HENRY, PIERRE, and STEPHANE MAZZOTTI, Laboratoire de Geologie de l'ENS (CNRS URA 1316), Paris, France; SIEGFRIED LALLEMANT, Universite de Cergy-Pontoise, Cergy, France; JUICHIRO ASHI, Geological Institute, University of Tokyo, Tokyo, Japan; KO-ICHI NAKAMURA, Marine Geology Department, Geological Survey of Japan, Tsukuba, Japan; and KAZUO KOBAYASHI, JAMSTEC, Yokosuka, Japan

Abstract: Tectonically Controlled Fluid Venting on the Eastern Nankai Margin: Results of Kaiko-Tokai Project

The Tokai region of the Nankai margin is characterized by the widespread occurrence of sea floor venting sites with associated benthic communities and authigenic carbonates. We here concentrate on two very active venting zones associated with active thrusting: the frontal thrust sites on the first imbricate of the accretionary wedge and the Tokai thrust sites at the wedge-backstop limit. The relationship between venting sites and tectonic structures is determined from a combination of multichannel seismics, high resolution side-scan sonar images, deep towed 3.5 kHz sounder profiles and submersible surveys. Venting sites are typically found on the hanging wall of active thrusts and within 1-1.5 km of the fault outcrop. This distribution may be interpreted in two very different ways. It may be the consequence of diffusive vertical upward flow coming from the compaction of the footwall. Alternatively, it may reflect focusing of flow along the fault zones at depths in the sediment. Numerical modeling shows that a fault zone with permeability of about 10^-16m² could channel fluid fluxes of about 1 m³/yr per meter of fault below 1 km depth but would loose its efficiency at shallower levels. At the deformation front, observations imply that coarse sand layers play a major role in channeling fluids to the sea floor vents. Several active vents are nevertheless found on the outcrop of major thrusts and negative polarity reflections are observed below about 1 km depth. This evidence suggests that thrusts are active fluid conduits in spite of the coarse turbiditic sedimentation.

AAPG Search and Discovery Article #90920©1999 AAPG Pacific Section Meeting, Monterey, California