Caribbean Tsunami and Earthquake Hazards Studies- Models

Science Center Objects

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. 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.

 

Side view of the uniformly tilted carbonate platform and the narrow shelf north of Puerto Rico

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.