The Precambrian, Caledonian and Variscan Framework to NW Europe

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The Precambrian, Caledonian and Variscan framework to NW Europe

M. P. COWARD Department of Geology, Imperial College, London SW7 2BP, UK

Abstract: During the Precambrian and the Palaeozoic, the tectonics of NW Europe were dominated by the sequential accretion of different terrains on to the North American Craton, e.g. old continental crust of the Scandinavian Craton, magmatic arcs of the Avalon-Brabant Massif, Pentevrian continental crust and the Brioverian magmatic arc. Terrains were locally bounded by thrust packages, by NW-SE trending strike-slip - transform faults parallel to the accretion direction, and by NE-SW trending strike-slip faults defining localized oblique collision or, more generally, boundaries to zones of lateral continental extrusion and escape. Close analogies can be made with the Tertiary Makran- Himalaya-Tibet collisional zones. The Laxfordian/Caledonian/Variscan thrusts and more importantly the large-scale strike-slip faults, imposed a complex heterogeneity to the crust which critically influenced the subsequent extension directions and the siting of basin bounding faults and tectonic inversion, from Devonian times to the present day.

The relatively recent availability of seismic data tectonic episodes, i.e. the Laxfordian (1800- has led to enormous advances in our under- 1700 Ma), the Caledonian (500-400 Ma) and standing of the deep geology of the British the Variscan (400-300 Ma). Other compress- Isles. In particular the deep seismic surveys ive and strike-slip orogenic events affected obtained by the BIRPS Group (British Insti- NW Europe but their effects were largely tutions' Reflection Profiling Syndicate) have obliterated by the major compressive events shown variations in crustal thickness and middle listed above. to deep crustal tectonic fabric in the northern Figure 1 shows the distribution of these and western offshore regions of Britain and tectonic events in four principal domains in throughout the North Sea (see for example Britain. Domain 1 consists of Lewisian rocks of Cheadle et al. 1987; Freeman et al. 1988; the NW Caledonian foreland, which with their Klemperer 1988). The combinations of shallow upper Proterozoic cover were originally part of commercial seismic data, deep level BIRPS the N American-Laurentian craton. Their seismic data and conventional structural and dominant crustal fabric is of Laxfordian age. stratigraphic field data, allow new models to be Domain 2 comprises rocks which were de- derived for the tectonic development of Britain. formed and metamorphosed during the Cale- They show us the relationship between deep donian tectonic event and can be subdivided level tectonic fabrics and surface structures, into several sub-domains based on the orienta- which combine to produce the overall tectonic tion and age of the Caledonian fabric. Domain framework to Britain and adjacent parts of NW 3 is the SE foreland of the Caledonian fold belt Europe. This paper aims to describe this struc- and consists of a Late Precambrian magmatic tural framework and (i) review the pre-Mesozoic arc, known as the Acadian or Cadomian basement kinematics in Britain and adjacent magmatic arc or Brabant Massif. Domain 4 parts of NW Europe, based on the the available consists of rocks deformed and weakly meta- seismic data, previous published structural data morphosed in the Variscan tectonic events and reviews, and new work by the author, and during Devonian to Carboniferous times. (ii) discuss how different fabric intensities and styles influence subsequent Upper Palaeozoic and Mesozoic-Cenozoic basin development. The Lewisian Gneisses of Domain 1, NW The pre-Mesozoic basement rocks of the Scotland and their Proterozoic cover British Isles range from the Lewisian gneisses of NW Scotland, dated at c.2900 Ma (Moorbath The Lewisian rocks of NW Scotland are et al. 1969), to the Devonian and Carboniferous quartzo-feldspathic gneisses and metasediments sediments involved in the Variscan fold and that were affected by several episodes of late thrust belts of southern Britain. Much of the Archaean to early Proterozoic deformation and tectonic framework of NW Europe was metamorphism. According to the now generally developed during three major compressive accepted chronology (e.g. Sutton & Watson

From HARDMAN, R. F. P. & BROOKS, J. (eds), 1990, Tectonic Events Responsible for Britain's Oil and Gas Reserves, Geological Society Special Publication No 55, pp 1-34. Downloaded from http://sp.lyellcollection.org/ by guest on September 24, 2021

2 M.P. COWARD

100 km i •

Laurent ian creton Proterozoic gneissic basement

DOMAIN 1

~ Dalradian -collapsed late PreCambrlan passive margin, deformed & metamorphosed /oev ,~l in by/Silurianearly Ordov,cian,(ag~ uplifted450-410 &Ma) c~led J Southern Uplands fore arc basin /~ & accretionary wedge of Ordovician Moines - Proterozoic to early Devonian metasedlments & basement , re deformed in Silurian / /~ ~ Solway line cooled by 400 N~ .... projection of deep crustal ...... " structure Welsh-Lake District Caledorlides Ordovician Silurian magmatic arc & local sedimentary basins, Lower ~ Palmozoic inversion, main deformation ...... end Silurian & mid Devonian DOMAIN 2 "-~ ~. ~ / I

Brabant Massif - late Pre Cambrian island arc DOMAIN 3

Varlscan fold & / DOMAIN 4 thrust belt / ,_..J-- EBB ophiolite

Fig. 1. Structural domains of Britain. See text for discussion.

1951; Park c. 1970) the gneisses formed at c. which consists of over 3 km of late Proterozoic 29(~) Ma during high grade, locally granulite- metasediments and metavolcanics (O'Nions et facies metamorphism associated with imbri- al. 1983), was thrust on to the Lewisian gneisses cation and deep burial of sediments and granitic in the Gairloch region. Laxfordian granulite- rocks. Slow cooling of the gneisses was followed facies metamorphic rocks were uplifted in South by reactivation during the lnverian episode Harris on the Outer Hebrides, so that through- (2600-2400 Ma), associated with the devel- out the Lewisian outcrop large shear zones with opment of upright folds and steeply dipping relatively flat lying fabrics which have been NW-SE shear zones (Coward & Park 1973). subsequently deformed by upright folds can be Scourian and Inverian age structures are cut by observed (Coward 8,: Park 1987). These upright a swarm of dolerite dykes, the Scourie dyke folds probably detach on shear zones in the suite of Sutton & Watson (1951). The sub- middle to lower crust (Coward 1990). Large sequent Laxfordian deformation was hetero- scale moderately to steeply dipping shear zones geneous and, on the mainland of Scotland, the at Laxford Bridge, Gairloch and South Harris Lewisian rocks can be divided into three zones, (Fig. 2) are either the tilted boundaries to gently a central zone where the Scourian structures dipping shears or they represent oblique to remain well preserved, and northern and lateral ramps where the shear climbed to differ- southern zones, where Laxfordian reworking ent crustal levels perpendicular to thrust was more intense and dykes and Scourian movement. fabrics have been reorientated into con- The early Laxfordian crustal thickening was cordance (Fig. 2). On the Outer Hebrides the followed by relaxation and extension, producing degree of reworking was even more intense, ductile shears with amphibolite-facies miner- with only small pods remaining in which alogy (Coward 1990). This extension may have Scourian fabrics are preserved. formed by collapse of the thickened Laxfordian The Laxfordian deformation involved large- orogenic zone, similar to the mode of crustal scale crustal thickening with an overthrust spreading in modern orogenic belts (e.g. Dewey direction of SE to NW. The Loch Maree Group, 1988). Granitic crustal melts are prominent Downloaded from http://sp.lyellcollection.org/ by guest on September 24, 2021

PRECAMBRIAN AND PALAEOZOIC FRAMEWORK TO NW EUROPE 3

LEWIS

Laxford shear

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~"U~'ST~/ "~/ "~~ ~l granit.... • ~ . ~ ~ scoo.an structure ~"~_.~ Laxfordian ., j ~ Loch Marree meta.volcanic metamorphics - Laxfordiansteep shear / ~ ~ Outer Isles faull /~ ~ Moinethrust zone normal fault

Fig. 2. Structures of the Lewisian Complex of NW Scotland and their influence on basin development in the Minches and Outer Isles Basins. Basement cut-out zones arc shown by heavy stipple. From Coward et al. 1990. along the major shear zones near Laxford and Palaeozoic K/Ar ages obtained from the on South Harris (Fig. 2); they post-date the phyllonites of the fault zone (D. Rex in Sibson compressional shears and at Laxford they are 1977). However many of the mylonites show synchronous with the extension. This extension small-scale folds and shear bands indicating an was probably responsible for the uplift and extensional down-dip shear couple (Sibson emergence of Laxfordian high-grade metamor- 1977), and White & Glasser (1987) suggest that phic rocks and may also be responsible for the the phylionites that occur close to the east coast restoration of the Laxfordian crust to its present of the southern Hebrides may be entirely the thickness, shown on the BIRPS surveys to be product of low-angle extensional faulting. This about 29 km (e.g. Brewer & Smythe 1984). extension presumably post-dates the thrust Cooling ages obtained from Laxfordian rocks movements on the Outer Isles fault; it may be range from c.1600-1400 Ma (Moorbath & Proterozoic, but is more likely to be of early Park 1971) and represent either the slow un- Palaeozoic age. roofing of the Lewisian complex following the The Stoer Group and the younger Torridonian main phase of extension or subsequent weak Group, which overlie the Lewisian gneisses, are phases of uplift. Numerous faults with mylonite thick clastic sequences which give Rb/Sr ages of and pseudotachylite occur on the Outer c.970 and 777 Ma respectively (Moorbath Hebrides and may be of late Proterozoic age. In 1969). The upper Torridon Group of sediments the Gairloch region of the Scottish mainland forms a thick (>6 km) sedimentary wedge and there are NW-trending pseudotachylite-bearing Sr isotope data suggest derivation from a Lax- shear zones which pre-date the late Proterozoic fordian crust (Moorbath et al. 1969). Stewart Torridonian sediments. The Outer Isles fault (1982) suggested that the Torridonian basin zone was probably initiated as a Laxfordian (or developed from the erosion of uplited fault later Proterozoic) structure and was sub- blocks formed as a result of regional extension. sequently reworked as a Caledonian thrust, as However on the mainland normal fault arrays shown from the range in Proterozoic to early of Torridonian age have not been detected. Downloaded from http://sp.lyellcollection.org/ by guest on September 24, 2021

4 M.P. COWARD

Cheshire et al. (1983) and Kilenyi & Stanley land, NW of the Great Glen, consist of Pro- (1985) extrapolate the Torridonian sequence terozoic metasediments (the Moines) with from the mainland to the Minches and consider minor basic and acid intrusives. They are in- that the basal part of the half graben in the tensely foliated and metamorphosed to upper Minches is filled by Torridonian sediments. greenschist and amphibolite facies and give syn- They suggest that extensional movements on tectonic metamorphic ages of c.460 Ma and the Outer Isles fault zone occurred during the mineral ages which probably reflect later cooling late Proterozoic, as this fault system marks the and uplift at 430-400 Ma (Brewer et al. 1979; western boundary of the Minches Basin. How- Johnson et al. 1985). Along the N coast of ever the correlation between the Torridonian Scotland the Moines show a complex interlayer- sediments seen onshore and the basal sediments ing of intensely foliated metasediments, meta- of the Minches Basin is by no means certain. basite intrusions and earlier basement, due to Other seismic interpretations (e.g. Enfield in isoclinal folding and closely spaced WNW- Coward et al. 1989) place the Torridonian sedi- directed thrust imbrication (Butler & Coward ments as basement to the Minches half graben 1984; Barr et aL 1986). The foliation dips c. 150 and suggest that the lowest sediments in the half to the E or SE in the western part of the section graben are of Devonian age. The provenance of but has steeper dips, sometimes nearly vertical, the thick Torridonian sediments seen onshore in the east. was to the NW, presumably from an area of The Moines were thrust to the WNW over a uplift along the Outer Hebrides (Williams 1969) foreland consisting of Lewisian basement, Tor- and the palaeoflow direction was from NW to ridonian arkosic sandstones and Cambro- SE. This is opposite to the expected palaeoflow Ordovician shelf sediments. In the northern direction if the Outer Isles fault had been a part of the thrust zone in Sutherland, a mini- master bounding fault to the Minches Basin in mum overthrust displacement of 54 km is shown Torridonian times, it is more likely that the by the restoration of imbricated Middle and Torridonian sediments were unrelated to re- Upper Cambrian sediments (Butler & Coward gional crustal extension, but were deposited in 1984) and the Moine Thrust cannot have cut up a deep basin associated with late Proterozoic through basement or lower Cambrian sediments crustal thickening. within 54 km of its present outcrop trace. How- During Caledonian compression there was ever, seismic profiles from offshore northern movement along the Outer Isles fault (Sibson Scotland (e.g. Fig. 3) show dipping reflectors in 1977), generally reactivating the Laxfordian the middle crust which are probably related to fabrics. During post-Caledonian times, there the Moine Thrust and to shear zones in its was further reactivation by extensional faulting hanging-wall. These reflectors are much farther to produce deep Devonian half grabens NE of W than would be predicted from onshore the Outer Hebrides and at least some, if not geology. Presumably this offset is the result of most, of the extension in parts of the Minches a NW trending lateral ramp, close to the N Basin and rifting in basins N of the Scottish Scottish coast. mainland (Fig. 2). The Outer Isles fault was The Great Glen Fault defines the eastern subsequently reactivated several times during limit of the Moines. Its age, sense and amount the late Palaeozoic and Mesozoic (Coward et al. of offset are disputed. A pre-Devonian, sinistral 1989). displacement of c.2000 km was suggested by Morris (1974) and Van der Voo & Scotese (1981) based on palaeomagnetic interpretations, The Caledonides of Domain 2 but the general consensus (Mykura 1976; Smith The Caledonian orogenic belt extends from & Watson 1983) suggests a maximum of a few northern Norway and Greenland to the southern hundred kilometres sinistral displacement at Appalachians and formed as a result of the the end of the Caledonian orogeny, with later closure of an early Atlantic (Iapetus) ocean Permo-Carboniferous dextral movements of a during the early Palaeozoic. The main tectonic few tens of kilometres (e.g. Ziegler 1982; units in the British Isles are shown in Fig. 1. Watson 1985). The orogenic belt formed by the accretion of magmatic arcs and continental fragments on to The British Caledonides SE of the Great the North American continental craton. Glen Fault SE of the Great Glen Fault, the full history The Caledonides NW of the Great Glen of the Caledonian orogen is preserved and The Caledonides of the NW Highlands of Scot- involved (i) the development of a rift basin Downloaded from http://sp.lyellcollection.org/ by guest on September 24, 2021

PRECAMBRIAN AND PALAEOZOIC FRAMEWORK TO NW EUROPE 5

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6 M.P. COWARD leading to a passive oceanic margin during the structural orientations is typical of inversion late Precambrian to Cambrian; (ii) the Early tectonics (cf. Gillcrist et al. 1987), but unusual Ordovician collapse of this passive margin on such a large scale. No basement rocks were during the early stages of reversal in plate involved in these large-scale fold structures. motion; (iii) the obduction of ophiolitic material The NW boundary of the Dalradian sequence is during the Early Ordovician; (iv) the develop- sometimes marked by a major shear zone, such ment of a magmatic arc; (v) the development of as the Port Skerrols Thrust, which carries a fore-arc basin and (vi) the final docking of this younger Dalradian rocks on to older Grampian fore-arc basin and its associated accretionary Moines and probably represents the reactivated sediment wedge with a southern magmatic arc basin-bounding fault. The early large-scale fold during the Late Silurian. This deformation in- fan is cut by NW verging ductile thrusts such as volved both dip-slip and strike-slip displace- the Boundary Slide (e.g. Roberts & Treagus ments. Some authors emphasise the importance 1979). These folds and ductile thrusts are associ- of strike-slip movements relative to dip-slip ated with the thickening of the sedimentary displacements. Thus Bluck (1985), Soper & pile and were followed by local high-grade Hutton (1984) and Hutton (1987) favour the metamorphism associated with this tectonic strike-slip accretion of up to six allochthonous burial (Wells & Richardson 1979). terranes, similar to the terrane accretion in the Subduction continued to the SE of the High- western USA (e.g. Coney 1989). lands, to generate arc-related magmatism and SE of the Great Glen, in the Grampian High- produce large bodies of gabbro and granite in lands, the Grampian Moines comprise late Pro- the Highlands, above the NW dipping sub- terozoic metasandstones which pass up, without duction zone (Fig. 4b). Flakes of ophiolitic any major unconformity, into the Dairadian material were obducted over the thickened sequence. The Grampian Moines, which may Dalradian sedimentary pile (Dewey & Shackle- be the time equivalents of the Torridon Group, ton 1984), producing the Unst ophiolite com- rest on but are interleaved tectonically with plex on Shetland and the Highland Border slices of basement, dated at c. 11(X) Ma (Piasecki Group of ophiolitic debris at the northern edge & van Breeman 1979). The lower Dalradian of the Midland Valley. S of the Midland Valley, consists of syn-rift and post-rift sequences of the Ballantrae ophiolite was obducted over oli- sandstones, shales and limestones deposited on stostrome material and formed the basement to the NW side of a rift basin (Anderton 1982; a later fore-arc basin and its accretionary sedi- 1985). Early Dalradian facies and thickness ment prism in the Southern Uplands (Mc- variations can be considered as due to deposition Kerrow et al. 1977; McKerrow 1987). The in different fault blocks, defined by SE-dipping accretionary prism finally docked with a mag- listric or straight faults and associated NW matic arc on the southern side of the lapetus trending transfer faults (Harte et al. 1984). One margin during Late Silurian to Early Devonian such transfer fault is observed as a major linea- times (McKerrow 1987). ment on regional gravity and magnetic data All the structures SE of the Dalradian High- (Fettes et al. 1986; Hall 1987). As extension lands are dominated by SE-verging thrusts and accelerated, subsidence rates increased and the folds. These range in age from the accretionary upper Dalradian sediments were deposited in a thrust structures of the Southern Upland fore- series of turbidite basins (Anderton 1985; Soper arc basin, to the late collisional structures of the & Anderton 1984). Thinning of the lithosphere Highland Boundary Fault, the Lake District was associated with intense local igneous ac- and Wales. Their dip probably partly reflects tivity. The uppermost Dalradian sediments, the dip of the lapetus subduction zone. Beamish which are of Arenig age and occur only in the & Smythe (1986) and Freeman et al. (1988) southern Highlands, were deposited as a distal map reflectors in the middle to lower crust, facies away from the basin margin, probably on which they associate with the suture. The Cale- the continental slope. donian suture is often taken to lie along the The extensional faults were reactivated in a Solway-Cheviot line (Fig. 1), although the major compressional episode of Ordovician age most prominant crustal reflectors lie a few tens resulting in large-scale positive structural in- of kilometres to the south. The reflectors pre- version. The distal sediments of the upper sumably represent mid-crustal shear zones Dalradian were intensely folded into a large associated with, if not on, the suture. fanning fold complex, whose axial surface varies In the Southern Uplands and Lake District, in dip across the Highlands. A simplified cross deformation was associated with low-grade section through the Dalradian structures is metamorphism and also the development of a shown in Figs 4b & 5. The wide range of locally strong penetrative cleavage. Assuming Downloaded from http://sp.lyellcollection.org/ by guest on September 24, 2021

PRECAMBRIAN AND PALAEOZOIC FRAMEWORK TO NW EUROPE

PRESENT EROSION LEVEL "SEA LEVEL" EARLY SILURIAN TOPOGRAPHY

UI 4UKm

END ORDOVICIAN " EARLY SILURIANTOPOGRAPHY

b ~ ...... 8~...... ~...... ,_

KEY

~Ordovician - Silurian sedimenls ' ' "':" "'~,":-:"i("-""~":.";'.;~ o 40km Ocean floor rocks . ~//~" ~ ~ Late - Precan'lbrian to early Ofdovician sediments ~ Laurentian plate ~1 Scandinavian plate

largely Grenville gneisses, may have been C no later sediments preserved considerable shortening obducled high if originally a passive margin grade rocks NW ~ 4i, - Finmark SE ~~.~ma"-

~duchle deformation / shortemng at almost tcm/year ~,--/~ for Scandinavian plate

Fig. 4(a). Schematic cross section through the Scottish-Norwegian Caledonides during thc late Silurian, assuming restoration of the Great Glen Fault system, based on cross sections by Barret al. (1986), Butler & Coward (1984), Hossack & Cooper (1987), Coward (1983), McGeary &Warncr (1985). FT, Flannan Thrust;' OIT, Outer Isles Thrust; ST, Sole Thrust of the Moinc thrust zone; MT, Moine Thrust; SBS, Sgurr Beag Slide. The shortening estimates arc from Butler & Coward (1984) and Hossack & Cooper (1987). The line of section is approximatcly along linc A-A' of Fig. 9. (b) Schematic section through thc Calcdonides SE of the Great Glen, during the Early Silurian. Dalradian structures arc simplified from regional structural studies (e.g. Roberts & Treagus 1979), the southern structures are simplified from McKcrrow et al. (1977). PST, Port Skerrols Thrust; IBS, lltay Boundary Siidc; HBT, Highland Boundary Fault. The line of section is approximately along line B-B' of Fig. 9. (c) Simplified linc drawing of (a) above, discussing some of the principal tectonic features. Vertical line shading: obductcd ophiolitic rock.

Grampian granites S. Uplands granites NW SE

Lake District ~' - back arc basin during 0,~ 30¢mI Ordovlclan - Silurian _ inverted during Caledonian

Fig. 5. Simplified section from the Great Glen Fault to the English Midlands, showing the present attitude of the structures -- modified from Freeman et al. (1988). The line of section is approximately along B-B' of Fig. 9. Downloaded from http://sp.lyellcollection.org/ by guest on September 24, 2021

8 M.P. COWARD an allochthonous source for sediments in the the indentation of different thicknesses of crust, accretionary prism, Bluck (1985) and Mc- as a result of early Palaeozoic basin devel- Kerrow (1987) argue that important strike-slip opment. movements occurred in the region of the Southern Uplands and along the trend of the The Caledonides of southern Scandinavia Highland Boundary Fault during and following Caledonian collison. However, no steep strike- There is a marked difference in the character of slip shear zones have been recognized from the Caledonian structures on both sides of the surface mapping or from deep seismic data North Sea. In Scandinavia, Caledonian tectonics (e.g. Freeman et al. 1988) and any major strike- involved collision between the N American cra- slip movements must have been accommodated ton and a large southern Scandinavia craton. on NW-dipping faults or have been sub- The latter comprises Archaean to Proterozoic sequently destroyed by these faults. gneisses overlain by late Precambrian and SE of the suture the basement consists of Lower Palaeozoic sediments. Plate collision uppermost Precambrian intrusive and volcanic began during the Ordovician, but several rocks and volcaniclastic sediments, overlain and hundred kilometres of crustal shortening and intruded by lower Palaeozoic magmatic ma- SE directed overthrusting continued until the terial. The Lower Palaeozoic sediments vary in late Silurian. The SW boundary of the Scan- thickness, associated with basin development dinavian craton lay approximately along the and there were important phases of folding, eastern margin of the Central Graben of the local uplift and inversion, probably related to North Sea, along what is often considered to be the growth of the magmatic arc (Fig. 6). The the extension of the Tornquist Line (Fig. 7) (see major deformation in Britain, south of the Bergstrom 1984; Kumpas 1984; Pegrum 1984). suture zone, occurred at end Silurian to early This zone formed a fundamental crustal bound- Devonian times and was associated with colli- ary in the Caledonides, between the continent- sional thickening of the magmatic arc and its continent collision of the Scandinavian Cale- related sedimentary basins (Figs 5 & 6). The donides to the NE and the continent-arc Caledonian foliations change strike across collision of the British and North American England and Wales, tracing out festoon-like Caledonides to the SW. patterns (Fig. 1) and these may be related to Figure 7 shows a sketch map of the Tornquist

Carmel Head Thrust SE NW

Snowdonia Bala System Berwyns

20 km t

Welsh basin - extended due to roll-back of subduction zone

volcanics

20km Palaeozoic cover --->~

Fig. 6. Top: Simplified section through the Welsh Caledonides to show the SE verging folds and thrusts and Palaeozoic thickness variations. Bottom: Diagramatic section through the Welsh Caledonides to illustrate the model of back-arc extension above a SE-dipping subduction zone, antithetic to the NW-dipping subduction zone below the Southern Uplands and Grampian Highlands (Fig. 4b). This model explains the different early Palaeozoic phases of stretching and thickness -- facies variations, the phases of tectonic inversion and the periods of volcanic activity (see Watson & Dunning 1979 and references therein). Downloaded from http://sp.lyellcollection.org/ by guest on September 24, 2021

PRECAMBRIAN AND PALAEOZOIC FRAMEWORK TO NW EUROPE 9

Fig. 7. Map of southern Scandinavia to show the position of the Tornquist zones and the Trans European Fault (from EUGENO Working Group 1988). See text for discussion. zone in southern Scandinavia and northern Front across southern Denmark. Therefore the Germany, based on the European Geotraverse TEF is probably a zone of strike-slip movement, data (EUGENO-S Working Group 1988). possibly with some local transpression or trans- The western continuation of the Tornquist Line, tension. (Figs 9 & 10, modified by movement the Sorgenfrei-Tornquist Line, appears as a on the Great Glen Fault, Fig. 11.) A modern tear fault of Palaeozoic to Tertiary age and in analogy would be the Chaman-Quetta fault particular is associated with tectonic inversion zone in western Pakistan (Dewey et al. 1988), in the late Cretaceous and early Tertiary. Scan- which marks the surface expression of the trans- dinavian crust, dated by mineral ages from deep form boundary of the indenting Indian plate well cores at 800-9(~) Ma, similar to the ages (Fig. 12). East of this fault zone there was of the Sveconorwegian orogenic belt of southern continent-continent collision between the Sweden (Fig. 7), continues to an approximately Indian and Asian plates. West of the fault there WNW trending zone which cuts across southern was continent-arc collision in southern Denmark and northern Germany. This zone, Afghanistan and in the Makran. termed the Trans European Fracture Zone (EUGENO-S Working Group 1988), is a The Caledonide orogen direct continuation of the eastern and main segment of the Tornquist Line in eastern Europe Figures 9 & 10 suggest the possible relationship (Fig. 7). The Trans European Fracture Zone between the Scandinavian Caledonides, the (TEF) is drawn just south of the limit of Cale- British Caledonides, SE of the Great Glen and donian deformation. North of the Caledonian the Caledonides to the NW of the Great Glen. Front, Lower Palaeozoic sediments sit in a small The Moines and Moine Thrust underwent at graben overlying Precambrian basement. South least 90 km shortening (Butler & Coward 1984) of the Front, the Lower Palaeozoic rocks carry between 460 and 4IX) Ma. This is long after the a penetrative cleavage and low-grade metamor- cessation of fold and thrust activity in Dalradian phism. They give Ar/Ar mineral ages of 530- rocks to the SE of the Great Glen. Thrust 400 Ma, with a peak of 450-440 Ma (Frost et tectonics in the Moines occurred at the same al. 1981; Zeigler 1982; Liboriussen et ai. 1987). time as the growth of the accretionary prism in As the Caledonian rocks are only observed in the Southern Uplands. Hence it is difficult to well cores, the tectonic transport directions can- find the driving mechanism for the Caledonian not be determined from kinematic indicators, deformation NW of the Great Glen, if the but only from the regional tectonic pattern. Moines were deformed in their present position Elsewhere in the Scandinavian Caledonides, relative to the rest of Britain (e.g. Coward the overthrust direction was to the ESE, parallel 1983). A more favourable explanation is that to the strike of the TEF and the Caledonian the thrust structures of the Moines formed as Downloaded from http://sp.lyellcollection.org/ by guest on September 24, 2021

NORTH GERMAN BASIN RINGKeBING-FYN HIGH SW JURASSIC CALEDONIAN NORWEGIAN DANISH BASIN NE

i~i~ ,..'..': ' " • :.: " ~ 7..":: '

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CA,,SON,,,,,oo~SLOWER PALAEOZOIC ~11111111111tL"~ tl llr sVECOFENNIAN .... BASEMENT LOW~A;EAL~EOZOIC SORGEN,REI-TORNOUISTZONE

20 ¸

100 km // TRANS EUROPEAN FAULT ZONE

Fig. 8. Section across the Trans European Fault, the edge of the Caledonian deformation and the Sorgenfrei Tornquist Zone -- from EUGENO Working Group (1988). The section line is shown in Fig. 7.

/ a / ,

! / *C~ONTINENT - CONTINENT COLLISION I

o~°-j

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EXPULSION

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Fig. 9. Simplified map of the Caledonides after restoration of the Great Glen Fault. Downloaded from http://sp.lyellcollection.org/ by guest on September 24, 2021

PRECAMBRIAN AND PALAEOZOIC FRAMEWORK TO NW EUROPE 11

LAURENTIAN PLATE j

SCANDINAVIAN JJ PLATE

J J

C ADOM IAN - AVALON IAN PLATE

Fig. 10. Diagram to show the different events in the Calcdonides, after restoration of the Grcat Glcn system. part of the continent -continent collision later Ordovician and Silurian at a shortening between the Scandinavian and North American rate of c. 1 cm/year (Hossack & Cooper 1987). cratons and wcrc then displaced sinistrally The amount of shortening across this part of during the late phases of Caledonian deforma- the Caledonides is not known, because of the tion. Figure 10 shows the principal subdivisions uncertain relative positions of the NW High- of this continental collision tectonic model. lands and Scandinavia, the unknown amounts Figure 4a shows a schematic section across the of strike-slip movement and the unknown Moine and Scandinavian collisionai belt at end amount of shortening that may have been taken Silurian times and compares this with the con- up by deformation of the original passive mar- temporaneous section across the Dalradian- gin. In northern Norway, kinematic movement Southern Uplands (Fig. 4b). Modern analogies indicators suggest considerable along strike would be comparative sections across the ductile displacements (Hossack & Cooper 1987) Himalayas- Pamirs in Pakistan- western China, and there are similar suggestions for SW- formed by continent-continent collision directed thrust movements in Shetland and NE between India and Asia, and across the con- Scotland (Fig. 9, with data from Flinn 1985; tinent-arc collision zone in southern Afgha- Coward 1983). nistan and eastern Iran (Fig. 12). Figure 10 shows the style of deformation across the suggested Moine-Scandinavian The post-Caledonian Devonian Basins orogenic belt. Ocean closure and obduction of Caledonian compressional deformation was high-grade rocks began during the early Cam- closely followed by extension, forming Devonian brian and slices of ophiolitic material are pre- basins in, for example, the Minches, the served in the higher thrust sheets ,of western Orkneys and Caithness, the Moray Firth, E and Norway (Nicholson 1979). There may have been W Shetland, western Norway, the Midland a quiescence in plate convergence during the Valley and Northumberland Basin and the late Cambrian and early Ordovician, but the Danish sector of the North Sea. Many of these major shortening in the NW Highland Cale- basins formed by reactivation of earlier Cale- donides and in Norway took place during the donian fabrics, so that the Devonian faults have Downloaded from http://sp.lyellcollection.org/ by guest on September 24, 2021

1.2 M.P. COWARD

WEST ORKNEY AND MINCH BASINS

displacement in late Middle ORS to produce ieastern basins displacement in early Upper ORS to Juxtapose basins EAST SHETLANO BASIN

• ~ ~i: ~,k~::~ ,'. ~¢)~!k~ i .... :z'~'Dq>!i';i,~ i, " MIDLAND VALLEY BASIN

Fig. 11. Simplified map to show the generauon of ORS basins east of the Great Glen Fault, associated with sinistral displacement on the fault.

similar strikes and dips to Caledonian thrusts. The Orcadian and West Orkney Basins However not all Devonian faults were dip-slip; strike-slip movements occurred on both gently The Devonian (or Old Red Sandstone-ORS) and steeply dipping faults. Another analogy can deposits of Caithness, the Orkney Isles and the be made with the Tibetan-Himalayan region Walls Peninsula of SW Shetland unconformably (e.g. Dewey 1988; Dewey et al. 1988), where overlie an irregular landscape of Caledonian much of the late Tertiary extension occurred by metamorphic rocks. They were deposited in a dip-slip reactivation of earlier major thrusts large intermontane basin known as the Orcadian (Royden & Burchfiel 1987; Coward et al. 1987) Basin (Donovan et al. 1974). The Lower ORS but there was also considerable strike-slip fault (Siegenian-Emsian) has a restricted distri- movement and extension along the strike of the bution and in western Caithness passes up into belt. The Devonian basins of Britain and NW Middle ORS (Eifelian-Givetian) without any Europe show the local development of inter- important stratigraphic break. The base of the montane basins which appear to have subsided Middle ORS oversteps the Lower ORS and gradually to become more lacustrine and some- onlaps basement rocks to the west. Uncon- times marine. They show several unconformities formities are present at the base of the Middle and locally intense deformation related to ORS on the Orkney mainland and in southern phases of tectonic inversion. Caithness, where the Lower ORS is considered Downloaded from http://sp.lyellcollection.org/ by guest on September 24, 2021

PRECAMBRIAN AND PALAEOZOIC FRAMEWORK TO NW EUROPE 13

Asian __ -~I~/~' are considered to be of Frasnian age (Rogers et P'a'e al. 1989). The deep structure of the Orcadian Basin can be seen from seismic data in the West Orkney Basin (WOB), offshore northern Scotland (e.g. Fig. 3). Up to 4 km of sediments infill the larger

Makran ~l/'/- ~ f/ of the half-graben and the average sediment thickness is in the order of 2 km. The basin Arlabia"~~~ n [ <- .Indian Plate ~ ~/~~\/ faults and sediment fill can be traced close to Plate// v/ ~ "~ the northern Scottish and Orkney coasts and correlations made with middle ORS deposits onshore (Coward & Enfield 1987). The WOB E \/ shows considerable extension, with maximum stretching factors of up to 1.6. However most of f plateconvergence (cm/yr) the sediments seen on the seismic data show J oceansubduction evidence of faulting and no post-rift sediments 500 km ~ thrustfront are preserved. The latter may have been re- ~ majorstrike slip fault moved by subsequent late Devonian or Car- Fig. 12. Map of the Markran-Himalayas-Tibet boniferous tectonic inversion, or alternatively collision zone, between the Indian and Asian plates, they were never deposited in this basin. for comparison with Caledonides collision -- see text The NW edge of the WOB is formed by for discussion - From Coward et al. (1987) and faults which are strongly curved in plan and Dewey et al. (1988). possibly curved in section, while the east part of the basin is formed by faults which are approximately straight in plan and section. On the to have been uplifted, eroded and folded prior deep seismic data (e.g. the MOIST line, see to deposition of the middle ORS. Brewer & Smythe 1984; Cheadle et al. 1987 and The Lower ORS of the Orcadian Basin is the DRUM line, see McGeary & Warner 1985) characterized by coarse lenticular breccias and all these faults dip to the ESE but cannot be conglomerates, interfingered with finer bedded traced to a depth greater than 18-20 kin. They sandstones. These sediments were deposited are parallel to reflectors in the basement, inter- within small isolated, mainly playa-filled interpreted as Caledonian shear zones. On com- montane basins (Mykura 1976). Local thick mercial speculative data (Coward & Enfield fanglomerates suggest deposition in basins 1987; Coward et al. 1989), faults with listric bounded by active fault scarps. The Middle geometries, which shallow towards a thin pack- ORS in Caithness and Orkney began with the age of reflectors dipping 10°-15 ° to the ESE, deposition of relatively quiescent lacustrine can be mapped. These shallow reflectors are sandstones up to 4 km thick in Caithness. There considered to represent structure within the was a gradual regression in later middle ORS Caledonian basement, possibly the offshore times to dominantly alluvial sedimentation. continuation of the Moine Thrust Zone (Co- Palaeoflow was from SW to NE, parallel to the ward et al. 1989). Thus much of the extension in general trend of the basin (Foster 1972), al- the WOB developed on faults that flatten onto though locally the sediment supply was from a gently dipping detachment, interpreted as the the NW as shown by the presence of Torridonian Moine Thrust, or onto major shear zones in the and Cambrian clasts. Moines. Half-graben observed offshore on seis- There is an important unconformity between mic data are also observed onshore (Coward et the Middle and Upper ORS throughout the al. 1989) and in each case coincide with the Orcadian Basin. On the Walls Peninsula of SW position of a major ductile shear zone (Coward Shetland, a 9 km thick sequence of middle ORS et al. 1989), the extensional faults occurring a thinly bedded sandstones was deformed by short distance into the hanging-wall of the polyphase folding before the intrusion of the ductile shears. Sandwich plutonic complex, dated at 360 _+ 11 The dominant effects of the Middle ORS Ma (Mykura 1976). On the Orkneys the onset inversion appear to be concentrated around the of upper ORS sedimentation was marked by an faults in the Orkneys and Shetland Isles. This unconformity followed by calc-aikaline lavas, inversion may be due to the final effects of dated at c.370 Ma (Halliday et al. 1977), fol- Caledonian compression, or to strike-slip tec- lowed by high-energy fluvial deposits, locally tonics associated with movements on the proto- with aeolian dunes. These upper ORS deposits Great Glen Fault. From the trends of the folds Downloaded from http://sp.lyellcollection.org/ by guest on September 24, 2021

14 M.P. COWARD on West Shetland and structures within the al. (1989) suggest a small (<100 km) displace- fault zones on the Orkneys, the shear couple ment. Strike-slip movements may be related to along the Devonian Walls Boundary Fault- a late transpressive phase of Caledonian de- Great Glen Fault would be sinistral (Enfield & formation (Soper & Hutton 1984; Hutton 1987) Coward 1987; Coward et al. 1989). Sinistral or alternatively to the effects of Acadian de- strike-slip displacement would not only generate formation in southern Britain and/or the Appa- tectonic inversion, with NE trending folds, but lachians, which result in indentation tectonics also NE-SW extension and could be the cause and lateral expulsion of parts of the Caledonian of local anomalous NE extension directions belt (Fig. 13). observed in the Bressay-Sumburgh Basin, in eastern Shetland (Fig. 11 see also Coward et al. The Devonian Basins of western Norway 1989). The amount of sinistrai strike-slip displacement along the Great Glen Fault must Two sets of Devonian basins occur W of remain speculative. From the offsets of the Norway: (i) the Hitra, basins, controlled by the Caledonian Front on Shetland and the Scottish Hitra Fault, which is probably a sinistral splay mainland, Flinn (1985) suggests an offset of from the Great Glen system, and (ii) the Solund I(X)-2(X) km. A large sinistral offset is required Basins which reactivate earlier Caledonian if the configuration of the Caledonian thrust thrusts. The Solund to Hornelen Basins, shown belt is anything like that shown in Fig. 9. How- in Fig. 14, comprise thick E-dipping Devonian ever from correlations of ORS facies across the sequences resting on Caledonian basement in Great Glen Fault in the Moray Firth, Rogers et the hanging-walls of listric low-angle faults

North Sea t East Shetland & Midland Valley pull Laurentia -middle apart basins Proterozoic crust

Lizard ~ Channel pull apart basin & ocean crust late Precambrian magmatic arc

,~ Caledonian thrust block [ ] region of Devonian gravity collapse

f subduction

._._...~_, 5 O0 km

Icartian

Devonian collision Avalon # Appalachian ~ Meguma collision

MIDDLE DEVONIAN TECTONICS

Fig. 13. Simplified map to show the Devonian tectonics of the North Sea and adjacent regions. During Devonian to early Carboniferous times, NW directed subduction occurred beneath NW France-southern Britain, generating back-arc extension during the early Carboniferous (Leeder 1982). However, in the Appalachians there was continued continent-continent collision (the Acadian phase), causing lateral extrusion of the British Caledonides, generating pull-apart basins along the major strike-slip zones. This model is similar to that proposed for Tertiary -- Recent tectonics in Tibet and .Anatolia (e.g. Dewey et al. 1987, 1989.). Downloaded from http://sp.lyellcollection.org/ by guest on September 24, 2021

PRECAMBRIAN AND PALAEOZOIC FRAMEWORK TO NW EUROPE 15

/ HORNELEN . : . . ; ~, -... : /" -.. .,. ! / (~,..\.~ ;. ;... "' " ~ ,~,..'~. / /" /" N /

STANDAL FAULT ~HASTEINEN /

\ / KVAMSHSTEN .. .,, , ,lOkm t "qt,~, - / J

/•" Devonian sandstones & conglomerate

.....'" // basal unconformity /. ,,, .," Caledonian thrust sheets ,,." 1 J Western Gneiss Complex

/ ...... boundary of extensional flow / •-: : /i /' f ~ low angle detachment late fault

"~~ ~ Devonian westward sheer criteria .,..,'" Caledonian eastward shear criteria

Fig. 14. Map of the Devonian basins of western Norway, from Scrannc & Scgurct (1987) and Scrannc (1988).

(Hossack 1984; Norton 1986; Seranne & Seguret be interpreted as rotated high-angle structures. 1987; Seranne 1988). In the footwalls of these Sediment facies vary from coarse sandstones large faults the basement shows less intense and conglomerates close to the fault trace, to Caledonian deformation, that is the Devonian alluvial sands in the centres of the listric half- extensional faults lie close to the boundary graben (Seranne 1988). Low-grade metamor- between Precambrian rocks, which were phic minerals in the basin fill and deformation strongly affected by Caledonian deformation, textures suggest that the basins were originally and rocks which were far less affected. The buried to a depth of c. l0 km (Seranne 1988), faults are parallel or nearly parallel to ductile that is the basins were originally much larger. fabrics in the basement. Kinematic indicators in They presumably formed by collapse of the the basement rocks show that the Caledonian Norwegian Caledonides, reactivating the major thrusts involved ductile shearing towards the basal Caledonian shear (Seranne 1988; Seranne ESE while, in the Devonian basin fill, the brittle et al. 1989). As the Caledonides were extended extension was opposite, to the WNW (Seranne and unroofed, the basins were uplifted and 1988; Seranne et al. 1989). subsequently eroded. The irregular listric form The Devonian displacements, in excess of of the faults on the map may be partly original 50 km, occurred along faults which, because of or due to cross folding associated with Devonian their low cut-off angles with bedding, initially sinistral shear on the Hitra strike-slip system. must have had a gentle dip and therefore cannot Downloaded from http://sp.lyellcollection.org/ by guest on September 24, 2021

16 M.P. COWARD

The Devonian Basins of the Midland south. The source of the alluvial fans was from Valley and the Scottish Borders the NW. Within the Lower ORS, there are large-scale The late Silurian to early Devonian evolution folds which trend NE-SW and have steep of the Midland Valley is characterized by the south-facing limbs in the footwall of the High- continued subsidence of the Caledonian foreland Boundary Fault. However the region NW arc basin (e.g. Leeder 1982). Subduction con- of the Fault must have been an area of positive tinued throughout the early Devonian as shown relief during Silurian and early Devonian times, by the volcanics and granites north of the High- as suggested by the source of some ORS pebbles land Boundary Fault and in the Southern Up- and the pre-Devonian cooling history of the lands, and by the thick volcanic sequences in Dalradian rocks (Dempster 1985). The early to the Midland Valley. Uplift of these volcanic middle ORS movements on the Highland regions provided the source for thick fluvial red Boundary Fault and the related folds to the SE bed molasse deposits, locally up to 9 km thick may be due to reworking of Caledonian fault in the Strathmore region on the northern flanks structures in the basement. The fold axial traces of the Midland Valley (Armstrong & Paterson are parallel to the trend of the Highland 1970; Dewey 1982; Haughton 1988). Within the Boundary Fault and kinematic indicators in the Midland Valley there are thick calc-alkaline, ORS structures suggest dominantly dip-slip rhyolitic and basaltic volcanic sequences. In the thrust movements towards the SE. NW-SE Stonehaven region of the Midland Valley (Fig. trending faults within the Midland Valley abut 15), Upper Silurian (Downtonian) breccias, against the Highland Boundary Fault, suggest- sandstones and sandy mudstones pass con- ing that they either predate the thrust move- formably upwards into the thick sequence of ments or, more plausibly, are strike-slip transfer lower ORS conglomerates, sandstones and faults of a similar age. In the hanging-wall to volcanics. The conglomerates, which inter- the Highland Boundary Fault, Dalradian re- digitate with the sandstones, thin towards the cumbent structures, including isoclinal folds and , sj

,~" ~ Invetbervie /I HIGHLAND BOUNDARY FAULT / ' '2kin /

\I I i I i / VOLCANIC ROCKS MUDSTONE SANDSTONE CONGLOMERATE N FAULT

20 km t

Fig. 15. Map of the NE part of the Midland Valley of Scotland, to show the distribution of the main folds and faults. Section a-b shows the large SE-verging folds in the ORS, formed possibly by reactivation of SE-verging basement structures. Downloaded from http://sp.lyellcollection.org/ by guest on September 24, 2021

PRECAMBRIAN AND PALAEOZOIC FRAMEWORK TO NW EUROPE 17

cleavages, are deformed into a hanging-wall more northerly shear systems of western Nor- antiform, so that the early structures appear to way. Either there has been over 350 km of post- be downward-facing (Shackleton 1958). The Devonian displacement on a continuation of shape of the Highland Boundary Fault at depth the Tornquist Line, to offset these shear systems is unknown but the reflectors, seen on the deep (Pegrum 1984), or the strike-slip faults must seismic line from offshore SE Scotland (e.g. bend to produce a wide zone of transtension in Freeman et al. 1988) farther south, all dip at the northern North Sea. The Great Glen Fault moderate angles to the north, suggesting that marks one edge of this diffuse transtensional this might also be the dip of the ORS thrusts. zone, while the other boundary would lie along Strike-slip movements accompanied the final the Viking Graben. The extension direction in closure of the Iapetus Ocean and small-scale this proto-North Sea would be NE-SW. ORS structures in the Southern Uplands and in deposits form the reservoir to the Buchan Field Northern Ireland indicate that widespread sin- off NE Scotland and ORS rocks have been istral wrench faulting continued both after the drilled in wells on the East Shetland Platform. imbrication of the fore-arc basin sediments and Based on the interpretation of deep seismic after the intrusion of a younger dyke suite at lines. Beach (1986) suggested that there may be 400 Ma. Clasts in the Lower Devonian sedi- 10-12 km of Devonian sediments beneath ments cannot be matched with the adjacent parts of the North Viking Graben, with only a Caledonian basement (Bluck 1985; Haughton very thin remnant of the original Precambrian 1989) and some clasts have a provenance from basement. If correct, this interpretation (Beach the south and east, i.e. from within the Midland 1986) suggests very large localized extension of Valley. Lower ORS clasts have Sr isotope com- the Caledonian crust. positions which are more radiogenic than Southern Upland sediments and less radiogenic than Dalradian rocks (Haughton 1989), sug- Devonian of the Anglo-Welsh Basins gesting that all Dalradian, Southern Upland SE of the Welsh Caledonides, the Lower ORS and Midland Valley sediments were derived rests conformably on marine Upper Silurian from different basement sources. Haughton sediments, except in SW Wales, where there is (1989) suggests that a basement similar to that an angular unconformity. A Lower ORS re- of southern Greenland was a source for the gression, shown by the transition from slow Midland Valley sediments but a basement shelf sedimentation through tidal/intertidal similar to the Grenville of the NE Appalachians deposits up into river channel deposits, indicates acted as a source for the Southern Upland a southerly migrating strandlinc of beaches and flysch. Thus some of the basin development in barriers (Allen 1974). During the middle ORS the Midland Valley and Northumberland there was uplift and broad-scale folding, as Trough may have involved large scale wrench represented by the middle ORS conglomerates movements, with displacements of several of West Wales and the absence of sedimen- hundred kilometres. The enormous thickness of tation, in small intermontane basins, as in NE the sediments may reflect their deposition in Anglesey, the ORS rocks were folded and local pull-apart or transtensional basins. cleaved. Throughout the ORS of the Anglo- During the middle Devonian, the Midland Welsh Basin the palaeoflow direction was Valley was a region of uplift, followed by grad- towards the south. The Lower ORS sediments ual slow subsidence throughout the late include detrital metamorphic minerals but, in Devonian to Westphalian. in the Scottish the Upper ORS, igneous and sedimentary rocks Borders, the Lower Devonian is represented by dominate (Allen 1974). This suggests uplift of volcanic sequences, followed by fluviatile Upper the Anglo-Welsh Caledonides during the ORS, infilling a region of moderate relief. As in middle Devonian, cutting off the supply of the WOB, this Middle Devonian uplift may be metamorphic minerals from the Scottish Cale- due to the final phases of Caledonian com- donides. In the upper ORS there was a tran- pression, or to the continued sinistral but now sition to marine conditions by the late Devonian transpressional, strike-slip movements. - early Carboniferous. Fig. 13 summarizes the large-scale implications of the sinistral transtensional model. Devonian Environments The NE-SW trending zones of strike-slip displacement of Devonian age have not been During the Devonian the sediments are domi- recognized in southern Scandinavia. Sinistral nated by molasse red bed facies, generally displacements in the Midland Valley have to deposited in intermontane basins. No true fore- link to similar displacement on the Hitra or land basin deposits occur in the British ORS; Downloaded from http://sp.lyellcollection.org/ by guest on September 24, 2021

18 M.P. COWARD

there are no coarsening upward sequences and Middle Devonian uplift and folding may record the basal ORS deposits often rest on deformed a late pulse of Caledonian compression, related Palaeozoic rocks. Even in the Anglo-Welsh to the final cessation of subduction. During the basins, SE of the Caledonian fold belt, the late Devonian there was continued gradual sub- sediments suggest gradual infilling or uplift of sidence followed by marine incursions and re- the basin during early Devonian times, rather newed extensional faulting during the early than the rapid deepening expected in a fore- Carboniferous. land basin. The deep sedimentary basins of the Orcadian, Shetland, West Norway and Mid- land Valley formed by large-scale extensional Carboniferous Extension and Basin collapse of the thickened Caledonian crust, Development enhanced by localized pull-apart basin development in strike-slip zones. There is probably a The Carboniferous basins of NW Europe can large transtensional basin in the northern North be divided into two segments, separated by the Sea, of which the East Shetland Basin, parts of NW-SE-trending Dowsing Fault Zone. To the Moray Firth and the ORS sediments in the the west, the Caledonian basement was broken Buchan Field may be part. into a series of fault blocks, with pulses of The strike-slip movements can be related to NW-SE trending extension in the Tournaisian the late stages of Caledonian compression, pos- and Visean. To the NE, there was a morpholo- sibly to lateral translation of crust away from gically smooth and almost horizontal deposi- zones of more intense late Caledonian com- tional area throughout much of the central and pression in the Appalachian belt to the SW southern North Sea. (Fig. 13). The strike-slip displacements are The NW-trending Dowsing Fault is one of a large, analagous to those of the strike-slip zones series of faults which acted as transfer fault in Tibet which formed by the indentation of zones during early Carboniferous extension India into Asia (Fig. 12). The widespread (Fig. 16). Dips of haif-graben bounding faults

SU SOUTHERN UPLAND FAULT S SOLWAY FIRTH ~~ ~ 100 km D DENT C CRAVEN B BOWLAND G GOYT

l\\\ NORTHSEA BASIN ANGLO~-- SCOTTISH

f fault i gas fields \ Fig. 16. Map to illustrate the principal early Carboniferous extensional and strike-slip faults. TEF, Trans European Fault. The distribution of the main gas fields (after Glennie 1986; Taylor 1986) shows the regions of greatest Permian subsidence. Downloaded from http://sp.lyellcollection.org/ by guest on September 24, 2021

PRECAMBRIAN AND PALAEOZOIC FRAMEWORK TO NW EUROPE 19 change across these transfer zones. Thus in the and Anglesey. The wide distribution of Dinan- western Pennines, the South Craven Fault was tian limestones and mudstones in Ireland sug- active during the Visean, transferring extension gests that the regional extension was similar to from a NW-dipping fault at the SE margin of that of Britain. the Bowland Basin, to a SE-dipping Middle The Brabant Massif remained as a tectonic Craven Fault (Gawthorpe 1987) (Fig. 16). There high throughout the early Carboniferous and were also smaller transfer fault zones within was onlapped by Namurian and Westphalian individual basins transferring extension from sediments, giving a steers head pattern of onlap one set of antithetic or synthetic faults to similar to that of other thermal subsidence another set (Gawthorpe 1987). phases (e.g. Dewey 1982). South of this massif, West of the Dowsing Fault Zone, important thick Carboniferous sequences occur in South basin bounding faults occur along the northern Wales, the Mendips, SE England and the edge of the Solway Firth, the Dent Fault system Ardennes, along the northern edge of the Rheic and its continuation along the Lunedale Fault, Basin. Extension factors must have been similar the Craven-Bowland system and the NE edge to those of northern England, with over 2 km of of the Goyt Trough. Visean limestones vary in Westphalian sediments preserved in South thickness from c.3 km in the Solway Basin to Wales. A minor but important phase of tectonic only 500 metres over the Alston fault block inversion occurred at the beginning of the (Ord et al. 1988). In the Bowland Basin, basin- Namurian, which resulted in folding the Lower wide debris flows and slump deposits demon- Carboniferous sediments on the hanging-walls strate two main episodes of extension during of some extensional faults. The South Wales the late Chadian and the late Asbian coalfield is different from the Westphalian coal- (Gawthorpe 1987). fields of northern England, in that rivers flowed Most of the half-graben were infilled by the into the basin from both north and south, in- early Namurian, when steady subsidence and dicating the presence of a Bristol ChanneI-N the influx of a fluvio-deltaic sequence of sand- Devon Landmass (Leeder 1982). Some of the stones, coals and marine shales began. Cyclic Westphalian sediments contain fragments of alternations of marine and terrestial sediments spilite and phyllite suggesting a source from the from the Dinantian through to the Westphalian Variscan fold and thrust belt to the south. The may reflect pulses of subsidence together with uppermost Carboniferous sediments may be eustatic changes of sea-level associated with considered as a Variscan molasse. the earliest of the Permo-Carboniferous Gon- The Dowsing Fault Zone marks the NE edge dwana glaciations (e.g. Ramsbottom 1974; of the Brabant Massif and, in the southern Heckel, this volume). The main development North Sea and northern Germany, Carbon- of coals was in the Westphalian, when coastal iferous sediments were deposited in a broad plain sediments prograded southwards across basin, the NE boundary of which lies along the Britain. An indication of the amount of sub- Tornquist Line and its continuation as the sidence is given by the 1 km thickness of West- Trans-European Fault Zone (Fig. 16). In the phalian rocks in the Midland Valley and the North Sea the Westphalian reaches a thickness 3 km thickness in the Lancashire-North Staf- of 1.2 km in the Sole Pit Basin, close to the fordshire coalfield (Ramsbottam et al. 1978). Dowsing Fault, while in N Germany the These syn-rift and post-rift sediment thicknesses Namurian and Westphalian together reach over in northern England suggest stretching factors 2 km in thickness (Ziegler 1982). These figures in the order of 2, assuming a simple homo- suggest a lower stretching factor than that of the geneous lithospheric stretching model (Leeder Anglo-Scottish basins, though as the basin is 1982; Dewey 1982, after McKenzie 1978). In wide, the amount of stretching may be very Scotland the stretching factor may be lower, similar. The difference in basin kinematics may although here estimates of stretch obtained reflect different Caledonian inheritance, in that from sediment thicknesses may be erroneous, the Anglo-Scottish and Irish basins overlay crust due to the presence of Dinantian volcanics and which had been deformed and thickened during hence a different sediment density and geo- the Silurian-Devonian times and the main thermal gradient. However the total stretch stretch was concentrated in the region between across the Anglo-S~:ottish Dinantian basins must the lapetus Suture and the older Cadomian have been in the order of 100 km. basement. The Carboniferous extension in the In southern Ireland, Dinantian basins occur North Sea seems to have concentrated farther on the west coast in Clare and in Munster. In south, possibly along a continuation of the North Wales normal faults formed parallel to North German Caledonides. the Caledonian fabric along the Menai Straits Downloaded from http://sp.lyellcollection.org/ by guest on September 24, 2021

20 M.P. COWARD

The Variscan Tectonics of Domain 4 by a gently dipping schistosity and gneissic banding, with overthrust sheets of eclogitic and The Variscan tectonics of NW Europe have granulitic material (Behr et al. 1980). The tec- been interpreted as (i) a thin-skinned fold and tonic transport direction was towards the NW, thrust belt (Shackleton et al. 1982; B1RPS & as determined from linear fabrics and folds with ECORS 1986), overthrusting Lower to Upper strongly curvilinear axes (author's own unpub- Palaeozoic rocks to the NW, and (ii) the lished observations). To the NW of this zone, northern margin of a strike-slip orogen (Bad- the Rheno-Hercynian and Saxo-Thuringen ham 1982; Sanderson 1984). The main belt of zones show lower-grade metamorphism but Variscan structures can be traced from South similarly NW directed shears and thrusts. Wales to the Ardennes of northern France, Together they indicate several hundred kilo- Belgium and southern Germany. To the west, metres of crustal shortening. Within the Rheno- they continue into the southern Appalachians. Hercynian zone upright to NW verging folds In Britain there seems to be a spatial and and thrusts probably decouple on a mid-crustal temporal division between the Caledonide and detatchment, as determined from deep seismic Variscide orogenies. However in Germany and data (Meissner et al. 1981; Giese 1983). In eastern Europe, Variscan deformation ranges Bavaria there are strike-slip faults and mylonite in age from Devonian to late Carboniferous. In zones, which developed after the main thrust the southern Appalachians the Variscan and transport. These structures trend NW, parallel Caledonian deformations appear to be one to the thrust transport direction in the Rhenish semi-continuous process of arc accretion on to Massif and may be coeval with the thrusts the American Craton. in this Massif, as they do not cut across the external thrust zones. Variscan tectonics of Germany and In NW France, there may have been less northern France crustal shortening during the Variscan, as the major thrusting is concentrated in southern The internal, or Moldanubian, zone of the Brittany and in the Massif Central. The rocks Variscides (Fig. 17) shows structures dominated of northern Brittany and Normandy comprise

%iiiiiiiiiiiiiiiiiiiiiiiii .~ _ (" 6o,9 Culm Basin t~ o~e ~, Carrick ~a~ "~v =f.,~q J ""'~e =

~-v ~ N~ ~Faill.du Midi ,4 ~''~~ "/~'~"~'/~ "~-~(ioe

North J7 Armoric Shear Zo 1

South A Shea MOLDANUBIANZONE ~ ,~'~-~ i I VOSGES~y ~ Bayerische Pfahl

/NN "\ CARBONIFEROUSCULM FACIES / /~ "~ ~ ~r~ DEVONIAN& LOWER CARBONIFEROUS / ~::ii;::~i';."~i!~ii::iiiii::iiii~::i~iii~iii~Jiiii!!iii ~ PRE-DEVONIAN THRUST , ,1OOkm ~ STRIKESLIP SHEARZONE

Fig. 17. Map to show the distribution of tectonic zones in the Variscides of NW Europe (after Coward & Smallwood 1984; Holder & Leveridge 1986a). See text for discussion. Downloaded from http://sp.lyellcollection.org/ by guest on September 24, 2021

PRECAMBRIAN AND PALAEOZOIC FRAMEWORK TO NW EUROPE 21

Proterozoic basement (Autran et al. 1980), part by a broad strike-slip zone trending from Kent of an Icartian basement block (see Fig. 13), to the Ardennes but as the thrust transport which was deformed c.600 Ma ago by large direction was dominantly towards the NW, scale shear zones and then unconformably across much of southern England and Wales, overlain by relatively undeformed late Precam- the Variscan Front is an oblique thrust structure brian to Lower Palaeozoic cover sediments. (Figs 17 & 18). There must have been a zone of strike-slip The sediments involved in the British Vari- deformation along the eastern edge of the scides range in age from Devonian to late Car- Armorican Massif, close to the present trend of boniferous. Olistoliths in the Devonian flysch of the Bray Fault Zone, which separates the zone South Cornwall contain shelf facies of Ordovi- of thrust tectonics in the German Variscides, cian to Devonian sediments, comparable with from the relatively weaker zone of deformation similar rocks in France. However no older in northern Brittany (Coward & Smallwood Lower Palaeozoic rocks occur in SW England 1984; Holder & Leveridge 1986a). Isotopic data and there are no overthrust sheets of older from the German Variscides (e.g. Autran et al. basement rock as in the southern Appalachians 1980) show no evidence for Precambrian base- (e.g. Hatcher 1987). The Devonian sediments ment and the rocks themselves comprise late range from continental to shallow marine ORS Precambrian to Palaeozoic sediments resting in North Devon, to large thicknesses of reef and on the SE continuation of the Cadomian- detrital limestones in South Devon. In Dyfed in Brabant magmatic arc. Presumably therefore, South Wales, Powell (1989) described faults the Bray Fault Zone lies close to the edge of across which Devonian and Carboniferous sedi- this old crustal block and acted as a major ments change markedly in thickness and facies, transform zone during Variscan tectonics, simi- indicating extension during the Siegenian- lar to the Tornquist Zone in northern Europe in Emsian and the Visean. In southern Cornwall, Caledonian times. Devonian sediments are characterised by distal turbidites, probably deposited on thinned crust or the edge of a continental shelf. Dewey (1982) The Variscides of Southern Britain estimates a stretching factor of c.2 for N Devon, The Variscides of southern Britain form a con- while the enormous thickness of Devonian sedi- tinuation of the outer, or Rhenish, zone of the ments in South Cornwall, possibly up to 12 km European Variscan belt (e.g. Holder & (e.g. Holder & Leveridge 1986b), suggests that Leveridge 1986a). They are bounded in the east they were deposited on highly attenuated con-

Worcester graben ~ SouthWales JOHNSTON coal field )) / I/~,,/ t

":~"::':":'::':': ...... ':~:: ...... London platform :: i ~:~ ~" ,,~', o o POST EARLY STEPHAN AN POS FOLDS & THRUSTS ,':"~i:)': ::i ::::: i i r~=We~'ocr :

o nt

FOLDS & S-VERGING THRUSTS i[ ill ,i O o

~i ' ~ill~li~,~Lkllllll,~~'~Lj coal rank PURBECK v CARBONIFEROUS * "~ "~ high volatile PHYLLITES TORBAY medium ,, low ~ Culm facies anthracite =." DEVONIAN THRUSTS ~ granite ophiolite o well '~ LIZARD ~ major Variscan thrust L ---, 50km ~ probable Variscan fault, reactivated

Fig. 18. Map of the principal structures in the Variscides of southern Britain (from Taylor 1986), showing the regions of thick-skinned thrust tectonics: the Mendip-Hog's Back and Cannington-Portsdown Faults, the regions underlain by Culm facies rocks and the rank of the Westphalian coal deposits. Note that the coal rank defines NE-trending zones, oblique to the Variscan Front, but approximately perpendicular to Caledonian and Variscan thrust directions and hence probably reflects tectonic inversion of deeper level structures. Downloaded from http://sp.lyellcollection.org/ by guest on September 24, 2021

22 M.P. COWARD tinental crust or on ocean floor. The Lizard boniferous slates give middle to late Carboni- ophiolite complex in South Cornwall, which is ferous ages (340-320 Ma). In N Devon and probably part of this ocean basin, gives a Sm/ Cornwall, the Namurian to Westphalian sedi- Nd isotope age of 375 -- 34 Ma, suggesting a ments are represented by the flysch-like Culm. Middle Devonian age of ocean floor generation These have been interpreted as syn-orogenic (Davies 1984). Similarly Kennack Gneisses, deposits derived from the Variscan thrusts (e.g. which comprise mixed granite and basaltic Isaac et ai. 1982). However there are two con- magmas, give an Rb/Sr whole-rock isochron trasting sequences, to the north, in the West- age of 369 - 12 Ma (Styles & Rundle 1984) and ward Ho region of North Devon, the rocks are are probably related to the ophiolite. Thus dominantly deltaic and derived from a northerly during the Devonian there must have been source, while a more general basinal facies stretching of the southern part of the Brabant consists of mudstone and turbiditic sandstones Massif in order to generate deep water flysch with westward or eastward axial palaeocurrent deposits and ocean-floor volcanics. Barnes & directions. Much of the basinal debris was prob- Andrews (1986) suggest that the Lizard rep- ably introduced from the northern delta resents new oceanic crust formed within the slopes. Thus during or following Devonian southern Cornwall basin. By rotating the cumu- thrust activity in South Devon, a large flysch late layering in the Lizard gabbros to the hori- basin developed in North Devon. This Culm zontal and similarly rotating the dyke complex, facies can be traced across southern Britain so that their palaeomagnetic pole position coin- from borehole data (Fig. 18 from Taylor 1986). cides closely with the mean UK pole for the The Devonian and Carboniferous rocks of Silurian to Devonian, Hailwood et al. (1984) the Bristol Channel must have been uplifted at suggest an original NNW-spreading axis. Any end-Westphalian times, as southerly derived other orientation requires rejection of the debris flowed into the South Wales basin during palaeomagnetic data (Barnes & Andrews 1986). late Westphalian times. There is also a pro- If correct, this indicates that the Devonian nounced angular unconformity between West- extension direction was approximately NE- phalian and Stephanian rocks in the Bristol SW, oblique to the Variscan Front across Channel region. Pre-Stephanian basin inversion southern Britain, but close to the trend of late and broad scale folding characterises much of Precambrian shear zones in northern Brittany the NE margin of the Brabant Massif and the (Brun & Bal6 1989; Strachan et al. 1989). Pos- southern North Sea basin. However the major sibly the Lizard ophiolite is part of a pull-part phase of folding and thrusting in South Wales basin on a NE-SW trending shear system, and the Mendips was post-Stephanian. Thin- similar to those operating in the Devonian skinned thrust zones characterise the deform- basins of northern Britain. The sense of shear is ation in the Wcstphalian sandstones, shales and unknown. However the indentation model for coals of Pembrokeshire and parts of the South Devonian strike-slip basins, given in Fig. 14, Wales coalfield (Trotter 1949; Coward & suggests that there should have been dextral Smaliwood 1984). Williams & Chapman (1986) displacement in southern Britain during early suggest a thin-skinned interpretation for the to middle Devonian times. Mendips thrust zone, with the thrusts detatching The Variscan structure of SW Britain can be in ORS sediments. However Chadwick et al. interpreted in terms of thin-skinned thrust tec- (1983) propose a thick-skinned thrust model for tonics (Shackleton et al. 1982). K/Ar mineral the Mendips, based on the presence of mode- ages (Dodson & Rex 1971) suggest a diachro- rately dipping seismic events in the upper to neity of deformation. Ages of 365-345 Ma middle crust. In Pembrokeshire there is evi- have been obtained from the slates of South dence for local thin-skinned thrusting in the Cornwall, suggesting that the Lizard ophiolite Upper Carboniferous, breached by thick- was obducted and phyllites formed before the skinned thrusts, such as the Johnston Thrust, end of the Devonian. Holwill (1966) records which involve Precambrian and Palaeozoic pebbles of deformed Givetian limestone from basement (Coward & Smallwood 1984). There the Upper Devonian sediments of Torbay (see are facies and thickness changes in the Palaeo- also Coward & McClay 1983). Presumably these zoic rocks across these thick-skinned thrusts, were derived from an uplifted thrust mass to which are probably therefore reactivated earlier the south. The overthrust direction, as obtained normal faults (Coward & Smallwood 1984; from lineations and sheath folds in the strongly Powell 1989). deformed Lower Devonian sediments of South East of the Mendips there is evidence to show Cornwall, was towards the NW. that the Variscan thrusts were reactivated sev- However, the majority of Devonian and Car- eral times in the Mesozoic to produce basin Downloaded from http://sp.lyellcollection.org/ by guest on September 24, 2021

PRECAMBRIAN AND PALAEOZOIC FRAMEWORK TO NW EUROPE 23 bounding faults, which were then tectonically terize the basin-bounding faults of northern inverted in the early Tertiary. Seismic data England and western Scotland (e.g. Gawthrope published by Kenolty et al. (1981) show that 1987). NE trending folds, which are often tight, a concealed thick-skinned Variscan thrust con- occur in the Pendle-Skipton area of northern trols the Vale of Pewsey anticline and probably England associated with inversion of the the Ham and Kingslere anticlines. Similarly the Skipton-Clitheroe Basin (Gawthorpe 1987). Hog's Back monocline probably overlies a Similarly, the folds in the Munster and Clare Variscan thrust (Smalley & Westbrook 1982; Basins of southern Ireland probably represent Taylor 1986). The trend of these moderately basin inversion rather than thin-skinned tecto- dipping thick-skinned thrusts, which form the nics as suggested previously (e.g. Cooper et al. Variscan Front in southern England, is shown 1986). on Fig. 18. Many of the NE-trending fault zones of Scot- The Culm basin of North Devon shows a land show indications of dextral displacement pattern of upright to north-verging folds in the during the Late Carboniferous and Permian. north, passing to south-verging to recumbent The E-W trend of normal faults and the NNE folds and thrusts to the south. This major fold trend of folds in the Midland Valley suggests a fan, which is about 50 km across, appears to be dextral strike-slip shear couple to the region carried south on the gently dipping shears which between the Highland Border and Southern outcrop in the TintageI-Polzeath area. Large Upland Faults. In the Shetlands the Walls south-verging back-folds also occur in South Boundary Fault is a complex zone, with ambi- Devon, steepening the earlier north-verging guous shear criteria, although the dominant thrusts to vertical (Coward & McCiay 1983; small scale shear indicators suggest that the Coward & Smallwood 1984). l_~ate Carboni- main displacement is dextral. Flinn (1969; Flinn ferous mineral ages of 310-290 Ma have been et al. 1979) suggests a dextral displacement of obtained from the region of back-folding in S 65 km, based on a correlation of magnetic Devon and Carboniferous to Permian ages of anomalies. 290-270 Ma have been obtained from the In East Shetland, tectonic inversion affects folded Culm rocks (Dodson & Rex 1971). This the Devonian sediments on the hanging-walls suggests that the back-folds and back-thrusts of the NE-trending faults. Kinks and thrusts represent the late stages of Variscan crustal affect the ORS on the Sumburgh Peninsula and shortening. large folds trend NNW across the region from Shackleton et al. (1982) make a rough esti- Sumburgh to Lerwick, probably developed on mate of over 150 km shortening across Devon the hanging-walls of reactivated normal faults. and Cornwall. The thrust transport direction From their trend, these folds were formed by was almost generally towards the NW or NNW E-W compression, or more likely, were associ- as determined from kinematic indicators on ated with dextral strike-slip movements on the fault planes and the trend of associated the Shetland fault system (Coward & Enfield tear faults (Shackleton et al. 1982; Coward & 1977; Coward et al. 1989). McClay 1983). Fold hinges are generally normal to the thrust direction but are locally rotated The Variscan Orogen into the NNW trend in more intensely deformed zones. In N Devon, South Wales and the Throughout much of NW Europe and the North Mendips, where thick-skinned thrust tectonics American Appalachians, the Variscan orogeny is evident, the structures trend approximately was but another episode in the history of pro- east-west, oblique to the general NW directed gressive NW directed accretion of magmatic movements in the thin-skinned zones. This arcs and older continental fragments on to the oblique trend may reflect the original trend to stable North American Craton. in southern pre-Variscan basement structures which reac- Britain however this history was punctuated by tivated to give the thick-skinned thrusts. In spasmodic crustal extension, particularly during Pembrokeshire there is evidence for c. 40 ° the early Devonian and early Carboniferous. rotation associated with these oblique struc- The early Devonian extension direction is un- tures, based on palaeomagnetic data (Mc- certain although the faults had a WNW trend Clelland-Brown 1983). from South Wales to Kent and controlled the position of the margin of subsequent intense Variscan inversion of the Carboniferous Variscan compression. From the trend of dykes in the Lizard complex, the extension direction basins can be shown to be NE-SW and this could be End-Carboniferous uplifts and folds charac- associated with a pull-apart basin in a major Downloaded from http://sp.lyellcollection.org/ by guest on September 24, 2021

24 M.P. COWARD

strike-slip shear as shown in Fig. 13. The the Variscan back-thrusts in the Rusey- Devonian normal faults along the Variscan Tintagel region. These extensional faults offset Front could be oblique structures associated many of the earlier compressional structures, with this shear couple. including the region where back-folds refold The early Carboniferous extension was more the earlier NW verging Variscan structures, widespread across NW Europe, producing rift leading to a complex confrontation of NW and basins in northern Britain and western Ireland SE verging folds. and a deep Culm facies basin from Cornwall to Some of this extension may be associated North Germany. Leeder (1982) suggests that with Carboniferous-Permian granite intrusion this stretching was related to the development of the Cornubian Batholith. Granite sheets in- of a back arc basin, associated with NW-directed trude several of the normal faults in the St lves subduction of the Variscan Ocean beneath NW district of Cornwall and many of the Cornish tin Europe. lodes are related to the same NW-SE exten- Tectonic inversion occurred during the middle sion (Moore 1975). Seismic refraction studies to late Devonian, obducting the Lizard ophi- by Brooks et al. (1984) and Doody & Brooks olite and uplifting Middle Devonian limestones, (1986) suggest that the Cornish granites have a to provide a source for some Upper Devonian flat base at about 10 km depth, that is they sediments. The Bristol Channel landmass was intruded as a horizontal sheet along the Vari- probably uplifted earlier, providing a sediment scan fabric, or were sliced across by a thrust source for the Siegenian conglomerates of SW or an extensional detachment (Shackleton et Wales. Minor inversion occurred during the al. 1982). The extensional structures in SW early Namurian, folding the Lower Carboni- England can be considered as Variscan anal- ferous rocks on the hanging-walls of the earlier ogues for the tectonics of the Basin and Range normal faults. The main period of compression in the western USA; they form a zone of con- however occurred during the Westphalian- tinental spreading above a crust weakened by Stephanian, producing the thrust systems of tectonic thickening and by the addition of Variscan orogenic belt. granitic rocks (e.g. Sonder et al. 1987). South of Cornwall, the deep Plymouth Bay Basin contains a sequence of Permo-Triassic sediments over l0 km thick (BIRPS & ECORS Permo-Triassic subsidence 1986). The basin is not fault bounded, although Deep Permo-Triassic basins formed in Cardigan some interpretations show small faults at the Bay, the Celtic Sea and Western Approaches. base of the Permian strata (Pinet et aL 1987). They trend NE-SE perpendicular to the The basin overlies south-dipping reflectors, Caledonide and Variscan thrust directions and correlated with the Carrick, Lizard and Start approximately parallel to the Caledonide fa- Thrusts onshore (Day & Edwards 1983; BIRPS brics, although oblique to the Variscan thrusts & ECORS 1986; Holder & Leveridge 1986b). of South Wales. The basins are cut by NW- Although the basin is deep, neither the top of SE trending tear faults which link with strike- the reflective lower crust nor the Moho are slip faults onshore in SW England. These tear depressed or uplifted beneath the basin. A faults cannot be traced onshore southern Ireland simple stretching model cannot be applied to or northern France and hence must transfer the Plymouth Bay Basin; instead it is considered displacement on to the basin-bounding faults to have developed above a deep crustal ramp, offshore. The extension direction was NW- probably where the flat detachments beneath SE parallel to these tear faults and to small the Cornish stretched upper crust ramp down lateral ramps which offset the main bounding to lower crustal levels. The onlap of sediments faults, especially along the SE margin of Car- towards the NW, as seen on the Swat data (e.g. digan Bay. The tear faults and lateral ramps are BIRPS & ECORS 1986), supports this model. steep to vertical in dip and can be recognized on Volcanic rocks of Stephanian to Asselian age seismic data from the offset of prominant re- were extruded in graben and pull-apart struc- flecting horizons and from associated flower- tures in the North German-Polish plain (Zeigler like secondary faults (Coward & Trudgill 1989). 1982), probably related to NW-SE move- Basement structures to the Celtic Sea graben ments on the Tornquist Line. These movements can be examined onshore in SW England, where which appear dominantly dextral are probably numerous extension faults dip NW. On the associated with Late Carboniferous inversion Devon and Cornish coasts there are both brittle of basins in northern Britain; the Tornquist and ductile extensional shears, including gently Line marks the NE boundary of these struc- dipping low-angle detachments which rework tures. Similar dextral movements can be Downloaded from http://sp.lyellcollection.org/ by guest on September 24, 2021

PRECAMBRIAN AND PALAEOZOIC FRAMEWORK TO NW EUROPE 25 postulated for the Dowsing Fault Zone, though dient and hence been a much cooler basin than here there are no volcanics. the Mesozoic basins of the North Sea. In the North Sea there were two main areas of Permian subsidence (Glennie 1986), probably reflecting continued thermal subsidence follow- 2. Control of the positions of the later ing Carboniferous stretching. The Northern and basins Southern Permian Basins are separated by the Mid-North Sea-Rinkobing Fyn High, which (a) Modification of the strength of the lithos- probably represents the region of lowest Car- phere. Crustal thinning initially weakens the boniferous stretching. crust but, following thermal equilibration, the zone of thinned crust will become stronger than the adjacent zones of less thinned crust (Dewey Discussion: Influence of basement 1982; Gillcrist et al. 1987). Similarly crustal thickening will initially strengthen the lithos- structures on subsequent stretching phere, but as the lithosphere thermally equi- The Precambrian and Palaeozoic structures of librates and the lower crust becomes hotter, it NW Europe control subsequent basin develop- will weaken and hence lead to crustal spreading ment in several ways. as in the Basin and Range (Sonder et al. 1987) or Tibet (Houseman & England 1986; Dewey 1988). Thus the zones of more intense Cale- 1. Control of subsidence patterns donian thickening in Scotland and Norway became the sites of intense Devonian extension (a) Generation of thermal subsidence. Crustal (Norton 1986; Seranne & Seguret 1987; Coward stretching during the early Carboniferous et al. 1989). largely controlled the position and intensity of later Carboniferous and Permian subsidence. (b) Generation of fimdamental faults or zones of Much of thc Permian subsidence in the North middle to upper crustal fabric which fail easily to Sea is probably thc result of thermal equili- produce basin bounding faults. Several late bration following Carboniferous extension. In Palaeozoic and Mesozoic basins occur on the North England a region of middle to late Car- hanging-walls of earlier large scale thrusts. boniferous thermal subsidence overlies the zone Examples are the Outer Isles and Minches of most pronounced Dinantian stretching, Basins on the hanging-wall of the Outer Isles although here the subsidence pattern was Fault (Brewer & Smythe 1984; Enfield & modified by a late Carboniferous episode of Coward 1987), basins on the hanging-walls of locally intense tectonic basin inversion. Caledonian thrusts along and to the south of the Solway Line (Hall et al. 1984), and the Celtic (b) Generation of crust and iithospheric mantle Sea and Bristol Channel Basins on the hanging- of anomalous thickness, in North Britain walls of Variscan thrusts offshore southern Devonian stretching affected crust previously Ireland and South Wales (Cheadle et al. 1987; thickened during the Caledonian Orogeny. At Brooks et al. 1988). the peak of Caledonian compression and meta- Large strike-slip fault systems, such as Torn- morphism, the crust of the NW Highlands was quist Line, the Bray and the Great Glen Faults, c.50 km thick (e.g. Coward et al. 1990) and appear to have acted as basement-bounding hence considerable crustal stretching was tear faults throughout several episodes of necessary to reduce the topography to sea level. extension and inversion. Thus strike-slip faults Early Devonian basins must have been canni- in SW England developed during Variscan balised during subsequent Devonian stretching, compression and were then reactivated during so that only the later stages of the stretching Permo-Triassic extension and Mesozoic-Ceno- history are preserved in the alluvial to lacustrine zoic extension and inversion (Coward & Trud- facies of the West Orkney Basin. As the litho- gill 1989). sphere had probably not thermally equilibrated As shown by the SHET deep seismic survey following Caledonian thickening, the thermal across the Walls Boundary Fault system north history of the WOB must have been very differ- of Shetland (McGeary 1987), a major tear fault ent from that of, for example, the Mesozoic may produce a slight offset in the Moho. This basins of the North Sea, where Jurassic stretch- and other major tear faults offset zones of dif- ing affected lithosphere previously thinned ferent crustal fabric and strength, in that (i) the during the Triassic (Barton & Wood 1984). The fabric may change strike and dip across the WOB must have had a lower geothermal gra- strike-slip fault and (ii) zones of weaker crust Downloaded from http://sp.lyellcollection.org/ by guest on September 24, 2021

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PRECAMBRIAN AND PALAEOZOIC FRAMEWORK TO NW EUROPE 27

Strike Slip Fault ~ of basin-bounding fault -,,I j, /

tectonic fabric widens

with depth

Fig. 20. Simplified block diagram to illustrate how major crustal-scale tear faults produce an anisotropy to the crust and tramlinc subsequent dcformation (modified from Daly 1986). formed, for example, of more quartz-rich ma- Atlantic transform faults generally followed terial may abut against stronger crust formed of Variscan structures and many of the Triassic- more feldspathic rock. The position and dip of Jurassic graben of western Europe are bounded middle to deep crustal reflectors differ across by Variscan tear faults. the Great Glen-Walls Boundary Fault on The Great Glen Fault must offset the lower both the SHET survey, north of Scotland crustal decoupling zone of the West Orkney (McGeary 1987) and the WINCH survey, SW Basin. It therefore acts as a buttress to any of Scotland (Hall et al. 1984). Furthermore a further movement on this deep level decoupling strike-slip fault may itself produce a steeply zone and hence locks that part of the extensional inclined ductile fabric at depth, as shown by fault system. This may explain the presence of a field studies of ductile shear zones in high-grade region of thick, relatively stable crust east of the metamorphic rocks (e.g. Coward & Park 1987). Great Glen-Walls Boundary Fault system, i.e. Hence major strike-slip faults produce irregular east of the Orkney and Shetland Islands, where corrugations of the brittle-ductile layering in Mesozoic extension is slight. It may also explain the lithosphere and also produce a 3D strength why (i) the Great Glen Fault forms a lateral anisotropy (Fig. 2(}). This anisotropy produces boundary to the Moray Firth Basin, probably an easy slip direction parallel to the large strike- controlling its opening direction and (ii) the slip fault and a more difficult slip direction northern continuation of the Great Glen system, perpendicular to the fault (Daly 1986). Hence along the More- Hitra Fault, forms the northern subsequent crustal thickening or thinning is boundary to the Viking Graben. likely to be 'tramlined' (see Daly 1986; Gillcrist et al. 1987) by these fundamental faults. (3) Control of the dip of the master faults it can be argued that the steeply dipping within the basin crustal anisotropies formed by major tear faults control the subsequent basin extension direction Following Navier-Coulomb failure criteria, a in the stretched crust and hence the trend of the rock may fail along older fractures, rather than transform structures as the basins grow. Thus new fracture systems, if their cohesion, friction earlier tear faults may essentially control the and orientations are correct. The ranges of later plate divergence direction. It is only when orientations of reactivated fractures depend on true oceanic crust forms that this influence of the cohesion and friction and the 3D stress crustal anisotropy ceases and the transform configuration. They have been calculated for a faults change trend with time. Thus during the range of stress ellipsoid shapes and orientations Jurassic, the Atlantic Ocean developed along a (Jaeger & Cook 1979; Gillcrist et al. 1987). wrench-rift system, from the Gulf of Mexico to Thus in a region which has undergone thrust Tethys. The continental break up and the early tectonics, such as the Caledonides of Scotland Downloaded from http://sp.lyellcollection.org/ by guest on September 24, 2021

28 M.P. COWARD

~ EXTENSION, fl-t.5

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/ I HANGING'WALL V :SUUSIOENCE ~CUT'OFF 8 • 1'38 ...... ~ 2o0/00 i

SYNTHtTIC d LISTRtC FAULT ONLAP OF SEDIMENT REACTIVATE0 , TO~NAROS ~ASEMENT HIOH ANTITHETIC ~--..J UPLIFT otigil~al laull Ir,~nd l 090/i.i0 ',,I oi-roo

e'. 30" 8.178 Fig. 22. Map of a region of block faulting, whcre the dip of thc tear fault controls the axis of block Fig. 21. Simplified sections to show how a step in a rotation. In this model, the later N-S extension is fault can generate extra strains in the hanging-wall or parallel to an existing fault dipping 3t)° E. The later footwall. Thc bend may bc produced from the faults originally had an E-W strike and 60 ° dip but rcworking of an earlier anisotropy, e.g. an carlicr havc been rotated into their present oblique thrust fabric with variable dips duc to local back- orientation by thc extension of B = 1.5. The resultant steepening (scc Butler & Coward 1984). In this displacement direction on the fault is marked. The particular example, the initial fault dip was taken to dotted line shows the tilt axis of the rotated blocks, be 55 °, and cxtcnsion took placc by block rotation. separating relative subsidence from relative uplift, The hanging-wall is assumed to deform by antithetic ignoring isostatic effects. Note this model assumes no fault slip. The uplift of the footwall and hence the boundary conditions, in that E-W extension is amount of erosion, has been calculated using the allowed during the later fault activity. If E-W subsidence modcl of McKcnzic (1978) and Barr extension were not allowed, the earlier fault would (1987), where steepen, causing tectonic basin inversion. subsidence = 3.4 (1 - 1/B) where B is the stretching factor. An average scdiment (Gillcrist et al. 1987) and hence produce zones density is assumed; sediment is shown stippled. of oblique normal faults. Furthermore, where the earlier thrusts and ductile shears vary in orientation, as is to be expected in any major and Norway, there may be a range of fabric thrust zone, then the dip of the faults will also orientations, from less than 30 ° dip, to over 75 ° , vary. Thus adjacent block-bounding faults may which may be favourable for subsequent reac- have different strikes and dips. Similarly a frac- tivation. The Devonian low-angle faults of ture may change strike and/or dip direction Norway (Seranne & Seguret 1987) and parts of with depth, depending on the variation in the West Orkney Basin (Coward et al. 1989) orientation of the earlier basement fabric. Thus originated from the reactivation of gently dip- block-bounding faults may steepen or shallow ping Caledonian shear zones. with depth, similar to listric or stepped faults in Where the basement fabrics are oblique to growth fault environments (Fig. 21). Extension the subsequent extension direction, they may along a bent fault surface will lead to strains in be in a favourable orientation for reactivation, the hanging-wall and/or footwall and therefore depending on the shape of the stress ellipsoid to a localized roll-over fold geometry and to Downloaded from http://sp.lyellcollection.org/ by guest on September 24, 2021

PRECAMBRIAN AND PALAEOZOIC FRAMEWORK TO NW EUROPE 29

J J / J / / / 7. I/ /'% ...... , .. NW trending shear ~i $

$ ~f Northern

" ~.( Basin

J J / f .....

1~ NW dip to Caledonide mid-crustal fabric

Southern SE Permian ~ varied .. ' • Basin []~] SE . Variscide major tear fault / -.-,-- Mesozoic basin-bounding fault t'ono,on / ---v-- thrust i i i , .... S edge of Caledonides continental slope II/I L i L , 200 km

I I ' i i I I illll Devon Fault system

Fig. 23. Map showing the tectonic framework to NW Europe, including the dip of the Caledonian and Variscan mid-crustal fabrics and the trend of the major strikc-slip faults. The important Mcsozoic basin- bounding faults are shown (from Ziegler 1982). systems of synthetic and antithetic faults (Fig. fracture has a moderate to gentle dip, the 21). Thus in regions of strong basement fabric, extensional fault blocks cannot rotate about a as in the northern North Sea or west of Britain, horizontal axis but will tend to rotate about an faults may change strike or dip with depth, inclined axis parallel to the pole to the earlier leading to complex local strain patterns. fracture. This rotation will cause the extensional faults and the cut-off lines to trend oblique to the extension direction (Fig. 22). It may explain 4. Control of the dip of the main basin- some of the oblique fault trends and tilts of fault blocks in parts of the Viking Graben. Here for bounding tear faults example, fault blocks forming the Brent and If the extension direction is parallel or closely Statfjord fields show simple tilting away from parallel to an earlier fracture in the crust, this the main block bounding fault, while the blocks may reactivate to produce a tear fault, as dis- forming the Ninian and Gullfaks fields show cussed above. Where the early fracture is steep, more complex tilts, highly oblique to the main and later extension takes place by block faulting bounding faults. A knowledge of the basement (e.g. Morton & Black 1975; Jackson & White structure beneath this part of the Viking Graben 1989), these blocks will rotate about the pole to may help explain the different fault and the earlier fracture. However, where the early rotational history of these fault blocks. Downloaded from http://sp.lyellcollection.org/ by guest on September 24, 2021

30 M.P. COWARD

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