HR: 1340h
AN: T23A-0561    [Abstracts]
TI: Permeability structure of a serpentinite-bearing fault zone along the Gokasho-Arashima Tectonic Line, Central Japan and its implication for thermal pressurization
AU: * Sone, H
EM: hirokisone@kueps.kyoto-u.ac.jp
AF: Kyoto University, Department of Geology and Mineralogy, Graduate School of Science, Kitashirakawa Oiwakecho, Sakyo-ku, Kyoto, 606-8181 Japan
AU: Noda, H
EM: nodahiroyuki@kueps.kyoto-u.ac.jp
AF: Kyoto University, Department of Geology and Mineralogy, Graduate School of Science, Kitashirakawa Oiwakecho, Sakyo-ku, Kyoto, 606-8181 Japan
AU: Shimamoto, T
EM: shima@kueps.kyoto-u.ac.jp
AF: Kyoto University, Department of Geology and Mineralogy, Graduate School of Science, Kitashirakawa Oiwakecho, Sakyo-ku, Kyoto, 606-8181 Japan
AB: Serpentinites are often trapped in large-scale fault zone, but no thorough studies have been conducted on internal and permeability structures of serpentinite-bearing fault zones. Kurosegawa tectonic zone, extending over several hundred kilometers across Shikoku Island and Kii Peninsula in Japan, is well-known for bearing numerous large serpentinite and other tectonic blocks. Gokasho-Arashima tectonic line is a fault in this zone in the eastern part of Mie Prefecture, central Japan. A large serpentinite body up to 600 m in thickness is adjacent to Cretaceous Matsuo Group, composed mostly of alternated sandstone and shale, across this tectonic line. Fault zone of this tectonic line at Matsuo outcrop consists of (1) fractured sedimentary rocks and fault breccia derived mostly from Matsuo Group ($>$ 30 m in width), (2) clayey fault gouge of about 50 mm in width, (3) foliated gouge of about 0.6 - 0.9 m in width, and (4) brecciated serpentinites ($>$ 250 m). Permeability has been measured with an intravessel deformation-fluid-flow apparatus in Kyoto on fault rocks taken from each structural division (1) to (4) above, using steady-flow method with Nitrogen gas as the pore fluids. Measurements were done during confining pressure cycling (from 5 MPa up to 120 MPa, and then back to 5 MPa, corresponding roughly to 0.3 - 7.5 km in depth. Permeability values at 100 MPa are 10-15 to 10-17 m2 for zone (1) above, around 3 x 10-18 m2 for zones (2) and (3), and greater than about 10-16 m2 for zone (4). Thus the fault zone is characterized by fault gouge (fault core) with low to intermediate permeability and by permeable damage zone. We also have measured constitutive properties of this fault zone as studied with a biaxial frictional testing machine and will report on these in our presentation. The possible effect of thermal pressurization in this fault zone was evaluated using the same method as Noda et al. (2003), a numerical analysis based on Lachenbruch (1980), fully incorporating changes in permeability, porosity and physical properties of water with build-up of pore pressure. The effect of large shearing deformation is not taken into account. Initial parameters were chosen from measured data at an effective pressure corresponding to 3 km in depth and analyses were done for deformation-zone widths of 10, 40 and 50 mm. In all cases thermal pressurization caused slip-weakening with characteristic weakening distance of a few meters. Permeability of fault gouge is not very low, so that thermal pressurization is not very effective for the present serpentinite-bearing fault zone. Key words: serpentinite-bearing fault zone, thermal pressurization, dynamic fault motion, fault rock, permeability
DE: 5114 Permeability and porosity
DE: 7209 Earthquake dynamics and mechanics
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting