LISPB REVISITED - A NEW LOOK UNDER THE CALEDONIDES OF NORTHERN BRITAIN

The Lithospheric Seismic Profile in Britain (LISPB), shot in 1974, included a 700 km long north-south profile of the crust and upper mantle of northern Britain. The wide-angle seismic data from LISPB was of exceptional quality and coverage making a reinterpretation using current modelling programs worthwhile. This reinterpretation shows that the original crustal model published by Bamford et al. (1978) satisfies the observed first arrival traveltimes, but that further detail of crustal velocity structure may be obtained by modelling first and later arrivals using synthetic seismograms. The new model shows strong variations in crustal velocity structure that appear to be correlated with geological and tectonic structures mapped at the surface. The new lithospheric model is compatible with the observed gravity field when converted to appropriate densities, and the variations in loading along the base of the density model show that the lithosphere is close to local isostatic equilibrium. Profiles of the variation of velocity and density with depth from the new model are compared with measurements of Lewisian rocks made at confining pressures up to 8 kbar, allowing the seismic model to be interpreted geologically. A synthetic normal incidence profile of the new model is superimposed on the nearby NEC deep seismic reflection profile, and the features imaged by the two techniques are compared.