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Earth Planets Space, Vol. 54 (No. 11), pp. 1103-1108, 2002

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

Norihiro Nakamura1 and Hiroyuki Nagahama2

1Research Center for Higher Education, Tohoku University, Sendai 981-8576, Japan
2Institute of Geology and Paleontology, Tohoku University, Sendai 980-8578, Japan

(Received January 15, 2002; Revised August 6, 2002; Accepted August 28, 2002)

Abstract: 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.


Corresponding author E-mail: n-naka@mail.cc.tohoku.ac.jp


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