A teleseismic receiver function study of the crustal structure of the British Isles

摘要

The onshore crustal and upper mantle velocity structure of the British Isles has been investigated by teleseismic receiver function analysis. The results of the study augment the dense offshore and sparse onshore models of the structure beneath the area. In total almost 1500 receiver functions have been analysed, which have been calculated using teleseismic data from 34 broadband and short-period, three-component seismic recording instruments. The crustal structure has primarily been investigated using 1D grid search and forward modelling techniques, returning crustal thicknesses, bulk crystal Vp/Vs ratio and velocity-depth models. Upper mantle structures have been investigated by applying Ps moveout corrections and migration techniques to the observed broadband receiver functions. H-k stacking reveals crustal thicknesses between 25-36 km and Vp /Vs ratios between 1.6-1.9. The crustal thicknesses correlate with the results of previous seismic reflection and refraction profile to within +/- 2km. The exceptions are the stations close to the lapetus suture where the receiver function crustal thicknesses are up to 5 km less than the seismic refraction Moho. This mismatch has been attributed to the presence of underplated magmatic material at the base of the crust. 1D forward modelling has revealed sub-crustal structures. In northern Scotland these correspond with the Flannan and W-reflectors. The isolated sub-crustal structure at station GIM on the Isle if Man may be related to the closure of the lapetus ocean. Ps conversion from the 410 km and 660 km discontinuities have been identified in the Ps moveout corrected receiver functions. The differential delay time between the phases is close to the global average of 24s, indicating that there is no significant thermal anomaly in the mantle transition zone beneath the British Isles. A discontinuity at ~220 km has been identified as the Lehmann discontinuity. A 30 km step in the Lehmann discontinuity close to the lapetus suture may be interpreted as juxtaposition of Laurentian and Avalonian mantle.