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

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 , where : strain rate, τ: 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, , derives an equation δ = 4[t/p · A exp(−Q/RT)]1/2, where δ: thickness of active layer of viscous deformation, ν: kinematic viscosity, and ρ: 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.

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Correspondence to Hidemi Tanaka.

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Tanaka, H., Shibazaki, B., Shigematsu, N. et al. Growth of plastic shear zone and its duration inferred from theoretical consideration and observation of an ancient shear zone in the granitic crust. Earth Planet Sp 54, 1207–1210 (2002). https://doi.org/10.1186/BF03353321

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Keywords

  • Shear Zone
  • Creep Deformation
  • Steady State Creep
  • Plastic Regime
  • Viscous Deformation