Carbonate platforms along the southeast Bahamas-Hispaniola collision zone

Abstract

The southeast Bahama banks have been colliding with the island of Hispaniola along a restraining bend within the northern Caribbean plate boundary zone since the late Miocene. A SeaMARC II/seismic reflection investigation was conducted along the deep-water flanks of Mouchoir Bank, and Navidad Bank to evaluate the response of these carbonate platforms to active plate tectonic collision. Overall, our data provide evidence of large-scale platform margin retreat which contrasts sharply with the prograding platform margins in the tectonically passive northwest Bahamas. North of Silver Bank there is a large (∼5,000 km 2), deep-water (3,000–4,000 m) plateau that records the drowning and step-back (∼50 km) of an Early Cretaceous carbonate platform margin. Intact (uneroded) segments of this Cretaceou platform margin may occur along the seaward edge of the deep-water plateau (Bahama Escarpment) separated by large (20 km across) erosional reentrants. A coalescing system of broad (up to 10 km), shallow (<200 m relief) canyons, akin to mountainous avalanche chutes, funnel debris into the reentrants from the flank of Silver Bank. The reentrants are interpreted as zones of structural weakness produced by passage of the Atlantic spreading center along a transform plate boundary during the Late Jurassic-Early Cretaceous. Drowning and step-back appear to be part of a global response of carbonate platforms to a mid-Cretaceous tectonic pulse of ocean crust formation, and relative sea-level rise. The southern margin of Mouchoir Bank has also been drowned and has stepped-back by about 30 km. This event occurred during the Late Tertiary prior to the mid-Pliocene and is attributed to tectonic tilting and subsidence resulting from oblique underthrusting of the southeast Bahamas beneath Hispaniola. The southern margin of Mouchoir Bank is also characterized by large (∼20 km across) amphitheater-shaped “scallops” that expose Tertiary shallow-water limestones along their deep-water flanks. Seaward of one scallop is a large (25 km wide, 50 km long) mass movement with flow lines preserved on the seafloor. We interpret these scallops as catastrophic collapse structures triggered by large earthquakes generated along the collision zone since the Late Tertiary. South of Navidad and Silver banks is a very complex seafloor morphology resulting in part from the accretion of a portion of the Bahamas onto Hispaniola. There is also a series of E-W oriented down-to-basin normal faults that have caused the platform margin to retreat. Extensional stress along this predominantly transpressive plate boundary may be a consequence of rotation of the nearby Puerto Rico block or lithospheric bending during oblique subduction. This study provides an initial framework within which to view the actualistic response of carbonate platforms to plate tectonic collision.