LISPB DELTA, a lithospheric seismic profile in Britain: analysis and interpretation of the Wales and southern England section
The Lithospheric Seismic Profile in Britain (LISPB), shot in 1974, included a 310 km profile LISPB DELTA, crossing the Palaeozoic Welsh Basin, the western extent of the Midland microcraton and the Cornubian zone of southern England. This first comprehensive analysis of these data has produced a sub-horizontally layered seismic and associated gravity model that correlates well with surface geology. A north–south decrease in crustal velocity and density corresponds to the change from Avalonian crust into the Rheno-Hercynian zone at the south end of the profile. High velocities and densities in the lowest crustal layer beneath north Wales are proposed to result from Cenozoic and possibly Ordovician igneous intrusive rocks, the former derived from an upwelling plume associated with the opening of the North Atlantic. Examination of the load distribution throughout the model shows that it is strongly correlated with the earthquake distribution along LISPB DELTA. Earthquake focal depth also correlates with heat flow. A simple heat-flow profile has been derived, and the seismic velocity model used to constrain crustal heat production values. A long-wavelength excursion from published data can be explained in terms of an increase in mantle heat flow resulting from a previously identified deep thermal anomaly beneath the Irish Sea.