194-6 Exhumation of the Orocopia Schist and the development of the Eastern California Shear Zone, California

Session: Broad Applications of Thermochronology to Understanding Geologic Rates and Processes Through the Sedimentary Record (Posters)

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Abstract:

The Eastern California Shear Zone (ECSZ) is a distributed zone of faults and folds that currently accommodates 20–25% of Pacific-North America plate motion. It consists of an active western belt (modern ECSZ) and a mostly inactive eastern belt (paleo-ECSZ). Development of the ECSZ followed a period of regional NE-SW extension throughout southwestern North America that also exhumed slivers of Pelona-Orocopia-Rand schist. Competing models suggest both early (>10 Ma) and late (<5 Ma) initiation of deformation in the paleo-ECSZ, and documenting this cryptic earlier history is essential for understanding the timing and nature of the transition from NE-SW extension during early growth of the transform plate margin.

This study utilizes zircon (U-Th)/He thermochronology to refine the timing of cooling and exhumation in southeastern California, providing insight into the tectonic setting during initial evolution of the paleo-ECSZ. Samples were collected from metamorphic basement units in Picacho State Recreation Area, including the Orocopia schist, mafic orthogneiss, and Winterhaven Formation, which are overlain by 23 Ma volcanic rocks and the <23 Ma to <9.8 Ma Bear Canyon conglomerate. These units were selected based on the criteria that they were emplaced and deformed during initial evolution of the paleo-ECSZ.  Zircon (U-Th)/He data and HeFTy modeling will constrain the latest phase of exhumation in this region. These new thermochronologic data will be compared with existing zircon (U-Th)/He data from other Pelona-Orocopia-Rand schist exposures to evaluate regional cooling histories and exhumation mechanisms.  Preliminary data comes from San Emigdio Mountains (73-96 Ma; n=8), Sierra de Salinas (62-70 Ma; n=8), Tehachapi (36-61 Ma; n=8), and Quartz Hill (14-76 Ma; n=8) exposures to provide a regional tectonic framework. Modeling of these data and acquisition of new data from Picacho State Recreation Area will place important timing constraints on the upper crustal exhumation history of schist outcrops. These findings contribute to broader regional models of transform boundary evolution and crustal deformation along the Pacific-North America plate boundary.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-9528

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