Results of the seismic investigation of the Puerto Rico trench and outer ridge, carried out in 1959 by Lamont Geological Observatory, Woods Hole Oceanographic Institution, A. and M. College of Texas, and Hudson Laboratories, are presented as a crustal section. Three layers above the mantle having compressional velocities of 2.2, 5.3, and 6.6 km/sec are defined continuously from the Nares basin to within 20 km of the north wall of the trench. Extension of the seismic section to the north wall indicates that these three layers may crop out. A layer of velocity 4.2 km/sec is clearly defined in the region of the Nares basin but pinches out to the south. The depth to the M discontinuity, velocity 7.7 to 8.3 km/sec, varies from 12.2 km to a minimum of 9.8 km. South of the trench a layer of velocity 4.6 km/sec overlies one of 6.6 km/sec. Velocities determined under the trench are not concordant with those found under the outer ridge, A layer of velocity 5.7 km/sec overlies material of velocity 7.4 km/sec; depths to these horizons are 10 and 14 km, respectively. It is suggested that the trench section is displaced downward at least 2 km relative to the outer ridge and that the materials underlying the trench have subsequently been altered. A gravity profile has been computed from layer densities and thicknesses derived from the seismic data. The shape of this computed profile agrees well with the measured free‐air anomaly except at the axis of the trench, where the computed values are 50 to 70 mgal too high.

No abstract

Several refraction profiles on the outer ridge north of Puerto Rico, combined with earlier studies, support the presence of an east‐west trending structural high or welt on the surface of the mantle. The determinations of shallow mantle depths lie along an east‐west line north of a closed free air (20 mgal) gravity high. The trend of the postulated ridge parallels the axis of the Puerto Rico trench and intersects the northwest‐trending magnetic lineations reported by other investigators at an angle of 50°.

No abstract

No abstract