Page 14 - An Atlas of Carboniferous Basin Evolution in Northern England

Regional structural framework 7

Norway and Greenland. This suggests that at least two plate margin processes Slivers of MORB basalts under the Giessen-Selke nappe (Wederpohl et al.
were prevalent in late Devonian-Carboniferous times; an extensional system 1983) provide further evidence that late Devonian oceanic crust was generated
propagating southwards from the northern Boreal Ocean and a dominantly in the Rheno-Hercynian Basin. The age of final break-up, when the rift became
contractional system encroaching northwards on southern Britain (Knott et al. oceanic, is thought to be late Givetian (375 Ma).
1993; Maynard et al. 1997). Models such as early Boreal extension causing A period of renewed rifting in the Frasnian was focused along the centre of
east-west rifting (Haszeldine 1988) or two-phase models involving discrete the Rheno-Hercynian Basin (Houchen 1988). This phase of rifting was possibly
events such as back-arc rifting followed by thermal subsidence (Leeder 1988) associated with dextral shear, as a result of oblique subduction of the Rheic
have evolved from this basic plate tectonic framework. Maynard et al. (1997) Ocean. In the Gramscatho Basin in southern Cornwall, stretching at a right-
suggest the above are rather too simplistic and argue for a model which handed offset in the resulting dextral transform system increased extension to
rationalises the complexity of structural settings encountered in the Variscan the point at which dyke intrusion began and finally led to the generation of the
province of NW Europe. Building on the work of Coward (1990), Maynard et Lizard ophiolite complex. Palaeomagnetic data suggest a spreading axis
al. (1997) propose a model of crustal escape/extrusion in which the extruding oriented NNW-SSE (Hailwood et al. 1984; Barnes & Andrews 1986). This
crust was able to extend north-south due to release of confining forces as it presents a problem as this orientation is largely incompatible with north-south
moved eastwards. Within this context, the present-day plate-tectonic setting of extension unless the oceanic crust was generated along a leaky transform
the eastern Mediterranean offers a modern analogue to the late Devonian to system linking actively spreading segments of the main back-arc system.
early Permian development of NW Europe in terms of size and range of local Isotopic studies of the Lizard ophiolite complex in western Rheno-Hercynia
structural styles. suggest oceanic crust generation at 375 Ma followed by obduction between 370
and 355 Ma (Davies 1984). However, older dates of up to 400 Ma (Halliday &
Mitchell 1976; Styles & Rundle 1984) would place its generation during the
Variscan extension earlier Early Devonian normal extension.

In the late Devonian-early Carboniferous, northern England lay in an
equatorial position inboard of the southern margin of Laurasia marked by
the Rheno-Hercynian Ocean (Fig. 6). Along the northern margin of the Variscan compression
Rheno-Hercynian Ocean an extensive carbonate platform, covering the area
between SW England and eastern Poland, developed during the early Compression and subduction of the Rheic Ocean continued into the late
Carboniferous. Rifting was affecting the northern margins of the Rheno- Dinantian (Fraser & Gawthorpe 1990). A schematic cross-section, illustrating
Hercynian Basin by the early-mid Devonian (Sellwood & Thomas 1986) either the degree of shortening in the British Variscides, is shown in Figure 7. The
as a result of simple back-arc extension related to the NW-directed subduction timing of the transition between overall extension and overall compression
of the Rheic Ocean beneath the Armorican microcraton (Burg & Matte 1978, (from subduction and back-arc spreading to collision) is uncertain and is likely
Leeder 1987a, 1988) or the continued extrusion of crust eastwards from the to have been diachronous. Burg & Matte (1978) considered the Rheic Ocean to
developing Appalachian orogen to the west (Maynard et al. 1997). To the have closed towards the end of the Devonian, whereas Leeder (1987a) argued
south, in the Massif Central region, calc-alkaline volcanic activity was driven for continued subduction and associated back-arc extension throughout the
by the subducting Rheic oceanic crust (Fig. 6). The presence of a subduction early Carboniferous in order to explain early Dinantian rifting in northern
zone to the south of the South Armorican shear zone is supported by England. Maynard et al. (1997) suggest that by late Dinantian times the width
dismembered ophiolitic material thrust southwards in the Massif Central on of the Rheic Ocean was such that continental collision would have commenced.
the south side of the suture (Burg & Matte 1978). Further evidence for Further evidence is provided by the deepwater flysch-like deposits (Culm) of
Fig. 5. Caledonian tectonic provinces of England and Wales (after Fraser et al. 1990 and subduction includes the presence of blueschists on the He de Groix and further southern England which were derived from a southerly source suggesting the
Fraser & Gawthorpe 1990; based on data from Turner 1949; Soper et al. 1987; Pharaoh west in the Vendee (Guiraud et al. 1987) and the associated high-temperature/ early presence of Variscan mountains and the initial development of the
et al. 1987). low-pressure assemblages in Brittany (Lefort 1979). Variscan foreland basin. Early thrusting, marked by the obduction of the
The back-arc nature of the Rheno-Hercynian Ocean is supported by the Lizard complex in the early Carboniferous (365-345 Ma Fammenian-
chemistry of tholeiitic basalts in Cornwall (Floyd 1982) and also by the Chadian) (Barnes & Andrews 1986; Dodson & Rex 1971), provides further
the extensional basins in northern England. association of 'oceanic' volcanic rocks with thick greywackes (Matte 1986). evidence that the change from extensional to compressional tectonics in the
Plate reconstructions for the late Devonian-Carboniferous can provide a Early (Early-Mid Devonian) extension within this back-arc setting was south of the British Isles probably took place in the late Dinantian. The
basis for reconciling these apparently contradictory models (Knott et al. 1993; dominantly north-south oriented (e.g. Benton and Ritec Faults in South associated deformation migrated northwards with the western parts of the
Maynard et al. 1997). The plate reconstruction presented here (Fig. 6) was Wales, Houchen 1988). Sellwood & Thomas (1986) have suggested that the South Devon and Trevone Basins inverting in the late Visean (Sellwood &
developed as a starting point in the series of maps constructed by Knott et al. basins on the northern margin of the rift in Devon and Cornwall developed as Thomas 1986) and minor inversion in the East Midlands basins at the end of
(1993) in an analysis of the late Palaeozoic and Mesozoic evolution of the a series of half graben controlled by a set of down-to-the-south normal faults the Brigantian (Fraser et al. 1990).
North Atlantic continental margin. Knott et al. (1993) had the benefit of (Fig. 7) as evidenced by the formation of the Trevone and South Devon Basins In Westphalian C/D times, the final collision of Gondwana with both
Bathonian to Recent magnetic anomaly data as an external constraint on the in the region. It has been suggested that the South Devon Basin has certain Laurentia to the west and Iberia to the south took place. This marked the
relative motions of the plates. The constraints of magnetic anomaly data are oceanic affinities and the crustal-scale cross-section interprets the zone as lying culmination of the Variscan deformation and the inversion of the late
not available for pre-Mid Jurassic plate reconstructions and more reliance has very near to the continent-ocean transition after break-up (Chandler & Isaac Devonian-early Carboniferous rift basins of northern England. In common
to be placed on palaeomagnetic and regional tectono-stratigraphic data. 1982) (Fig. 7). The Upper Devonian to Lower Carboniferous facies in the half- with the late extensional phase, there appears to be an important component of
The late Devonian-Carboniferous reconstruction in Figure 6 illustrates the graben trend from elastics in the early syn-rift, to reefal and pelagic carbonates dextral shear during compression. Sanderson (1984) considered that compres-
northwards movement of Gondwana throughout the mid to late Devonian, with black shale/cherts towards the end of the rift phase. The reefs typically sion took place in a dextral transpression regime. Several major faults show
progressively closing the Rheic and Rheno-Hercynian oceanic basins. Also developed on the footwalls of the rotated fault blocks (Fig. 7). There are clearly evidence of Variscan dextral shear, principally the Bray Fault (Matte 1986) and
shown in the reconstruction are the early attempts of the Boreal rift to similarities here with the Mesozoic Biscay margin, where Kimmeridgian reefs the North and South Armorican shear zones (Cogne 1960; Chauris 1969; Watts
propagate southwards along the line of the Caledonian collision zone between developed on tilted footwall blocks prior to break-up (Montadert et al. 1977). & Williams 1979; Jegouzo 1981). In SW England, Coward & Smallwood (1984)