Earth Planets Space, Vol. 54 (No. 11), pp. 1207-1210, 2002

## Growth of plastic shear zone and its duration inferred from theoretical consideration and observation of an ancient shear zone in the granitic crust

Hidemi Tanaka1, Bunichiro Shibazaki2, Norio Shigematsu3, Koichiro Fujimoto3, Tomoyuki Ohtani3, Yukari Miyashita3, Tomoaki Tomita4, Kentaro Omura5, Yoji Kobayashi4, and Jun Kameda1

1Department of Earth and Planetary Science, Graduate School of Science, University of Tokyo, Tokyo 113-0033, Japan
2International Institute of Seismology and Earthquake Engineering, Building Research Institute, Tatehara 1, Tsukuba, Ibaraki 305-0082, Japan
3Geological Survey of Japan, AIST, Tsukuba, Ibaraki 305-8567, Japan
4Institute of Geoscience, University of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan
5National Research Institute for Earth Science and Disaster Prevention, Tsukuba, Ibaraki 305-0006, Japan

(Received April 5, 2002; Revised August 24, 2002; Accepted October 6, 2002)

Abstract: A new model for growth of plastic shear zone is proposed based on the basis of a theory of fluid dynamics coupled with a rheological constitutive function, and is applied to a natural shear zone. Mylonite, ultramylonite and other ductile fault rocks are well known to deform in a plastic flow regime. The rheological behavior of these kinds of rocks has been well documented as a non-linear viscous body, which is empirically described as = Atn exp(-Q/RT), where : strain rate, t: shear stress, Q: activation energy, R: universal gas constant, T: absolute temperature, and A and n are constants. Strain rate- and temperature-dependent viscosity is obtained by differentiating the equation, and simplified by substituting n = 1. Then, substitution of the equation into a diffusion equation, d = 4, derives an equation d = 4[t/r · Aexp(-Q/RT)]1/2, where d: thickness of active layer of viscous deformation, n: kinematic viscosity, and r: density. The duration of creep deformation along the ancient plastic shear zone (thickness: 0.076 m) is estimated to be around 760 s, in a temperature range from 300 to 500°C. This estimation is rather good agreement with intermittent creep during inter-seismic period, than steady state creep or co-seismic slip.

Corresponding author E-mail: tanaka@eps.s.u-tokyo.ac.jp

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