Marine Geophysical Measurements on the Southern Boundary of the Caribbean Sea
M. M. Ball, C. G. A. Harrison, P. R. Supko, W. Bock, N. J. Maloney, 1971. "Marine Geophysical Measurements on the Southern Boundary of the Caribbean Sea", Caribbean Geophysical, Tectonic, and Petrologic Studies, Thomas W. Donnelly
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Marine geophysical measurements in Unare Bay, Venezuela, reveal a system of east-west-striking normal faults. The normal faulting ranges upward in scale from slump faults bounding 1 to 2 km broad terraces on Tortuga-Margarita Bank, through large growth faults in the sedimentary fill of the Cariaco Basin, to the faults bounding the horsts and grabens that control the island and strait topography on the south boundary of the Caribbean. It is believed that the slump faulting can trigger gravity slides that in turn could result in thrust sheets such as those seen onshore adjacent to the marine study area.
There is indication from magnetic measurements of a belt of basic igneous intrusives extending across Unare Bay from north of Cabo Cordera to the straits between Araya and Cubagua. A segment of this belt, west of Araya, is offset to the south. This may possibly be due to north-south strike-slip offset. Gravity observations show a lack of correlation of gravity maxima with the locus of basic intrusives. This is probably due to the intrusive bodies underlying sediment-filled structural lows.
Igneous or metamorphic rocks occur at shallow depths (less than 1 km) under most of Tortuga-Margarita Bank.
The south boundary of the Caribbean is considered to be a type of continental margin. The system of flattened normal faults bounding tilted blocks is a result of crustal extension associated with the origin of the Caribbean Sea. Smaller slump faults and gravity slides are due to the tilting of blocks on the larger normal faults.
The relative lack of seismicity of the Caribbean's south boundary with apparent simultaneous welding of the South American Continent to both the Caribbean and Atlantic can be explained by invoking left-lateral strike-slip motion in the Atlantic floor that is taken up by underthrusting in the Lesser Antilles island arc. Geometric reconstruction of Atlantic spreading reveals a shear requirement that matches in sense and magnitude the left-lateral shear indicated by offsets on the Mid-Atlantic Ridge. From this it follows that ridge offsets are true transcurrent rather than transform faults, and that these faults, coupled with spreading, are the mechanism for opening the Caribbean Sea.