Structure of the Crust Beneath the Caledonian Foreland and Caledonian Belt of the North Scottish Shelf Region

Abstract

This paper presents those results from the sea-to-land North Atlantic Seismic Project of 1972 which apply to the North Scottish shelf region. Time-term analysis based on first arrivals yielded Pg of 6·10±0·15 km s−1,P* of 6·48±0·06 km s−1 and Pn of 7·99±0·02 km s−1. The presence of a P* refractor yielding first arrivals distinguishes the northern British crust from that south of the Caledonian belt. Beneath the Caledonian foreland, the P* refractor varies in estimated depth between 2 and 16 km and its depth correlates with the Bouguer anomalies; it most closely approaches the surface beneath a 90 mgal Bouguer anomaly high west of the Shetland Islands, and is interpreted in terms of widespread occurrence of Lewisian granulites beneath the region. The Moho beneath the foreland occurs at a remarkably uniform estimated depth of 26±2 km. The P* refractor has been identified beneath the western part of the Caledonian fold belt and beneath the Moray Firth, and may occur more extensively. The crust beneath the fold belt is of the order of 20 per cent thicker than beneath the foreland, except beneath Moray Firth where the crustal thinning to the present estimate of about 23 km may have occurred during the post-Caledonian formation of the Moray Firth basin.