Caribbean Tsunami and Earthquake Hazards Studies

Model

Figure 1: (A) Side view of the uniformly tilted carbonate platform and the narrow shelf north of Puerto Rico. (B) Graphical representation of the method used to calculate the duration of the tilt event. Click on image for larger view.

The Puerto Rico trench exhibits great water depth, extremely low gravity anomaly, and a tilted carbonate platform between (reconstructed) elevations of +1300 m and -4000 m. we suggest that these features are manifestations of large vertical movements of a segment of the Puerto Rico Trench, its forearc, and the island of Puerto Rico that took place 3.3 m.y. ago over a time period as short as 14-40 kyr. These vertical movements are explained by a sudden increase in the slab's descent angle that caused the trench to subside and the island to rise. The increased dip could have been caused by shearing or even by a complete tear of the descending North American slab, although the exact nature of this deformation is unknown. The rapid (14-40 kyr) and uniform tilt along a 250-km-long section of the trench is compatible with scales of mantle flow and plate bending. The proposed shear zone or tear is inferred from seismic, morphological, and gravity observations to start at the trench at 64.5W and trend southwestward toward eastern Puerto Rico. The tensile stresses necessary to deform or tear the slab could have been generated by increased curvature of the trench following a counterclockwise rotation of the upper plate and by the subduction of a large seamount.

Figure 2: Sketches of the proposed model. (a) North-south cross-section at the longitude of Puerto Rico showing the change in slab dip and topography from their initial profile (dashed lines) to their present profile (solid lines). Solid and dotted lines denote present and initial asthenospheric corner flow due to subduction. A flow from the underside of the subducting plate through a gap between the two segments through a gap between the two segments. (b) 3-D view to the NE of the subducting North American oceanic plate and the thicker Bahamas crust beneath the NE corner of the Caribbean plate. The island Puerto Rico (P.R.) and the Virgin Islands (V.I.) are shown schematically. Orange arrows denote tensile stresses due to the curvature of the subducting slab and tensile stress due to the subduction of Main Ridge. Note the steeper dip of the slab under Puerto Rico relative to under the Virgin Islands. The nature of the boundary between the two slab segments is unknown and is represented here as a sharp cut for simplicity. Grey arrow - A possible asthenospheric flow through a gap between the two slab-segments from the underside of the subducting plate into the corner between the subducting and overlying plate. Thin dashed line - The PRT. Black arrow - Relative direction of plate motion. Click on image for larger view.

The model proposed here provides a tectonic framework for the NE Caribbean plate boundary, which will help in the assessment of earthquake and tsunami hazards for Puerto Rico, and the British and U.S. Virgin Islands. Beyond the regional interest, it shows that geological phenomena of the scale observed here can arise from local crustal interactions through coupling between lithosphere and asthenosphere and between horizontal and vertical tectonic forces. Finally, the contrast between the collapsed trench and uplifted island in the Puerto Rico section of the subduction zone, and the adjacent more normal subduction zone of the Virgin Islands, provides constraints on dynamic models of subduction zones. A more detailed study of the history of the collapse of the carbonate platform may help constrain the rheological properties of the slab and its surrounding asthenosphere, and may also provide constraints on rates of reef and platform growth during sea level rise.