MIOCENE EXTENSIONAL DEVELOPMENT OF THE HARQUAHALA MOUNTAINS CORE COMPLEX, WEST-CENTRAL ARIZONA: MAGNITUDE AND RATES OF SLIP ALONG THE EAGLE EYE DETACHMENT

Metamorphic core complexes (MCC) in the Colorado River extensional corridor (CREC) contain numerous exposures of low-angle normal faults (detachments) that exhumed mid-crustal mylonites. This study focuses on the Harquahala Mountains MCC, at the southern edge of the Whipple tilt domain, where extension was accommodated along the Eagle Eye detachment (EED). We present new geo-thermochronometry data on Miocene deformation in the Harquahala Mountains to determine timing, displacement magnitude, and slip rates along the EED. Zircon and apatite (U-Th)/He ages from 31 samples along a ~55 km extension-parallel transect (~N60E) display 3 distinct trends when plotted versus distance from the EED: 1) samples in the SW, furthest from the EED, show the oldest ZHe ages of ~50-40 Ma; 2) a middle segment where ZHe ages decrease from ~40 to 22 Ma over ~15 km; 3) a third segment of 22 ZHe ages ranging from ~22-16 Ma record rapid cooling during detachment slip. The second and third segments of ZHe ages are clearly separated by an inflection point ~34 km SW of the EED, recording a minimum displacement estimate and initial exhumation of deeply buried rocks at ~22-20 Ma. Linear regression of ZHe and AHe ages yields slip rates of 8.9 +5.5,-2.5 km/Myr and 4.7 +0.9, -0.7 km/Myr, respectively.

A lithologic correlation has been proposed between distinct plutonic footwall rocks in the Little Harquahala Mountains (LHQ1) and preserved clasts (AR1) within Tertiary breccia (Tcb) exposed near Bullard Peak at the NE end of the Harcuvar Mountains (Reynolds and Spencer, 1985). Underlying ash-flow tuffs constrain the maximum age for Tcb suggesting active syn-extensional deposition by ~23 Ma. Identical U-Pb ages of 164.3 ± 1.4 (LHQ1) and 164.4 ± 1.1 Ma (AR1) and lithologic similarity between these units strongly support this correlation, indicating ~45 km of offset across the detachment system. Exhumation of the zircon partial retention zone in the breakaway zone requires ~8-12 km of displacement (for a 60-35° fault dip) in addition to the ~34 km derived from the spatial extent of reset ZHe ages, yielding ~45 km of total displacement. The EED displacement estimate shows clear agreement between lithologic and geo-thermochronometric offset constraints, thus providing a rare opportunity to fully resolve timing, rates, and total displacement magnitudes in the CREC.