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more links to Puerto Rico researchN R Grindlay, F Delano, W Rosado (all at Dept. of Geology, UPR, Mayaguez, PR
00681-5000; 787-265-3845, email: nancy@bigwave.upr.clu.edu); P Mann
(UTIG, Austin, TX 78759-8397); J Dolan
(Dept. of Earth Sciences, USC, Los Angles, CA 90089-0740); J-P van Gestel, S Muszala (both
at Dept. of Geol. Sci., UT-Austin, Austin TX 78712); A Munoz (Instituto Espanol de
Oceanografia, 28002 Madrid, Spain); J Charles III (W Virginia Wesleyan College, WV)
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 the northeastern North American-Caribbean plate boundary.
Obliquely colliding bathymetric 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 a 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, a transverse 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 motion 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 recent high-angle 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 along the base of the scarp formed by the subsided carbonate margin 90-120 km
from 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.