CARIBBEAN RESEARCH PROJECT: PROGRESS REPORT
Progress is reported in mapping the Eastern Caribbean area; 24 Ph.D. theses have been completed or are under way. One major objective is to interpret the available geophysical data in light of the geologic history of the past 100 million years.

Mapping of a series of quadrangles from Caracas to Tinaco in Venezuela has provided a cross section through the Cordillera de la Costa and Serrania del Interior. Several episodes of mid-Cretaceous metamorphism, rather than a single episode, are now recognized.

The concept that island arcs represent an early stage of alpine-type mountain building is rejected. Alpine-type mountains are developed along the thin margin of continental crust, island arcs on an oceanic type of crust. The two represent different kinds of reaction to the same type of tectonic forces. Germano-type mountains develop similarly in areas of continental crust of normal thickness. All three types can be seen along a single tectonic axis from the Antillean islands (island arc) into the coastal ranges of Venezuela (alpine-type) and thence to the Venezuelan Andes (germano-type).

Laboratory measurements of seismic velocities in fresh peridotite (8.4 km/sec) through partially serpentinized peridotite to serpentinite (5.7 km/sec) indicate that the so-called basaltic crust of the oceans may be peridotite two-thirds serpentinized. If so, Mattson's basement rocks which are serpentinites may be altered mantle rocks exposed at the surface. On the other hand MacKenzie has shown that true intrusive peridotites emplaced at temperatures above a minimum of 800°C. exist.