MéLANGE AS A PLATE BOUNDARY FAULT ROCK- EARTHQUAKE AND SLOW SLIP

Mélange formation is a key issue to understand the convergent plate boundary processes since it was realized as a fossil Benioff zone in subduction zone during the founding stage of plate tectonics (Ernst, 1970). Although definition of tectonic, sedimentary, or diapiric (injective) mélange was a matter of controversy in the initial period, the research focus shifted more mechano-chemical processes of the mélange formation.

I review a recent research progress of mélange and discuss the plate boundary processes, especially seismogenesis and its updip and downdip limits. Most of on-land mélanges in Japanese islands were once buried beyond 150℃, reached to maximum more than 250-350℃ in depth, and came back to the surface. Therefore they record the plate boundary processes in detail in that temperature range, which coincides with the seismogenic zone as suggest by Hyndman and Wang (1993).

Earthquake fault rocks of pseudotachylites have been discovered from several parts of mélange, especially roof thrust of the duplexed mélange packages (Ikesawa et al., 2003). However, the discovered earthqake fault rocks are not only the pseudotachylyte but also fluidized cataclasites with heating evidence. This new type of earthquake fault rocks suggests that thermal pressurization is a key mechanism for dynamic weakening of fluid dominated plate boundaries in subduction zone. Dynamic chemical reaction was also detected from the fault rock. This phenomenon indicates an abrupt change in redox state in association with sudden fluid flow.

Mélange including the earthquake fault rocks is characterized by “block-in-matrix” as traditionally well-known. Deformation mechanisms for the tectonic mélange are dominantly cataclasis with pressure solution. Relationship of these mechanisms with recently discovered slow slip in subduction zone must be a main focus of the next stage of research of mélange.

Ernst, W.G. 1970, J.Geophts. Res., 75, 886-901.

Hyndman, R.D. and Wang, K., 1993, J.Geophts. Res., 98, 142039-142060.

Ikesawa, E., Sakaguchi, A., and Kimura, G., 2003, Geology, Geology, 31, 637-640.