Microstructural analysis on experimentally sheared limestone gouge What causes velocity weakening?

Abstract

A recent experimental study on fault gouges prepared from limestone samples taken from the Longmen Shan Fault Zone, which hosted the Great Wenchuan Earthquake of 2008, showed unstable, velocity weakening slip in low velocity experiments performed at temperatures of 100-150°C but stable, velocity strengthening slip at lower temperatures. Velocity weakening is the process of shear strength reduction with increasing sliding velocities and is a requirement for seismogenesis. The frictional properties were studied macroscopically with help of the Rate and State dependent Friction law. However, the resulting data do not enable identification of the microphysical processes which cause velocity weakening in the limestone gouge. To determine the process causing velocity weakening, the simulated fault gouge has been examined under the light and electron microscope, with magnifications ranging from 5 to 150.000 times. Calcite twins and fractures were observed and the density of these structures seems to increase with temperature. The 150°C limestone sample shows a more chaotic microstructure, with the R-shear bands being less common, broader and less recognizable compared to the 25°C-100°C sheared limestone gouges. Calcite twinning is also very common in the high temperature experiment. Quantitative analysis of the grain size distribution shows that the smallest grain sizes (minimum grain size ~50 nm) barely change whereas large polycrystalline clusters (~15 µm) do show a decrease in size. Comparison of the mechanical and microstructural data with previous microphysical models for the velocity dependence of slip suggests that possible microphysical reasons for velocity weakening includes competition between frictional viscous flow, dilatation, -and compaction due to granular rearrangement plus pressure solution and/or minor crystal plastic flow (twinning). The very small grain size developed in the samples might speed up processes like diffusion, allowing such mechanisms to control slip at relatively high rates.