The Oceanographer fracture zone -- I. Crustal structure from seismic refraction studies

Abstract

Summary. This paper presents the results of four lOOkm long seismic refraction lines carried out in and around the active transform of the Oceanographer fracture zone, which offsets the Mid-Atlantic Ridge rightlaterally by 130 km near 35N. The seismic data are interpreted using standard travel-time inversions, the time-term method and two-dimensional ray-tracing techniques. The results show that beneath the Oceanographer fracture zone the crust is typically only 4-5 km thick. It consists of a thin, upper crustal layer, in which the P-wave velocity increases rapidly with depth, and a main crustal layer with velocity increasing from 6.4 km s-l at the top to 6.8 km s-' at the base of the crust. Some degree of crustal thinning is found to extend for distances of 30-40 km on either side of the fracture zone axis. In addition to this a small area (10-15 km long) of crust less than 2.5 km thick is found beneath the centre of the transform valley. Crust less than 4 km thick is also observed on a smaller offset fracture zone 85 km to the south of the Oceanographer, but no evidence is found for significantly thin crust beneath another small offset fracture zone 65 km to the north of it. We conclude that the moderate thnning of the crust at the fracture zone is due to a progressive reduction in the crustal production rates of the oceanic spreading centres as they approach the transform fault. However, we attribute the occurrence locally of extremely thin crust to a different process, in which a thin lid of mafic and ultramafic rocks overlying unaltered upper mantle material is accreted at the junction of a spreading centre and a transform fault. This may happen if the accretion rate at the end of the spreading centre becomes.so slow that the usual processes which give rise to 'normal' oceanic crust can no longer operate. It therefore seems that there are two distinct types of crust either of which may be found at different points along a fracture zone. This might account for the extreme variability in crustal thickness found in different fracture zones by previous studies.