A seismic refraction study of crustal structure between the Faeroe Isles and Scotland

Abstract

A seismic refraction project known as the North Atlantic Seismic Project (NASP) was carried out in the NE North Atlantic between Scotland and Iceland during July 1972. This thesis presents the results and interpretation of the data obtained between Scotland and the Faeroe Islands. The first arrival travel time data was analysed by firstly fitting straight line segments by least squares, and secondly by time term analysis. The shot-station configuration of the project favoured time term analysis as this method combines the large quantity of data obtained, and the interpretation is not limited by apparent velocities. Amplitude measurements were made on some of the data in order to positively identify the large amplitude secondary arrivals observed as the reflected phase from the Moho, PmP. This phase was used to supplement the crustal structure information obtained from the first arrivals, and theoretical travel times have been calculated for these reflections. Little use has-been made of S wave arrivals. Two main crustal layers were established beneath the Scottish shelf with a Moho depth of 25-26 km. The upper basement layer is at a depth of about 2-3 km beneath mainly Palaeozoic sediments (5.0 km s(^-1)) and has a P wave velocity of 6.1 km s(^-1). It is interpreted as Lewisian schists and gneisses. The lower layer at a depth of about 9 km has a P wave velocity of 6.48 km s(-1) and is interpreted in terms of granulite facies Lewisian material. A normal Moho Pn velocity of 7.99 km s(^-1) was found. A continental crustal thickness of about 30 km was determined beneath the Faeroe Plateau. There is quite an abrupt transition in the basement material between the north west and south-east regions of the Plateau. The material in the north-west has a velocity of about 6.1 km s(^-1) and is interpreted as normal continental metamorphic rocks such as gneisses. In the south-east the velocity of about 5.5- 5.6 km(^-1) is interpreted in terms of slates, and the transition between the two as a change in metamorphic grade. The crustal structure was not well determined beneath the Faeroe/Shetland Channel but the region appears to be underlain by anomalous oceanic material with a Moho depth of 11-18 km. Two main crustal layers were identified with velocities of about 4.65 km s(^-1) and 6.l6 km s(^-1), and at depths of about 2.2 km and 7.9 km respectively.