The nature of the Variscan basement in southeast England: evidence from integrated potential field modelling

Abstract

The Variscides of southeast England are buried beneath post-Carboniferous cover. Interpretations of the basement are based mainly on deep boreholes. Geophysical signatures from the basement are contained within the regional gravity and magnetic data. A gravity stripping exercise has been undertaken to remove the gravitational effect of the post-Variscan cover to generate a residual gravity map. This map is interpreted along with integrated potential field modelling along four long interconnected profiles and compared with a revised pre-Permian subcrop map. The magnetic evidence suggests that Precambrian magnetic basement of the Midlands Microcraton has been buried southwards by north-vergent Variscan thusting over the foreland. North of the Variscan Front, short-wavelength anomalies superimposed upon this deep Precambrian source are due to shallower Silurian and Carboniferous volcanic rocks. Many residual gravity lows within the Rhenohercynian zone may be related to thick, low-density Devonian basins. In the English Channel a change in geophysical signature occurs north of the Portland–Wight Fault, coinciding with phyllites in the basement. Models are presented in which the English Channel magnetic anomalies originate within the pre-Permian basement. Comparisons with anomalies in the Southwestern Approaches suggest that the Portland–Wight Thrust is a terrane boundary, possibly a subduction-related suture, implying southerly directed Variscan subduction.