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Puerto Rico Cruise OverviewFrom Paul Mann:
17 June 1996
Everything is going well. We have been out since last Saturday morning We are having our share of MR1 towfish problems but hopefully this will settle down. SCS is good.
18 June 1996
Things are going reasonably well. One thing we are still waiting on is clearance to enter the waters of the Dominican Republic. At the moment this is not critical as we have a large area to do in Puerto Rico and the British Virgin Islands for which the clearance has gone through.
19 June 1996
We have now surveyed 4 complete NNE lines from the Puerto Rican shelf on the Caribbean plate to the crest of the outer bulge of the downgoing North America plate. Our survey started almost immediately after leaving San Juan harbor. The main features of the 60 X 2175 km long box include:
1. The Mio-Pliocene carbonate cap of Puerto Rico dipping seaward and drowned in late Pliocene time when the Puerto Rico trench to the north formed. This 1 km thick section crops out along the northern flank of the island in a karst terrane. The MR1 reveals an intricate system of canyons some perhaps formed as sapping features from fresh waters percolating through the limestone from the high rainfall/runoff area in Puerto Rico. The sharp flexure of the platform between the shelf and slope is controlled by a vertical fault.
2. A major scarp that truncates the carbonate section and is the site of major landsliding and slumping down into the deep part of the Puerto Rico trench. One goal is to see if this feature is fault controlled and is the along strike extension of the plate boundary strike-slip fault in northern Dominican Republic on which a large magnitude groundbreaking earthquake occurred 750 years ago. If the PR margin fault is the same main plate boundary fault it may pose a seismic hazard to the heavily populated north coast of Puerto Rico.
3. The Main Ridge and adjacent basins. These sit at depths of about 5 to 8 km at the base of the scarp. The Main Ridge, named by Ewing in the early 60s, trends obliquely to the margin, an observation that led McCann and Sykes to propose it represents an upward bulge above an obliquely subducting fracture zone ridge that is subparallel. Because of MR1 problems we have not gotten a good look at this; seismic does not show any layering so its possibly metamorphosed arc basement dredged more to the west and cropping out in the Dom. Rep. or blocks of accreted Atlantic crust. The 1985 UTIG MA thesis by Treadgold in the NAT area suggest a slight misalignment betweenthe Main Ridge and the fracture zone with which has been correlated.
4. A scarp separating the Main Ridge from the deep part of the Puerto Rico trench. Gloria data revealed lineaments that were interpreted as strike-slip faults. This can be tested with higher resolution images from MR1 including their eastern extension in waters of the BVI and Dom. Rep. that were not part of the Gloria EEZ survey.
5. The abyssal part of the Puerto Rico trench. The traditional interpretation of this 8.5 km deep hole and site othe worlds largest negative freeair anomaly is that the Atlantic slab defined by earthquakes beneath Puerto Rico is subducted along this trench. Our crossings like those reported by Ewing in 1965 show that the turbidite trench fill is undeformed. One possibility is that this "Marianas style" trench along which Cret. age ocean is subducting is not strongly enough coupled to deform a recent fill. The other possibility is that other faults (eg, scarps between here and the Main Ridge) are now taking up the plate motion.
6. The outer slope of the Puerto Rico trench and forebulge. Large displacement normal faults bring the oceaniccrust down sharply as seen in other trenches of this type.
Our plan is to work on the Main Ridge problem, work out the active fault pattern moving to the east into UK waters of the BVI and then back west to the Dom. Rep. where the clearance just came though today.
The MR1 is back on line after some problems related to a frayed cable. The UT grad students (Steve Muszala, Jean-Paul van Gestel are doing well) along with 3 other undergrads from other universities. Jean-Paul is processing the seismic collected so far using Sioseis.
28 June 1996
We are now about 2/3 of the way through the cruise. Our MR1 towfish problems have been repaired so we are getting some good sidescan images and bathymetry of even the deepest water areas (8+ km). The towfish problems included a bad transmitter, a noise problem linked to the installation of an unshielded cable after the last MR1 cruise, and time varying gain problems traced to the signal preamplifier. Because all of this translated into lost time and data quality, we have petitioned Dave Epp at NSF to give us three extra science days. He is sympathetic and considering this request now. With the extra days we could cover about 90% of our originally proposed study area. We are also negotiating with Hawaii to have them purchase the Hydrosweep bathymetric data for us for the entire survey area. We did not budget Hydrosweep in our original proposal that was intended to rely only on the MR1 bathymetry. However, in these circumstances, the Hydrosweep data could be merged with the MR1 data to provide high quality bathymetric coverage as well as lower resolution "sidescan" data that could act as a base to fill in gaps in the MR1 sidescan coverage.
Scientifically, we have determined the following:
1) The Puerto Rico trench with a little deformed turbidite trench fill does appear to be the trace of the Puerto Rico Benioff zone as the top of the downgoing Atlantic plate can be traced on single channel data. Tilting of features in the deepwater area north of the Puerto Rico shelf (Main Ridge, Median Ridge, Marginal basin of Ewing et al.) is consistent with southward subduction of Atlantic ocean crust at the trench.
2) A zone of active strike-slip faults lies just landward of the trench. These faults which must sole into the Benioff zone dipping beneath them apparently accommodate part of the North America-Caribbean plate motion by the process of "strain partitioning".
3) The Oligocene-early Pliocene carbonate platform best described from onland exposures, well and seismic reflection data from northern Puerto Rico extends as a continuous feature far to the east as seem as our easternmost seismic line in the Virgin Passage separating St. Thomas (USVI) from the island of Culebra off the east coast of Puerto Rico. This regionally extensive platform underwent a drowning and northward tilting event after about 3 my ago that previous workers have linked to an episode of tectonic erosion related to subduction at the Puerto Rico trench. Our data which will provide the best map of the distribution, attitude of this datum plane should be able to better evaluate the tectonic erosion hypothesis.
The remaining week of the survey in the western area we are in now will: 1) continue mapping out the drowned carbonate platform in the area of the Dominican Republic and Mona Passage; 2) attempt to map out the main strike-slip fault (Septentrional FZ) landward of the zone of strike-slips faults in the trench area 3) attempt to map a zone of normal faults between Puerto Rico and eastern Dominican Republic. GPS data suggests that Puerto Rico is moving eastward about 1 cm faster than the Dominican Republic relative to North America. The discrepancy is apparently being accommodated by this zone of diffuse extension.
Everyone is doing well. The UT and Univ. of Puerto Rico students have finished a round of presentations on various aspects of the geology and geophysical tools we are using and now are helping with the cruise report.
Mon, 8 Jul 96 02:22:34 GMT
We are now in a holding pattern at the entrance of San Juan harbor which we should get into about 11 pm. Because of the approaching hurricane, we had to finish up the survey about 18 hours earlier than expected. The ETA of the hurricane is about 1 pm on Monday in San Juan. The Ewing has been told to stay in a state of readiness and leave if the Coast Guard declares a "Condition II" and clears the harbor of all vessels. In this event, the Ewing will probably go around south of the island and wait for it to pass. Most of us have flights scheduled out about the time the hurricane it to arrive tomorrow.
Despite the last minute hurricane problems, the last week of the cruise was extremely productive. TThe MR1 worked well and the SCS system continued to perform well as it has done throughout the cruise. We made several solid discoveries:
1) We mapped the Septentrional strike-slip fault as a continuous feature along the north coast of Puerto Rico. Bending of the fault off the northwest coast of Puerto Rico appears responsible for the "Mona Block", a bathymetric high known tto consist of blueschist metamorphics through dredging and Alvin dives by Heezen et al. in the 1970s.
2) A parallel zone of strike-slip faults first mapped by Masson and Scanlon using Gloria along the south wall of the Puerto Rico trench may interact in a left-stepping manner with the Septentrional fault zone and contributed to the formation of the large bathymetric depression of the Puerto Rico trench. The fact that the edge of Puerto Rico carbonate margin drowned to depths of 5 km mimics the strike changes in the Septentrional fault zone indicates that transtension along it may be the primary mechanism for the regional subsidence in the trench and along the north coast of Puerto Rico.
2) Our Mona Passage survey revealed an active, 150 km wide zone of east-west extension that has extended and drowned the Miocene-early Pliocene carbonate platform. Previous workers had proposed that extension was limited to the Mona Canyon area off the west coast of Puerto Rico but these new data extend the zone to the area off the SE coast of the Dominican Republic ("Yuma basin"). While there have been plentiful seismic reflection lines collected mostly on transits through the Mona Passage, ours were the first to shoot lines in EW direction suitable for imaging the NS oriented normal faults. Diffuse opening across this extensional province explains why Puerto Rico can move faster at a rate of 2 cm/yr relative to North America than Hispaniola to the west which has become pinned against the SE extension of the Bahama platform.
If the flights go as planned, I should be back Monday night.
Cheers, Paul.
