(Top image) Shaded relief of the new multibeam bathymetry along the Puerto
Rico Trench illuminated from the northwest. Thin contours indicate
bathymetry at 500-m intervals. (Bottom image) Combined bathymetry map of
the multibeam bathymetry data, single-beam bathymetry compilation around
Puerto Rico, Lidar data near shore, and topography of Puerto Rico.
Contour interval is 500 m. Thick barbed white lines denote thrust faults;
thick white lines denote normal faults; thin white line denotes strike-slip
fault; thin black line denotes northern edge of tilted carbonate platform
and southern edge on land; dashed lines indicate head scarp of slope
failures; dotted line indicates debris toe; blue lines indicate fissures
in the seafloor; A - pull-apart basin; B - location of probable extinct
mud volcano observed on backscatter images. Large box and a grey arrow
show location and viewing direction of the figure shown in the tsunami
section. Click on image for larger view.
To help understand the origin of the unusual bathymetry,
gravity, and vertical tectonics of the plate boundary and to provide
constraints for hazard assessment, the morphology of the entire 770-km-long
trench from the Dominican Republic in the west to Anguilla in the east
was mapped with multibeam echosounder during three cruises in 2002 and 2003.
Parts of the Puerto Rico Trench were previously surveyed with side-scan sonar
[Grindlay et al.,1997] and multibeam echosounder [Dillon et al., 1998; Grindlay
et al., 1997], often at lower resolution and with line orientation and
spacing that did not provide complete bathymetric coverage.
The bathymetry (seafloor topography) data were collected
using the SeaBeam 2112 multibeam system aboard the National Oceanic and
Atmospheric Administration (NOAA) ship Ron Brown, with sufficient swath
overlap and proper line orientation for hydrographic survey.The data were
gridded at 150-m grid size following resolution tests.Vertical resolution
is estimated to be 0.5-1% of the water depth (10-80 m; L.Mayer,
oral communication,2003). Backscatter mosaic images derived from the
multibeam bathymetry data aided in interpretation.The total mapped area is
100,000 km2, slightly smaller than the area of the state of Virginia.
The trench can be divided into two parts at about
65-66W.The western part includes the deepest sector of the trench,
and is associated with the most oblique convergence.This sector is
10-15 km wide and 8300-8340 m deep relative to mean sea level
(figure). It is remarkably flat and is covered by nonreflective pelagic
sediments.Seismic profiles show it to be underlain by rotated blocks of
the NOAM plate that indicate trench subsidence.The unusual depth extends
southward over parts of the forearc. The trench floor narrows to the west
and abruptly shallows to 4700 m as it turns into the Hispaniola Trench,
where convergence is more perpendicular. The eastern part of the trench
is shallower by 700 m and more rugged than the deep western part.The
subducting NOAM plate is observed in seismic lines to be broken into blocks,
but the descending blocks are not rotated.
Detailed seafloor mapping of complete geological provinces,
such as the one reported here, provide critical perspective on their
origin and development and provide base maps for studies in other
disciplines. Here and in related publications, the maps are used to
investigate the causes of the subsidence and deformation of this unusually
deep part of the Atlantic Ocean and to identify earthquake and tsunami
hazards. Earthquake hazard from strike-slip motion in the forearc may be
small, although other potential sources of earthquakes in the region may
exist.Tsunami hazard to the northern coast of Puerto Rico and the Virgin
Islands from submarine slope failures appears to be high.
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