The thermal and mechanical development of the Wessex Basin, southern England

Abstract

Summary The Wessex Basin of southern England is an E-W-trending Late Palaeozoic to Tertiary extensional basin formed on Early Palaeozoic Variscan crust. Extension occurred over a protracted period from Late Carboniferous to Cretaceous times resulting in the evolution of four main depocentres or sub-basins; the Channel basin, Winterborne-Kingston Trough, Vale of Pewsey, and Weald basins. Each sub-basin was controlled fundamentally by the normal reactivation of basement thrust faults and wrench or transfer faults. The basement faults represent primary structures associated with the fold-and-thrust terrain of the Devonian-Carboniferous Variscan Orogeny. Subsequent, but synchronous, reactivation of both thrust and transfer faults tended to compartmentalize the basement, leading to discrete depocentres and the production of general, rhomboidal-shaped, sub-basins (characteristic of pull-apart basins in strike-slip terrains). To investigate the thermo-mechanical properties of the lithosphere during basin formation, 50 boreholes within the Wessex Basin were back-stripped to isolate the driving tectonic subsidence responsible for basin formation. The resultant driving subsidence is characterized by a gentle, sometimes negative, exponential form, with superimposed rapid, discrete, subsidence events. As the geological development of the Wessex Basin was characterized by polyphase extension and the utilization of crustal detachments, the lithospheric model used to determine δ (crustal extension above the detachment) and β (stretching below the detachment) included the effects of depth-dependent stretching and finite re-rifting. From the back-stripping analysis, the basin-wide-averaged upper-crustal extension and lower-crustal/sub-crustal lithospheric stretching were 11 and 5% respectively. The distribution of δ is strongly influenced by the location of growth faults, and hence basement thrust-fault reactivation, suggesting that δ is reflecting the brittle failure and collapse of hanging wall blocks. Repeated slip and block collapse resulted in the polyphase extension responsible for renewed basin subsidence. In contrast, β is uniformly distributed across the basin, suggestive of a ductile failure of the lower crust and responsible for the low-magnitude thermal subsidence. Strain compatibility between the upper-crust and lower-crust/sub-crustal lithosphere is maintained partly within the basin (5%), and partly external to the basin. As the Variscan basal thrust was the fundamental relay (in the sense of Wernicke) responsible for the Wessex Basin formation, we postulate that strain balancing induced a cumulative (essentially thermal) isostatic uplift (of ≈ 60 m) under the northern margin of the Armorican Massif.