DISTINGUISHING MIOCENE MYLONITIZATION FROM LATE CRETACEOUS–EARLY TERTIARY MYLONITIZATION IN THE HARCUVAR MOUNTAINS METAMORPHIC CORE COMPLEX, WEST-CENTRAL ARIZONA

Geologic mapping and petrographic analysis of mylonites in the Harcuvar Mountains metamorphic core complex, west-central Arizona, document parallel shear zones that formed under amphibolite-facies and greenschist-facies conditions. The eastern half of the core complex is dominated by mylonitic Late Cretaceous leucogranite that has traditionally been interpreted to record top-to-the-NE-directed shear during early Miocene extension. Microstructures and deformation mechanisms from dozens of oriented thin sections suggest most mylonitic fabrics in the interior of the range formed under amphibolite-facies conditions (>500° C), whereas the flanks of the range records greenschist-facies mylonitization that appears to overprint the amphibolite-facies fabrics. On the northern flank of the range near Burnt Well, greenschist-facies mylonitization is restricted to clastic metasedimentary rocks. Locally these metasedimentary mylonites record foliation-parallel cataclastic deformation and co-seismic frictional melting, indicating that shearing progressed through the brittle-plastic transition. Metasedimentary mylonites are absent along the southern flank of the range near Bullard Peak, but greenschist-facies fabrics in crystalline mylonites are present along the Bullard detachment fault. Our mapping and cross sections demonstrate that the greenschist-facies mylonite zone is ~150-250 meters thick on both the northern and southern flanks of the range despite different rheologies. The amphibolite-facies leucogranite mylonite structurally below this shear zone is >1 km thick. The orientation of mylonitic stretching lineations and the top-to-the-NE sense of shear in the greenschist-facies and amphibolite-facies shear zones are identical. Based on the known thermal history of the range, we interpret the greenschist-facies shear zone to have formed during Miocene crustal extension and the amphibolite-facies shear zone to record Late Cretaceous to early Tertiary extension. The parallel geometry of these shear zones suggests that Miocene mid-crustal extension reactivated the older shear zone.