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.