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, dextral 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 30 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 pendant. The amount of displacement on the zone is uncertain, but matching fault slivers of Pennsylvanian(?) marble in the Northern Ritter Range pendant to similar exposures north of the Mount Morrison pendant suggests a minimum dextral offset of 20 km. The Gem Lake shear zone was active in early Late Cretaceous time, from before 91 Ma (the age of the syntectonic granodiorite of Kuna Crest) to at least 80 Ma (40Ar/39Ar age of syndeformational mica from the shear zone in the Ritter Range pendant). Deformation in the Gem Lake shear zone is characterized by combined dextral simple shear and subvertical stretching, which are variably partitioned in anastomosing high-strain zones. In the shear zone at Gem Lake, predominantly dextral deformation is indicated by porphyroclast asymmetries, S-C fabric, and asymmetric crenulations; a component of subvertical stretching is indicated by a moderately to steeply plunging stretching lineation. In a segment of the shear zone at Kuna Crest, 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 regional system of shear zones in the eastern Sierra Nevada, the Sierra Crest shear zone system. This dextral transpressional system was active prior 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 zones as far north as Saddlebag Lake pendant and at least as far south as Oak Creek pendant and the Owens Valley, indicating a possible strike length greater than 150 km.