Permeabilities, fluid pressures, and flow rates in the Barbados Ridge Complex

Abstract

Recent measurements from Ocean Drilling Program leg 110 and Deep Sea Drilling Project leg 78a indicate that pore pressures near the toe of the Barbados accretionary prism may be dose to lithostatic and that the d6collement is a zone with relatively high rams of fluid flow and methane transport. We used a numerical model of fluid flow to estimate intrinsic permeabilities, pore pressures, and flow velocities that are consistent with these observations. Model results suggest that the permeability of the d6collement may be 3-5 orders of magnitude greater than that of adjacent prism sediments. ff permeabilities in the prism vary with depth in a manner similar to those in sedimentary basins, the average intrinsic permeability of the d6collement, k d , must be about 10 -14 m 2. When k a is 10 -13 m 2, high pore pressures do not develop near the deformation front in the model. ff k a is 10 -15 m 2, simulated pressures are unrealistically high in both the prism and underthrust sediments arcward of the deformation front. Water originating from compaction in the d6collement and underthrust sediments flows laterally seaward, while water expelled from prism sediments flows upward to the ocean floor. However, flow velocities are small, and the net motion of pore water in prism and undenhrust sediments is arcward relative to the deformation front because of tectonic transport. Pore water migrates seaward in spite of tectonic transport only in discrete zones with higher permeability, in this case the d6collement. brmODUCT•ON 1982]. Both Ocean Drilling Project (ODP) leg 110 and Deep Sea Drilling Project (DSDP) leg 78a recovered sediment samples In accretionary complexes, gravitational and tectonic forces from the complex. These samples document decreases in porosity deform highly porous sediments as they are either accreted onto as basin sediments are incorporated into the accretionary the overriding plate or carried downward with the underthrust complex. Observations from DSDP leg 78A also suggest that plate. Fluids expelled from the compacting sediments influence pore pressures may reach near-lithostatic values just above the many aspects of subduction zone geology, including heat d6collement within 5 km of the deformation front [Moore and transport [e.g., Langseth and Hobart, 1984], diagenesis and Biju-Duval, 1984]. In addition, some pore water samples metamorphism [e.g., Etheridge et al., 1983], and benthic biology collected during ODP leg 110 contained anomalous methane, [e.g., Kulm et a/.,1986]. In addition, relative rates of sediment chloride, and temperature distributions, suggesting preferential loading and fluid dissipation determine the magnitude and fluid flow along fault zones, especially within the d6collement distribution of excess fluid pore pressures. These pore pressures [Moore et al., 1987; Gieskes et al., 1989]. affect the shape of the accretionary wedge [e.g., Davis et al., In this paper, we present a nmerical model of fluid flow in the 1983] as well as thrust fault and fold geometries [Hubbert and Barbados R...

Hydrostatic consolidation tests of undeformed, clay-rich samples from the Barbados accretionary prism, Leg 156

“…Our results are consistent with the lower permeability population. Our permeability measurements are also the same as those inferred by Screaton et al (1990) and Wuthrich et al (1990), given the assumption of near-lithostatic fluid pressures at Site 542.…”

Evaluation of hydrogeologic properties of the Barbados accretionary prism: a synthesis of Leg 156 results

“…Numerical modeling suggests that permeabilities within the décollement zone must be many orders of magnitude above background values to produce the observed geochemical anomalies (Bekins et al, 1995) or the inferred near-lithostatic pore pressures (Screaton et al, 1990). Fundamental difficulties lie in the extrapolation of lab measurements to large-scale systems in such low-permeability environments, particularly when flow systems may be transient in nature (Neuzil, 1986).…”

A numerical model for coupled fluid flow and matrix deformation with applications to disequilibrium compaction and delta stability

“…For example, during continental collision, overthrusting and tectonic shortening cause compaction-induced groundwater flow [Oliver, 1986]. In subduction zones, fluid flow occurs in the accretionary wedge owing to pore space collapse due to mechanical shearing and loading of the subducting oceanic plate [Screaton et al, 1990] and by dehydration of minerals in the descending slab during prograde metamorphism [Peacock, 1990;Richard et al, 2007]. In extensional environments groundwater flow is induced within fault bounded continental rift basins by various mechanisms, seismogenic pumping [Sibson, 1994], compaction [Harrison and Summa, 1991], and density [Hanson, 1992], and topographic [Person and Garven, 1994] driven variations.…”