ABSTRACT
Shearing experiments on halite, calcite and mixed halite-calcite layers of about 0.7 mm-thick have been performed to study mechanical properties of fault, using a high temperature biaxial testing machine. Ambient temperature was increased in linear proportion to the normal stress on the simulated fault to study the effect of geothermal gradient. Results from halite shear zones provide an experimental strength profile of the lithosphere, which can be divided into brittle, intermediate, and fully-plastic regimes, with the peak strength slightly below the middle of the intermediate regime. Stick-slip (unstable fault motion) was recognized down approximately to the strength peak at which the velocity dependency of friction changes from the potentially unstable velocity weakening to the velocity strengthening behavior. Because both power and exponential laws can fit to the data of halite shear zones in the fully-plastic regime, the transitional mechanisms between high and low temperature plastic deformations may operate in this regime. Results from similar experiments on mixed halite-calcite shear zones indicate that the strength profile and the behavior of the shear zone are determined nearly solely by halite, the weaker mineral, even when halite content is as small as 5 vol. % at large displacements near the residual frictional strength. Existing fault models all contradict with these results and a new fault-zone model is proposed.
