The middle and upper Miocene Esmeralda Formation of western Nevada was deposited in a continental basin that crops out over an area of about 2,000 km2. The formation consists of thin, westerly derived sedimentary rocks in the western three-quarters of the outcrop area and of thick (3+ km) easterly derived sedimentary rocks in the eastern quarter. Megabreccias along the eastern margin of the basin are interpreted as landslide deposits derived from fault scarps. The basin was probably a half-graben with a major fault or faults on the east side. The position of the basin, its size, and the inferred major syndepositional faults on its east side are all unrelated to present-day topography and the distribution of major faults in the area and indicate a change in paleogeography and structural pattern since the late Miocene. In the eastern part of the basin, this change was accompanied by deformation that includes low-angle-fault detachment of the Esmeralda Formation from underlying amphibolite-grade Late Proterozoic and lower Paleozoic rocks in the Mineral Ridge-Weepah Hills area and uplift, folding, faulting, tilting, and surface exposure of the entire 3+ km thickness of the formation. Middle and upper Miocene basins in the northern part of the Basin and Range province commonly have been attributed to the onset of basin-range tectonism. The tectonic history of the Esmeralda Formation, however, indicates that some of these basins do not occupy the same area, nor are they related to the same syndepositional faults, as present basins. The extensional basin of the Esmeralda Formation can be viewed either as an early structure in an evolving, but kinematically related, extensional terrane in which the distribution of basins and faults changed gradually with time, or as the product of an extensional event kinematically different from that which produced present structures in the same area. The latter hypothesis is favored because of the marked contrasts in the paleogeography and structures associated with the Esmeralda basin compared with modern basins.
