A crustal section across the Puerto Rico trench

Abstract

A crustal section across the Puerto Rico Trench, from 450 km north to 250 km south of San Juan, was deduced from seismic refraction and gravity data. The result, a refinement of previous work, was made possible through more complete seismic refraction coverage and a program for high-speed electronic computation of two-dimensional gravity problems. On the basis of refraction data, the crust was divided into five layers having compressional wave velocities of 1.54, 2.1, 3.8, 5.6, and 7.0 km/sec. Densities taken from a density-velocity curve compiled by Nafe and Drake are 1.03, 2.0, 2.4, 2.7, and 3.0 gm/cc, respectively. Depths to the Mohorovicic discontinuity were computed from the gravity data using sub-crustal density of 3.4 gm/cc, which corresponds to 8.2 km/sec on the density-velocity curve. (A similar calculation was made using a sub-crustal density of 3.3 gm/cc). Depth to M under the trench is about 20 km decreasing sharply on both sides. Northwards, it reaches a minimum of about 10 km under the Outer Ridge and then deepens gradually to about 13 km beneath the southern margin of the Nares Basin. South of the trench M rises under the Puerto Rico Shelf to about 17 km and then deepens sharply to about 30 km beneath Puerto Rico. South of Puerto Rico the depth decreases again to about 14 km under the Venezuelan Basin. Depths to M were also obtained by using Airy isostatic anomalies and assuming constant crustal density of 2.67 gm/cc over a mantle of density 3.27 gm/cc. The crustal section thus deduced differs significantly from that obtained when the density structure within the crust was considered.