Geochemical and Isotopic Evidence for Fluid Flow in the Western Nankai Subduction Zone, Japan

Abstract

At the Western Nankai Trough subduction zone at ODP Site 808, chemical concentration and isotopic ratio depth profiles of D, O, Sr, and He do not support fluid flow along the décollement nor at the frontal thrust. They do, however, support continuous or periodic lateral fluid flow: (1) at the base of the Shikoku Basin volcanic-rich sediment member, situated -140 m above the décollement, and particularly (2) below the décollement. The latter must have been rather vigorous, as it was capable of transporting clay minerals over great distances. The fluid at ~ 140 m above the décollement is characterized by lower than seawater concentrations of Cl-(>18% seawater dilution). It is 18 O-rich and D-poor and has a non-radiogenic, oceanic, or volcanic arc Sr isotopic signature. It originates from "volcanic" clay diagenesis. The fluid below the décollement has also less Cl~ than seawater (>20% dilution), is more enriched in 18 O and depleted in D than fluid, but its Sr isotopic signature is radiogenic, continentalterrigenous. The source of this fluid is located arcward, is deep-seated, where illitization of the subducted clay minerals, a mixture of terrigenous and volcanic clays, occurs. The 3 He/ 4 He ratio below the décollement points to an -25% mantle contribution. The nature of the physical and chemical discontinuities across the décollement suggests it is overpressured and is forming a leaky "dynamic seal" for fluid flow. In contrast with the situation at Barbados and Peru, where the major tectonic features are mineralized, here, although the complex is extremely fractured and faulted, mineralized macroscopic veins, fractures, and faults are absent. Instead, mineralized microstructures are widespread, indicating a diffuse mode of dewatering.

Clay-Mineral Provenance, Sediment Dispersal Patterns, and Mudrock Diagenesis in the Nankai Accretionary Prism, Southwest Japan

“…One possible migration pathway is the dtcollement zone, within which permeabilities are enhanced by a penetrative scaly fabric of closely spaced fractures (Moore 1989;Moore and Vrolijk 1992). Direct evidence for active fluid advection along the dtcollement is lacking, but Kastner et al (1993) demonstrated how an intense transient pulse could have injected low-salinity fluid roughly 300,000 years ago followed by diffusive downward migration. However, the dtcollement pathway is not necessarily mandated by the data.…”

“…In the case of the Nankai accretionary prism, we know that the depth interval containing US clays coincides with a decrease in interstitial water chlorinity (Shipboard Scientific Party 1991;Kastner et al 1993). Chloride content begins to decrease gradually from maxima of approximately 590 mM at 560-580 mbsf to minima of 450 mM at 1040-1080 mbsf, and there is a deflection in the gradient at about 820 mbsf (Figure 9).…”

“…Finally, stratigraphic intervals with abundant layers of volcanic ash (560-820 mbsf) show large variations in dissolved silica concentrations, together with more subtle anomalies in dissolved calcium, lithium and strontium. Collectively, the porewater data demonstrate that sediment diagenesis and fluid-sediment exchange have occurred within the accretionary prism Kastner et al 1993). It is important to evaluate how specific clay minerals have influenced this hydrogeochemical budget.…”

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“…Pore water profiles did not clearly indicate active fluid flow along the décol-lement or along the frontal thrust, and fractures within the décollement zone have not been mineralized Maltman et al, 1992;Moore, Taira, Klaus, et al, 2001); however, at >400-500 mbsf, pore water freshening is detectable in Cl -concentrations that are lower than seawater values (Figures F6, F7; cf. Kastner et al, 1993). The observed freshening is partly explained by in situ diagenesis (compaction and thermally driven smectite dehydration) but also suggests some episodic lateral fluid flow along one or more sediment horizons (Moore, Taira, Klaus, et al, 2001;Saffer and Bekins, 1998;Underwood et al, 1993).…”

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“…Pore water profiles did not clearly indicate active fluid flow along the décollement or along the frontal thrust, and fractures within the décollement zone were found to be not mineralized Maltman et al, 1992;Moore, Taira, Klaus, et al, 2001); however, at >400-500 mbsf, pore water freshening is detectable in Cl − concentrations that are lower than seawater values (cf. Kastner et al, 1993). The observed freshening is partly explained by in situ diagenesis (i.e., compaction and thermally driven smectite dehydration) but also suggests some episodic lateral fluid flow along one or more sediment horizons (Moore, Taira, Klaus, et al, 2001;Saffer and Bekins, 1998;Underwood et al, 1993).…”