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Late Pleistocene Kinematics of the Great Southern Puerto Rico Fault Zone, Puerto Rico

Description: 

At the Northern Caribbean Plate Boundary Zone, geodetic data show differential motion between the Puerto Rico-Virgin Islands (PRVI) microplate and neighboring Hispaniola. The precise location of this microplate boundary is unclear over geologic timescales. This differential motion may be accommodated through distributed deformation on multiple faults located throughout southwestern Puerto Rico, with lateral motion and/or shortening predicted by various tectonic models. Recently, several onshore faults in southern Puerto Rico were recognized as Quaternary-active. However, the kinematics of these faults—particularly any lateral component—remain largely unconstrained. This is due in part to extensive erosional and anthropogenic landscape modification, steep relief, and frequent landsliding, limiting the preservation of geomorphic features that could serve as recorders of fault motion. Here we constrain the kinematics of the Great Southern Puerto Rico fault zone (GSPRFZ), an onshore fault considered for inclusion in the upcoming 2025 USGS National Seismic Hazard Model update for Puerto Rico and the U.S. Virgin Islands. Integrating <1-m lidar-derived topography, historical air photos, and field mapping, we identify a series of ~50–1200-m-long fault scarps and lineaments that trend northwest-southeast and extend for ~25 km across the southern coastal plain. Fault scarps are primarily south-facing, cut across topography, and displace Quaternary deposits and landforms. We document multiple offset geomorphic markers, including channel thalwegs and interfluves formed in deposits previously mapped as Quaternary piedmont alluvial plain. We observe both vertical (south-side-down) and right- lateral m-scale displacements, which indicate that the GSPRFZ may accommodate right-lateral oblique motion in the late Pleistocene, consistent with northeast motion of PRVI away from Hispaniola. Additional studies are needed to determine the kinematics of other onshore Puerto Rico faults and better understand how these faults may be accommodating deformation at the PRVI-Hispaniola microplate boundary.

Session: Cryptic Faults: Advances in Characterizing Low Strain Rate and Environmentally Obscured Faults [Poster Session]

Type: Poster

Date: 5/1/2024

Presentation Time: 08:00 AM (local time)

Presenting Author: Emerson

Student Presenter: No

Invited Presentation: 

Authors