Open Access

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

  • Hidemi Tanaka1Email author,
  • Bunichiro Shibazaki2,
  • Norio Shigematsu3,
  • Koichiro Fujimoto3,
  • Tomoyuki Ohtani3,
  • Yukari Miyashita3,
  • Tomoaki Tomita4,
  • Kentaro Omura5,
  • Yoji Kobayashi4 and
  • Jun Kameda1
Earth, Planets and Space201454:BF03353321

https://doi.org/10.1186/BF03353321

Received: 5 April 2002

Accepted: 6 October 2002

Published: 26 June 2014

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.