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The Precambrian, Caledonian and Variscan Framework to NW Europe

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The Precambrian, Caledonian and Variscan Framework to NW Europe Downloaded from http://sp.lyellcollection.org/ by guest on September 24, 2021 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 HARRIS(~ f; NOF~TH SKYE ~"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
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