MIXED MODE DEFORMATION IN THE TRANSPRESSIONAL GRANT-PARKER SHEAR ZONE, EASTERN SIERRA NEVADA, CA

Characterizing the geomechanics of arc shear zones is important for understanding physical processes that occur within arc crust. We present field and microstructural observations of a NW-trending transpressional dextral shear zone likely active during Cretaceous Sierra Nevada batholith construction, with evidence of both ductile and brittle deformation. The Grant-Parker shear zone lies roughly parallel to the Cretaceous Gem Lake shear zone (~5 km to the SW and similar in character) suggesting that the Grant-Parker shear zone is part of the Sierra Crest shear zone system. The Grant-Parker shear zone is localized mostly in the Cretaceous Aeolian Buttes and June Lake plutons, with only sparse deformation in the older, late Triassic Lee Vining Canyon pluton, suggesting that strain localized within the younger intrusions. We infer that deformation may have occurred during or just after emplacement of the two Cretaceous plutons (~80-90 Ma), which overlaps with deformation in the nearby Gem Lake shear zone.

The Grant-Parker shear zone is exposed for ~7 km, striking W-NW with a maximum deformation zone width < 1 km wide, typically narrower (~300m). Meter-scale mylonites strike NW and dip steeply to subvertically to the SW, and have stretching lineations trending SE and plunging between ~40-75°. Transpressional shear-sense indicators include en echelon gashes and sigma and delta clasts of feldspars observed in thin section which usually indicate north-side down dextral compression, and possible signs of reversal. S-C’ shear bands are recognized particularly within zones of distributed deformation adjacent to higher strain mylonitic bands. Pseudotachylite is clearly recognizable in an outcrop with distinctive injection veins, and is concordant with the mylonitic fabrics. Whole rock geochemical analysis of the pseudotachylite corroborates a frictional melting origin. Thin distributed seams of likely pseudotachylite occur in other outcrops, and are offset by later brittle deformation characterized by slickensides on blocks of outcrops, generally subhorizontal, and often trending E-W. Thus the Grant-Parker shear zone records mixed mode deformation, with periods of ductile deformation punctuated by seismic events, within plutons emplaced during the main Cretaceous episode of Sierra Nevada arc magmatism.