END-CARBONIFEROUS FOLD-THRUST STRUCTURES, OXFORDSHIRE, UK - IMPLICATIONS FOR THE STRUCTURAL EVOLUTION OF THE LATE VARISCAN FORELAND OF SOUTH-CENTRAL ENGLAND

Interpretation of commercial seismic reflection data from the west of Banbury, south-central England, has provided information on the late C arboniferous structural evolution of the Oxfordshire Syncline. End-Car boniferous 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 earlie r compressional structures. In contrast to the WNW-ESE trend of struct ures to the south, in the Variscan foreland fold-thrust bell, major fa ults and folds in Oxfordshire are approximately N-S trending. The stru ctures seen to deform Westphalian D-Stephanian-aged sequences on the s eismic profiles are a combination of steep (30-40 degrees dip) Fault-b ounded uplifts and less dominant thin-skinned tectonics, apparently sh owing transport to the east. In the south, some thin-skinned structure s are present, and in the north cover sequences are uplifted in a narr ow zone of deformation on reactivated basement Faults. The amount of s hortening also changes along strike, from c. 1600 m in the south to c. 950 m in the north. Reactivation of major basement faults with differ ential basement shortening accounts for the main structures. Localized detachment in the young and weak sediments close to the palaeo-land s urface explains the thin-skinned structures seen on some of the seismi c lines. These end-Carboniferous structures in Oxfordshire and structu res in the Malvern-Abberley Hills represent the structural limits of a large N-S-striking intra-Stephanian uplift in the late Variscan forel and. It is suggested that the structures formed during a discrete phas e of intra-Stephanian E-W-directed regional shortening prior to the fi nal phase of movement in the Variscan belt.