Tectonic evolution of the Tobago Trough forearc basin
Authors: Speed R. , Torrini R. , Smith P. L. 
Published: February 2008

The Tobago Trough is a modern marine forearc basin of the Lesser Antilles arc. It lies above an undrilled, mainly conformable sedimentary succession at least 10 km thick and probable oceanic basement. The strata occur in a forearc basin belt between the crystalline arc platform and the accretionary forearc for hundreds of kilometers north and southwest of the Tobago Trough. Seismic data permit division of Tobago Trough strata into four sequences: A+, AB, BC, and C−. A+ is Quaternary; AB, early Pleistocene to late Miocene; BC, Miocene; and C− includes Miocene probably through middle Eocene and perhaps rocks as old as mid‐Cretaceous or older. Thickness maps of the four sequences indicate major configurational changes in basins that preceded the modern one. A southern C− basin extended southwest to the Carupano Basin north of Venezuela. The northern C− basin extended and perhaps thickened northeast of its current limit of recognition at the crosscutting inner forearc deformation belt (IFDB). The southern C− basin continued unchanged through BC time, whereas the northern BC basin was confined to the region west of the IFDB. The depoaxis of the BC basin lay close to the western basin flank. In contrast, sequences AB and A+ accumulated in an asymmetric basin with maximum subsidence at its eastern margin, at the foot of the IFDB. A stratigraphic model of Eocene and younger layers in the Tobago Trough is derived from seismic attributes, Cenozoic rocks on nearby islands, well data in the Carupano Basin, and piston cores. The model proposes that (1) sequences C− and lower BC are volcanogenic hemipelagite, including ashfall and fine turbidite to the north and, possibly, continent‐derived turbidite to the south; (2) upper BC and AB are mainly proximal basin flank‐derived turbidite to the north, perhaps grading south to mainly continent‐derived hemipelagite; and (3) A+ is mainly continent‐derived hemipelagite and volcanogenic ashfall. Vertical velocity distributions in Tobago Trough strata suggest mainly compaction‐related increases at an undercompacted rate for the upper 3–4 km. Local increases in velocity gradient at these depths could represent defluidization, possibly at layer‐parallel detachments extending from the IFDB. Depositional rates in the Tobago Trough may have increased monotonically through later Cenozoic time; they have always greatly exceeded deep oceanic rates. The northwestern margin of the C− basin was a preexisting irregular edifice that included the arc platform and the St. Lucia Ridge. The flank edifice and deep basin floor were covered by onlap of forearc basin strata in a water‐filled basin and, at the same time, underwent southeast‐down rotation. The C− tilting was without evident local deformation and was perhaps due to intrusion at a currently incompletely defined locus to the northwest. The initiation of the BC depoaxis at the western edge of the northern basin records the Miocene development of a clastic wedge of arc platform provenance, perhaps due to the onset of Neogene volcanism and (or) emergence of the arc platform. The shift of the depoaxis in AB and A+ times to the eastern basin margin reflects loading of the lithosphere by the tectonically thickened IFDB. The Miocene and younger features of the Tobago Trough probably reflect a superposition of tectonism associated with the development of the Neogene Lesser Antilles arc on an older arc system. The Neogene structures crosscut the older ones and are not simply the result of progressive growth of the accretionary forearc.