In northwestern Arizona, the high-standing, relatively unextended Colorado Plateau abruptly gives way across a system of major west-dipping normal faults to a highly extended part of the Basin and Range province known as the northern Colorado River extensional corridor. The transition from unextended to highly extended upper crust is unusually sharp within this region, contrasting with a broad transition zone elsewhere. The southern White Hills lie near the eastern margin of the extensional corridor in northwestern Arizona and contain a large east-tilted half graben that chronicles Miocene extension and constrains the timing of structural demarcation between the Colorado Plateau and Basin and Range province during Neogene time. This growth-fault basin is bounded on the east by the west-dipping Cyclopic and Cerbat Mountains fault zones. Greater tilts in the hanging walls suggest that these faults have listric geometries. The stratigraphy in the half graben consists of Miocene vol canic rocks intercalated with an eastward-thickening wedge of synextensional fanglomerates. Tilts in the Miocene units decrease up section from ~75° to 5°. Recent ⁴⁰Ar/³⁹Ar dating (11 new dates) of variably tilted volcanic rocks in the growth-fault basin and regional relations constrain the timing of east-west extension between ca. 16.6 and <9 Ma, with peak extension from ca. 16.6 to 15.2 Ma. Capping 8.7 Ma basalts are tilted 5°–10° and record the waning stages of extension. Thus, the sharp boundary between the Colorado Plateau and Basin and Range began developing by ca. 16.5 Ma and has changed little since ca. 9 Ma. Major extension and basin development significantly lowered base level within the extensional corridor and induced headward erosion into the western margin of the Colorado Plateau, which ultimately facilitated development of the western Grand Canyon. Abundant clasts of 1.7 Ga megacrystic granite in the eastward-thickening fanglomerates within the growth-fault basin suggest a partial provenance from the Garnet Mountain area along or near the western margin of the Colorado Plateau beginning as early as ca. 16 Ma and continuing to ca. 9 Ma.