BOLIVAR Project: A New Model for Grenada and Tobago Basin Evolution

The Grenada basin, located in the SE Caribbean, is bounded to the northwest by the Aves Ridge and to the southeast by the Lesser Antilles Arc and Tobago basin. Existing tectonic models for Grenada basin evolution are based on the assumption that the Grenada basin fits into the traditional backarc model, with the Grenada basin formed by rifting of the Lesser Antilles arc away from the Aves Ridge. However our analysis of new seismic reflection and refraction data, acquired during the 2004 BOLIVAR program, suggests that the Grenada and Tobago basins were connected as a single basin during the Paleogene. Uplift of the Lesser Antilles arc and associated platform initiated during early to middle Miocene; the arc formed a barrier to sedimentation between the two basins by the late Miocene. We suggest a new tectonic model for evolution of these basins: 1) Paleogene extension of at least 70 km of the preexisting forearc of the Great Arc of the Caribbean (Aves Ridge) by the mechanisms of slab rollback and flexural subsidence. 2) Flexural and thermal subsidence ceases in the middle Eocene, producing a wide, deep-marine forearc basin encompassing the present-day Grenada and Tobago basins. 3) Oblique plate convergence between the Caribbean and South American plates causes a backthrust response in the weakened and thinned crust of the Grenada/Tobago forearc basin during the late Oligocene to middle Miocene. 4) Magmatism in the Lesser Antilles arc builds a ridge on the inverted forearc that becomes a major sediment barrier between the Grenada and Tobago basins during post-middle Miocene.