Date of Award:
Master of Science (MS)
James P. Evans
The Chelungpu thrust fault, Taiwan, and the Mozumi right-lateral fault, Japan, provide an opportunity to characterize active faults in clastic sedimentary rocks and provide constraints to seismologic models. The northern Chelungpu fault has a 10-30 m wide primary damage zone characterized by dense fractures and chemical alteration. The southern Chelungpu fault has a 25-70 m wide primary damage zone characterized by dense fractures, alteration, intensely sheared rock, and secondary faults. The complexity of the damage zone, geochemistry, and clay mineralogy of the southern fault zone reflects its greater maturity (~1 Ma) relative to the northern fault zone (~46-100 Ka). A transition exists from smectite in exhumed fault core to illite-rich fault core at depth (200 - 1000 m) due to co-seismic fluid flow and radiated seismic energy. Clay composition plays a role in fault weakening.
Microstructures in deformed Mozumi siltstone indicate syn-tectonic fluid pressurization and flow, and shear concentrated in sericite-rich matrix. Kaolinite and illite clays dominate the host rock and fault breccia; illite, smectite, and kaolinite dominate clay-rich fault breccia. Whole-rock geochemistry shows a depletion of most oxides in fault rocks relative to unaltered host rock (up to ~90%). Resistivity values are depressed by 0-50 ohm-m, and νp and νs are decreased by ~0.30 km/s and ~0.40 km/s across the main fault relative to wall rock, and an average of ~0.70 km/s and ~1.0 km/s relative to host rock, respectively. Calculated values of Young’s modulus and Poisson’s ratio of fault rocks range from 16.2 to 44.9 GPa and 0.263 to 0.393, respectively. The protolith has a calculated Young’s modulus of 55.4 GPa and a Poisson’s ratio of 0.242. Lowest values of Young’s modulus and highest values of Poisson’s ratio correspond to fault breccia with high fluid content, and are offset from the most altered and damaged fault rocks. Fluid-rich pockets, and thus alteration, apparently migrate through the fault zone and may facilitate creep on the Mozumi fault because these fluid rich rocks are unable to sustain the shear stresses needed for brittle failure. The Chelungpu and Mozumi faults illustrate the temporally dynamic and heterogeneous nature of active fault zones.
Isaacs, Angela J., "Characterizing Deformation, Damage Parameters, and Clay Composition in Fault Zones: Insights from the Chelungpu Thrust, Taiwan, and Mozumi Right Lateral Fault, Japan" (2005). All Graduate Theses and Dissertations. 6059.