Neotectonics of Puerto Rico and the Virgin Islands, northeastern Caribbean, from GPS geodesy

The boundary between the North American and Caribbean plates is characteriz ed primarily by left-lateral motion along predominantly east-west striking faults. Seismicity and marine geophysical survey data are consistent with a t least two, and possibly three, microplates in the diffuse boundary zone i n the northeastern Caribbean: (1) the Gonave, (2) the Hispaniola, and (3) t he Puerto Rico-northern Virgin Islands (PRVI). We discuss results from GPS geodetic measurements acquired since 1994 to test the microplate hypothesis , define PRVI translation and rotation within the boundary zone, and constr ain PRVI neotectonics. GPS-derived velocities are analyzed with respect to both North American and Caribbean plate reference frames. Integrated displa cements across PRVI are limited to a few millimeters per year, consistent w ith a rigid PRVI and permitting calculation of an average velocity for PRVI . The motions of PRVI relative to North America and the Caribbean are 16.9/-1.1 mm/yr toward N68 degreesE+/-3 degrees (1 sigma) and 2.4+/-1.4 mm/yr t oward S79 degreesW+/-26 degrees (1 sigma), respectively. In contrast with s ome recent models, ongoing rotation of PRVI about a nearby (< 25<degrees> d istant) vertical axis is not supported by the geodetic data. In addition, w e argue against eastward tectonic escape of PRVI and favor a simple, progre ssive increase in velocity across the plate boundary zone, requiring that t he summed magnitude of strike-slip fault slip rates will equal the total pl ate motion rate between the Caribbean and North America. GPS data are consi stent with components of left-lateral strike-slip faulting along the Muerto s trough south of Puerto Rico and shortening across the Puerto Rico trench. Comparison of GPS velocities for PRVI with respect to North America with t otal North America-Caribbean relative motion suggests up to 85% of North Am erican-Caribbean plate motion is accommodated by the Puerto Rico trench and offshore faults north of Puerto Rico. Differences in GPS-derived velocitie s from Hispaniola and PRVI yield east-west extension across the N-S trendin g Mona rift of a few millimeters per year when estimated elastic strain acc umulation effects along the north Hispaniola deformed belt and the Septentr ional fault zone are considered. The opening rate implies an age of the Mon a rift of 2-3 million years, agreeing with marine geophysical data that sup port a young age for the structure.