Triassic–Jurassic development of the St. George’s Channel basin, offshore Wales, UK

Abstract

Isopach maps and backstripped tectonic subsidence plots from the Triassic–Jurassic of the St. George’s Channel basin reveal significant departures from current models of extensional basin development: (i) rifting was highly protracted with discrete increments of crustal extension spanning ca. 120 My, (ii) substantial syn-kinematic sediment on the footwalls of major faults records pure shear (sub-seismic scale) deformation to have been important during regional Triassic extension. Forward modelling of depth-converted seismic interpretations is used to test the notion of combined pure and simple shear and to quantify the distribution of crustal extension between large and sub-seismic scale faults. The models indicate that Triassic subsidence was generated by ca. 2.5 km extension on major faults and 10 km extension by sub-seismic scale faulting. In fault-bounded compartments, Jurassic extension was dominated by large-scale faults, whereas in the areas of overlap between major fault segments, Jurassic extension was taken up chiefly by sub-seismic scale faulting. Comparison of measured vitrinite reflectance (VR) and VR modelled from heatflow history yielded by the forward models supports this scheme of episodic, pure shear-dominated extension. This research highlights the inherent difficulty in using seismic interpretation in isolation from quantitative subsidence and/or thermal history data to identify syn- and post-kinematic sedimentary sequences. Furthermore, it provides an opportunity to more fully reconstruct Mesozoic basin development in the West-of-UK area which, in the Cheshire, Central and East Irish Sea basins, is largely precluded by erosion of post-Triassic strata.