TOBAGO, WEST INDIES, AS A GUIDE FOR THE MESOZOIC EVOLUTION OF THE GREAT ARC OF THE CARIBBEAN

Tobago, West Indies, exposes a natural, oblique cross-section across a composite Mesozoic ocean-arc terrane. Two distinct suites of oceanic-arc rocks form the hanging wall and footwall, respectively, of the normal-sense, plastic-to-brittle Central Tobago fault system. The hanging-wall block consists of an Albian plutonic-volcanic-dike suite that is chiefly nonmetamorphosed to prehnite-pumpellyite facies. The footwall block consists of the lower greenschist-facies North Coast Schist, which is locally metamorphosed to amphibolite-facies rocks in a normal-sense, dynamothermal aureole (i.e., high-temperature shear zone) associated with the foundering of Albian ultramafic-mafic cumulates during crustal extension late in the emplacement history of the plutonic rocks. The early history of the North Coast Schist indicates elongation in the X strain axis during low-grade regional metamorphism. These relationships are interpreted to indicate an early history of arc-parallel shear strain in an oceanic-arc setting during oblique, easterly dipping subduction followed by a reversal in subduction-zone polarity (i.e., westerly dipping) prior to the Albian (i.e., pre-105 Ma). The low-grade metamorphism in the North Coast Schist thus pre-dated the polarity-reversal event, whereas the Albian oceanic-arc magmatism manifested by the Tobago Volcanic Group, intrusive plutonic rocks, and mafic dike swarm post-dated the reversal event. This geologic history provides an instructive guide for interpreting the Mesozoic evolution of the Great Arc of the Caribbean. Thus the early phase of the Great Arc involved oblique, west-facing subduction followed by a subduction-zone polarity reversal event in the interval 120-105 Ma. Eastward migration of this newly formed, east-facing subduction system was accompanied by crustal extension in the forearc and tectonic erosion (and high P/low T metamorphism) of older arc rocks such as the Villa de Cura Group (Venezuela). The reason for the subduction polarity reversal is controversial, but timing relationships as manifested on Tobago strongly suggest that the collision of an early west-facing subduction system with the Late Cretaceous Colombian-Caribbean plateau province (<90 Ma) is not a viable mechanism to cause the subduction-zone polarity reversal.