DYNAMIC ROLE OF TECTONIC MéLANGE IN RELATION TO MEGA SEISMOGENESIS – SPACE AND TIME PARTITION OF DEFORMATION IN SUBDUCTION PLATE BOUNDARY

Tectonic mélange is essentially a fault rock regarding to its definition. It is, however, difficult to imagine so because of its relatively broad occurrence and complexity of superimposed deformations, which limited us in a way of understanding that the mélange is a passive reproduct by some deformation. We examined different mélanges in the Shimanto accretionary complex in southwest Japan to assess the dynamic role of mélange in subduction plate boundary. Comparing mélanges from different maximum burial depth, we extracted successive change in deformation with subduction. Here we present an integrated result of our recent progress tectonic mélange study from the Japanese subduction zone.

Studied mélanges, the lower and upper Mugi mélange and Makimine mélange, have their maximum temperature of about 150, 200 and 340 °C, respectively. This range of temperature corresponds to up-dip to down-dip limit of seismogenic zone. These mélanges are exposed as a pile of thrust sheets. They are composed dominantly of shale and sandstone showing block-in-matrix structure, and basalt in the bottom of each thrust sheet. Such a deformed pile of ghost ocean stratigraphy suggests that these rocks have deformed in the plate boundary and underplated. Pseudotachylyte was found from the roof fault of the Mugi mélange. Paleo thermal structure reconstructed from vitrinite reflectance study showed that there is no temperature gap at the roof fault, which indicates that the roof fault was active when the fault plane was parallel to the isotherm. Therefore, we conclude that this roof fault was the former seismogenic décollement (Kitamura et al., 2005, Tectonics and Kimura et al., this meeting). The formation temperature of the veins in the sandstone boudin’s neck is 125-195 °C (Matsumura et al., 2003, Geology), which indicates that the deformation of sandstone happens around up-dip limit of seismogenic zone.

We performed analyses of anisotropy of magnetic susceptibility (AMS) and boudin size distribution to obtain structural information of shales and sandstones, respectively. Results show that the deformation in the shales is dominantly vertical compaction in the shallow seismogenic zone and changes to simple shear deformation. The size and the aspect ratio of sandstone boudins decrease successively with subduction. These observations suggest that the mélange itself continues to deform within seismogenic zone. On the other hand, formation of the sandstone block is not a single process. Firstly, sandstone layer pinches out because of viscosity contrast, which is followed by cataclastic deformation. When the simple shear component increases, Riedel shear becomes also a sandstone breaker.

Our investigation revealed that the tectonic mélange is a broad fault zone of the plate boundary and keep deforming within seismogenic zone, where the main slip zone is their seismogenic roof fault. Consequently, there is a clear partitioning in deformation between rapid and episodic deformation at the roof fault and slow and continuous deformation in the mélange body. Such a slow and tiny deformation may relate to low frequency swarm in the down-dip limit of seismogenic zone. Also the changing multiple forming processes of the mélange will be a key to understand variety of earthquakes (e.g. very low frequency earthquake).