Background

The Puerto Rico trench with water depths exceeding 8400 meters, is the
deepest place in the Atlantic Ocean. Its depth is comparable to the deep
trenches in the Pacific Ocean. Trenches in the Pacific are located in
places where one tectonic plate subducts or slides under another one.
The Puerto Rico trench, in contrast, is located at a boundary between
two plates that slide past each other with only a small component of
subduction. The trench is less deep, where the component of subduction
is larger. The unusually deep seafloor is not limited to the trench, but
it also extends farther south toward Puerto Rico. The Puerto Rico trench
is also associated with the most negative gravity anomaly on earth, -380
milliGal, which indicates the presence of an active downward force.
Finally, a carbonate platform, which was originally deposited in flat
layers near sea level is now tilted northward at a uniform angle. Its
northward edge is at a depth of 4500 meters, and its southern edge can
be found on land in Puerto Rico at an elevation of a few hundreds
meters. Many tectonic models have been proposed to explain these unusual
observations, and marine exploration efforts of the type reported here
are necessary to discriminate between them.

Many earthquakes and tsunamis resulting from these plate tectonic
movements have occurred in historical time in the northeastern
Caribbean. As population in this region continues to grow, future such
events will pose serious hazard to the 4 million U.S. citizens of Puerto
Rico and the Virgin Islands. The hazards to these islands are mainly in
the form of submarine faults. Our expedition has discovered major new
faults and submarine slides, often in areas, where we did not expect to
find them based on present tectonic models.

Discoveries:

View to the east of a newly active discovered east-west fault system was
on the Caribbean plate close to the trench. Vertical exaggeration is
15:1. The fault is at least 200 km long and traverses the entire mapped
area. The fault must have been recently active as can be seen from the
fresh scarps that cut through hills. A pull-apart basin, 8350 meters
deep developed where the fault system steps over to the northeast (See
interpretation map). The fault likely accommodates the east-west
relative motion between the plates, whereas the dipping frontal fault in
the trench accommodates the orthogonal dip slip component of the motion
between the plates. Partitioning of the oblique convergence between the
plates to orthogonal dip slip and a parallel (or strike-) slip motion is
typical of most subduction zones and serves to reduce the amount of
force required to slide the plates along their inclined interface. It
is, therefore highly unusual to find the strike-slip fault system so
close to the trench, which might indicate that the friction between the
North American and Caribbean plate is very low. This, in turn may
suggest that the subduction zone is not capable of generating very large
dip-slip earthquakes.

View to the west of two landslides on the North American plate. Vertical
exaggeration is 15:1. The slide in the front, which is 15 km wide is
characterized by a "ski-jump" profile. The slide in the back is 25 km
across. The bottom of the "ski jump" is flat and is highly
non-reflective suggesting that it filled with soft pelagic sediments.
Because the slide in the back had time to be filled with sediments, it
is likely older than the frontal slide. Landslides have been previously
mapped on the southern slope of the Puerto Rico trench on the Caribbean
plate in relation to the tilt and collapse of the carbonate platform.
However, this is the first time that landslides were discovered on the
descending North American plate north of the trench. The presence of
slides on this plate suggests that not only did the Caribbean plate
collapse, but so did also the North American plate near the trench,
suggesting that a single process may be responsible for the collapse of
the entire plate boundary.

View to the east of the topography (left) and a backscatter image
(right) of an unusual landslide. This landslide produced a debris field
about 10 km long with diverging streaks of high reflectivity (white). We
do not know what produced these streaks. This landslide is also unusual
in that it is located at 7800 meter depth and its source appears to be a
small hill. The left parts of the hill and the landslide are missing and
appear to have been cut off by the newly discovered strike slip fault
(see A above) and by the 8350 meter deep pull-apart basin that it
produced. Had the basin been there before the slide, the debris would
have filled the basin instead of flowing to the east. The fault may have
also shifted the main part of the hill that served as a source for this
slide.

The Mona rift system shown in this map is very different from previous
interpretations based on crude maps of the area. The rift system
accommodates a small amount of extension between the Dominican Republic
and Puerto Rico, which are both on the Caribbean plate. The extension
formed extremely deep and narrow (7-8 km wide) basins with a flat floor.
It was previously thought that a single basin on the left side of the
map extended all the way to the trench. We discovered, however, that
this basin is only 23 km long and another basin to the northeast is part
of the rift system. This yet unnamed basin is 8200 meters deep. There
are other parallel normal faults that are not associated with the deep
basin and may accommodate part of the extension. Equally surprising is
the fact that the newly-discovered strike slip fault near the trench
continues westward across the rift system, whereas previously it was
thought to be a barrier to east-west strike-slip faults.

View to the ?? of possible strike-slip fault system farther south of the
trench. At least one of these faults, named the 19degree North fault was
suggested to accommodate the east-west motion between the North American
and the Caribbean plates. Its location at the base of the slope and
close to Puerto Rico was a cause of concern as a source for ground
shaking and a trigger for tsunami causing slides. However, this fault,
if it exists, and other possible ones appear to be inactive based on the
bathymetry and back scatter image although they appear to cause some of
the topographic lineaments, and perhaps control the northern and
southern extents of the Mona rift basins.

We thank Captain Dreves and the crew of the NOAA ship Ronald H. Brown
for their professional and dedicated support of the expedition. This
expedition was funded by NOAA Ocean Exploration Office. The scientific
crew included Dr. Uri ten Brink (Chief Scientist, USGS- Woods Hole Field
Office), Lt. Shepard Smith, NOAA (University of New Hampshire), Charles
Worley (USGS-Woods Hole), and Tom Stepka (NOAA/OMAO).