EPISODIC AND CONSTANT FLOW MODELS FOR THE ORIGIN OF LOW-CHLORIDE WATERS IN A MODERN ACCRETIONARY COMPLEX

Some low-chloride pore waters observed in accretionary complexes are t hought to result from clay dehydration and subsequent migration of the released water along faults or sand layers. We test this hypothesis w ith a two-dimensional flow and transport model for a cross section of the northern Barbados accretionary complex. The model flow system is d riven by consolidation of the accreted sediments and by fluids from sm ectite clay dehydration. Steady state simulations result in concentrat ions that are too high along the decollement fault and too low near th e seafloor. In a transient model we simulate buildup and release of fl uids by assuming that strain or hydrofracture along the fault causes a n instantaneous increase in decollement permeability of 2-3 orders of magnitude. With such an increase, the observed concentrations can be a chieved in 100-1000 years. Also pressures along the fault rise to near lithostatic values in 10-100 years and remain high for 1000-10,000 ye ars. This pressure rise may represent a mechanism for sustaining high fault permeabilities long after the initial increase.