PSEUDOTACHYLYTES IN FAULTS OF THE MT. ABBOT QUADRANGLE, SIERRA NEVADA: IMPLICATIONS FOR SEISMIC SLIP

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. Prior to this finding, these faults have been used to conceptualize mechanical models of fault evolution, and recently they have been proposed as analogues of fault dynamics during seismogenic slip. The discovery of pseudotachylyte in the Mt Abbot faults lends legitimacy to using these faults as a natural laboratory for the study of earthquake processes. The pseudotachylytes are less than 0.3 mm thick and are found in faults accommodating less than 20 cm of offset in association with other fault rocks and mineral fillings. Field and microstructural evidence indicate that the faults exploited pre-existing mineralized joints and show the following overprinting structures: quartz veins, mylonites (> 400 oC), cataclasites and pseudotachylytes (~250 oC) more or less coeval with epidote veins, and zeolite veins (< 200 oC). The initial shear deformation (mylonites) of preexisting joints occurred under aseismic (i.e., ductile) conditions; cataclasis and pseudotachylyte generation were dominant in the later stages of shearing; and only a fraction of the total offset was associated with seismic slip. The presence of pseudotachylyte in sub-millimeter thick zones lends supports to 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 product of seismic slip in similar fault rocks, and pseudotachylytes may be more common than typically thought.