A SYNTHESIS OF THE YOUNGEST DETACHMENT FAULT-RELATED COOLING AGES IN THE LOWER COLORADO RIVER EXTENSIONAL CORRIDOR: SIMILARITIES AND DIFFERENCES BETWEEN THE THERMAL HISTORY OF METAMORPHIC CORE COMPLEXES

Extensive (U-Th)/He data collected from metamorphic core complexes in the lower Colorado River extensional corridor (CREC) over the last ~15 years have provided new information on the timing of large magnitude extension along the regionally correlative detachment system in west-central Arizona and eastern California. We present new data from the White Tank and South Mountains of central Arizona and compare these results to existing studies to evaluate how coherent the structural and thermal histories of individual metamorphic core complexes are within regionally extensive continental detachment fault systems. We focus on (U-Th)/He ages from the structurally deepest parts of the mylonitic footwalls.

A single sample from the White Tank Granite (²⁰⁶Pb/²³⁸U age of 71.9 ± 1.5 Ma) at the NE end of the White Tank Mountains yielded a ZHe age of 15.9 ± 1.6 Ma (n=3). South Mountain samples SM1 and SM2 are located ~10 km and <1 km SW of the South Mountain detachment fault. SM1 and SM2 yielded ZHe ages of 16.9 ± 1.5 Ma (n=4) and 15.0 ± 2.0 Ma (n=3), respectively, and mean AHe ages of 12.0 ± 2.9 Ma (n=3) and 11.5 ± 1.1 Ma (n=5), respectively.

Three samples within ~1.5 km SW of the Eagle Eye detachment in the NE Harquahala Mountains, yielded mean AHe sample ages of ~12.6 ± 0.3 (n=2), 15.9 ± 1.9 (n=4), and 16.9 ± 0.7 Ma (n=5), whereas 4 mean ZHe sample ages are 15.2 ± 0.9 (n=5), 15.4 ± 1.4 (n=9), 16.1 ± 1.1 (n=5), and 16.2 ± 0.8 Ma (n=5). Mean AHe sample ages from within ~4.5 km SW the Bullard detachment in the NE Harcuvar Mountains are ~13.1 (n=1), 13.3 ± 2.1 (n=2), 13.9 ± 2.6 (n=4), and 16.1 ± 2.3 Ma (n=3) (recalculated mean ages ± 1σ from Carter et al., 2004). The 2 northeasternmost samples from the Buckskin Mountains yielded a single AHe age of 13.3 ±1.3 Ma (n=3) and 2 ZHe ages of 12.2 ± 1.6 Ma (n=3) and 13.0 ± 0.7 Ma (n=2) (Singleton et al., 2014). Although most ages have a similar range, (U-Th)/He ages from the Buckskin and the South Mountains are slightly younger than other core complexes. Both the Buckskin and South Mountains experienced substantial early Miocene plutonism, which may not have initiated detachment faulting but appears to have facilitated more prolonged detachment fault slip or elevated fluid flow that resulted in younger cooling ages. A regional evaluation of this large thermochronometric data set may provide new insights into the structural development of the CREC.