Pseudotachylyte in the Bench Canyon Shear Zone, central Sierra Nevada, California: Frictional melting in the brittle and semi-brittle fields

Many aspects of pseudotachylyte are controversial, particularly whether it is the product of intense comminution (e.g. ultracataclasite'') or frictional melting. Ubiquitous exposures of pseudotachylyte in the Bench Canyon shear Zone (BCSZ), central Sierra Nevada, California, provide an excellent opportunity for further study. Scanning electron microscopy (SEM) reveals vesicles, amygdules, crystallites and embayments of microxenocrysts, textures which are supportive of a melt origin for pseudotachylyte in the BCAZ. EDS and microprobe analyses indicate strong compositional contrasts between pseudotachylyte and granodiorite host; one explanation for this is preferential melting in order of individual mineral melting points.