Pseudotachylite Bearing Cretaceous Fault in the Saddlebag Lake Pendant, Central Sierra Nevada, CA

Over the past several years the undergraduate researchers and mentors in the University of Southern California’s Undergraduate Team Research program has mapped the northern continuation of the Gem Lake shear zone from Gem Lake to Virginia Canyon near the north end of the Saddlebag pendant. In the center of this dominantly dextral, ductile shear zone we now recognize a pseudotachylite bearing brittle fault that often juxtaposes Triassic metavolcanics to the east of the fault with a Jurassic metasedimentary package to the west of the fault. Kinematic indicators such as slickenlines, steps, and offset dikes found within the brittle fault zone also suggest dextral oblique motion, similar to the motion of the ductile shear zone. The brittle fault dips steeply and strikes N-NW with the fault zone width varying from narrow (sub m scale) to a 100-200 m wide fracture zone as seen in the Sawmill area. Jurrasic metasediments (> 177Ma) and Cretaceous metavolcanics (110-95Ma) lie to the West of the fault and Triassic metavolcanics (219Ma) lie to the East of the fault in the Virginia Canyon, Saddlebag Lake, and Sawmill areas. The absence of ~45 million years of Jurassic metavolcanics along the contact of the fault in each area, suggests tectonic removal of the sequence. Pseudotachylite, quartz vein rich breccias, gouge, fault scarps, and truncated Cathedral Peak dikes (~88 Ma) originating from the Tuolumne Batholith (TB), are common features associated with the brittle fault. The truncated, 88 Ma Cathedral Peak dikes plus nearby biotite cooling ages of 82 Ma indicate that displacement on the brittle fault continued well after TB emplacement and cooling and likely continued after ~80 Ma. The pseudotachylite suggests earthquakes occurred on the brittle fault during the Cretaceous. Movement also occurred along the fault at fairly shallow depths as indicated by the presence of vugs, or cavities with free euhedral crystal growth, within the quartz vein breccias. In the Sawmill Canyon area, located immediately southwest of Saddlebag Lake, the fault zone widens and is rich with evidence of brittle faulting including the quartz vein breccias similar to other study areas and localized pseudotachylite veins and breccias with angular rock fragments varying from <1 to > 4 cm in size. The fault branches off into several different strands within this zone, each associated with the quartz breccias and pseudotachylite common in the area. Outcrop scale kink bands found along some of the fault strands, suggesting late brittle faulting with the decrease of regional strain. Brittle faulting in the Sawmill area may be further complicated by large-scale boudinage associated with the faulting in the area. Some of the contacts in this area identified as fault strands may in part be large, fluid-filled cracks associated with bending (tension due to scar folding?) during boudinage of the metasedimentary package and nearby margin of the Tuolumne batholith in the area.