Clay gouge |
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| Institution: | 1. Department of Chemistry, Biology and Marine Science, Faculty of Science, University of the Ryukyus, 1, Senbaru, Nishihara, Okinawa 903-0213, Japan;2. Graduate School of Environmental Studies, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8601, Japan;3. Department of Chemical Oceanography, Atmosphere and Ocean Research Institute, The University of Tokyo, 5-1-5, Kashiwanoha, Kashiwa-shi, Chiba 277-8564, Japan;4. Department of Geoscience, Taiwan National University, Taipei, Taiwan;5. Department of Earth and Planetary Sciences, Graduate School of Sciences, Kyushu University, 6-10-1, Hakozaki, Fukuoka 812-8581, Japan;6. School of Marine Science and Technology, Tokai University, 3-20-1, Orito, Shimizu-ku, Shizuoka 424-8610, Japan |
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| Abstract: | Clays are a common component of fault gouge, but their genesis and importance in fault evolution is poorly understood. We present preliminary evidence that clays participate in extensive mineral reactions and microfabric changes during faulting. Rather than thinking of clay reactions as a consequence of mechanical processes or fault localization following diagenetically altered horizons, we see the interplay between clay mineral reactions and mechanical processes as a single, integrated process. Furthermore, faulting may lower kinetic barriers to low-temperature (～100°C) mineral reactions that are common in sedimentary rocks.Our most striking example of fault diagenesis-deformation is a profile of %illite in mixed-layer illite/smectite in shales beneath the Lewis Thrust, Canada. Whereas burial diagenesis caused minimal smectite-to-illite reaction, shales within meters of the thrust are almost completely converted to illite. The consequences of these changes are manifested in geochemical, geochronologic and microfabric characteristics of clay gouge. In this example, faulting has helped overcome a kinetic barrier in the smectite-to-illite reaction without appreciable addition of heat. In another example we infer that dissolution–precipitation reactions continue during faulting even when smectite has already been completely transformed to illite.If mineral reactions intimately interact with mechanical processes in shallow-crustal faults, then our current understanding of the mechanical and hydraulic properties of fault zones may be incomplete. Syndeformational mineral reactions and associated fabric changes could make faults much weaker than would be expected from evaluation of the static mineral assemblage of gouge and single crystal properties. Syndeformational mineral reactions may promote fault slip (affecting earthquake activity) in gouge-bearing faults under stress conditions considerably lower than predicted from static mineral properties. In addition, fault-induced dissolution-precipitation reactions may contribute to fault localization. |
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