Tectonic significance and formation processes of pseudotachylite-bearing shear zones near June Lake, CA
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Authors
Hart, Olivia
Issue Date
2024
Type
Thesis
Language
en_US
Keywords
June Lake , Pseudotachylite , Structure , Tectonics
Alternative Title
Abstract
The Grant-Parker shear zone in Mono Basin, CA is a ~NNW striking, pseudotachylite-bearing shear zone developed within the Late Cretaceous Aeolian Buttes pluton along the eastern edge of the Sierra Nevada Batholith. Previous studies have linked this shear zone to either a ring fracture and the Mono craters or Cretaceous intra-arc deformation. If this shear zone is part of a ring fracture, the structure could indicate the presence of a large magma body feeding the Holocene eruptions of the Mono Craters. If formed from intra-arc deformation, the shear zone could provide insight into how strain is partitioned in the seismogenic upper crust. Yet, the geometry, kinematics, and timing of the Grant-Parker shear zone have not yet been rigorously studied. To better understand the tectonic significance of the shear zone and the formation process of the pseudotachylite, I combined field research with analytical and numerical methods and characterized the structure and dynamics of the shear zone. Field observations show the shear zone is ~100-200 m wide, ~8 km long, and contains mylonite, cataclastic flows, pseudotachylite, and slip surfaces. Kinematics largely show reverse and dextral motion, though some localities show abundant sinistral-sense motion. Cross-cutting relationships and U-Pb zircon dating constrain the age of the shear zone to ~96-97 Ma. Microstructural and scanning electron microscope analysis confirm both the kinematics observed in the field and the presence of melt-origin pseudotachylite. Al-in-hornblende and plagioclase thermobarometry suggest an emplacement pressure of ~1.3 kbar and a solidus temperature of ~615°C. Quartz c-axis patterns and paleo-piezometry show ductile deformation occurred in a temperature range of >600 to ~300 °C at a flow stress of ~80-90 MPa. A thermo-kinematic simulation shows the shallowly emplaced Aeolian Buttes pluton cooled to background crustal temperatures in under 1 Myr. Mylonite, cataclastic flows, and pseudotachylite are interpreted to form during this <1 Myr cooling period. The pseudotachylite was formed within the brittle regime under a differential stress of ~113-318 MPa. Using an energy balance equation, I estimate fault slips of ~2-5 m (if strike-slip faulting) or ~1 m (if thrust faulting). Moment magnitudes associated with these displacements are likely ~7M. Based on the above observations and comparison with other shear zones in the central Sierra Nevada, I conclude that the shear zone is not part of the hypothetical ring fracture related to the Mono craters. Instead, I suggest the Grant-Parker shear zone reflects the Late Cretaceous kinematic transition from contraction to dextral transpression in response to the shift from arc-normal to oblique convergence between the Farallon and North American plates. The existence of the pseudotachylite within the shear zone suggests such a transition can cause large earthquakes in the seismogenic upper crust.