Dating the duration of deformation along a major shear zone within the Eastern Peninsular Ranges Batholith

Adam Brackman, Jeremy A. Torres, Joshua Schwartz, Elena A. Miranda, & Sinan O. Akciz

Published August 15, 2019, SCEC Contribution #9866, 2019 SCEC Annual Meeting Poster #100 (PDF)

Poster Image: 
Understanding the formation and evolution of continental crust in Southern California is a complex inverse problem due to the ever-evolving tectonics of the region. Determining an accurate date for the formation of the unaltered protolith of the Eastern Peninsular Ranges (EPR) Batholith will help constrain the maximum age of deformation in the Borrego Springs Shear Zone (BSSZ) and Santa Rosa Mylonite Zone (SRMZ) that transect the batholith. These new age constraints will in turn contribute to the development of the SCEC Community Rheology Model by better describing the duration of deformation and the properties of these shear zones that serve as exposed analogs to the shear zones that extend beneath the brittle faults of the southern San Andreas Fault system. Three samples were selected from mylonitic shear zone rocks and two samples were chosen from cross-cutting dikes within the EPR; all were mechanically separated to obtain zircon grains for U-Pb zircon geochronology. We collected cathodoluminescence images via scanning electron microscope (SEM) and laser ablation sector field inductively coupled plasma mass spectrometer (LA-SF-ICPMS) data to present new 206Pb/238U zircon ages. In deformed host rocks, we targeted core and rim domains to capture the timing of igneous crystallization and subsequent metamorphism associated with ductile deformation. These analyses help provide a better picture of the geologic history of the region and aid in understanding the ductile shear zone components of the San Andreas Fault system and the complexity of the North American-Pacific tectonic boundary.

Brackman, A., Torres, J. A., Schwartz, J., Miranda, E. A., & Akciz, S. O. (2019, 08). Dating the duration of deformation along a major shear zone within the Eastern Peninsular Ranges Batholith. Poster Presentation at 2019 SCEC Annual Meeting.

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