CRETACEOUS AND MIOCENE HISTORY OF PLUTONISM, DIKING, AND DEFORMATION IN THE CHEMEHUEVI METAMORPHIC CORE COMPLEX

The Chemehuevi Mountains, southeastern California, constitute a metamorphic core complex (MCC) that is part of a belt of MCCs along the North American Cordillera. The Chemehuevi MCC was constructed through Cretaceous and Miocene plutonism, deformation, and diking, with the exact timing constraints between pulses of magmatism and deformation still uncertain. We conducted detailed field transects from the regional detachment fault to the structurally deeper core of the footwall to better understand the plutonic and deformational history of the Chemehuevi Mountains. In progress U-Pb zircon and apatite dating of distinct plutons and dikes bracket the timing of magmatism and diking, and by observing relationships between deformed and undeformed dikes, we can constrain ages on deformational events. Preliminary results reveal that Cretaceous penetrative shear zones, migmatization, and pegmatitic diking appear to affect the 1.5 Ga basement gneiss wall rock; however, the Cretaceous peraluminous pluton lacks mylonitic deformation. A spatially extensive Miocene pluton along the northern margin of the Chemehuevi Mountains intruded both Cretaceous pluton and basement gneisses. Meter-scale, localized shear zones are associated with the Miocene intrusive bodies, along with possibly Miocene mylonitic and undeformed dioritic dike cross-cutting the existing ductile fabrics. We did not observe extensive decameter-scale mylonitic zones as seen in nearby MCCs in the region. These footwall rocks were cut by major low-angle detachment normal-faults, which were associated with strong chloritic alteration and fracture zone at the lower plate. We did not observe any correlation between Cretaceous or Miocene ductile shear zones and Miocene detachment faults, which suggests that the Miocene normal faults exhumed lower plate material without forming mylonitic shear zones. Our results have implications for the mid-crustal condition during Mesozoic orogenesis and regional development of MCCs in the Colorado River extensional corridor, and more broadly within the North American Cordillera.