KINEMATIC MODEL OF THE NORTHERN PUERTO RICO FAULT ZONE: TRANSPRESSION, STRAIN PARTITIONING, AND LATERAL EXTRUSION

The Northern Puerto Rico Fault Zone (NPRFZ) is one of two major fault zones in Puerto Rico that were active from middle to late Eocene along the Caribbean-North American plate boundary zone. The fault zone is a region of highly deformed rock that extends north of the San Lorenzo Batholith to the northwest towards the north of the Utuado Pluton. Although the fault zone has been mapped, there is no kinematic model that explains the observed finite strain. In this study, I use structural data of published geologic maps to construct a kinematic model for the NPRFZ. The NPRFZ is defined by left-lateral and minor right-lateral strike-slip faults that strike E-W and SE-NW, and by folds with axial planes that strike sub-parallel to the faults. The W- and NW-trending left-lateral faults form zones of anastomosing and intersecting faults that cut through Late Cretaceous to Eocene rocks composed of bedded volcanic and calcareous sedimentary rocks, lava flows, and massive limestone. The faults are about 20-60 km long and may have accommodated at least 15 km of displacement based on fault length-displacement relationships. NW and NE trending conjugate oblique faults cut the strike-slip faults in some locations. Within the western part of the NPRFZ, faults, which previously were described as forming horsts and grabens, may be high-angle reverse faults. To the northeast of the fault zone, rocks contain mostly NW-trending right-lateral faults and fold axes that strike NE-SW and N-S. The orientations of structural data suggest that the region was affected by transpression and that strain was partitioned into pure and simple shear components. The pure shear component is expressed as E- and NW-trending fold axes, NW and NE trending conjugate oblique faults, and NW-trending right-lateral faults in the northeast. The shortening axis that produced these structures trends N and NE. The simple shear component is expressed as major W- and NW-trending left-lateral faults with an E- and NE-directed shortening axis. The right-lateral faults parallel to the strike-slip faults in the NPRFZ and the NW-trending right-lateral faults in the northeast may have accommodated as a result of lateral extrusion. Strain was localized north of the Utuado Pluton and San Lorenzo Batholith possibly due to the lower strength of the volcanic and sedimentary rocks.