A seismic refraction survey of the earth's crust beneath the lesser antilles

Abstract

Seismic refraction data from the Lesser Antilles Seismic Project of April 1972 are analysed to delineate the crustal layers beneath the Lesser Antilles Island Arc, Grenada Trough, Tobago Trough and Barbados Ridge. First arrival travel times from 23 of the LASP recording stations are analysed by the classical reversed-profile, modified ‘plus-minus’ and time-term methods of interpretation. Two crustal layers of velocities about 6.2km s(^-1) and 7.0km s(^-1) are found beneath the Lesser Antilles ridge. It is suggested that the upper layer is of largely andesitic composition and that the lower layer represents the old oceanic crust which has been intruded with basic magma from the upper mantle beneath the arc. The presence of two crustal layers is common to most island arcs. Shear-wave velocities and Poisson's ratio for the upper crust beneath the arc are estimated, and these suggest that no large magma chambers are present above about 5km. The crust of the Tobago Trough is shown to consist of a layer of velocity 7.0km s (^-1), overlain by a thick pile of sediments and ? metamorphics, This structure shows closer affinities to 'Atlantic' crust than to 'Caribbean' crust. The results from the Grenada Trough are shown to be consistent with previous interpretations of the crustal structure. These results indicate that there may have been an earlier period of subduction at a site some distance west of the present Lesser Antilles subduction zone. This may be related to the possible formation of the Aves Ridge as an active island arc in late Cretaceous – early Tertiary times.