Some low-chloride pore waters observed in accretionary complexes are thought to result from clay dehydration and subsequent migration of the released water along faults or sand layers. We test this hypothesis with a two-dimensional flow and transport model for a cross section of the northern Barbados accretionary complex. The model flow system is driven by consolidation of the accreted sediments and by fluids from smectite clay dehydration. Steady state simulations result in concentrations that are too high along the d6collement fault and too low near the seafloor. In a transient model we simulate buildup and release of fluids by assuming that strain or hydrofracture along the fault causes an instantaneous increase in d6collement permeability of 2-3 orders of magnitude. With such an increase, the observed concentrations can be achieved 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 years. This pressure rise may represent a mechanism for sustaining high fault permeabilities long after the initial increase. A steady source of pore fluids enters an accretionary complex with the accreted seafloor sediments. These fluids may then affect a variety of geologic processes. For example, high pore pressures affect fault strength and seismicity [Magee and Zoback, 1993], and fluid-rock interactions influence metamorphic reactions [Peacock, 1990]. In a thorough review of fluid Paper number 95WR02569. 0043-1397/95/95WR-02569505.00 flow in accretionary prisms, Moore and Vrolijk [1992] described two processes that drive the flow system. One is the very high mechanical loading rate on the saturated sediments as they are either incorporated into or underthrust below the complex. This loading rate is up to 15 times the maximum due to sedimentary basin subsidence [Moore and Vrolijk, 1992; Neuzil, 1995]. The resulting consolidation is often limited by low permeabilities, leading to near-lithostatic pore pressures. A second process is the dehydration of minerals and generation of hydrocarbons as sediments buried deep in the complex are subjected to higher temperatures. Anomalously low chloride and high methane concentrations in pore waters suggest that this is occurring in many accretionary complexes [Kastner et al., 1991]. The elevated temperatures and exotic chemical signatures of the pore fluids indicate that the flow systems supplying the vents may extend, in some cases, for 50-100 km. However, the source of heat and solutes is still poorly understood. Figure lc shows pore water chloride and methane concentration changes with depth at Ocean Drilling Program (ODP) site 671 in the Barbados Ridge complex. The pore waters are almost 10% lower in chloride than seawater at the depth of the d6collement fault. In addition, more recent chloride concentrations from the dficollement at ODP site 948 (near site 671) were 20% lower than that of seawater [Kastner et al., 1994]. Gieskes et al. [1990] discussed clay membrane filtration, hydrate dissolution, and the de...

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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...

Low chloride pore fluids observed along faults in clay‐rich accretionary complexes are commonly attributed to the release of interlayer water during the smectite to illite transformation. However, to date, there has been no thorough analysis of the location and quantity of fluids that may be generated by this mechanism. To address this problem, a temperature and time dependent rate expression describing the dehydration reaction was coupled to a kinematic model of the northern Barbados accretionary complex. Temperatures in the complex were estimated by modeling heat flow through the prism sediments as they thicken arcward. The sediments' temperature‐time histories were computed using a model for the velocities of sediment motion through the complex. In this model the prism sediments follow uniformly diverging paths from the toe, while the underthrust sediments undergo uniaxial strain. The model predictions are validated against clay mineralogy data from Barbados Island mudstones. Our results show that the location of the peak dehydration rate is 20 km farther arcward in the underthrust sediments than in the prism complex. The fresh water produced by the reaction results in pore waters that are 10–30% fresher at a distance of 50–70 km from the prism toe. This constraint indicates a probable fluid migration path of more than 50 km from the reaction zone to the sites where freshened pore fluids have been observed. The peak rate of fluid production when expressed as the volume of fluid per volume of sediment per second is 2×l0−15.

In this paper we describe an approach that uses indicator geostatistics to interpret qualitative borehole logs and compute experimental variograms for complex alluvial sediments. Borehole descriptions are first transformed into binary indicator values based on inferred relative permeability from the borehole descriptions. The resulting indicator data can then be used to compute variograms and construct three‐dimensional variogram models. The ranges of computed indicator variograms for a groundwater contamination site in Santa Clara Valley, California, are very sensitive to the orientation of the search plane. These variograms are consistent with known stratigraphie features and describe the spatial structure of deposits from different depositional environments. Indicator kriging weighs all the available data on the basis of a three‐dimensional, anisotropic variogram model and provides an estimate of uncertainty in the hydrostratigraphic correlation. Kriged indicator values represent probabilities that sediments at a specific location fall into one of two indicator categories. The location of the 0.5 indicator contour is approximately the boundary between high‐ and low‐permeability sediments that might be constructed in a geologic cross section.