Geology and structure of the North Boqueron Bay-Punta Montalva Fault System

The North Boqueron 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. The fault zone consists of a series of Cretaceous bedrock faults that reactivated and deformed Miocene limestone and Quaternary alluvial fan sediments. The fault zone is seismically active (ML < 5.0) with numerous locally felt earthquakes. Focal mechanism solutions and structural field data suggest strain partitioning with predominantly east-west left-lateral displacements with small normal faults oriented mostly toward the northeast. Evidence for recent displacement consists of fractures and small normal faults oriented mostly northeast found in intermittent streams that cut through the Quaternary alluvial fan deposits along the southern margin of the Lajas Valley, Areas of preferred erosion, within the alluvial fan, trend toward the west-northwest parallel to the on-land projection of the North Boqueron Bay Fault. Beyond the faulted alluvial fan and southeast of the Lajas Valley, the Northern Boqueron Bay Fault joins with the Punta Montalva Fault. The Punta Montalva Fault is defined by a strong topographic WNW lineament along which stream channels are displaced left laterally 200 meters and Miocene strata are steeply tilted to the south. Along the western end of the fault zone in northern Boqueron Bay, the older strata are only tilted 3° south and are covered by flat lying Holocene sediments. Focal mechanisms solutions along the western end suggest NW-SE shortening, which is inconsistent with left lateral strain partitioning along the fault zone. The limited deformation of older strata and inconsistent strain partitioning may be explained by a westerly propagation of the fault system from the southwest end. The limited geomorphic structural expression along the North Boqueron Bay Fault segment could also be because most of the displacement along the fault zone is older than the Holocene and that the rate of displacement is low, such that the development of fault escarpments and deformation all along the fault zone has yet to occur.