Fluid expulsion from the Cascadia accretionary prism: evidence from porosity distribution, direct measurements, and GLORIA imagery

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Abstract
Fluid expulsion from the Cascadia accretionary prism off Oregon results from porosity reduction by compaction, and by cementation as methane-rich pore waters precipitate diagenetic carbonate deposits near the sediment-water interface. Porosity changes suggest that dewatering begins 5-6 km west of the base of the slope, in a proto-deformation zone, GLORIA imagery of surficial carbonate deposits confirms that fluid is actively expelled from this zone; there is no such evidence further west in Cascadia Basin. Within the uncertainties of the data, porosities do not decrease landward beneath the prism. This pattern is consistent with imbricate thrust faulting on the slope which provides the vertical load to induce compactive dewatering, and may physically import as much as 50% of the total fluid volume in the section. A simple vertical compaction model suggests that significant pore water volumes have been expelled from the lower slope, but at flux rates (10 -11 -10 -12 m 3 m -2 s -1 ) which are orders of magnitude less than those measured at individual vent sites (10 -6 m 3 m -2 s -1 ). Faulting clearly controls some fluid expulsion, but GLORIA data suggest that repeated local discharge, cementation, and abandonment lead to dispersed accumulations of diagenetic carbonate.