Geophysical investigations in the eastern Caribbean: Summary of 1955 and 1956 cruises

Abstract

This paper is concerned with an intensive examination of an island arc and deep sea trench and its associated interior basin. The area investigated was between 10 degrees -25 degrees N. and 53 degrees -70 degrees W. The 1955 work was conducted from the research vessels Atlantis and Caryn and the 1956 work from the research vessels Atlantis and Bear. The instrumentation for these cruises was much the same as that for other marine seismic refraction cruises. Both cruise ships, in this investigation, were equipped to record and shoot. Both vessels made frequent bathythermograph lowerings to provide a record of ocean temperature vs. depth from the ocean surface down to a depth of 900 ft.; this information was used to compute the velocity of propagation of the direct water wave. Except for the Cayman trough in the northwestern part, the deepest water found inside the island arc of the Caribbean is in the Venezuelan basin. In some places the depth exceeds 2,500 fathoms and in others the depth is greater than 2,900 fathoms. The average oceanic structure is 5 km. of water, 1 km. of low-velocity sediments, and 5 km. of crustal rocks (velocity about 6.5 km./sec.) overlying the mantle (velocity about 1.8 km./sec.). The crustal structure in the deepest part was found to be similar to the oceanic structure with significant differences. The high-velocity crustal rock is not included in a single layer with the same velocity throughout, but is so constituted that the lower part has a higher velocity than the upper part. The crustal materials are thicker in general than the normal oceanic crust and thicken appreciably out from the central portion of the basin. The lower velocity materials (1.7-5.5 km./sec.) overlying the crust are thicker than in the Atlantic basin. The authors suggest that the entire area has been extensively intruded by a differentiate migrating upward from deep in the mantle, and that the raised portions were the areas where most of the material reached the surface. The deep sea trench resulted from the expanded Caribbean crust having been overthrust onto the oceanic crust. This overthrust depressed the high-velocity layers on the oceanic side of the arc creating the deep sea trench and the associated negative anomaly belt.