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The Parguera Fault: Quaternary Reactivation of a Fault in Southwest Puerto Rico

Description: 

Puerto Rico is located in a seismically active oblique plate boundary and has experienced historical moderate to large earthquakes. However, the location and kinematics of active faults on Puerto Rico remain mostly unclear. Here, we present evidence of a previously unrecognized east-west-trending normal fault in southwest Puerto Rico, which we name the Parguera fault. At the northwestern edge of seismicity from the 2019-ongoing southwest Puerto Rico seismic sequence, near the coastal town of La Parguera, we mapped a set of east-west oriented scarps that cross multiple generations of alluvial fans. Our mapping consists of both remote, sub-meter resolution lidar-derived topographic interpretations and field observations. We also apply optically stimulated luminescence (OSL) geochronology and subsurface datasets (ground penetrating radar and aeromagnetic data).

The scarps mapped on the lidar-derived topography are discontinuous, cross elevation contours, and extend at least 12 km laterally, with up to a 2 m, primarily down-to-the-south vertical offset of geomorphic surfaces. In some places, the scarp coincides with a pre-existing unnamed bedrock fault that separates Cretaceous basalts and Parguera limestone or juxtaposes different Cretaceous basalt units. The scarp location is also consistent with discontinuities in subsurface data, supporting the presence of a fault at depth, where aeromagnetic data highlight discontinuities in the underlying basement rocks coincident with the mapped scarps. Based on field observations, the faulted alluvial fans may be correlative with previously mapped Holocene-age fans along the southern flank of the Lajas Valley, just a few km to the north, which we hope to confirm with pending OSL ages. We interpret that the scarps across possible Holocene-age fans, supported by subsurface data, indicate the presence of a previously unrecognized Quaternary-active normal fault that may represent reactivation of an older bedrock fault. Evaluation of this feature will serve to improve our understanding of the seismic hazards and tectonic history of Puerto Rico.

Session: Cryptic Faults: Advances in Characterizing Low Strain Rate and Environmentally Obscured Faults - II

Type: Oral

Date: 5/1/2024

Presentation Time: 11:30 AM (local time)

Presenting Author: Jessica

Student Presenter: No

Invited Presentation: 

Authors