Two east-west seismic reflection profiles crossing Antelope Valley, Smokey Valley, Railroad Valley, and Big Sand Springs Valley in east-central Nevada demonstrate the evolution of Tertiary basin extension in the Basin and Range Province from broad sags to narrow, fault-bounded basins. Reprocessing of a 480 channel, 60 fold, dynamite-source experiment enabled good imaging of basin stratigraphy. These data suggest two distinct phases of basin development occurred, separated by a regional unconformity. The early phase is characterized by development of a broad symmetric basin riddled with many small offset normal faults. The later phase shows a narrowing of the basin and subsidence along one dominant structure, an apparent planar normal fault. The unconformity separating the two phases of extension marks a transition from broad subsidence to local asymmetric tilting that took place over a short period of time relative to sedimentation rates. Antelope Valley, Smokey Valley, and Railroad Valley clearly show evidence for two-phase development. Big Sand Springs Valley represents only the later phase of extension. The absence of dating of stratigraphic units within the basins precludes us from determining if the abrupt tectonic transition within the basins resulted from differences in local strain rates or amounts, or was due to changes in regional stress fields.