Crustal velocity structure of the British Isles; a comparison of receiver functions and wide-angle seismic data

Magmatic underplating at the base of the continental crust is thought to be an important consequence of mantle plume activity. Geochemical evidence supports the hypothesis of underplating beneath the British Tertiary Igneous Province during Palaeocene times, a process that has been invoked to explain at least part of the observed pattern of Cenozoic denudation and consequent offshore deposition. This study uses receiver functions to independently test the results of modelling of four wide-angle seismic lines across the British Isles, several of which indicate the presence of a lens of high-velocity material at the base of the crust. This lens has been interpreted as evidence of Palaeogene magmatic underplating. Receiver functions were calculated for three permanent and two temporary three-component broad-band seismometers. In order to detect P-to-S conversions from intracrustal velocity discontinuities, high frequencies were preserved in the calculation of the receiver functions. Receiver functions were modelled using the delay times of the P-to-S converted arrivals only, with constraints on the VP and VP/VS structure imposed from controlled source data. This pragmatic method of interpreting receiver function data agrees well with the results of applying an established inversion technique. Resulting crustal velocity models are broadly consistent with the velocity models from the wide-angle lines, showing evidence for a four-layer crust with laterally varying Moho depths of between 33 ± 2.5 km beneath southwest Ireland, 32 ± 2.5 km beneath northern and eastern Ireland, 31 ± 2 km beneath the Isle of Man and 35 ± 3 km beneath northern England. While these velocity models are consistent with the presence of a lower crustal layer, receiver functions are insensitive to the P-wave velocity structure, and so unfortunately they cannot be used to independently constrain the P-wave velocity of the lower crust, which is an important diagnostic of the presence of magmatic underplating.