SEISMIC-REFLECTION EVIDENCE FOR 2-PHASE DEVELOPMENT OF TERTIARY BASINS FROM EAST-CENTRAL NEVADA

Two east-west seismic reflection profiles crossing Antelope Valley, Sm okey Valley, Railroad Valley, and Big Sand Springs Valley in east-cent ral Nevada demonstrate the evolution of Tertiary basin extension in th e Basin and Range Province from broad sags to narrow, fault-bounded ba sins. Reprocessing of a 480 channel, 60 fold, dynamite-source experime nt 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 br oad symmetric basin riddled with many small offset normal faults. The later phase shows a narrowing of the basin and subsidence along one do minant structure, an apparent planar normal fault. The unconformity se parating the two phases of extension marks a transition from broad sub sidence to local asymmetric tilting that took place over a short perio d of time relative to sedimentation rates. Antelope Valley, Smokey Val ley, and Railroad Valley clearly show evidence for two-phase developme nt. Big Sand Springs Valley represents only the later phase of extensi on. The absence of dating of stratigraphic units within the basins pre cludes us from determining if the abrupt tectonic transition within th e basins resulted from differences in local strain rates or amounts, o r was due to changes in regional stress fields.