Three‐dimensional texture of natural pseudotachylyte: Pseudotachylyte formation mechanism in hydrous accretionary complex |
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| Authors: | Yohei Hamada Gaku Kimura Jun Kameda Asuka Yamaguchi Mari Hamahashi Rina Fukuchi Yujin Kitamura Shin'ya Okamoto |
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| Affiliation: | 1. Kochi Institute for Core Sample Research, Japan Agency for Marine‐Earth Science and Technology, 200 Monobe Otsu, Nankoku‐city, Kochi 783‐8502, Japan;2. Office of Liaison and Cooperative Research, Tokyo University of Marine Science and Technology, 4‐5‐7, Konan, Minato–ku, Tokyo 108–8477, Japan;3. Department of Natural History Sciences, Graduate School of Science, Hokkaido University, N10‐W8, Kita‐ku, Sapporo‐shi, Hokkaido 060‐0808, Japan;4. Atmosphere and Ocean Research Institute, The University of Tokyo, 5‐1‐5 Kashiwanoha, Kashiwa‐shi, Chiba 277‐8564, Japan;5. Department of Earth and Environmental Sciences, Graduate School of Science and Engineering, Kagoshima University, 1‐21‐35 Korimoto, Kagoshima 890‐0065, Japan |
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| Abstract: | Melt‐origin pseudotachylyte is the most reliable seismogenic fault rock. It is commonly believed that pseudotachylyte generation is rare in the plate subduction zone where interstitial fluids are abundant and can trigger dynamic fault‐weakening mechanisms such as thermal pressurization. Some recent studies, however, have discovered pseudotachylyte‐bearing faults in exhumed ancient accretionary complexes, indicating that frictional melting also occurrs during earthquakes in subduction zones. To clarify the pseudotachylyte generation mechanism and the variation of slip behavior in the plate subduction zone, a pseudotachylyte found in the exhumed fossil accretionary complex (the Shimanto Belt, Nobeoka, Japan) was re‐focused and microscopic and three‐dimensional observations of the pseudotachylyte‐bearing fault were performed based on optical, electron, and X‐ray microscope images. Based on the patterns contained in the fragment, the pseudotachylyte is divided into four domains, although no clear domain boundaries or layering structures are not found. Three‐dimensional observation also suggests that the pseudotachylyte were fragmented or isolated by cataclasite or carbonate breccia. The pseudotachylyte was rather injected into the surrounding carbonate breccia, which is composed of angular fragments of the host rock and a matrix of tiny crystalline carbonate. The pseudotachylyte volume was extracted from the X‐ray microscope image and the heat abundance consumed by the pseudotachylyte generation was estimated at 2.18 MJ/m2, which can be supplied during a slip of approximately 0.5 m. These observations and calculations, together with the results of the previous investigations, suggest hydrofracturing and rapid carbonate precipitation that preceded or accompanied the frictional melting. Dynamic hydrofracturing during a slip can be caused by rapid fluid pressurization, and can induce abrupt decrease in fluid pressure while drastically enhancing the shear strength of the shear zone. Consequently, frictional heating would be reactivated and generate the pseudotachylyte. These deformation processes can explain pseudotachylyte generation in hydrous faults with the impermeable wall rock. |
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| Keywords: | fault dynamic lubrication hydrofracturing micro X‐ray CT Nobeoka Thrust plate subduction zone pseudotachylyte |
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