Episodic and constant flow models for the origin of low‐chloride waters in a modern accretionary complex

Abstract

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...

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).…”

“…Brown and Ransom (1996) suggest that porosity values in the Barbados accre- Figure 5. Plot of porosity vs. permeability (after Bekins, 1995), based on data for argillaceous formations compiled by Neuzil (1994;shaded region), Leg 110 permeability measurements are from Taylor and Leonard (1990;black circles), and porosity-permeability functions are from Bekins, et al (1995;dashed and solid lines). Measured permeabilities from Leg 156 samples are plotted as open diamonds.…”

Episodic fluid flow in the Nankai accretionary complex: Timescale, geochemistry, flow rates, and fluid budget

“…In particular, the study of geochemical anomalies observed in boreholes at accretionary systems provides a means to quantify fluid sources and hydrologic properties [Bekins et al, 1995]. Here we combine measured chloride concentrations at site 808 with other drilling observations and seismic data from the Nankai accretionaxy complex to evaluate several aspects of the hydrologic system.…”

Introduction to Logging-While-Drilling Investigations of Faulting, Fluid Flow, and Seismic Images of the Northern Barbados Subduction Zone

“…Anomalies in pore-water chemistry (Gieskes et al, 1990;Kastner et al, 1997) and temperature (Fisher and Hounslow, 1990) indicate focused fluid flow along fault zones and in sand layers. Models simulating this fluid expulsion from the prism suggest that the flow is transient (Bekins et al, 1995). The faults are characterized by suprahydrostatic, and locally nearly lithostatic, fluid pressures Labaume et al, 1997;Screaton et al, 1997;Zwart et al, 1997).…”