The Lincoln Ranch basin in west-central Arizona preserves a >1.8-km-thick section of Oligocene–Miocene sedimentary rocks deposited during large-magnitude slip along the Buckskin detachment fault. We present new geothermochronometric and stratigraphic data from the middle Miocene Sandtrap Conglomerate to constrain synextensional basin development and slip rates along the Buckskin detachment fault. The Sandtrap Conglomerate consists of a >1.25-km-thick section of pebble to boulder, polymict conglomerate dominated by mylonite clasts derived from the Buckskin detachment fault footwall. U-Pb dating of underlying ash-fall tuffs indicates a maximum depositional age of 14.7 ± 0.6 Ma. Apatite (U-Th)/He dating (AHe) from boulder clasts within the Sandtrap Conglomerate yielded mean ages of ca. 15.9 ± 3.6 and 15.7 ± 1.5 Ma within the lowest ∼100 m, and slightly younger ages up section to minimum ages of ca. 13–12 Ma, which together with the U-Pb maximum depositional age indicate Sandtrap Conglomerate deposition between ca. 15 Ma and ca. 12 ± 2 Ma and average sedimentation rates of ∼250–1250 m/m.y. The transition from the underlying sandstone to Sandtrap Conglomerate deposition coincides with an abrupt increase in mylonitic clasts and a shift from SE- to NW-directed paleocurrents that are orthogonal to footwall corrugations. We interpret this transition to record initiation of a tertiary breakaway fault that resulted in extensive subaerial exposure of the Ives Peak footwall corrugation. (U-Th)/He ages and mylonitic clast lithologies best match footwall rocks ∼10–16 km to the SW, indicating average fault slip rates of 2.9–4.6 km/m.y. during the last ∼3.5 m.y. of extension along the Buckskin detachment fault.