Crustal Structure in the Vicinity of South‐west England

Summary This paper describes the interpretation of results from a sea‐to‐land crustal seismic experiment based on south‐west England and extending to Ireland and Brittany. Only P g and P n were observed as first arrivals and these have been subjected to time‐term analysis yielding estimates of 5.82 and 8‐05 km s ‐1 respectively. A remarkably uniform and consistent structure was observed along line 1 of the project, which overlies the granite batholith of south‐west England and its seaward extension to the WSW. Along line 1, the crustal structure was determined using first arrival data supplemented by the well displayed wide angle reflection P m P , yielding the following estimates: P g = 5.85 km s ‐1 ; P n = 8.07 km s ‐1 ; mean crustal velocity = 6.20 km s ‐1 ; crustal thickness = 27 km; dip of Moho insignificant. No discontinuities were detected within the crust, but there is a gradual increase in velocity with depth between the base of the batholith (10 km depth) and the Moho. The upper crustal channel wave P g was observed along this line. Line 2 extends from South Ireland to Land's End, and the time‐terms are about 10 per cent larger than along line 1. P g time‐terms along the northern part of line 2 reflect the presence of a sedimentary basin known from gravity anomalies, but corrected P n time‐terms suggest that the Moho is not detectably upwarped or downwarped beneath this basin. Line 3 extends from Land's End to Brittany, and displays a similar crustal thickness to that of line 1, except that the Moho shallows slightly beneath the sedimentary trough which crosses this line.