End-Carboniferous fold-thrust structures, Oxfordshire, UK: implications for the structural evolution of the late Variscan foreland of south-central England

Abstract

Interpretation of commercial seismic reflection data from the west of Banbury, south-central England, has provided information on the late Carboniferous structural evolution of the Oxfordshire Syncline. End-Carboniferous structures are preserved in the footwall of the main basin-bounding faults of the Permo-Triassic Worcester Graben. Restoration of major pre-Triassic normal faults has allowed reconstruction of earlier compressional structures. In contrast to the WNW-ESE trend of structures to the south, in the Variscan foreland fold-thrust belt, major faults and folds in Oxfordshire are approximately N-S trending. The structures seen to deform Westphalian D-Stephanian-aged sequences on the seismic profiles are a combination of steep (30-40° dip) fault-bounded uplifts and less dominant thin-skinned tectonics, apparently showing transport to the east. In the south, some thin-skinned structures are present, and in the north cover sequences are uplifted in a narrow zone of deformation on reactivated basement faults. The amount of shortening also changes along strike, from c. 1600 m in the south to c. 950 m in the north. Reactivation of major basement faults with differential basement shortening accounts for the main structures. Localized detachment in the young and weak sediments close to the palaeo-land surface explains the thin-skinned structures seen on some of the seismic lines. These end-Carboniferous structures in Oxfordshire and structures in the Malvern-Abberley Hills represent the structural limits of a large N-S-striking intra-Stephanian uplift in the late Variscan foreland. It is suggested that the structures formed during a discrete phase of intra-Stephanian E-W-directed regional shortening prior to the final phase of movement in the Variscan belt.

Seismostratigraphic analysis of Paleozoic sequences of the Midlands Microcraton

“…Permian in the centre of the Worcester Graben is the result of erosion of this extensive high zone by unconformities at Shelveian, Acadian and two Variscan episodes, rather than a simple Variscan uplift and later inversion of what is now the Permian Basin by Variscan thrusts reaching the pre-Permian surface at the Malverns in the west and the zone of the Inkberrow Fault in the east, as proposed by Chadwick (1993) and Peace & Besly (1997). This is in agreement with Smith (1987), who inferred a Precambrian subcrop beneath the Devonian in the Worcester Graben area.…”

“…These authors had access to good-quality seismic lines acquired by Clyde Petroleum in the 1980s over the area between Stow-on-the-Wold and just north of Chipping Camden, where the late Variscan faulting and folding is most intense. Further seismic data now available demonstrate that the intense structuration dies out very rapidly south of this area (see line RG-004 in Butler & Jamieson 2013), although Peace & Besly (1997) demonstrated reverse faulting in the Upton borehole, some 12 km south of Stowon-the-Wold. Comparison between Figures 8 and 11, which are about 10 km apart at the OTB location, shows that it also dies out northwards.…”

“…The Oxfordshire Thrust Belt (OTB) was named by Peace & Besly (1997) to encompass the intense late Variscan deformation seen in parts of the eastern boundary of the Worcester Graben. These authors had access to good-quality seismic lines acquired by Clyde Petroleum in the 1980s over the area between Stow-on-the-Wold and just north of Chipping Camden, where the late Variscan faulting and folding is most intense.…”

Variscan structures in the Kent Coalfield, southeast England, and their regional significance

“…There may have been additional communication through Oxfordshire into the southern province in Berkshire, where Westphalian A-C Coal Measures are known (Foster et al 1989). The High was overstepped by later Westphalian sequences; Peace & Besly (1997) describe the structural evolution of the Variscan foreland and the High through late Carboniferous times.…”

“…The location of the front is, however, ambiguous, depending on the criteria used to define this limit. It is useful to discriminate here between the general north/northwest-verging system, that is, 'Variscan', and certain north-south striking, endCarboniferous, thrusts and folds found in (and well beyond) the immediate Variscan foreland (Corfield et al1996;Peace & Besly, 1997). The most widely used criterion is the external limit of a linked thin-skinned thrust system or décollement.…”