Near the western margin of the Basin and Range Province in an area encompassing some 1,500 km2 between Mono Lake, California, and Yerington, Nevada, six structural basins contain thick accumulations of Miocene-Pliocene sedimentary and volcanic rocks. From approximately 22 to 18 m.y. ago, the area was a highland from which ignimbrite flows of Oligocene age were generally eroded. Subsequent eruptions of andesitic rocks blanketed the area with flows and breccia. Between about 12.5 and 9 to 8 m.y. ago, the area became an integrated basin of sedimentation in which some 2,500 m of strata accumulated. During this period, faulting, along west and northwest trends, and volcanism occurred. Within the basin, surface environments varied from fluvial to lacustrine, and basin margins fluctuated, the maximum extent of the basin having been reached about 10.5 m.y. ago, but a single integrated basin persisted. By approximately 7.5 m.y. ago, the region had been disintegrated by normal faulting into existing structural blocks. Faults of this episode generally trend northeast, east, and northwest. Relative tectonic quiescence ensued for about 4 m.y. During this time a well-graded erosional surface evolved and was locally covered by basic volcanic flows and silicic protrusions, commonly emplaced along faults of the earlier episode. Broad upwarping and block faulting during the Quaternary Period produced the present topography. In contrast to trends of faulting prior to 7.5 m.y. ago, Quaternary normal faults have a north orientation. These faults terminate en echelon in structural warps or by abrupt decrease in displacement to define a northeast-trending lineament across the area, parallel to the Mono Basin–Excelsior zone to the south and the Carson lineament to the north.

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