The Variscan Orogeny: 10 the development and deformation of Devonian/Carboniferous basins in SW England and South Wales B. E. LEVERIDGE & A. J. HARTLEY The upper Palaeozoic Orogenic Province of SW England is Shiefergebirge of Germany, some 800 km to the east, is estab- a part of a belt of Devonian and Carboniferous basins that lished (e.g. Franke & Engel 1982; Holder & Leveridge 1986a) extended from Devon and Cornwall through to Germany, some and together they make up a major part of the Rhenohercynian 800 km to the east. Their complex sequence of basin devel- Zone. That is the northernmost of the palaeogeographical opment and phases of deformation, described in this chapter and tectonic zones proposed by Kossmat (1927), which, with cumulatively comprise the Variscan Orogeny in this region. the Saxothuringian and Moldanubian zones, constitute the Synchronously with the Devonian events within the Variscan orogenic belt in Germany and central Europe (Fig. 10.2). Orogen, the mainly fluvial facies of the Old Red Sandstone Bard et al. (1980) and Matte (1986) proposed interpretation filled basins in the Avalonian continent north of the Variscan of Kossmat’s zones in terms of plate tectonics. At that time front (Chapter 6). During the succeeding Carboniferous, basins postulated settings for SW England were diverse; an ensialic within the continent were mainly extensional in origin, until a back arc basin (Floyd 1982), part of a trans-Europe oblique period in the late Carboniferous when many basement faults shear zone (Badham 1982), and an intracontinental fold belt were inverted (Chapter 7) resulting in uplift of the basin fill, that (Matthews 1984). The Gramscatho flysch (Hendriks 1949) and initiated a new palaeogeography at the start of the Permian. associated Lizard ophiolite (Strong et al. 1975; Styles & Kirby The South Wales Basin represents a transitional zone 1980) in southern Cornwall were seen by Barnes & Andrews between the mobile Variscan belt and the continent to the (1986) as representing a small intracontinental basin generated north. This transitional position is reflected in the Devonian in an E–W strike–slip zone across northern Europe. In contrast by the interdigitationof the Old Red Sandstone facies and Holder & Leveridge (1986b) interpreted those as products of marine sediments at the northern margins of the Variscan the closure of a more extensive oceanized basin. Correlation basins (Chapter 6). Throughout the Dinantian and Namurian and continuity of stratigraphies and structures between SW the succession within the South Wales basin had much in England and central Europe (Holder & Leveridge 1986a) common with successions in basins within the continent to the appeared to enhance the concept of an E–W linear Europe-wide north (Chapter 7). It was not until the Silesian that Variscan Rhenohercynian oceanized zone and collision belt (Franke deformation affected basin development and caused its 1989). By extrapolation of Kossmat’s zones through western deformation (see this chapter). Europe into Iberia (Fig. 10.2), Franke (1989, 1992) and Matte et al. (1990) defined sublinear microplates and separating collision belts, with southerly subduction zones in the Rhenohercynian and Saxothuringian and northerly subduction The Variscan of SW England beneath the Moldanubian. Dextral interplate movements and The Variscan orogen in Britain and mainland Europe small oblique basins were reinvoked, however, for SW England by Holdsworth (1989). He speculated that the E–W ‘Start– The Upper Palaeozoic massif of SW England (Fig. 10.1), is Perranporth Line’ might represent a terrane boundary, between situated on the northern margin and forms an integral part of the ‘Old Red Sandstone’ continent to the north and an the Variscan orogen of central and western Europe. The orogen Armorican microplate to the south, with transtension actively is some 1000 km wide, and represents tectonic activity from the controlling intervening Devonian basin evolution. Early Palaeozoic to the Early Permian, and was in large part The Rhenohercynian Zone in continental Europe is now contemporaneous, and interactive with the Caledonides (e.g. considered to lie at or near the southern margin of the Eastern Soper 1986b). Correlation of the province with the Rheinisches Avalonia plate (Franke 2000), the southerly of the three plates, 226 B. E. LEVERIDGE & A. J. HARTLEY Fig. 10.1. Geological sketch map, based on BGS maps, showing the basins of SW England. with Laurentia and Baltica, which collided to form the by Franke et al. (1995). In Germany the ENE–WSE-trending Caledonides (e.g. Soper 1986; McKerrow et al. 1991). Avalonia Rhenohercynian suture lies between the Northern Phyllite Zone and Armorica were derived from Gondwana when in a and the Mid German Crystalline High (of the Saxothuringian southern peri-polar position in the Lower Palaeozoic, drifted Zone) and within those tectonic units there are Ordovician north at differing rates (e.g. Trench & Torsvik 1991) and reas- and Silurian arc volcanics trending NE–SW. Those have been sembled later in the Palaeozoic in equatorial regions. Armorica attributed to the closure of the Rheic Ocean (Franke & Onken comprises an assemblage of continental microplates, with 1995; Franke et al. 1995; Oncken 1997). Closure is thought to intervening marginal, basinal and ophiolite remnants, of the have been at the end of the Lower Palaeozoic (Tait 1999) imme- Saxothuringian and Moldanubian zones and their correlatives, diately preceding Rhenohercynian rifting in the early Devonian which amalgamated during that process (Tait et al. 1997). The along or close to the Rheic suture (Franke 2000). major Variscan oceanic separation, the Rheic Ocean, was pro- McKerrow et al. (2000a) have accepted a close coincidence posed by Scotese et al. (1979), Cocks & Fortey (1982), and Van of the Rheic suture and the Rhenohercynian suture in SW Der Voo (1983), to lie between the ‘ORS’ (Eastern Avalonia) England. Geological evidence confirming such coincidence, continent and Armorica on the basis of palaeomagnetic and or the presence of a pre-Devonian Rheic suture immediately biostratigraphical evidence. More recent palaeomagnetic data south of or within the province (beneath the Start–Perranporth have supported that view (see Tait et al. 1997). Recognition zone with its locally strong secondary Variscan structures?) is of an oceanic suture at the southern margin of the awaited. Rhenohercynian (Holder & Leveridge 1986a; Franke 1989) led some to propose that it was that of the Rheic Ocean (e.g. Oczlon Devonian and Carboniferous basins of SW England 1993). The lack of a paired metamorphic belt, and apparently restricted related arc magmatism, indicated to Franke (1989) The Devonian and Carboniferous rocks of SW England are that it was a narrow ocean, up to 500 km being proposed disposed within six juxtaposed E–W-trending basins (Fig. 10.1). THE VARISCAN OROGENY 227 Fig. 10.2. European Variscan tectonic zonation, after Franke (1989), based on Kossmat (1927), as amended by Holder and Leveridge (1994). BCBF, Bristol Channel/Bray Fault Zone. The evolution of those basins broadly reflects: (i) episodic 1977). The Lizard ophiolite, in association with Gramscatho Early Devonian–Dinantian continental rifting that developed Basin tectonostratigraphy, has been interpreted as evidence that a broad, complex, passive margin with oceanic lithosphere in such rifting led to the oceanization of that basin (e.g. Barnes the south; and (ii) Late Devonian continental collision and & Andrews 1986; Holder & Leveridge 1986b). The ‘Basin and Carboniferous deformation of the passive margin. Rise’ concept, imported to SW England from the Rheinishes In the southern part of the area now represented by the Schiefergebirge of Germany (Goldring 1962), explained the Gramscatho Basin and Lizard and Start complexes (Fig. 10.1), common Mid–Late Devonian association of contemporaneous there was an intermediate stage. Here early rifting and the shallow-marine platform and basinal deposits between the generation of oceanic lithosphere were succeeded by the forma- Gramscatho and Culm basins (e.g. House & Selwood 1966; tion of an active continental margin by the early Mid-Devonian. House 1975; Selwood et al. 1984). Most of these sequences were As a consequence, lithostratigraphical units here provide formerly assigned to the Trevone Basin (Matthews 1977) in the evidence of convergence-related deformation during Mid–Late west or the South Devon Basin (Selwood & Durrance 1982) in Devonian times prior to continental collision. the east (e.g. Selwood 1990). However, three major E–W Devo- Although the Gramscatho Basin formed during Early nian basins, including previously unassigned Lower Devonian Devonian rifting, its infill (Figs 10.1 & 10.6) was largely derived successions north of the Gramscatho Basin, are now identified during Mid–Late Devonian convergence, from its active south- (Leveridge et al. 2002); the Looe, South Devon and Tavy basins ern continental margin. However, remnants of the earlier rift (Fig. 10.1). history, including oceanic lithosphere, are preserved. The The Gramscatho, Looe, South Devon and Tavy basins Devonian–Dinantian infill of the other basins to the north developed sequentially northwards from the Early Devonian, (Figs 10.1, 10.8 & 10.10) reflect sedimentation and magmatic and rifting culminated with formation of the Dinantian Culm activity during the Early Devonian-Dinantian rifting alone. The Basin. Pre-rift basement is not exposed, but is probably present Culm