Crust and upper mantle structure in the region of Barbados and the Lesser Antilles

Abstract

The Lesser Antilles form one of only two island arcs that occur in the Atlantic Ocean. Bathymetric, gravity, magnetic, and seismic reflection data were collected by HMS HECLA during 1971 in an area bounded by latitudes 12 54’ N and 13 54' N, and longitudes 57 W and 61 10' W, including the islands of St. Lucia, St. Vincent, and Barbados. These data are reduced and interpreted in conjunction with seismic refraction data from an experiment run in 1972 and data from other published and unpublished sources. The structure of the upper sedimentary layers is derived from the seismic reflection records. The crustal structure is modelled two dimensionally using a non-linear optimisation technique to fit the observed gravity and seismic refraction data. The island arc and the Barbados Ridge are examined in detail, and the nature of a ridge running eastward from St. Lucia into the Atlantic Ocean basin is investigated. Magnetic anomalies are treated by direct modelling, magnetic to gravity transformation, and analysis of the power spectrum. The seismicity of the eastern Caribbean is considered with respect to possible plate motions, and maps of focal depth and energy release are presented. The mechanism and causes of subduction beneath the Lesser Antilles are discussed. The possible gravity anomaly caused by subducted lithosphere is estimated and its effect on the determination of the crustal structure examined. The influence of the relative motions between the North and South American plates on the development of the Caribbean and the Lesser Antilles is studied. Some ideas on the origin and growth of the Barbados Ridge and the island arc are put forward. Geophysical data profiles, and computer programs for reduction and interpretation of data are presented in appendices.