Trace element anomaly in fault rock induced by coseismic hydrothermal reactions reproduced in laboratory friction experiments

Friction experiments at high velocities (0.1 to 0.4m/s) at room temperature under high pore fluid pressures (2 or 5MPa) and trace element analyses of the run products were carried out on simulated fault gouge derived from the Chelungpu fault, Taiwan. The friction coefficient decreased to similar to 0.1 during sliding, and the slip surface temperature reached 300 degrees C by 6m of slip displacement. The slip caused a small, but distinct decrease of Li in the deformed gouge when the slip surface temperature exceeded 300 degrees C. The observed Li depletion agreed with calculated results, indicating that it resulted from a hydrothermal reaction at a high temperature produced by short-duration (up to 40s) frictional heating. The geochemical data suggest a small fluid/rock ratio and low reaction temperature in the simulated fault, which is explained by higher normal stress or repeated earthquakes which elevated fluid-rock ratios in the natural fault.