TERRAPUB Earth, Planets and Space

Earth Planets Space, Vol. 56 (No. 12), pp. 1217-1223, 2004

Occurrence of mylonite zones and pseudotachylyte veins around the base of the upper crust: An example from the southern Hidaka metamorphic belt, Samani area, Hokkaido, Japan

Koji Shimada1, Hidemi Tanaka1, Tsuyoshi Toyoshima2, Tomohiro Obara3, and Tadafumi Niizato4

1Department of Earth and Planetary Science, The University of Tokyo, Hongo, Tokyo 113-0033, Japan
2Department of Geology, Niigata University, Igarashi, Niigata 950-2181, Japan
3Japan Oil Development Co. Ltd., Shinkawa, Tokyo 114-0033, Japan
4Horonobe Underground Research Center, Japan Nuclear Cycle Development Institute, Horonobe, Hokkaido 098-3297, Japan

(Received May 31, 2004; Revised December 28, 2004; Accepted December 29, 2004)

Abstract: Geological studies on exhumed pseudotachylyte-bearing mylonite zones in S-type tonalite were carried out in the southern Hidaka metamorphic belt, Hokkaido, Japan. Mylonitization is characterized by (1) development of composite planar fabrics, (2) grain size reduction, and (3) change in modal composition an increase in mica content and a decrease in quartz content from protolith to mylonite. Mylonite zones are heterogeneously concentrated in the host rocks. At microscopic scales, shear deformation is concentrated heterogeneously in finegrained layers along C-surfaces. Most of the pseudotachylyte layers are subparallel to the C-surface, and tend to overprint thick mylonite zones. The heterogeneous development of mylonite zones, which may be activated as layers of co-seismic slip, should be incorporated into numerical modeling of seismogenic zones.
Key words: Pseudotachylyte, mylonite, strain localization, Hidaka metamorphic belt.

Corresponding author E-mail: faultrockshimada@mac.com

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