Crustal and Mantle Structure Across Puerto Rico —From the Caribbean Plate to the Puerto Rico Trench— From an Onshore-Offshore Wide-Angle Seismic Transect

The Puerto Rico Trench (PRT) marks the oblique subduction of the North American Plate under the Greater Antilles Island Arc and the Caribbean Plate. In 2023 we conducted a geophysical survey of the PRT and across the island of Puerto Rico (PR) using the RV Langseth (cruises MGL2315 and MGL2316). PRISTINA (Puerto RIco Subduction Tectonics seismic INvestigAtion) consists of: a) 2D ultra-long-offset (13.65 km) multichannel seismic (MCS) reflection data; b) a N-S island-crossing wide-angle seismic profile sampling the incoming plate, PRT, Puerto Rico, Muertos thrust belt and Caribbean Plate instrumented with nodal land stations and ocean bottom seismometers (OBS); c) a wide-angle OBS profile crossing the PRT north of the British Virgin Islands; d) Four wide-angle fan profiles; e) Underway bathymetry, gravity and magnetics; and f) a temporary deployment of broadband stations in Puerto Rico.

Here we present a 2-D P-wave velocity (Vp) model along a 310-km-long, North-South-trending seismic transect at Longitude 66°30’W that illuminates the crustal and mantle structure of the Muertos Trough south of PR on the Caribbean Plate, the island of PR, and the PRT. The model is derived from traveltime tomography of active-source wide-angle data acquired with 30 ocean bottom seismometers (nominal spacing of 5-10 km), a temporary land deployment of 48 short-period nodal seismometers (nominal spacing of ~1 km), and 4 permanent broadband stations, all of which recorded marine airgun shots.

Our preliminary results show that along the southern section of the transect, an ~11.5-km-thick Caribbean crust underthrusts the frontal ~30 km of the Muertos Thrust Belt where 2-7-km-thick sediments (Vp<4 km/s) have been accreted against a crystalline terrain (Vp~5.5 km/s) forming the southern submarine slope of PR. Beneath Central-Southern PR, velocities of 6.5 kms are not reached until ~22 km depth, and Vp of 7 km/s at 32 km depth, suggesting an island arc crust of normal thickness but abnormally low Vp. Beneath Central-Northern PR high velocities (6.5-7 km/s) are found at 4-10 km depth, similar to global averages of island arc crustal structure. Along the northern section of the transect beneath the PRT, a narrow sedimentary wedge with very low Vp (3 km/s at 5 km depth) overlies a heavily faulted Cretaceous Atlantic lithosphere shallowly subducting (9°) beneath the northern submarine flank of PR, where Vp of 5-7 km/s suggests a crystalline nature.