In the North American Cordillera, metamorphic core complexes are generally interpreted to record a single phase of large-magnitude Tertiary extension. However, growing evidence suggests many Cordilleran core complexes underwent Cretaceous deformation that fundamentally influenced their Tertiary tectonic development. In particular, mylonites in mid-Tertiary core complexes may have reactivated Late Cretaceous extensional shear zones. The primary goal of this project is to determine the relationship between Late Cretaceous deformation and Miocene core complex development in the Harcuvar and Buckskin-Rawhide Mountains in western Arizona. This study tests the hypothesis that lower plate shear zones in these core complexes are composite structures that initiated during Late Cretaceous extension. Research will evaluate the age and tectonic significance of lower plate fabrics and assess how Late Cretaceous structures influenced the location, geometry, and style of Miocene core complex extension. Research goals will be accomplished through a multidisciplinary approach involving geologic mapping, microstructural analysis, U-Pb geochronology, 40Ar/39Ar thermochronology, and Ti-in-quartz geothermometry. The Harcuvar and Buckskin-Rawhide core complexes are outstanding sites to address this topic because several portions of the lower plate are not overprinted by Miocene strain and because clear cross-cutting relationships are present between mylonitic and non-mylonitic lower plate rocks. In addition, these are classic localities where some of the widely accepted models of core complex formation were developed.
Research on core complexes continues to play a central role in our understanding of fundamental tectonic processes, including the origins and mechanics of large-magnitude extension and the long-term behavior of major fault zones. The anticipated results of this research will provide insight into many of these extensional processes. For example, the locations of core complexes and the low-angle geometry of core complex structures may have been strongly influenced by older tectonic activity. In addition, if core complex extension initiated during the Late Cretaceous, the slip rate and displacement on mid-Tertiary detachment faults may be lower than originally believed. The discovery of Late Cretaceous extension in core complexes would also have important implications for understanding the tectonic evolution of the Cordillera during the Laramide orogeny.
