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
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
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
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.