The transition from cataclasis to intracrystalline plasticity in experimental shear zones

Abstract

The change with depth in the Earth's crust from cataclastic faulting to dislocation-dominated (intracrystalline-plastic) ductile flow is correlative with many geological and geophysical phenomena. An analogous transition is achieved in polycrystalline halite layers sheared at conditions of constant normal-stresses between 10 and 90 MPa, temperatures between 22 and 250 °C and shear-strain rates between 3(10^-1) and 3(10^-5) s^-1. From conditions of low temperatures, low normal stresses and high strain rates to conditions of high temperatures, high normal stresses and low strain rates the following transitions occur: frictional to pressure-insensitive flow, unstable to stable shearing, rate-weakening to rate-strengthening behavior, localized faulting to homogeneous flow, and cataclastic to crystal-plastic deformation mechanisms. These transitions do not necessarily coincide. The long-term shear strength of halite is described with a constitutive model that assumes at least three independent flow mechanisms are operative. The mechanisms are non-frictional (pressure-insensitive) intracrystalline-plastic flow by dislocation motion, frictional semi-brittle flow by the combined action of brittle and plastic mechanisms and frictional cataclastic faulting by dominantly brittle mechanisms. The constitutive model predicts the following mechanical and physical characteristics of faulting in the crust: 1) Maximum shear strength occurs at the transition from frictional to pressure-insensitive flow, which is a function of pressure and temperature gradients, and strain rate. 2) An abrupt change in the shear-strength gradient occurs in the frictional regime at the transition from cataclastic faulting to semi-brittle flow. This occurs at depths that are approximately one-third to two-thirds that of the maximum shear strength and approximately coincides with the transition from rate-weakening to rate-strengthening behavior. 3) If rate-weakening is a necessary condition for slip instability, then the base of the seismogenic layer occurs in the frictional regime well above the depth of maximum shear strength. 4) At steady-state in the cataclastic faulting regime, strain localization is associated with rate-weakening and the width of the comminuted zone should increase with an increase in depth or a decrease in strain rate. 5) The change from dominantly cataclastic to crystal-plastic deformation mechanisms, as indicated by the wear products (fault-rocks), occurs in the semi-brittle flow regime.