3-D GRAVITY ANALYSIS OF THE N.E. CARIBBEAN AND THE DEVELOPMENT OF THE PUERTO RICO TRENCH

A 500-km long section of the carbonate platform north of Puerto Rico and the Virgin Islands collapsed simultaneously sometime after 3.4 Ma to a maximum depth of 4.5 km. This sudden subsidence, which may be associated with the formation of the Puerto Rico Trench, is puzzling given that the direction (ENE) and the rate (~20 mm/y) of North American (NOAM)-Caribbean plate motion in this area has remained constant during the past 45 Ma. The collapse has been attributed to subduction erosion, to a tear in the downgoing NOAM plate in the area of maximum curvature, and to an interaction at depth between the flaps of the Caribbean and NOAM plates. We modeled the gravity field in the NE Caribbean in 3-D with the simplifying assumption that the sources of the anomaly are only due to the water-crust and crust-mantle interfaces. Consistent results were obtained by modeling in the space domain (CordellÕs method) and in the wave number domain (ParkerÕs method). A 2-D model along a 350-km-long seismic profile across this plate boundary was also compared with the 3-D model results. The gravity model indicates that a 25±5 km thick crust extends from Puerto Rico northward under the collapsed area to just south of the Puerto Rico Trench. The thick crust is not an artifact of excluding low-density sedimentary rocks from the model, because there is no evidence in seismic reflection data for an appreciable accretionary prism south of the trench. Moreover, dredging has recovered arc-related metamorphic rocks and limestone, similar to those found in Puerto Rico, at a depth of 7100 m south of the trench. The gravity model suggests that the entire crust north of Puerto Rico has been tilted northward. A whole-crustal tilt requires space to be created by a sudden removal of the foundation. One possibility is that the NOAM plate flap, which underlies Puerto Rico and the Virgin Islands has suddenly increased its dip or rolled back. Either of these options will pull down the overlying crust if the interface between them has high friction, which allows shear stresses to build up. The existence of a large negative gravity anomaly (-355 mGal), more negative than in typical trenches is consistent with this interpretation. Large magnitude earthquakes may occur at the interface between the Caribbean and the NOAM plate if the interface can support high shear stresses.