The Parguera Fault System: Possible Quaternary Reactivation of Bedrock Faults in Southwest Puerto Rico

We present evidence of a previously unrecognized east‐west‐trending normal fault, named the Parguera Fault, in southwest Puerto Rico. The fault is located at the northwestern edge of seismicity from the southwest Puerto Rico seismic sequence that began in 2019, near the coastal town of La Parguera. We integrated geomorphic and neotectonic mapping from lidar‐derived topography, field observations, and luminescence geochronology of natural exposures of the fault with observations from subsurface data sets (ground penetrating radar (GPR), seismic reflection, and aeromagnetic data) to better characterize the fault at depth. We mapped a set of east‐west oriented scarps that cross multiple generations of alluvial fans. 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 correlative 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 coincident with weak discontinuities in seismic reflection and GPR data and magnetic lineaments in the aeromagnetic data, supporting the presence of a fault at depth. Luminescence samples from alluvial fan deposits date to ∼32–7 ka. We interpret that the scarps crossing late Pleistocene and Holocene fans indicate potential Quaternary reactivation of a previously unrecognized normal or normal‐oblique fault. Deformation may be partitioned between the Punta Montalva and Parguera Faults in a distributed transtensional zone in southwest Puerto Rico.