Research paperShallower structure and geomorphology of the southern Puerto Rico offshore margin

- The southern Puerto Rico slope studied with new swath bathymetry and seismic data.
- The Muertos thrust belt is narrow and probably inactive south of St. Croix rise.
- The Investigator fault zone shows active dip-slip motion, but no strike-slip motion.
- New observations for future assessment of the natural resources offshore Puerto Rico.
- New observations for future assessment of the seismic-tsunami hazard in Puerto Rico.

Oblique convergence between the North American and Caribbean plates along the eastern Greater Antilles island arc has yielded the asymmetric Muertos thrust belt in the backarc 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 seismic reflection profiles, we studied 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 - which includes lower and upper thrust belts with distinct deformational styles and lateral continuity - and the shelf slope 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. In the western Anegada passage province, the Whiting basin and Whiting and Grappler ridges are formed by large dip-slip normal faults driven by a NW-SE-oriented extensional regime. The western St. Croix rise shows a complex structure where 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.