This abstract was published in the Transactions of the American Geophysical Union Fall Meeting, in San Francisco, California, T72C-06, Copyright 1996 by American Geophysical Union.

MR1 Sidescan Reveals Aftermath Effects of Late Neogene Oblique Collision of the Bahama Platform on the Puerto Rico Trench

Nancy R. Grindlay  Department of Earth Sciences, University of North Carolina at Wilmington, Wilmington, NC 28403-3297, USA, F. Delano, W. Rosado, Paul Mann  Institute for Geophysics, The University of Texas at Austin, 4412 Spicewood Springs Road, Bldg. 600, Austin, TX 78759-8500, USA, James F. Dolan  Department of Earth Sciences, University of Southern California, Los Angeles, CA 90089-0740, USA, Jean-Paul Van Gestel  Department of Geological Sciences and Institute for Geophysics, The University of Texas at Austin, Austin, TX 78712, USA, S. Muszala  Department of Geological Sciences and Institute for Geophysics, The University of Texas at Austin, Austin, TX 78712, USA, A. Munoz, J. Charles.

The Puerto Rico Trench (PRT) and northern margin of the Puerto Rico/Virgin Islands platform occupy a zone of tectonic transition between island arc/convergent and strike-slip tectonics along northeastern North American-Caribbean plate boundary. Obliquely colliding bathmymetric features on the downgoing North America plate are an additional complicating factor to be considered in a tectonic model for the area of the PRT, and formed the focus of a recent high-resolution sidescan (MR1), Hydrosweep, SCS, gravity and magnetic survey of the PRT aboard the R/V Ewing.

MR1 data and closely spaced SCS lines reveal that the Oligocene to early Pliocene carbonate cap is present up to depths of 6000 m indicating a massive subsidence event over the past 2 my. Previous studies have linked this event to an episode of tectonic erosion caused by the oblique collision of the Main-Barracuda Ridge, an aseismic ridge on the subducting North America plate. However, the limited and very localized deformation observed associated with the Main Ridge suggest that it has had relatively little impact on the island margin. We propose that the erosional episode is linked instead to the aftermath of the oblique collision of the SE end of the Bahama Platform. The active collision of the Bahama Platform is presently occurring along the NE margin of Hispaniola [Dolan and Wald, in press] so presumably the Puerto Rico margin experienced this collisional event in the late Neogene and is now in a state of post-collisional adjustment.

Relative mortion along this portion of the North American-Caribbean plate boundary appears to be accommodated mainly by strike-slip tectonism. All faults imaged in the PRT are high-angle recent faults consistent with active strike-slip motion parallel to this feature. These faults are parallel to a new fault imaged for the first time that extends over 250 km aling the vase of the scarp formed by the subsided carbonate margin and lies within 90-120 km of the north coast of Puerto Rico. Three distinct segments of the fault all display characteristics of an active left=lateral fault zone. Fine-scale alternation of releasing and restraining bends on the central segment of the fault established its left-slip character along with the orientation of the larger transtensional segments bounding it to the east and west. Only in the vicinity of the Hispaniola margin do the geophysical data reveal evidence of convergent tectonics seen in the back tilting and folding of turbidite-filled forearc basins.

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