OGAWA, Y., KUWANO, K., University of Tsukuba, Tsukuba, Japan; OBA, T., and KOIZUMI, I., Hokkaido University, Sapporo, Japan

Abstract: Carbonate-Cemented Sedimentary Breccia in the Northern Japan Trench Product of Subduction-Rellated Methane Seepage

Carbonate cement of sedimentary rocks as hard ground or chimneys have been known from various areas mostly in subduction-related thrust and strike-slip fault regimes. We found unique carbonate-veined and -cemented sedimentary breccias from the toe of the northern Japan trench landward slope along the Sanriku Escarpment at a depth of around 6400 m by JAMSTEC submersible dives. The area is not a type of accretionary prism, rather tectonically- and bottom current-eroded area, exposing various kinds of sedimentary rocks ranging from middle Miocene to Pliocene diatomaceous muddy/clayey (partly sandy) lithologies. Those rocks show ramarkable brecciation with multistage calcite veining and cementation. Breccias were found around Calyptogena communitiess (colonies), and were well interpreted as products of hydraulic fracturing. The d C-d O isotope measurement of the calcite suggested to be of biogenic origin at moderate temperature as similar to the examples from the Nankai trough.

We concluded that these occurrences and characteristics of the breccias are the products by long-lasting (since Miocene?) thrust-fault activity through which seepage of methane-bearing fluid contributes for carbonate veining and cementation together with brecciation. Such kinds of breccia are also recorded in some ancient accretionary complexes, and might be a good indicator of thrust-related deformation at convergent margins.

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