Aspects of Early Stages of Convergence: Barbados Forearc, ODP Leg 110

Elliott Taylor, J. Casey Moore, A. Mascle

ODP Leg 110 achieved a first-ever penetration of the detachment surface (decollement) between two converging plates. The Barbados forearc is the location of an eight-site transect drilled across this accretionary wedge and on the incoming Atlantic plate.

Our results show offscraped sediments form repeated thrust-faulted sequences at the accretionary toe, with little internal deformation other than scaly fabric generally confined to thrust-fault planes. The decollement is characterized by very scaly, early Miocene orange claystone. This decollement zone is about 40 m thick at Site 671. Hemipelagic to pelagic sediments slip relatively undeformed beneath the accretionary wedge. Pore-water chloride and methane geochemical anomalies define two fluid flow systems in this area: (1) methane-enriched fluids derived from deeper within the arc travel along the decollement and through subducted, sand-rich layers, and (2) locally derived methane-poor fluids move through thrust-fault surfaces within the accreted series.

A reference site was cored for comparison of structural, chemical, and physical properties of sediments not yet involved in the convergence process. The occurrence of minor scaly fabric, dewatering and shear veins, and pore-water geochemical anomalies here are clear indications of the propagation of tectonic collision 6 km seaward of the deformation front.

Later stages of accretion are typified by thrust packages of highly deformed and folded sediments. The high degree of deformation on the lower arc is accompanied by little loss of pore space. Major, out-of-sequence thrust faults are sources of landward-dipping seismic reflectors along the lower arc and form preferred dewatering paths for the offscraped sediments.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.