[S-SS30_29PO1] New perspective of great earthquakes along subduction zones

[SSS30-P29] 3D micro structural observation of pseudotachylyte

*Yohei HAMADA¹, Gaku KIMURA², Jun KAMEDA³, Asuka YAMAGUCHI⁴, Mari HAMAHASHI², Yujin KITAMURA⁵, Rina FUKUCHI², Ryoji KAWASAKI² (1.Kochi institute for Core Sample Research, JAMSTEC, 2.Department of Earth and Planetary Science, The University of Tokyo, 3.Earth and Planetary System Science Department of Natural History Science, 4.Atomosphere and Ocean Research Institute, The University of Tokyo, 5.Department of Earth and Environmental Sciences,Graduate School of Science and Engineering Kagoshima)

Keywords:pseudotachylyte, 3D micro structure, surface roughness

Pseudotachylyte, molten fault rock due to dynamic frictional heating, is a strong evidence of seismic fault slip [Sibson 1975]. Recent research reveals pseudotachylytes can be related with dynamic weakening mechanism such as melt lubrication [DiToro et al., 2006]. However, observations of internal structure of pseudotachylyte have been confined to 2D observations with optical-electron microscope. Here we performed X-ray 3D structural observation of natural pseudotachylyte developed close to the Nobeoka thrust which is a major Out of sequence thrust in fossil accretionary prism (Shimanto-belt). The Nobeoka thrust located in Kyusyu Island, south west Japan, bounding northern and southern Shimanto belt of Cretaceous-Tertiary accretionary complex. The thrust is considered to have been active during 40-48Ma at seismogenic depth of ~11kmbsf, experienced maximum temperature of which is 320 Cin the hanging wall and 250 C in the footwall. Thus, the Nobeoka thrust is examined that it was major OST in seismogenic zone of accretionary prism (Kondo et al., 2005; Hara and Kimura, 2008; Raimbourg et al., 2009). The pseudotachylyte bearing fault develops in the hanging wall of the Nobeoka thrust with 1 mm of width. Okamoto et al. (2007) reported that carbonate-matrix implosion breccia fill tensile cracks and inner periphery of the fault, interposing pseudotachylyte, based on optical microscopic observation. Though pseudotachylyte cut the implosion breccia, the fault jog consists only of the carbonate-matrix breccia. It may show the fault experienced dynamic pore water pressurizing accompanied by pseudotachylyte generation at its first frictional slip. Therefore, the fault is appropriate to structural investigation of dynamic fault weakening mechanism.We performed structural observation of this pseudotachylyte with scanning electron-microscope and 3D X-ray microscope. In the electron microscopic observation, we found that fragments of host rock unevenly distributed in the pseudotachylyte. The number of fragments is larger at lower part (footwall-side) than within the center of the pseudotachylyte. We also found open cracks along the fragments arrangement. It is considered to be cooling crack generated due to rapid cooling of molten rock. The 3D x-ray microscopic observation was performed with cylinder sample of 8 mm diameter. The spatial resolution of the x-ray microscope is 1 micro meter, and detailed 3D fault structure was imaged. We focused four planes, A: lower plane of lower fault filling vein, A': lower plane of pseudotachylyte, B: upper plane of upper vein, B': upper plane of pseudotachylyte. The surfaces configurations were extracted and its roughness was evaluated as calculated average roughness, Ra (theta), in each direction. We found that Ra has minimum value in the same direction in each plane, and the lineation strongly develops at the lower planes (A, A').From the above results, we discussed the faulting process as:1) Start faulting. strain concentrated in the footwall side and pore pressure was raised at the part.2) Hydraulic fracturing by high pore pressure, tensile cracks formation and fluid migration.3) Strength (friction) recovery by draining and formation of pseudotachylyte.