Thin pseudotachylytes in faults of the Mt. Abbot quadrangle, Sierra Nevada: Physical constraints for small seismic slip events

We document the occurrence of pseudotachylyte (solidified melt produced during seismic slip) along strike-slip faults in the Lake Edison granodiorite of the Mt. Abbot quadrangle, Sierra Nevada, California and provide constraints on ambient conditions during seismic faulting. The pseudotachylytes are less than 0.3 mm thick and are found in faults typically up to 1 cm in thickness. Total measured left-lateral offset along sampled faults is approximately 20 cm. Field and microstructural evidence indicate that the faults exploited pre-existing mineralized joints and show the following overprinting structures (with inferred ambient temperatures): mylonites are more or less coeval with quartz veins (>400 °C), cataclasites and pseudotachylytes (∼250 °C) more or less coeval with epidote veins, and zeolite veins (<200 °C). Based on observations of the microstructural textures of faults combined with theoretical heat transfer and energy budget calculations, we suggest that only a fraction (<30%) of the total offset was associated with seismic slip (i.e. pseudotachylyte). The presence of pseudotachylyte in sub-millimeter thick zones lends support for the concept of extreme shear localization during seismic slip. The elusive nature of these pseudotachylytes demonstrates that observations in outcrop and optical microscope are not sufficient to rule out frictional melting as a consequence of seismic slip in similar fault rocks.