BOLIVAR & GEODINOS: Investigations of the Southern Caribbean Plate Boundary

The southern Caribbean-South American plate boundary has many similarities to California's San Andreas system: 1) The CAR-SA system consists of a series of strands of active right lateral strike-slip faults extending >1000 km from the Antilles subduction zone. This system has several names and includes the El Pilar, Coche, San Sebastian, Moron, and Oca faults. 2) The CAR-SA relative velocity has been about 20 mm/yr of mostly right lateral motion since about 55 Ma, giving a total displacement on the CAR-SA plate boundary similar to that of the San Andreas system. 3) The plate boundary has about 10% convergence in western SA, with less as one moves eastward due to relative convergence between North and South America. 4) The CAR-SA system has fold and thrust belts best developed continentward of the strike-slip faults, similar to the San Andreas. 5) There is a big bend in the CAR plate boundary at approximately the same distance from the Antilles trench as the big bend in Southern California is from the Cascadia subduction zone. The tectonic origins of the CAR-SA plate boundary and the San Andreas are very different, however, despite the similarities between the systems. Rather than impingement of a ridge on a trench, the CAR-SA system is thought to have resulted from a continuous oblique collision of the southern end of a Cretaceous island arc system with the northern edge of South America. During this process the CAR island arc and the modern CAR plate overrode a proto-Caribbean plate and destroyed a Mesozoic passive margin on the northern edge of SA. BOLIVAR and GEODINOS are multi-disciplinary investigations of the lithosphere and deeper structures associated with the diffuse CAR-SA plate boundary zone. We review a number of observations regarding the plate boundary obtained or confirmed from these studies: 1) The Caribbean Large Igneous Province, being overridden by the Maracaibo block in western Venezuela, can be identified beneath Aruba and coastal Venezuela, and is associated with broad uplift of the coastal regions. This is likely a site of continental growth. 2) The accretionary wedge terranes of the Southern Caribbean Deformed Belt formed in the Neogene, and extend as far east as the Aves Ridge. They result from SA overriding the CAR LIP, which for a number of reasons, we do not regard as normal subduction. 3) Igneous rocks on the islands of the Leeward Antilles arc, Aruba to Los Testigos, show a steady decrease in age from west to east (94.7-37.4 Ma), suggesting that the islands have been progressively captured from the Antilles arc by the plate boundary during the prolonged island arc-continent collision. Terrane capture models thus far cannot completely explain the data. 4) High (> 6.5 km/s) P-velocity bodies are found in the shallow crust along the main strike-slip faults along much of the plate boundary. We interpret these as elements of the HP/LT metamorphic terranes found in the adjacent thrust belts of central Venezuela. This suggests to us that displacement partitioning in the trench and subsequent strike-slip both play important roles in exhumation of the HP/LT terranes. 5) Crustal thickness variations in the plate boundary region are large (> 10 km), of short spatial wavelength (< 100 km), and indicate that the highest elevations of the coastal mountain belts are not supported isostatically.