We present thermochronologic and geochronologic data that constrain the slip history of the Buckskin-Rawhide detachment fault in west central Arizona, one of the largest extensional fault systems in the North American Cordillera. (U-Th)/He zircon and apatite thermochronology, integrated with 40 Ar/ 39 Ar geochronology of postdetachment volcanic rocks, indicate that large-magnitude extension associated with the detachment fault initiated at~21-20 Ma and continued until~12-11 Ma in the southwestern portion of the Buckskin-Rawhide metamorphic core complex. (U-Th)/He footwall cooling ages from the breakaway zone in the western Bouse Hills to upper greenschist-facies mylonites in the southern Buckskin Mountains indicate that the slip rate on the detachment fault was 3 + 1.5/À1 km/Myr during the early Miocene. Space-time patterns of hanging wall tilting suggest that at 17-16 Ma, a secondary detachment fault breakaway developed 12 km northeast of the primary detachment fault breakaway. Proximal conglomerates deposited in a supradetachment basin adjacent to the secondary breakaway scarp were displaced 6-11 km northeast in the middle Miocene by the Buckskin-Rawhide detachment fault at a slip rate of 1.2-2.7 km/Myr. The total displacement across the detachment fault in the southwestern portion of the core complex is 24 ± 10 km, well short of the previous estimate of 66 ± 8 km across the entire core complex. Based on these data and new observations, we propose that total displacement on the Buckskin-Rawhide detachment fault system increases in the slip direction to~40-50 km at the northeastern end of the exposed footwall, corresponding to time-averaged slip rates that ranged from~2 km/Myr to ≤6 km/Myr across the entire core complex.
Magmatic arcs may play a major role in the initiation, behavior, and abandonment of intra-arc strike-slip systems. Here we present zircon U-Pb and (U-Th)/He geochronology/thermochronology with new mapping to relate Coastal Cordillera arc magmatism to sinistral shear along the Atacama fault system (AFS) in northern Chile. New dates from 18 intrusions along the AFS between 24.6°S and 27°S compiled with published data record a minor Early Jurassic magmatic pulse , broad latest Jurassic to Early Cretaceous (150-120 Ma) pulse, and a minor younger (120-105 Ma) pulse. Mylonitization occurred only along the margins of Early Cretaceous plutons and surrounding Paleozoic metasedimentary rock, whereas Jurassic plutons and metasedimentary rocks away from Early Cretaceous plutons lack mylonitic fabrics. Early Cretaceous magmatism facilitated AFS deformation by thermally weakening the crust with elevated geothermal gradients that enabled mylonitization to take place at~5-to 7-km depths and low stresses. Spatial variability of pluton emplacement produced significant rheological heterogeneity, giving rise to a highly segmented fault system that did not originate as a regional-scale shear zone. Synkinematic dikes (~120-117 Ma) cut mylonitic fabrics, and a postkinematic dike (~110 Ma) records the end stages of slip. The cessation of slip coincided with cooling below~180°C at~116-99 Ma as arc magmatism migrated eastward and geothermal gradients relaxed, coeval with a major reorganization in plate motion and the onset of seafloor spreading in the south Atlantic.
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