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[adkyn]

At about 30 Ma, the ridgeline between the Pacific and Farralon plates interacted with the North American plate, thus stopping the the subduction and starting the San Andreas. This lead to the major development of the Basin and Range province, which has faulted and widened Nevada by about 100%. Now, there was extension before that (mainly due to Sevier and Laramide orogenic collapse), but the bulk o fit happened at this time. Because the San Andreas 1) does not take up the entire slip between the Pacific and North American plates and 2) is not exactly geometrically aligned with the motion of the plates, approximately 75% of the slip is taken up (currently) on the faults outside of the San Andreas, including most faults in Nevada. The way that the plates are arranged and the geometry of the plates cause extension (mostly) in these areas.