Inversion and exhumation of the St. George's Channel basin offshore Wales UK

Abstract

The western UK basins of the Irish Sea have provided one of the best natural laboratories for investigating the causes and consequences of intracratonic uplift and erosion (exhumation). To date, the emphasis has been on igneous underplating as the chief process driving their exhumation. In this paper, we demonstrate that tectonic inversion (the shortening of formerly extensional basins and reactivation of their constituent faults) dominated the exhumation of the St. George's Channel basin, offshore Wales. Based on mapping of an extensive 2D seismic grid, evidence is presented for at least two major inversion episodes in the Late Cretaceous and the Neogene, plus minor shortening during the Eocene. Inversion was distinctly noncoaxial, especially during the Neogene when coeval transpression and transtension was focused at discrete bends and stepovers on the basin-bounding St. George's, Bala and Northwest Flank faults. That the principal mechanism driving these uplift episodes was inversion (as opposed to igneous underplating) is corroborated by analysis of thermal history data (apatite fission track and vitrinite reflectance). They reveal late Cretaceous and Neogene geothermal gradients that were comparable with that at the present day, i.e. no significant increase in basal heat flow. Sonic velocity profiles logged in hydrocarbon boreholes constrain the minimum thickness of the eroded section, which varies between c. 1000 m in the centre and c. 2240 m at the margins of the basin. Given the strength of evidence for tectonic inversion in the St. George's Channel basin, our favoured model invokes superimposition of the effects of inversion and igneous underplating to account for the complex exhumation history of the St. George's Channel basin in particular, and the western UK basins in general.