Tribochemical wearing in S-C mylonites and its implication to lithosphere stress level

摘要

A new approach for revealing the brittle origin of C -surfaces as localized high shear strain zones in S-C mylonites (mylonites with C -surfaces cutting through a mylonitic S -foliation) is presented. A compiled worldwide catalog of width ( W ) and displacement ( D ) data for shear zones indicates that ductile mylonites show a constant W / D ratio of 10^-0.3 and ratios of brittle 'cataclasites' vary in magnitude from 10^-1 to 10^-3, implying that the ratio is a diagnosis for discriminating ductile and brittle shear zones. A newly measured W - D data of shear displaced minerals along C -surfaces in granitic S-C mylonites from the Hatagawa shear zone in northeast Japan is added on the worldwide W - D catalog, being plotted on a brittle origin with the high W / D ratio of 10^-1.5. Using this result and a tribochemical wear theory which accounts for wear formation under hydrothermal conditions, C -surfaces in the S-C mylonite might have been formed by cataclastic deformation under the lithosphere stress level of ca. 300 MPa at temperature of 400°C with water for granite. This result suggests a high lithosphere stress level at the depth of the S-C mylonite formation where deformation is predominantly plastic.