TECTONIC SIGNIFICANCE AND FORMATION PROCESSES OF PSEUDOTACHYLITE NEAR JUNE LAKE, CA

Pseudotachylite is rare in the field but bears the information to understand regional tectonics and fault mechanics. The Aeolian Buttes pluton, located in the eastern Sierra Nevada near June Lake, CA, contains pseudotachylite and associated brittle-ductile shear fabrics. These fabrics provide insight into how brittle strain is localized in the seismogenic upper crust and elucidates the deformation and exhumation history of the study area.

In the field these pseudotachylites present as dark bands ranging from 1 mm to 10 cm thick and 10s to 100s of cm long. The pseudotachylites are associated with two sets of brittle slip surfaces: one steeply dipping and one shallowly dipping. The two shear planes crosscut each other and contain complex kinematic indicators. The overall strike of the brittle fabrics shows a curvature change from approximately E-W in the southern outcrop to N-W in the northern outcrop. Mylonite can often be traced from the strike of brittle fabrics, suggesting the shear formed at or near the brittle-ductile transition zone. We have obtained the U-Pb zircon ages of the Aeolian Buttes pluton (97.5 Ma) and two dikes crosscutting shear zones (97.3 and 96.4 Ma), pinpointing the timing of shear to 96-97 Ma.

Under the Scanning-Electron Microscope (SEM) the pseudotachylites show a cryptocrystalline matrix with microcrystallites of quartz, feldspar, amphibole, apatite, and zircon on the scale of 10s of microns. Quartz survivor clasts of ~100 microns account for ~5% of the grain composition and are evenly distributed in the matrix. These quartz clasts show embayed and scalloped edges, suggesting partial melting occurred at the quartz melting temperature of ~1700 C. The matrix also contains a large population of micron-scale, sub-spherical oxides, which were immiscible in the silica-rich melt. Together, these features are diagnostic of seismic origin pseudotachylite formed from the quenched melt created during shear heating.

To better understand the tectonic genesis of the pseudotachylites and associated shear zones, we will construct a cooling path from various thermochronometers and constrain the pressure-temperature-time history of the pluton. Scaling analysis will be used to constrain the heat budget and stress drop during pseudotachylite formation.