2015 GSA Annual Meeting in Baltimore, Maryland, USA (1-4 November 2015)

Paper No. 210-100
Presentation Time: 9:00 AM-6:30 PM


BENEFIELD, Jacqueline1, SINGLETON, John S.1, VOMOCIL, Terence1 and WONG, Martin S.2, (1)Department of Atmospheric, Oceanic, and Earth Sciences, George Mason University, 4400 University Drive, Fairfax, VA 22030, (2)Department of Geology, Colgate University, Hamilton, NY 13346, jbenefield93@gmail.com

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.