The Cenozoic uplift and earthquake belt of mainland Britain as a response to an underlying hot, low-density upper mantle

Abstract

A belt of hot, low-density uppermost mantle underlying mainland Britain down to at least 200 km depth, revealed by seismic tomography, may be the prime cause of the Cenozoic uplift and exhumation. We use finite-element modelling to demonstrate how isostatic uplift can occur in response to such a low-density hotspot beneath continental crust. To explain the narrow width of the uplift of northern Britain, the lower crust must be ductile (power-law rheology assumed) and the asymmetrical uplift may be bounded at least on the west side by a pre-existing fault or faults of appropriate polarity. Faulting has probably been reverse under NW–SE regional compression since the onset of the Cenozoic. With the assistance of continuing denudation, inferred gross Cenozoic exhumation of up to 3000 m can be explained. British earthquakes concentrate along a similar north–south belt, with the strongest events in the west. We suggest that the earthquakes result from the continuing tectonics associated with the hot upper mantle, the uplift it causes, and the weakened crust. The underlying low-density region gives rise to tensional loading stress in all directions and bending stresses are associated with the upper-crustal flexuring accompanying uplift. These large stresses supplement NW–SE regional compression. Available earthquake mechanisms are approximately consistent with this stress environment.