Stress-induced smectite dehydration: ramifications for patterns of freshening and fluid expulsion in the N-Barbados accretionary wedge

Presented is new experimental evidence that smectites can partially dehydrate from 18.5 Angstrom to 15.4 Angstrom hydrates when they are subjected to effective mineral framework stresses above 1.3 +/- 0.3 MPa. We show here that this process can substantially freshen saline pore fluids and impact our understanding of the hydrogeologic processes in convergent margins where smectites are abundant. The general anomalous freshening of pore fluids in the N. Barbados accretionary wedge was previously proposed to relate to the widespread lateral expulsion of fluids derived from mineral dehydration reactions at great depth. In contrast, our analysis suggests that a significant part of the widespread anomalous freshening relates to stress-induced smectite dehydration that occurred during the Ocean Drilling Program (ODP) pore water extraction process. Clear evidence for long-distance lateral flow appears to be limited to localized chloride and isotopic anomalies developed in a 20 m thick, locally mineralized region at the top of decollement horizon at Site 948. Our reinterpretation is consistent with the fluid flow in the decollement zone being (1) highly transient on time scales of several hundreds of years, and (2) channeled laterally into heterogeneous finger-like flow paths that are apparently not intersected, and/or sampled by drilling in most regions at the toe of the wedge. The incipient stress-induced dehydration of smectite may also be buffering in situ fluid pressures in the wedge, maintaining the effective stresses at similar to 1.3 MPa. (C) 1999 Elsevier Science B.V. AU rights reserved.