Geophysical investigation of the Puerto Rico trench and outer ridge

Abstract

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.

Vertical motions of the Puerto Rico Trench and Puerto Rico and their cause

“…Published crustal models to explain the unusual observation of the Puerto Rico trench cover almost the entire spectrum of tectonic processes. These models included transtension and rifting at the trench [ Bunce and Fahlquist , 1962; Speed and Larue , 1991], N‐S compression and buckling [ van Gestel et al , 1998], subsidence and uplift due to geometrical complications in a strike‐slip fault system [ Jany et al , 1990], and subsidence and uplift due to counterclockwise rotation of the Puerto Rico/Virgin Island (PRVI) block [ Masson and Scanlon , 1991]. Crustal forces that operate by repeated fault ruptures will result in a slower deformation rate than mantle flow, thus it will be difficult to explain the short duration of subsidence (tens to hundreds of kiloyears) by crustal extension, block rotation, or crustal compression.…”

Paleogeography and Geological History of Greater Antilles

“…It could have existed in some form before Cenomanian time if it was a dynamic feature, the locus of underthrusting of oceanic crust beneath continental crust. Bunce and Fahlquist (1962), Talwani (1964), and Bunce (1966, p. 172) suggested that the trench is a downfaulted block, but Bunce (1966, p. 172) suggested alternately that the trench is a downwarped feature. J. reported the southward tilt of the trench-floor sediments into Puerto Rico.…”

“…For various viewpoints on the tectonic development of the Puerto Rico Trench, the reader is referred to papers by Talwani and others (1959), Bunce and Fahlquist (1962), J. , Hersey (1962Hersey ( , 1966, Van Voorhis and Davis (1964), Donnelly (1964), Talwani (1964), Sykes and Ewing (1965), Hersey and Chase (1965), Bowin and others (1966), Bunce (1966), Bunce and Hersey (1966), and Monroe (1968).…”

Control of Intensity of Deformation in Puerto Rico by Mobile Serpentinized Peridotite Basement

“…Positions of the lines were calculated by the authors on the basis of the estimated crustal thicknesses and Bouguer anomalies of Shurbet and Ewing (1956). The crust may be serpentinite, in part, but it is certainly not Bunce and Fahlquist (1962). Numbers 1 through 5 refer to sequences shown in Figure 2.…”

“…Part C-D-E of Figure 5 is extended across the Puerto Rico Trench, using data from Bunce and Fahlquist (1962). Two possible positions of the base of the crust are shown in the A-B-C portion.…”