LATE-STAGE SLIP HISTORY OF THE BUCKSKIN-RAWHIDE DETACHMENT FAULT AND TEMPORAL EVOLUTION OF THE LINCOLN RANCH SUPRADETACHMENT BASIN: NEW CONSTRAINTS FROM THE MIDDLE MIOCENE SANDTRAP CONGLOMERATE

The Lincoln Ranch basin in west-central Arizona preserves a >1.8 km thick section of clastic sedimentary rocks deposited during large-magnitude slip along the Buckskin-Rawhide detachment fault. We present new geochronologic, thermochronometric, and stratigraphic data from the middle Miocene Sandtrap conglomerate, providing insights on the timing of syn-extensional basin development and slip rates on one of the largest Cordilleran detachment faults. The Sandtrap conglomerate (SC) consists of a >1.25 km thick section of clast-supported, pebble-boulder, polymict conglomerate dominated by mylonite clasts derived from the detachment fault footwall. Zircon U-Pb dating of three ash-fall tuff beds within sandstone below the SC yielded primarily inherited zircons with Proterozoic ages. The weighted mean ²⁰⁶Pb/²³⁸U age of the four youngest concordant ages is 14.7 ± 0.6 Ma, providing a maximum age constraint of ~14-15 Ma for the lowermost SC. The abrupt transition from lacustrine sandstone deposition to alluvial fan deposition of the SC coincided with a shift to a NW/N directed paleocurrent and the appearance of mylonitic clasts. We interpret this transition to record inception of a new detachment breakaway that exposed the mylonitic footwall along the Ives Peak corrugation. Conglomerate beds uniformly dip ~40-70° SW and abruptly transition to shallow dips ~1 km above the base of the section, indicating most tilting occurred during the late stages of detachment slip. Zircon and apatite (U-Th)/He ages (ZHe and AHe, respectively) were determined from eight mylonitic boulders throughout the SC. Mean AHe ages from within the lowest ~100 m are ~15.7 ± 1.7 and 15.9 ± 3.1 Ma, further supporting a maximum depositional age of ~15 Ma for the lowermost SC. (U-Th)/He ages young slightly up-section to minimum AHe ages of ~11.5-13 Ma. Geo-thermochronometric ages indicate that SC deposition was active between ~15-11.5 Ma at an average sedimentation rate of ~350-625 m/Myr. (U-Th)/He ages and lithologies of mylonitic clasts match footwall rocks ~10-20 km to the SW, suggesting a time-averaged slip rate of 3-10 km/Myr in the last ~3 Myrs of slip. The presence of growth strata in the uppermost part of the section suggests extension may have accelerated in the last ~1-2 Myrs of slip along the Buckskin-Rawhide detachment.