*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 Tanaka^{1}, Bunichiro Shibazaki^{2}, Norio Shigematsu^{3}, Koichiro Fujimoto^{3}, Tomoyuki Ohtani^{3}, Yukari Miyashita^{3}, Tomoaki Tomita^{4}, Kentaro Omura^{5}, Yoji Kobayashi^{4}, and Jun Kameda^{1}

^{1}Department of Earth and Planetary Science, Graduate School of Science, University of Tokyo, Tokyo 113-0033, Japan

^{2}International Institute of Seismology and Earthquake Engineering, Building Research Institute, Tatehara 1, Tsukuba, Ibaraki 305-0082, Japan

^{3}Geological Survey of Japan, AIST, Tsukuba, Ibaraki 305-8567, Japan

^{4}Institute of Geoscience, University of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan

^{5}National 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 = A*t*^{n} 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* · *A*exp(-*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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