The Architecture of a Fossilized Out-of Sequence Thrust in Ancient Accretionary Complex :Footwall of the Nobeoka Thrust, Japan

The Nobeoka Thrust in Kyushu, Japan is a large ancient fossilized out-of-sequence thrust exhumed from seismogenic depth of accretionary prism. Recent studies have been revealed the general features of the thrust: 1) the deformation is quite intensive due to repeated shear, extensional cracking, and a large amount of fluid influx and crack-fill veining (Kondo et al., 2005), 2) pseudotachylyte-bearing subsidiary faults were discovered in hanging-wall of vicinity of fault core of the thrust and temporal temperature change during faulting was detected (Okamoto et al., 2006, this meeting), 3) P- and S- wave velocity and density contrast between hanging-wall and footwall can explain the seismic reflection pattern of mega Out-of-sequence in modern Nankai Trough (Tsuji et al., 2006). Our new detailed structural analyses presents that the architecture of the footwall damage zone is strongly controlled by the mesoscale heterogeneity of thrusting-related shear fractures. Two vein sets, tension cracks and shear fractures characterize the footwall damage zone. Geometrical relationships between them suggest that both the structures are component of composite planar fabrics as observed in many studies. Microscopic observation of the tension cracks and shear fractures shows that crack-seal and slip along the shear fractures might have been intimately related. Tension cracks show repeated cracking and sealing, while shear fractures have shear concentrated zones composed of submicron scale quartz or amorphous silica grains, which is proposed to contribute to dynamic weakening for unstable slip. The number of crack-seal textures and thin silica layers is almost the same. Therefore observed relationship suggests one of the earthquake-related aspects for seismogenic out-of-sequence thrust in subduction zone. These typical architectures of the Nobeoka Thrust are expected to be found by NanTroSEIZE deep drilling to the modern Nankai Trough.