LATE EXTENSION INCEPTION AND RAPID FAULT-SLIP AT THE SOUTHERN EDGE OF THE LOWER COLORADO RIVER EXTENSIONAL CORRIDOR: HARQUAHALA MOUNTAINS CORE COMPLEX, WEST-CENTRAL ARIZONA

The Harquahala Mountains metamorphic core complex of west-central Arizona is located within the lower Colorado River extensional corridor (CREC) and bound by a low-angle normal fault, the Eagle Eye detachment (EED). The Harquahala core complex is the southernmost exposure of Tertiary mylonites exhumed along the regional low-angle normal fault system that connects the lower plate of the Chemehuevi, Whipple, Buckskin-Rawhide, and the Harcuvar Mountains, making it a key location to understand the timing and pattern of extension within the lower CREC as a complete system. A ~45 km transect of samples for (U-Th)/He dating (n=16) was collected from the Harquahala footwall along the regional extension direction of ~060° to determine the cooling history associated with core complex exhumation and slip along the Eagle Eye detachment. Zircon (U-Th)/He ages (ZHe) were plotted versus distance from the EED and display 3 main characteristics: 1) samples furthest away yield older, pre-extension cooling ages of ~43 Ma, 2) a decrease in ZHe ages through a preserved partial retention zone (PRZ), and 3) ten nearly invariant ages that decrease from 17.6 ± 0.3 to ~15-16 Ma over the last 22 km and record cooling due to slip on the EED. Initial apatite (U-Th)/He ages are systematically younger than ZHe ages and display the same pattern. The preserved inflection point between the PRZ ages and ZHe ages recording fault slip indicates extension began at ~17-18 Ma within the Harquahala Mountains, several million years later than the ~20-24 Ma estimates from other core complexes in the lower CREC. Nearly invariant ZHe ages attributed to fault-related cooling suggest rapid time-averaged slip rates of ~10-16 km/m.y. from 17.6 ± 0.3 Ma to ~15-16 Ma. Extension continued in the Harquahala Mountains to at least ~12-13 Ma based on an apatite (U-Th)/He age of 12.6 ± 0.3 Ma ~1 km from the Eagle Eye detachment. These results indicate extension began several million years later in the Harquahala Mountains than other core complexes in the lower CREC and was accommodated by rapid fault-slip along the Eagle Eye detachment.