40Ar/39Ar K–FELDSPAR THERMOCHRONOLOGY OF THE HARCUVAR CORE COMPLEX, WESTERN ARIZONA: NEW INSIGHT INTO THE TIMING OF EXTENSION AND DEGREE OF FOOTWALL TILT

Although core complexes are a fundamental mode of crustal extension, many aspects of their development remain uncertain, including the timing and rate of fault slip and whether detachment faults form and slip at their present low–angles. The Harcuvar core complex (western AZ), formed by top–NE slip on the Bullard low-angle normal fault and associated mylonitization. The Harcuvar range extends for >60 km in the slip direction and is thought to be an intact footwall block. This study presents new ⁴⁰Ar/³⁹Ar thermochronology from the Harcuvars that yields new insight into its tectonic development.

Both mica and K–feldspar ⁴⁰Ar/³⁹Ar ages show substantial gradients along the footwall in the slip direction. Biotite ages drop from 49 Ma to 31 Ma to 17 Ma from the SW to the NE footwall. K-feldspar in the SW footwall yield age spectra with a minor flat at 20 Ma followed by a steep climb up 45 Ma ages. K–feldspar age spectra from further NE are more dominated by Miocene ages, with K–feldspar in the NE–most footwall yielding nearly flat age spectra that range from 14–17 Ma. MDD K-feldspar thermal models suggest that the rapid footwall cooling began at 20 Ma and continued until at least 14 Ma.

The 20 Ma rapid cooling event likely marks the start of major extensional slip. Although prior work on hanging wall deposits suggests that slip began at 27 Ma, there is no thermal signal of this in our data, suggesting that this earlier extension was of limited magnitude or did not occur. The thermal gradients suggest that the footwall may be moderately SW–tilted, as the SW footwall resided at ~225°C at 20 Ma while the NE footwall was hotter than 350°C and possibly much hotter. A series of 45° NE dipping andesite dikes near Cunningham Pass also suggest that the footwall is significantly tilted, implying the detachment fault may have formed at a moderate to steep dip. Substantial footwall tilt may be incompatible with an intact footwall and may instead require that it is cut by younger faults. The lack of an age gradient in Ar/Ar data from the adjacent Rawhide–Buckskin core complex (e.g. Scott et al. 1998) may suggest that those data are influenced by Miocene reheating during intrusion of the Swansea plutonic suite. Thus, the new Ar/Ar data from the Harcuvar core complex likely places better constraints on the timing of extension and possible degree of footwall tilting in the region.