LATE CRETACEOUS TO EARLY TERTIARY EXTENSIONAL ORIGIN OF THE HARCUVAR AND BUCKSKIN-RAWHIDE METAMORPHIC CORE COMPLEXES, ARIZONA

Metamorphic core complexes are interpreted as sites of large–magnitude extension that involved a shear zone that evolved from ductile to brittle. In spite of their importance in extension, many aspects of core complexes remain controversial, including the timing of extension and the age and significance of lower plate mylonites. Here we focus on the timing of core complex development, which is broadly accepted to have occurred during the early to middle Miocene in the central and southern Basin and Range. However, new data from the Harcuvar and Rawhide–Buckskin core complexes in Arizona suggest that they originated as Late Cretaceous to early Tertiary extensional shear zones which were reactivated during Miocene extension.

Lower plate mylonites have NE–SW trending lineations and top-NE kinematics. In the majority of these mylonites, dynamic recrystallization occurred by grain boundary migration in quartz and subgrain rotation in feldspar, and EBSD data indicate that slip in quartz was dominated by prism <a>, suggesting mylonitization occurred at amphibolite facies (>500°C). New ⁴⁰Ar/³⁹Ar hornblende ages from the Harcuvar lower plate range from 67–43 Ma and young towards the NE, indicating that the lower plate cooled to <500°C no later than 43 Ma and that the NE lower plate was structurally deepest at that time. Therefore, amphibolite facies mylonites that dominate the range predate Miocene extension and accomodated extensional shear. Miocene shear is likely limited to greenschist-facies mylonites along a <300 m thick carapace of the range and thin (cm-scale) shear zones.

New U-Pb ages further constrain the timing of mylonitization. Syn-kinematic titanite is evident in a number of mylonitic samples and yields mean U-Pb ages from ca. 80–60 Ma, which we interpret to directly date mylonitization. A late-kinematic pegmatite dike yields a U-Pb zircon age of 64 Ma, suggesting that mylonitization was nearly over by that time. Together, these new results strongly suggest that these core complexes originated as Late Cretaceous to early Tertiary extensional shear zones. Miocene extension was superimposed on this existing shear zone, resulting in a composite extensional feature. These results may have important implications for understanding the tectonic significance of Cordilleran core complexes.