ABSTRACT

A study of refraction seismic velocities beneath the Caribbean Sea and nearby areas of the Atlantic Ocean reveals that the thickest crust occurs in the Antillean island belts and the Nicaraguan Rise. In the case of the Nicaraguan Rise, the maximum thickness is about 22 km and is located to the south of the present topographic crest. Isopachs of total crustal thickness indicate that the Caribbean crust is intermediate between average oceanic and continental crust.

The oldest dated rocks in the Caribbean are Jurassic, which is consistent with the idea that the region had its origin in early Mesozoic time as a result of rifting and subsequent drift between the American and Afro-European continental blocks.

Throughout Jurassic and Lower Cretaceous time the Nicaraguan Rise was a mobile belt of vast submarine lava flows and mafic intrusives. Vulcanism gradually decreased during the Upper Cretaceous, so that clastic and carbonate beds alternated with tuffs, agglomerates, and mixed volcanic-sedimentary units. The first phase of the Laramian orogeny occurred at the end of the Cretaceous, and it may have been at this time that a rift along the southern flank of the rise separated it from the Beata Ridge.

Normal marine sedimentation, clastic and chemical, was the rule during the Tertiary. Islands emerged and sank as movement occurred between crustal blocks. Tectonism beginning in Middle Miocene time severely altered the topography and the depositional pattern of the rise. The Cayman Trough rift occurred as a result of a left lateral fault along which there was at least 400 km of displacement. The rise was titled southward, thus shifting the topographic crest 80 to 150 km north.