Extensive underthrusting of undeformed sediment beneath the accretionary complex of the Lesser Antilles subduction zone

Abstract

Along the eastern margin of the Caribbean Plate, which overrides the Atlantic oceanic lithosphere at ∼20 km Myr−1 in an easterly direction1, east of the Lesser Antilles island arc, lies a wide accretionary complex (Barbados Ridge Complex); 260-km wide at the latitude of Barbados and >20-km thick beneath Barbados3,4, it increases in width and thickness from north to south2 (Fig. 1). The style of deformation varies considerably along the eastern margin of the complex2,3,5–11, and is related to the thickness and type of sediment on the Atlantic Ocean floor. A common feature is that a proportion of undeformed sediment from the ocean floor is brought beneath the deformed sediments of the accretionary complex, and in some areas a clearly defined seismic reflector separates the deformed and undeformed sediments, which has been interpreted as a decollement surface5,12,13. A seismic section from an investigation of the accretionary complex, conducted from RSS Discovery in 1980 using 12-channel seismic reflection and long range side-scan sonar (GLORIA), shows undeformed sedimentary rocks carried horizontally 74 km beneath deformed rocks forming the accretionary wedge. We report here that the low shear stresses required on the decollement between the deformed and undeformed rocks are made possible by porewater at pressures close to that of the lithostatic load.