Stratigraphic and structural relations indicate that a large-displacement east-dipping normal fault, the Seaman fault, is concealed under southern White River Valley, Nevada. The Seaman fault forms the breakaway of an eastward-rooting pre-middle Oligocene (pre-30.6 Ma) extensional detachment system and may connect northward to the breakaway of the Oligocene extensional system exposed in the Snake, Schell Creek, and Egan Ranges. We propose that this regional breakaway zone bounds a single eastward-rooting Oligocene extensional belt, the Snake-Stampede system, that affected much of eastern Nevada and western Utah. The southward migration with time of Tertiary volcanism in the Great Basin caused varying temporal relations between volcanism and deformation in the Snake-Stampede system; approximately synchronous extension is prevolcanic to the south but syn- to postvolcanic to the north. This pattern conflicts with widely accepted temporal and genetic links between volcanism and crustal extension. Basin and Range crustal extension probably neither triggered nor was triggered by magmatism, but instead volcanism and extension operated somewhat independently.

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