Selective melting processes as inferred from experimentally generated pseudotachylytes

Abstract

High-speed friction experiments have been conducted on gabbros and granites to better constrain the process of frictional melting during seismogenic fault motion. Experiments were done on cylindrical specimens of 25 mm in diameter under a normal stress of 1.0–1.5 MPa and at slip rates up to 2 m/s using a rotary-shear high-speed frictional testing machine. The experimentally-generated pseudotachylytes consist of a fused glassy matrix with abundant vesicles and angular or sub-angular to rounded fragments. These locally occur as injected network veins in the fractured rock. X-ray diffraction analysis has revealed the presence of as much as 40–70 wt% glass. The mineral contents of clasts in the experimental pseudotachylyte in granite as determined by X-ray diffraction analysis, indicate that quartz is the most resistant to melting, biotite the least, and feldspar is intermediate. Thus, the SiO 2 depletion in natural pseudotachylyte glass is likely to be due to the selective melting of constituent materials during frictional melting.