Structure and tectonics of the upper Cenozoic Puerto Rico Virgin Islands carbonate platform as determined from seismic reflection studies

The Puerto Rico-Virgin Islands carbonate platform was deposited over an are a of 18,000 km(2) from early Oligocene to Holocene on top of an inactive an d subsiding Cretaceous-earliest Oligocene island are. Regional single-chann el and multichannel seismic reflection lines presented in this study provid e the first information on the regional stratigraphy and structure of this platform that has previously been known mainly from onshore stratigraphic s ections of a relatively small (2250 km(2)) portion of the platform exposed by late Neogene tectonic uplift along the north coast of Puerto Rico. Seism ic reflection lines are used to map the thickness of the carbonate platform strata and to correlate this thickness with onshore outcrop and well data from northern and southern Puerto Rico, St. Croix (U.S. Virgin Islands), an d the Saba Bank. Limestone thickness variations from a little over 2 km to almost zero are used to subdivide the Puerto Rico-Virgin Islands platform i nto five distinct carbonate provinces: (1) north Puerto Rico area including the onshore exposures; (2) Virgin Islands area; (3) St. Croix and Saba Ban k area; (4) south Puerto Rico area; and (5) Mona Passage area. Carbonate th ickness and structural information from each area are used to test five pre viously proposed models for the deformation and vertical movements of the p latform. The most prominent feature of the platform in the Puerto Rico-Virg in Islands area is a large, east-west trending arch. The northern limb of t his arch exhibits a smoother, more uniform dip than the steeper, more abrup tly faulted, southern limb. The cope of the arch is responsible for the exp osure of are basement rocks on Puerto Rico. The origin of this arch, which occurs over a 300 km wide area, is best explained by north-south shortening and arching, caused by interaction at depth of subducted slabs of the Nort h America and Caribbean plates. Other important evidence for this model can be found in the Benioff zones observed in the earthquake profiles. Loading of the Caribbean plate results in downward flexing of the North America pl ate and causes the 4 km subsidence of the carbonate platform north of Puert o Rico.