An explosion seismology investigation of the continental margin west of the Hebrides, Scotland, at 58°N

Abstract

During summer 1975, a line of large shots was fired across the continental margin between the Rockall Trough and the Hebridean shelf along 58°N. Arrivals were observed at temporary seismic stations set up across Scotland and in northwestern Ireland. A clear P 2 phase was observed to cross the margin and a converted phase P 1 also seen on the records is interpreted as travelling in the sub-sedimentary oceanic crust of Rockall Trough and in the upper continental crust beneath the shelf. The continental crust beneath the Hebridean shelf is estimated to be 27 ± 2 km thick, with P g = 6.22 ± 0.03 km/s and P n = 8.01 ± 0.04 km/s as determined by time-term analysis. P g delays on the outer shelf are interpreted in terms of a seaward thickening wedge of Mesozoic sediments which pre-date the split. P n beneath the Rockall Trough was poorly determined at 8.20 ± 0.17 km/ s and the Moho is estimated to be 18 ± 2 km deep at 58°N. This and other seismic and gravity work indicates a northward thickening of the crust along the Rockall Trough, accounting for the northward decrease in the height of the slope. Our results, and those of gravity interpretations, indicate a relatively abrupt transition between continental and oceanic crust, possibly correlating with the lack of major shelf subsidence. This is attributed to a relatively cool origin for this margin. The main thinning of the continental crust beneath the slope is attributed to outslip of continental crustal material into and beneath the newly forming oceanic crust during the first few million years after the split, possibly enhanced by pre-split stretching.