Abstract

The Permian to Cretaceous tectonic evolution of the Wessex Basin was controlled by horizontal tensional and vertical isostatic forces within the lithosphere. The gross morphologies of its constituent structures were governed by the location of Variscan thrust and wrench faults in the upper and middle crust, which suffered extensional reactivation in tensional stress fields oriented approximately NW-SE. Several episodes of crustal extension can be resolved, in early Permian, early Triassic, early Jurassic and late Jurassic/early Cretaceous times. These were characterized by the rapid subsidence of fault-bounded basins and commonly, by erosion of adjacent upfaulted blocks. Superimposed upon the fault-controlled subsidence, dominant during periods of fault quiescence, and becoming increasingly important with time, a component of regional subsidence is considered to have a thermal origin. This suggests that crustal extension was accompanied by some form of, not necessarily uniform, lithospheric thinning. Subsidence analyses assuming local Airey isostasy give cumulative crustal extension factors of 20–28% beneath the grabens. A more reasonable assumption of regional Airey compensation indicates basinwide crustal extension of 13–17%. which is consistent with BIRPS offshore deep seismic reflection data.