Selective melting processes as inferred from experimentally generated pseudotachylytes

High-speed friction experiments have been conducted on gabbros and granites to better constrain the process of frictional melting during seismogenic f ault motion. Experiments were done on cylindrical specimens of 25 mm in dia meter under a normal stress of 1.0-1.5 MPa and at slip fates up to 2 m/s us ing a rotary-shear high-speed frictional testing machine. The experimentall y-generated pseudotachylytes consist of a fused glassy matrix with abundant vesicles and angular or sub-angular to rounded fragments. These locally oc cur as injected network veins in the fractured rock. X-ray diffraction anal ysis has revealed the presence of as much as 40-70 wt% glass. The mineral c ontents of clasts in the experimental pseudotachylyte in granite as determi ned by X-ray diffraction analysis, indicate that quartz is the most resista nt to melting, biotite the least, and feldspar is intermediate. Thus, the S iO2 depletion in natural pseudotachylyte glass is likely to be due to the s elective melting of constituent materials during frictional melting. (C) 19 98 Elsevier Science Ltd. All rights reserved.