Rheology of the Borrego Springs Shear Zone segment of the Eastern Peninsular Ranges Mylonite Zone: implications for the development of the Community Rheology Model

Elena A. Miranda, Miguel Zamora-Tamayo, Joshua Schwartz, Jennifer Bautista, & Sinan O. Akciz

Published August 15, 2019, SCEC Contribution #9863, 2019 SCEC Annual Meeting Poster #154

We define and constrain the rheology of mylonites in the Eastern Peninsular Ranges Mylonite Zone (EPRMZ) in order to place realistic constraints on the strength of ductile shear zones as part of the development of a Community Rheology Model (CRM) for Southern California. These exposed rocks serve as an analog for the deep crustal, ductile shear zones beneath the brittle-ductile transition (BDT) of faults belonging to the southern San Andreas Fault (SAF) system. Oriented field samples were collected from the Santa Rosa Mylonite Zone and Borrego Springs Shear Zone segments of the EPRMZ and were subsequently cut and polished into billets. We completed microstructural and electron backscatter diffraction (EBSD) analyses on a granodiorite mylonite sample from the Borrego Springs Shear Zone. The quartz grains in the samples display deformation microstructures indicative of subgrain rotation (SGR) recrystallization, and are characterized by an average grain diameter of 22.3 microns. These quartz grains show a strong crystallographic preferred orientation (CPO), with strong c-axis maxima indicative of rhomb <a> and prism <a> slip. The plagioclase grains in the sample have microstructures indicative of bulging grain boundary (BLG) recrystallization and some limited subgrain rotation recrystallization, and the grains have an average grain diameter of 9.9 microns. The plagioclase grains have a weak, non-random CPO indicative of slip on the {011} plane in the <110> direction. The fine-grained plagioclase grains derived from BLG recrystallization also show evidence of phase mixing with quartz and biotite in the tails of feldspar porphyroclasts. From the microstructural observations and CPO data, we interpret that both minerals primarily deform via dislocation creep at amphibolite-grade deformation temperatures (about 500-600˚C), but quartz deforms more easily by dislocation creep due to enhanced dislocation climb relative to plagioclase. In addition, phase mixing indicates that grain size sensitive deformation also contributes to shear zone development, and may significantly enhance strain localization. Our results indicate that the rheology of the Borrego Springs Shear Zone is characterized by both dislocation creep of quartz and grain size sensitive phase mixing, and that CRM models should incorporate a component of grain size sensitivity in order to avoid over-estimating shear zone strength.

Citation
Miranda, E. A., Zamora-Tamayo, M., Schwartz, J., Bautista, J., & Akciz, S. O. (2019, 08). Rheology of the Borrego Springs Shear Zone segment of the Eastern Peninsular Ranges Mylonite Zone: implications for the development of the Community Rheology Model. Poster Presentation at 2019 SCEC Annual Meeting.

Related Projects & Working Groups
Fault and Rupture Mechanics (FARM)