Tertiary basin development, volcanism, and structural tilting at the west edge of the Colorado River extensional corridor is examined using single-crystal 40Ar/39Ar ages from volcanic rocks in Ship, Piute, Little Piute, and northern Old Woman Mountains (Old Woman Mountains area). Complications in age determination resulting from xenocrystic contamination and common excess trapped Ar are readily circumvented using single-crystal analysis. Preferred ages indicate that the two thickest sections of Tertiary strata, in the Ship and Little Piute Mountains, were deposited between about 19-19.5 Ma and 17 Ma. The earliest Tertiary magmatism in the area is recorded by a 21 Ma dike from the northern Old Woman Mountains, but no volcanic units older than 19.3 Ma have been dated. This earliest diking correlates with the initial phases of volcanism to the east in the Colorado River trough. Westward tilting of the Little Piute Mountains occurred after 17.8 Ma (uppermost tuff), and final westward tilting of the Piute Mountains occurred after 18.4 Ma (Peach Springs Tuff) and before 16.6 Ma (flat-lying dacite). Major rotation of strata in these areas overlaps a major period of detachment faulting recognized in ranges in the Colorado River trough. Basins in the Old Woman Mountains area have stratigraphic thicknesses comparable to those observed in the Colorado River, but unlike basins in the Colorado River trough, an appreciable period of sedimentation preceded volcanism in the Old Woman Mountains area. Differences in sedimentation history and timing of volcanism between the two areas are related to differences in the magnitude of extensional strain and possibly the position of the regional mantle thermal anomaly giving rise to magmatism in the Colorado River extensional corridor. West tilting of strata in the northernmost Piute Mountains (Goffs Butte) occurred after 10.6 Ma and is the youngest documented tilting in the region at this latitude.
