Earth Planets Space, Vol. 56 (No. 12), pp. 1209-1215, 2004
T. Toyoshima1, T. Obara2, T. Niizato3, H. Tanaka4, K. Shimada4, M. Komatsu5, Y. Wada6, and T. Koyasu6
1Graduate School of Science and Technology, Niigata University, Niigata 950-2181, Japan
2Japan Oil Development Co., Ltd., Tokyo 104-0033, Japan
3Horonobe Underground Research Center, Japan Nuclear Cycle Development Institute, Horonobe, Hokkaido 098-3297, Japan
4Department of Earth and Planetary Science, Graduate School of Science, the University of Tokyo, Tokyo 113-0033, Japan
5Department of Earth Science, Ehime University, Matsuyama 790-8577, Japan
6Department of Geology, Niigata University, Niigata 950-2181, Japan
(Received June 5, 2004; Revised November 20, 2004; Accepted December 3, 2004)
Many pseudotachylytes and their related fault rocks are found in the Hidaka metamorphic belt representing an ancient crustal section. On the basis of field observations of the pseudotachylytes and related fault rocks, nature of seismogenic faulting in the Hidaka crust is examined. The field observations suggest the following conclusions. (1) Two structural types of pseudotachylytes are distinguished: layer-parallel and layer-oblique. The latter are scattered in the metamorphic belt, but the former occur only in the southern part of the metamorphic belt. (2) An abundance of the layer-parallel pseudotachylytes suggests that earthquakes occurred repeatedly and frequently in the southern part, where complicated and duplicated crustal structures occur with many lowtemperature thin mylonite zones. The southern part with such crustal structures was an ancient seismogenic area containing asperities and having a radius of a few tens of kilometers in the Hidaka crust. In the seismogenic area, the layer-parallel pseudotachylytes resulted from seismic slip on the mylonitic foliation within the lowtemperature mylonite zones with strong preferred orientation of micas. (3) The layer-parallel pseudotachylytes and the subsequent layer-oblique pseudotachylytes post-date the latest and very-low-temperature mylonitization in the metamorphic belt. The former pseudotachylytes formed just above the upper side of the brittle-plastic transition zone.
Key words: Pseudotachylyte, mylonite, cataclasite, asperity, earthquake, Hidaka metamorphic belt.