Fluid Flow within the Barbados Ridge Complex, Part I: Dewatering within the Toe of the Prism

Abstract

Sediment compaction at convergent margins expels pore fluids, which in turn influence many aspects of subduction zone geology. Drilling in the Barbados Ridge complex during ODP Leg 110 and DSDP Leg 78A provided information about sediment types, porosities, and the geometry of the complex. In this paper, we use these observations to estimate the rates of sediment porosity loss and accompanying fluid expulsion from the prism, the decollement, and the underthrust sediments. Rates of porosity loss depend on how rapidly the sediments move arcward through the complex. We compute rates of sediment movement in the prism and decollement as a function of distance from the deformation front. This calculation assumes that the rates of sediment movement decrease arcward as the result of porosity loss in the prism and decollement and thickening of the prism. According to computed rates of sediment movement for the prism, sediment deceleration is greatest within the first 3 to 5 km arcward from the deformation front. Similarly, most dewatering of the prism sediments takes place in this region. Beyond 5 km, rates of sediment dewatering become great est in the underthrust sediments, assuming these sediments are carried downward at a uniform rate with the oceanic plate.

Permeability and fluid flow in natural mudstones

“…Smith 1980;Morrow et al 1981Morrow et al , 1984Knipe 1993Knipe , 1997Dewhurst et al 1996a, b;Clennell et al 1998) as well as conduits for channelized migration of pore fluids (e.g. Moore et al 1988;Moore 1989;Screaton et al 1990;Brown et al 1994;Clennell et al 1998). These observations suggest that faults intermittently transmit fluid.…”

“…However, numerous accounts have been documented in foreland, basinal and accretionary wedge environments which illustrate considerable degrees of fluid migration (tens of kilometres) along fault zones in clay-rich sediments (e.g. Hubbert & Rubey 1959;Moore et al 1988Moore et al , 1995Moore 1989;Fisher & Hounslow 1990;Screaton et al 1990;Logan 1992;Brown et al 1994;Shipley et al 1994;Roberts & Nunn 1995). Therefore, even though theoretical and experimental research suggests that the permeability of a fault zone in clay-rich sediments should be lower than that of the surrounding wall rocks, it appears that extensive fluid flow may occur as a result of dilation and/or fracturing of the fault zone sediments.…”

“…Clennell et al this volume). Fluid flow models of the area (Screaton et al 1990) indicate that the permeability of the d6collement must have been 3-5 orders of magnitude greater than that of the surrounding sediments to account for the observed fluid flux.…”

Compaction in the Nankai and Barbados accretionary prisms: New insights from logging‐while‐drilling data

“…The more rapid fluid expulsion at Costa Rica may be explained by considerably larger compressibility of the subducted high-porosity surface sediments. Screaton et al [1990] suggested that the sediments of the Nankai Trough show greater porosity loss with depth than the sediments of the Barbados Ridge Complex. At 0.5 km depth in the Nankai prism, Bray and Karig [1988] calculated porosities of approximately 0.30: 46% of the surficial porosity.…”

Sediment Permeability at the Nankai Accretionary Prism, Site 808

“…Mechanical modeling (Carson and Berglund, 1986) and hydrological modeling (Shi andWang, 1985,1987;Screaton et al, 1990;Wuthrich et al, 1990) require measured boundary conditions of structural, constitutive, stress, and hydrologic conditions of these active margins to refine continued efforts.To quantitatively evaluate sediment consolidation state, in-situ measurements of both pore pressure and stress are needed; however, most studies of pore pressure and in-situ stress, to date, depend on indirect evidence. The existence of abnormal pore pressures in excess of hydrostatic around accretionary wedges has been inferred both from indirect observations and theoretical modeling (von Huene and Lee, 1983).…”