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Abstract

The Virgin Islands basin is a 4.5-km-deep passage that connects the Atlantic and Caribbean seas. A variety of models have been proposed to explain its tectonic origin, which range from right- and left-lateral pull-apart basins to a rotational-type basin or even a simple, orthogonal rift basin. This study integrates three data types to better understand the Miocene to recent kinematics of basin opening and its present-day tectonics known from a parallel zone of earthquakes and GPS results that span the basin from Puerto Rico to St. Croix (U.S. Virgin Islands). A grid of 400 km of 2D seismic lines provided courtesy of the Danish Galathea 3 expedition reveals the geometry of faults underlying the offshore basin to a depth of 7.5 seconds two-way time.

The basin is asymmetrical having more throw along the southeastern normal fault than the normal fault along its northwestern edge. The island of St. Croix, is the uplifted footwall of the southeastern normal fault while the island of Vieques, Puerto Rico, is the uplifted footwall of the northwestern fault. Seismic data show that both bounding normal faults are listric and have associated rollover anticlines in the basin center. A linear, possibly strike-slip fault system can be traced for a distance of 4.7 km in the center of the basin. Fourteen normal fault planes have been measured in Miocene and younger rocks on the footwall blocks of Vieques and St. Croix and revealed dip-slip normal faults having fault planes oriented northeast to east-northeast and parallel to the long axis of the offshore basin.

A GPS baseline between continuously recording sites in southwest Puerto Rico and St. Croix reveals that that the basin is presently opening in a direction of 100° - or roughly at right angles to its long axis - at a rate of 2.5 mm/yr. We conclude based on seismic data, striated fault planes, and GPS results that the present-day opening of the basin and perhaps its early evolution is the result of simple, orthogonal rifting in a northwest-southeast or west-northwest/east-southeast direction.

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