Expulsion of fluids from depth along a subduction-zone decollement horizon

Abstract

Results from Deep Sea Drilling Project Leg 78A at the Barbados Ridge suggested that high fluid pressures are found along the decollement zone separating the underthrusting Atlantic Ocean crust and the overthrusting Caribbean plate, but drilling failed to penetrate this major fault surface¹. On Leg 110 of the Ocean Drilling Program we cored through this fault surface, achieving the first penetration of a decollement separating two plates in a subduction zone. Here we report pore-water chemistry, temperature anomalies and structural observations which indicate that the deforming sediments de-water through fracture permeability associated with faults, and that the accretionary wedge and under-thrust sediments comprise distinct fluid domains separated by a permeability barrier above the decollement zone. Methane-bearing fluids derived from a probable thermogenic source travel ≥25 km beneath the shallowly inclined decollement.