284-4 Unraveling Metamorphic Core Complex Evolution in the Chemehuevi Mountains Utilizing Coupled U-Pb Petrochronology and Microstructural Analysis

Session: The Deformation-Metamorphism-Fluid Triplet Governing Plate Boundaries and Orogens

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Metamorphic core complexes (MCCs) are hallmark extensional structures that offer a window to understand temporal and spatial periods of deformation, fluid flow, and metamorphism. Recent work within the southern belt of MCCs in the North American Cordillera reveals some of the mylonitic deformational history is pre-Miocene, highlighting the need to constrain the timing of polyphase deformation within MCCs. In the Chemehuevi Mountains MCC (CHM), southeast California, previous work has suggested a phase of Late Cretaceous ductile shearing followed by Miocene detachment faulting. Contextualized deformation ages and kinematic trends in the region can elucidate the relationships and feedback between contraction, extension, and magmatism in the lithosphere during the Mesozoic to Cenozoic. In the CHM, exhumation and displacement are facilitated by the Chemehuevi detachment fault, which juxtaposes brittlely faulted Miocene sediments and volcanic rocks against lower-plate mylonitic and migmatitic Proterozoic basement. Prior low-temperature thermochronology yields dominantly Miocene ages that record detachment faulting and exhumation. Here, we present LA-ICP-MS U-Pb geochronology coupled with field observations and microstructural analyses in CHM to characterize the spatial and temporal evolution of magmatism, mylonitization, and extensional faulting. New zircon and apatite U-Pb ages reveal the presence of a Miocene pluton and a Cretaceous pluton that intrude the crystalline Proterozoic basement. Field observations and microstructures exhibit a similar top-to-the-NE shear sense in mylonites, however apatite U-Pb petrochronology suggests at least two discrete ductile deformational events since the Proterozoic. We interpret the Late Cretaceous-Paleocene apatite U-Pb dates (72-58 Ma) from the mylonites to be associated with Late Cretaceous plutonism and/or the Laramide orogeny. Conversely, the more localized Miocene shearing (26-16 Ma) is spatially concomitant with and associated with extension and pluton emplacement. The microstructures and timing of ductile shearing in CHM reveal a complex polyphase deformational history, including Late Cretaceous–Paleocene and Miocene deformation and magmatism. MCCs in the western USA uniquely expose a tectonic history of Mesozoic orogeny exhumed through Cenozoic extension, thus offering a view into mid-crustal conditions and kinematics of an ancient orogen.

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

doi: 10.1130/abs/2025AM-9188

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