The structure and evolution of the Wessex Basin, southern England: an example of inversion tectonics

Abstract

The Wessex Basin of southern England consists of four main sub-basins. Each sub-basin is thought to be fundamentally controlled by the normal reactivation of Hercynian basement thrust and wrench (or transfer) faults. Post-Hercynian polyphase reactivation of both east-west thrust and northwest-southeast transfer faults tended to compartmentalise the basement thereby leading to the initiation of discrete (Permian-Cretaceous) depocentres. While rifting was confined to the western portions of the area during the Permo-Triassic, easterly migration in the Mesozoic allowed the initiation of the Weald and Channel Basins. The sub-basins represent therefore, simple sinistral pull-apart basins opening along northwest-southeast faults. This same fault direction is postulated to have also played a major role in controlling and/or modifying the development of European basins in general.Major pre-Aptian uplift and erosion is largely attributed to the thermal effects (and hence the isostatic uplift of the basin) associated with continental rifting of the Bay of Biscay and the North Atlantic margins. Major inversion (and hence basin destruction) began in the Late Cretaceous-Early Tertiary with the uplift of the Channel, Weald and Vale of Pewsey Basins and culminated in the Oligocene-Miocene, synchronous with the Helvetic phase of Alpine deformation. Wessex Basin inversion was facilitated by the thrust reactivation of Variscan faults above an intracrustal detachment surface, the same surface which allowed upper-crustal extension and hence basin initiation in the Permo-Triassic.The structural development of the Wessex Basin, both during extension (basin formation phase) and compression (inversion phase), was ultimately controlled by the plate motion of Africa relative to Europe. Such plate motions successfully predict the observed NW-SE sinistral motion from the Stephanian to the Aptian, E-W sinistral motion during the Aptian-Cenomanian, and largely NW-SE dextral motion from the Cenomanian to the present day.