| The Beaverhead fault cuts Late Pleistocene alluvial fans on the western flank of the Beaverhead Mountains. The two southern most fault segments, Blue Dome and Nicholia, are the focus of this study. Geomorphic mappingused0.5m resolution LiDAR and field investigation to delineate five fan units (Qaf1-Qaf5). Twenty-five optically stimulated luminescence (OSL)ages on fan gravels are16–113ka. Regional alluvial fan surfaces formed under cooler and/or wetter late Pleistocene climates, and the fan ages suggest multiple periods of cooler and/or wetter climate during the last glaciation. Fault scarps cut Qaf2 –Qaf5 alluvial fans in the Nicholia segment, but fault scarps are absent along the Blue Dome segment. OxCal modeling using fan ages and rupture patterns constrains the most recent fault ruptures on the Nicholia segment to32 +/-7.3 kaand77+/-6.8 ka, and the most recent Blue Dome rupture appears to pre-date 110 ka. Key Words: Alluvial fans, rupture, earthquake, Beaverhead Mountains, fault, optically stimulated luminescence |