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

1993; Linguagem: Inglês

10.2973/odp.proc.sr.131.143.1993

Miriam Kastner, Henry Elderfield, William J. Jenkins, J.M. Gieskes, Toshitaka Gamo,

Geology and Paleoclimatology Research

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 decollement 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 decollement, and particularly (2) below the decollement. 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 decollement is characterized by lower than seawater concentrations of Cl- (>18% seawater dilution). It is 18O-rich and D-poor and has a non-radiogenic, oceanic, or arc Sr isotopic signature. It originates from volcanic clay diagenesis. The fluid below the decollement has also less Cl~ than seawater (>20% dilution), is more enriched in 18O 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 clays, occurs. The 3He/4He ratio below the decollement points to an -25% mantle contribution. The nature of the physical and chemical discontinuities across the decollement 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.