TECTONIC IMPLICATIONS OF DUCTILE SHEAR ZONES IN THE JURASSIC KEITHS DOME PLUTON, NORTHERN SIERRA NEVADA BATHOLITH, CALIFORNIA

Structural data were collected on four Jurassic and Cretaceous plutons near Lake Tahoe, to facilitate interpretation of regional structure and to evaluate changing tectonic regimes inferred for the arc to the south; transpressional during the Mid-Late Jurassic (Saleeby and Busby-Spera, 1992), and neutral to weakly extensional in the mid-Cretaceous (Tobisch et al., 1995). Steep (>70°), N-NW striking, <1 cm to >30 m wide ductile shear zones deform the Jurassic Desolation Valley and Keiths Dome plutons of the Crystal Range Suite, and Mesozoic metavolcanic and metasedimentary host rocks. The highest concentration (85% of all measured zones) is in the Keiths Dome pluton. Rakes of lineations in the zones are mostly >45°. West-side up, reverse motion is dominant, whereas the strike-slip component is inconsistent. Deformation temperatures ranging from ~300 °to >550° are suggested by fragmented plagioclase, bent biotite grains and small, recrystallized quartz in lower-T zones, and subgrains, grain boundary migration, and larger, equant, recrystallized quartz and feldspar in higher-T zones. The shear zones locally deform steep (≥77°), E-NE striking mafic dikes which cut the Keiths Dome pluton and other Jurassic plutons of the Crystal Range Suite, including the ~164 Ma Pyramid Peak granite (Loomis, 1983; Sabine, 1992). Temporal resolution of major ductile shear is restricted by limited geochronologic data; shear zones deform <164 Ma dikes and are truncated by the ~91 Ma Echo Lake intrusion. Orientations of the ductile shear zones in the Jurassic plutons and host rocks are largely concordant to those of the regional foliation, and discordant to pluton contacts in places, indicating that the shear zones record tectonic strain rather than emplacement. The strain field inferred from the N-NW striking reverse shear zones and E-NE striking subvertical dikes is one of E-NE contraction combined with N-NW subhorizontal extension (from dikes) and subvertical stretching (from lineations). This is compatible with regional transpression, such as proposed to the south in the Mid-Late Jurassic.