THE GEM LAKE SHEAR ZONE - CRETACEOUS DEXTRAL TRANSPRESSION IN THE NORTHERN RITTER RANGE PENDANT, EASTERN SIERRA-NEVADA, CALIFORNIA

The Gem Lake shear zone is a northwest striking, steeply dipping, dext ral transpressional shear zone that provides the first direct evidence for dextral deformation in wall-rock pendants in the central part of the Sierra Nevada batholith. The Gem Lake shear zone is a minimum of 3 0 km in length and extends at least from the north end of the Northern Ritter Range pendant to the southeast edge of the Ritter Range pendan t. The amount of displacement on the zone is uncertain, but matching f ault slivers of Pennsylvanian(?) marble in the Northern Ritter Range p endant to similar exposures north of the Mount Morrison pendant sugges ts a minimum dextral offset of 20 km. The Gem Lake shear zone was acti ve in early Late Cretaceous time, from before 91 Ma (the age of the sy ntectonic granodiorite of Kuna Crest) to at least 80 Ma (Ar-40/Ar-39 a ge of syndeformational mica from the shear zone in the Ritter Range pe ndant). Deformation in the Gem Lake shear zone is characterized by com bined dextral simple shear and subvertical stretching, which are varia bly partitioned in anastomosing high-strain zones. In the shear zone a t Gem Lake, predominantly dextral deformation is indicated by porphyro clast asymmetries, S-C fabric, and asymmetric crenulations; a componen t of subvertical stretching is indicated by a moderately to steeply pl unging stretching lineation. In a segment of the shear zone at Kuna Cr est, a strongly developed stretching lineation indicates predominantly subvertical stretching, with lesser component of dextral strike slip. The Gem Lake shear zone is considered to be part of a proposed region al system of shear zones in the eastern Sierra Nevada, the Sierra Cres t shear zone system. This dextral transpressional system was active pr ior to, and synchronous with, intrusion of the Late Cretaceous Sierra Nevada batholith; deformation ceased shortly after pluton emplacement. The Sierra Crest shear zone system includes possibly related shear zo nes as far north as Saddlebag Lake pendant and at least as far south a s Oak Creek pendant and the Owens Valley, indicating a possible strike length greater than 150 km.