Natural Resources Management and Environmental Sciences Department
BS in Earth Sciences
The Sierra Nevada batholith represents an archetypal continental arc, although the style of deformation throughout the history of the arc is only poorly understood, in part due to the voluminous nature of Late Cretaceous magmatism that has overprinted earlier structures. The purpose of this study is to gain a better understanding of pre-Late Cretaceous deformation in the Sierra Nevada batholith through a structural and geochronological study of the Jurassic White Fork pluton. A Jurassic age for the White Fork pluton is indicated by its inclusion of numerous 148 Ma Independence dikes, and geochronology on the White Fork pluton is limited to discordant U–Pb zircon dates of ~156 Ma. We determined new U–Pb zircon ages using laser ablation ICPMS on two granodiorites from the White Fork pluton and one quartz monzonite correlated with the Jurassic Diamond pluton. The ages were indistinguishable within error, yielding ages of 166.4 +3.5/-3.6 and 164.8 +3.3/-3.3 Ma for the granodiorite samples, and 167.0 +3.5/-3.6 Ma for the quartz monzonite. These ages are statistically identical to the adjacent 165 Ma Woods Lake Pluton, and similar to other Jurassic plutons exposed at this latitude in the Sierra Nevada. With respect to deformation, at least three distinct sets of ductile shear zones that form cm- to tens of m- wide anastomosing shear zones were identified in a transect across the White Fork pluton and into the Woods Lake pluton: steeply NE-dipping shear zones with steeply SE-plunging lineations and E-side-up shear sense, shallowly SE-dipping shear zones with down-dip lineations and thrust shear sense, and shallowly SW-dipping shear zones with down-dip lineations and thrust shear sense. All shear zones deform the Independence dikes and are absent from adjacent Late Cretaceous plutons in the region indicating an age range of 148–92 Ma for these ductile fabrics, and shallowly SW-dipping shear zones are locally observed cutting steeply NE-dipping shear zones suggesting that they occurred later in the strain history of the pluton. NE–SW shortening suggested by steeply NE-dipping, and shallowly SW-dipping shear zones is similar to strain observed in the nearby Sawmill Lake shear zone, and suggests that intra-arc contractional strain may have been more regional in nature in the Sierra Nevada batholith in the Early to Late Cretaceous.