THE NORTHERN BOQUERóN BAY- PUNTA MONTALVA FAULT ZONE; A THROUGH GOING ACTIVE FAULT SYSTEM IN SOUTHWESTERN PUERTO RICO

We propose that the North Boquerón Bay-Punta Montalva Fault Zone is an active fault system that cuts across the Lajas Valley in southwestern Puerto Rico. The fault zone has been recognized and mapped based upon detailed analysis of geophysical data, satellite images and field mapping. In this work we demonstrate the fault zone consists of series of Cretaceous bedrock faults that reactivated and deformed Miocene limestone and Quaternary alluvial fan sediments. The fault zone is seismically active (M_L < 5.0) with numerous locally felt earthquakes. Focal mechanism solutions suggest strain partitioning with predominantly east-west left-lateral displacements with small normal faults oriented mostly toward the northeast. Evidence for recent displacement can be found in intermittent streams that cut through the Quaternary alluvial fan deposits along the southern margin of the Lajas Valley, an E-W trending 30-km long fault-controlled depression. This evidence consists of fractures and small normal faults oriented mostly northeast. Areas of preferred erosion, within the alluvial fan, trend toward the west-northwest parallel to the on-land projection of the North Boquerón Bay Fault. Beyond the faulted alluvial fan and southeast of the Lajas Valley the Northern Boquerón Bay Fault aligns with the Punta Montalva Fault. Both faults show strong southward tilting of Miocene strata. On the western end the Northern Boquerón Bay Fault is covered with flat lying Holocene sediments whereas at the southern end the Punta Montalva Fault shows left lateral displacement of stream drainage on the order of a few hundred meters. This suggests that most of the displacement along the North Boquerón Bay Fault Zone and the Punta Montalva fault is older than the Holocene, and that the rate of displacement is low such that the development of fault escarpments all along the fault zone has yet to occur.