In the Yerington district, western Nevada, pre-Tertiary rocks are overlain by an Oligocene ignimbrite sequence and Miocene andesites. Basin and Range normal faulting began in Miocene time, as andesitic volcanism died out (17 to 18 m.y. ago), and has continued to the present. The faults dip east and are curved, concave upward, with net displacements in a nearly east-west direction. Movement on the curved faults has resulted in steep westward tilting of the Miocene andesites and of all older rocks. Alluvium and 8- to 11-m.y.-old basalt flows deposited during the period of faulting are tilted gently west. The oldest faults, which dipped steeply east when they were active, are now inactive and dip gently eastward as a result of westward tilting on other faults. Younger faults dip more steeply east, and the youngest faults, those responsible for present Basin and Range topography, are the steepest. More than 100 percent of east-west extension has taken place across the district because of normal faulting. The rate of extension was most rapid between 17 and 11 m.y. ago and was slower after 11 m.y. ago. The extension is deep seated rather than thin skinned and apparently involves thinning of the crust. Several theories of origin for Basin and Range structure can be rejected because of the field data at Yerington, and the theory that Basin and Range structure was caused by a continental spreading axis best fits the data. Basin and Range spreading seems to have been most active between the projections of the Mendocino and Murray fractures. It may have first started south of the Great Basin, when these fractures were farther south relative to the continent and when the oceanic spreading axis that had been between these fractures was interacting with the continent.

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