HR: 0800h
AN: S51B-0149    [Abstracts]
TI: Thrusting-related Fluid Flow in Subduction Zone: Geologic and Isotopic Evidence From Ancient Subduction Complex
AU: * Yamaguchi, A
EM: asuka@eps.s.u-tokyo.ac.jp
AF: Dept. Earth and Planetary Sci., Univ. Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-0033 Japan
AU: Ujiie, K
EM: ujiiek@jamstec.go.jp
AF: IFREE, JAMSTEC, 3173-25 Showa-machi, Kanazawa-ku, Yokohama, 236-0001 Japan
AU: Kimura, G
EM: gaku@eps.s.u-tokyo.ac.jp
AF: Dept. Earth and Planetary Sci., Univ. Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-0033 Japan
AU: Kimura, G
EM: gaku@eps.s.u-tokyo.ac.jp
AF: IFREE, JAMSTEC, 3173-25 Showa-machi, Kanazawa-ku, Yokohama, 236-0001 Japan
AB: Role of fluid in a seismogenic plate boundary of subduction zone remains poorly understood. To reveal the origin and behavior of syn-tectonic fluid in the ancient subduction complex, we examined occurrences of veins and carbon and oxygen isotope composition of vein calcite of the Mugi Melange in the Shimanto Belt. The melange preserves a relationship between rock deformation and fluid flow near the up-dip limit of seismogenic zone. The Mugi Melange is a late Cretaceous to Paleocene tectonic melange, and is characterized by blocks of sandstone and basalt in shale matrix. The Mugi Melange is divided into five units based on the repetition of basaltic layer of N-MORB, representing duplex-underplated assemblage. Paleo-temperature determined by vitrinite reflectance is 120-150 \deg C in the studied section, and fluid inclusion thermo-barometry indicates 125-245 \deg C and 92-149 MPa (Matsumura et al., 2003). This P-T condition is in the vicinity of up-dip limit of the seismogenic zone. Major deformational stage of the Mugi Melange is underthrusting (D1), underplating (D2), and uplifting (D3). There are various occurrences of quartz and calcite veins in melange formed during various stages. We classified three occurrences of veins named intra-boudin veins (IBV), network veins (NWV), and fault-parallel veins (FPV). IBV are observed only around the neck of boudined sandstone block in the black shale matrix, so they were precipitated when block-in-matrix structure was formed due to layer-parallel extension during D1. NWV are distributed in the damage zone of duplex ramp thrust, and are mainly perpendicular to shear surface of the thrust. Such occurrences suggest that NWV formed inter-faulting period of D2. FPV show implosion breccia-like texture (Sibson, 1986), and they are observed just below the basalt / shale boundary. Vein minerals are dominantly calcite. These features suggest that FPV would be precipitated as a result of co-slip fluid migration during oceanic crust underplating. We analyzed carbon and oxygen isotopic compositions of calcite obtained from these three occurrences of veins. Oxygen isotopic compositions are in the narrow range of +15.8 to +19.2 \permil (vs. V-SMOW) for all veins. \delta $^{18}$O values increase from IBV to FPV. Precipitation temperatures determined by fluid inclusion (Matsumura et al., 2003) are higher in NWV than in IBV. These results show that delta-18O values of fluids increase from IBV ($\sim$ +4 \permil) to FPV ($\sim$ +10 \permil). Assuming the same dehydration reaction of clay minerals of host rock and isotopic equilibrium, the dehydration temperature is lowest for IBV, moderate for NWV, and highest for FPV. In contrast to \delta $^{18}$O values, carbon isotopic compositions of vein calcite are broad, in -17.2 to -3.7 \permil (vs. V-PDB) for all veins. IBV and NWV include light carbon, while FPV take heavy one. This result suggests that organic carbon from shale and igneous carbon from basalt were mixed in the fault core. In conclusion, we suggest that the fluid dehydrated from both shale and basalt was accumulated in cracks in damage zone of plate boundary fault, and migrated to shallower portion in association with seismic event.
DE: 9355 Pacific Ocean
DE: 8010 Fractures and faults
DE: 8045 Role of fluids
DE: 8150 Plate boundary--general (3040)
DE: 1040 Isotopic composition/chemistry
SC: Seismology [S]
MN: 2004 AGU Fall Meeting