EVIDENCE FOR MIOCENE REACTIVATION OF A LATE CRETACEOUS TO EARLY TERTIARY SHEAR ZONE IN THE HARCUVAR AND BUCKSKIN-RAWHIDE METAMORPHIC CORE COMPLEXES, ARIZONA

Metamorphic core complexes are sites of large-magnitude extension, yet many aspects of their tectonic development remain controversial. Important questions remain about the timing, magnitude, rate, and geometry of extension, and the age and tectonic significance of lower plate mylonites. Most core complexes in the central and southern Basin and Range are thought to have formed during the Miocene. However, new results from the Harcuvar and Buckskin-Rawhide core complexes in western Arizona suggest that Late Cretaceous to early Tertiary (Laramide) deformation played a key role in their tectonic evolution.

Lower plate mylonites in the region have NE-SW-trending lineations and a top-NE sense of shear. In the Harcuvar and southern Buckskin Mountains, mylonites with top-NE shear commonly show quartz grain boundary migration and feldspar subgrain rotation recrystallization, suggesting mylonitization at >500 °C. EBSD analyses indicate rhomb and prism <a> or exclusively prism <a> slip in quartz, supporting this interpretation. A new ca. 43 Ma ⁴⁰Ar/³⁹Ar cooling age on hornblende from the NE Harcuvar footwall (deepest during the Miocene), suggests that the >500˚ C mylonites predate Miocene extension. These mylonites are intruded by a synkinematic ca. 64 Ma pegmatite dike, suggesting the top-NE amphibolite-facies mylonitization is Laramide in age. In the Harcuvar footwall, greenschist-facies, top-NE directed mylonitic fabrics that are likely Miocene appear to be limited to a <0.5 km thick carapace on the range flanks and minor cm-scale shear zones.

These results suggest that lower plate mylonitization in several portions of these core complexes is unrelated to Miocene detachment faulting. The tectonic significance of the pre-Miocene mylonites is unclear, but given their kinematic similarity to Miocene extension, they likely accommodated Laramide crustal extension, raising questions about whether the geometry and location of Miocene core complexes was tectonically inherited from older events. Evidence for high slip magnitudes and rates in the Miocene should be reevaluated in light of a more protracted exhumation history. Tectonic inheritance and polyphase extension may be common to many Cordilleran core complexes, which would have broad implications for our understanding of their tectonic significance.