mountain belt Late Palaeozoic-Early Mesozoic rift basins characterize the Variscan terranes of North America and western Europe and form the precursor basins for Atlantic rifting. In the eastern USA, the basins follow trends of Appalachian structures and appear to rework both thrust and strike-slip mylonites (Fig. 7). In the western UK, deep Permo-Triassic basins formed in Cardigan Bay, the Celtic Sea and the Western Approaches (Fig. 8). These basins trend NE/SW, perpendicular to Caledonian and Variscan thrust directions and approximately parallel to the strike of Caledonian fabrics, but slightly oblique to the strike of Variscan thrusts in south Wales. The basins are cut by NW/SE-trending tear faults which link with strike-slip faults in onshore southwest England. These tear faults cannot be traced into onshore Ireland to the northwest or into France to the southeast, and must have acted as transfer systems to basin opening (Coward & Trudgill 1989).

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PERMO-TRIASSIC BASINS OF NW EUROPE

17

The extension direction was NW/SE, parallel to the tear faults and to small lateral ramps which offset the main basin-bounding faults, especially along the southeast region of the Cardigan Bay Basin. In the latter region the tear faults are steep to vertical and can be recognized on seismic data from the offset of prominent reflecting horizons and from their associated flower-like secondary faults (Coward & Trudgill 1989). Similar strike-slip faults with flower structures occur in the Cockburn Basin (Smith pers. comm. 1994). The North Celtic Sea Graben is c. 350 km long but only 50 km wide. It is bounded on the northwest by a master fault which can be traced to middle crustal levels as a prominent reflector on SWAT deep seismic data (BIRPS & ECORS 1986). The geometry of the basin changes across a major tear-fault system to the northeast into the St George's and Cardigan Bay Basins, where the master fault dips to the northwest. To the southeast the North Celtic Sea Graben is separated from the shallower, but structurally more complex, South Celtic Sea Graben by the Pembroke Ridge. The South Celtic Sea Graben also changes character to the northeast across the major Devon tear fault, into the Bristol Channel Basin. From regional interpretations of seismic and well data (Naylor & Shannon 1982; Tucker & Otter 1987; Coward & Trudgill 1989), the North Celtic Sea Graben contains a thick sequence of Permo-Triassic continental deposits and evaporites overlain by Liassic and Middle Jurassic shales and limestones. On the SWAT 4 deep seismic lines (Fig. 9), the lower crust displays prominent to gently dipping short reflectors, in contrast to the more transparent upper crust. The position of the Moho is inferred to be at the base of the zone of prominent reflectors. The Moho is flat on the seismic sections, at c. 10 km depth TWT, deepening slightly beneath the North Celtic Sea Graben. However, on a true depth scale the crust may thin slightly beneath the Graben (Cheadle et al. 1987). The lower crust, i.e. the zone of prominent reflectors, thins from c. 5 km to c. 3 km to the southeast. There is a zone of southerly dipping reflectors in the mantle beneath the North Celtic Sea Graben observed on the SWAT 4 and 5 lines. These reflectors may represent some form of mantle shear. The upper crust shows weak, moderately prominent S-dipping reflectors along the length of SWAT-4 (Fig. 9). These reflectors can also be seen on some shallow commercial seismic data and are interpreted as Variscan structures, probably thrusts and associated fabrics, which may have been reactivated during Mesozoic extension (Ruffell & Coward 1992). On deep seismic data, the S-dipping reflectors do not cross, but appear to merge with, the zone of lower crust flat reflectors. Some authors (e.g. Gibbs 1985) argue that the most prominent of the S-dipping reflectors mark the Variscan thrust fault, which was later reactivated as a major extensional fault. There is no apparent thickening of the rift-phase sediments towards the S-dipping reflectors; the sediments appear to thicken towards the southeast margin of the basin. The South Celtic Sea Graben shows a similar history to that of the North Celtic Sea Graben, with continental rift-phase sediments in the Permo-Triassic and Middle Jurassic marine shales and limestones (Kamerling 1979; Van Hoorn 1987). The basement structures can be seen in onshore SW England, where numerous extensional faults dip to the NW (Shackleton et al. 1982). On the Devon and Cornwall coasts, both brittle and ductile extensional shears occur, including major ductile low-angle detachments which rework the Variscan back-thrusts in the Rusey-Tintagel area. The ductile detachment zone is c. 1 km thick, with numerous small-scale ductile shear

18

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" 10 km of Mesozoic sediments. The northwest part of the basin comprises SE-dipping Permo-Triassic sediments which dip towards the master bounding fault. There are several SE-dipping antithetic faults and some pinch-outs of sediment which may represent onlaps into the tilted half-grabens or small low-angle faults. In real terms the Moho is relatively flat, although in depth sections there is a small velocity pull-down beneath the thick sedimentary cover. The lower crustal reflectors are most prominent in the northwest part of the basin (as seen on SWAT 2) (Fig. 12). These structures are not cut by the basin-bounding faults. There is no evidence for a thermal subsidence phase beneath the main part of the Cardigan Bay Basin and presumably the zone of lower crustal and mantle stretching is situated elsewhere, to the northwest of Cardigan Bay. Alternatively, the lower lithospheric stretch may diffuse over a wide region so that the thermal subsidence was slight and evidence for it destroyed during Cretaceous to Tertiary inversion. Thus different models for stretching apply to adjacent basins linked by tear or transform faults. These tear faults control the distribution of Triassic extension (and subsequent inversion) and are probably reactivated Variscan shear zones. This same system bounds many of the early basins in the Atlantic and was probably also responsible for the development of Tethys to the southeast. Relaxation and extension of the Variscan thrust belt resulted in slip along major NW/SE-trending left-lateral strike-slip zones that extended across Europe as far east as the Black Sea (Ziegler 1988) (Fig. 5). The basins probably formed by late orogenic collapse. The presence of granites and volcanic rocks in SW England suggests high heat flow. Variscan subduction had ceased and so simple back-arc spreading models are not applicable. Some model of roll-back or delamination of the Variscan

21

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Fig. 12. Line drawing of SWAT 2 across the Cardigan Bay Basin (from BIRPS & ECORS 1986). Location shown on Fig. 8. subducted plate may explain the regional extension along the length of the Appalachian-Variscan mountain belt, similar to models used to explain Tertiary Basin and Range subsidence in the western USA.

Permo-Triassic of the North Sea Permian In the North Sea there were two main areas of subsidence forming the northern and southern Permian basins, separated by the Mid-North Sea-Ringkobing Fyn system of highs (Figs 5 & 13) (Glennie 1990). A small Permian basin also occurs in the Moray Firth. The early basin sediments are termed the Rotliegend, an old German miners' term for the red beds that underlie the Zechstein (Glennie 1990). Rotliegend sedimentation occurred during earliest Permian to Late Permian times; Zechstein deposition took place entirely within the Tartarian, the youngest stage of the Permian. Lower Permian volcanism (early Rotliegend) is most evident in north Germany and Poland and within the Horn-Bamble-Oslo Grabens (Fig. 6). The thickest Rotliegend sequences are preserved in northern Germany in the areas of most widespread lower Rotliegend volcanism, suggesting that thermal subsidence was important (Glennie 1990). Lower Rotliegend volcanics and sediments are preserved in the Oslo Graben-Bamble Trough-Horn Graben areas north of the Trans-European Fault Zone. Transfer zones between the Horn Graben and Bamble Trough trend NW/SE, suggesting that this was the Early Permian extension direction parallel to the Trans-European Fault Zone. Early Rotliegend igneous activity includes the intrusion of the Whin Sill in northern England and some of the dyke swarms in the Midland Valley of Scotland.

PERMO-TRIASSIC BASINS OF NW EUROPE

23

Fig. 13. Simplified map showing the limit of igneous activity and the trend of dykes in the North Sea.

In these areas, volcanism followed Westphalian inversion and pre-dated much of the Permo-Triassic extension, suggesting that the region was underlain by hot asthenosphere, possibly the edge of a NW European hot spot. The trend of the dyke swarms in the Midland Valley of Scotland and adjacent regions suggests local NE/SW extension, perpendicular to that in the Horn-Oslo Grabens. It should be noted that if the region was underlain by a hot spot, only small amounts of extension could lead to the upwelling of asthenospheric melt. Some of the Permo-Carboniferous regional uplift could be due to thermal doming, while mantle cooling could contribute to much of the subsequent Rotliegend subsidence. The Southern Permian Basin has a main depocentre trending WNW/ESE across the northern parts of west and east Germany, in which up to 1.5 km of shales and halite accumulated. The basin sediments onlap the Brabant Massif in the south and the Mid-North Sea High in the north (Fig. 14). Late Carboniferous inversion structures formed a topography which was infilled by the Stephanian Barren Red Beds and then the Rotliegend sandstones. The Brabant Massif was a positive feature throughout the Carboniferous, but the Mid-North Sea High shows evidence of broad inversion during the latest Carboniferous-Early Permian.

24

M. COWARD

Fig. 14. Schematic section through the South Permian Basin. From Fisher & Mudge (1990). The Polish Trough forms part of the southern depocentre of the Southern Permian Basin and is controlled by NW/SE-trending fault systems, with the northern margin being the Tornquist Line (Fig. 5). This NW/SE-trending rift basin, the PolishDobrodgea Rift, extends from Poland to the Black Sea. Rifting was associated with Permo-Triassic sea-floor spreading in the Black Sea (Ziegler 1988). Several models may explain the origin of the Southern Permian Basin: (i) Rifting Model. Badley et al. (1988) and Smith et al. (1993) argue that that Early Permian extension initiated the N/S-trending Viking and Central Graben systems and that the Southern Permian Basin was related to this stretching. This model is considered unlikely, as evidence for important Early Permian extension is lacking in the Southern Permian Basin. The Upper Carboniferous Barren Red Beds and the Rotliegend infill a topography resulting from Carboniferous inversion tectonics (Fig. 14). Thickness changes are a result of infilled topography rather than graben development. Permian rifting however, occurred in the Northern Permian Basin and west of Shetland. Some minor rifting events could have occurred in the north and east of the Southern Permian Basin. (ii) Flexural Basin Model. The Southern Permian Basin and its continuation into Poland and its offset to the Peri-Caspian Basin, lie on the foreland of the Variscan mountain belt. Lithospheric thickening could have produced a flexural foreland basin and account for some of the subsidence. However, the edge of the Permian Basin lies 200 km north of the main thrust front and the basin formation coincided with mountain belt erosion and uplift rather than lithospheric thickening. This model is therefore unlikely. (iii) Thermal Subsidence Model. Thermal subsidence following Carboniferous rifting and, more importantly, following Late Carboniferous volcanism could account for the large subsidence basin. Fig. 13 shows the probable dimensions of the Late Carboniferous thermal dome and the subsequent Permian subsidence basin. Similarly, models can be proposed for the origin of the Mid-North Sea High. This High post-dates Variscan inversion and foreland basin development. It overlies a region of intense Late Carboniferous intrusive activity and may represent a zone of thickened crust less liable to subsequent thermal subsidence. Wadi and dune sands accumulated on the southern flanks of the Southern Permian Basin, reflecting transport of sediment away from the Variscan source. In the Northern Permian Basin, aeolian and fluvial sediments were derived from the Caledonides (Glennie 1990). Within the Southern Permian Basin, the Permian winds

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,.,ll0OC to 14.5 pm with a narrow standard deviation and an age that records the timing of this rapid cooling event. Slower cooling results in significant track shortening, reduced mean lengths, larger standard deviations, and an apparently mixed age which, probably will not be related directly to a discrete event. More detailed explanations of the temperature dependence of the annealing process are to be found in Laslett et al. (1987), Duddy et al. (1988) and Green (1989b). A primary measure of the single grain age spread within a fission track data set is given by the age dispersion value (relative standard deviation) that accompanies the central (modal) age (Galbraith 1992). A dispersion value (expressed as a percentage variation) greater than 10% indicates a spread in single grain ages about the modal age, beyond that expected from a single age population. In such cases the spread will be due either to multiple provenance ages, or to partial annealing that has exploited the compositional variation present in the sample, enhancing the existing spread in

Fig. 2. Hypothetical representation of changing single grain age distributions within a sediment in response to progressive annealing (and cooling in the lowest diagram). The horizontal axis represents increasing relative precision of single grain age; the vertical axis represents the standard error of each measurement (depicted by the 2o- error bars on selected points). The nominal stratigraphic age of the sediment is depicted by the shaded sector, the arc of which forms part of the radial age scale (Ma). Single grain ages are located by drawing a line from the origin through the data point to the radial age scale.

THERMAL HISTORIES OF BASINS FROM FISSION TRACK ANALYSIS

45

grain ages. A graphical method used to display the single grain ages and their associated errors is the radial plot of Galbraith (1990) (Fig. 2). The arcuate axis on the right represents fission track age and is scaled in Ma. Grain age precision is plotted on the x-axis, with those grains plotted closer to the arcuate axis being the more precise. Single grain ages are obtained by drawing a radius from the origin on the y-axis on the left through the crystal age to intersect the accurate scale. The usefulness of radial plots in observing the effects of increasing track annealing/ temperatures is illustrated by the three plots in Fig. 2. The upper plot shows an unannealed sample in which all grain ages are either older or equal to the stratigraphic/intrusion age. Moderate annealing is evident in the middle plot, where some grain ages are younger than the stratigraphic age, even at the 2a error level. As annealing increases, compositional effects are enhanced and the spread in single grain apparent ages widens, as measured by the age dispersion value. In the bottom plot of Fig. 2, extreme annealing removes the combined influences of compositional variation and inherited tracks, bringing the grain ages together to form a single age population with reduced age dispersion. The final stage in the interpretative procedure uses a forward modelling program with a Monte Carlo approach to predict FT age, mean length and standard deviation parameters, using the annealing algorithm of Laslett et al. (1987). Figure 5 illustrates the Monte Carlo approach used to assess the temperature history of a sample from the southwestern peninsula (see below). Fission track ages quoted in this study are central ages d:l standard error, followed by the percentage age dispersion.

Regional fission track results Southwestern peninsula The Permo-Triassic red-bed sequence in Devon and Somerset began with the deposition of fan breccias, fluvial and aeolian sands resting unconformably on folded Devonian and Carboniferous basement. Deposition was initially within local intermontane basins, becoming more extensive as the surrounding uplands were progressively denuded. Locally derived detritus is in evidence, including granite from the Cornubian batholith emplaced during the late Carboniferous. The general absence of palaeontological evidence in the Permian rocks has led to a poor stratigraphic resolution. A mid-Triassic fauna is the oldest stratigraphically useful material, although Warrington & Scrivener (1990) have confirmed a Late Permian sequence in Devon. Samples collected for FT analysis were assigned stratigraphic ages according to Warrington & Scrivener (1990). Table 1 details the resultant fission track data from the Permian and Triassic samples, whilst Fig. 3 shows the sample locations and FT central ages. The Permian and Triassic sediments sampled range in age from the Dawlish Sandstone Formation (Late Permian) to the Mercia Mudstone Formation (Middle to Late Triassic). The FT ages range from 278 ~ 14 to 191 • 10 Ma, with mean track lengths of < 14 lain. Comparisons of the FT ages with the nominal stratigraphic ages (~255-230 Ma) indicate that some sediments may have experienced minor postdepositional annealing, The radial plot in Fig. 4a from the Dawlish Sandstone Formation (Tatarian) is typical of the Permian samples. The FT age of 256 + 19 Ma

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