Shallower structure and geomorphology of the southern Puerto Rico offshore margin

Oblique convergence between the North American and Caribbean plates along the eastern
Greater Antilles island arc has yielded the compressive Muertos margin in the backarc region.
The Muertos margin is characterized by an asymmetric thrust belt with overall opposite
vergence to the subduction system in the forearc region. Offshore south of Puerto Rico, this
thrust belt disappears and is replaced by a succession of NE─SW- and E─W-trending deep
basins and steep ridges that characterize the western Anegada passage, resulting in a complex
deformation pattern. Using new high-resolution multibeam bathymetry and a dense data set
of mostly new seismic reflection profiles, we studied the along- and across-strike variations of
the geomorphology and shallower structure of the southern Puerto Rico offshore margin. We
have identified four morphotectonic provinces: the Puerto Rican sub-basin and Muertos
trough, the Muertos margin, the insular shelf and the western Anegada passage. The Muertos
margin province shows two distinct slope sub-provinces: the active Muertos thrust belt formed
by N─S-compression between the island arc and the Caribbean plate’s interior ─which includes
lower and upper thrust belts with distinct deformational styles and lateral continuity─ and the
shelf slope covered by a thick package of layered sediments highly-incised by a dense canyon
network. This network is disrupted by the Investigator fault zone consisting of a 130 km-long
E─W-trending band of active extensional deformation. The Investigator fault zone shows
differential surface expression caused by along-strike changes in the magnitude and
distribution of the deformation, though this deformation is driven by a N─S-oriented
extension, and if there is any amount of strike-slip it is insignificant. In the western Anegada
passage province, the Whiting basin and Whiting and Grappler ridges are formed by large dipslip
normal faults driven by a NW─SE-oriented extensional regime. The western St. Croix rise
shows a complex structure as attested by the existence of NE─SW- and E─W-oriented normal
faults. The NE─SW-trending NW-dipping normal faults observed at the summit of the rise
predate the E─W-bounding faults that could accommodate the extensional deformation at the
Present. This study provides detailed observations on the active tectonic and sedimentary
processes to help future studies assessing the natural resources and the seismic and
tsunamigenic hazard in the Puerto Rico region.