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For several decades it has been recognized that the extent to the east of lower Paleozoic eugeosynclinal rocks (Fig. 1) is greater in northern Elko County than elsewhere in Nevada. It was early recognized that these rocks are allochthonous; in some places it was demonstrable that they form the upper plate of the Roberts Mountains thrust and rest on eastern-facies (carbonate-clastic) lower Paleozoic rocks. Recent detailed mapping has shown that the Roberts Mountains thrust is not deflected to the east in central Elko County, as had been earlier supposed, but continues with a generally northward trend through the Mountain City quadrangle. Western-facies and overlap-assemblage rocks present to the east are the result of younger thrusting southward and eastward. Ages of younger thrusting include late Paleozoic, early Mesozoic, and post-Early Jurassic. These observations render superfluous explanations that require a deformation of the Roberts Mountains thrust to bring it to a northeasterly trend through north-central Elko County. We propose to account for the presence of the Roberts Mountains thrust in southern Twin Falls County, Idaho, by the presence of an east-trending right-lateral rift. This rift is now buried beneath Tertiary lava, with about 120 km of post-Devonian displacement, in southern Owyhee County, Idaho. Subsequent to the right-lateral movement, we propose that the Idaho block was translated still farther east and rotated counterclockwise about 30°. The compression in northern Elko County resulting from this rotation may be responsible for the east-west trends of the formations just south of the Idaho line, which condition the east-west trend of Mesozoic plutons in northern Elko County. Since these plutons are as old as Jurassic, the tectonism responsible may be Late Triassic and Early Jurassic. It is also responsible for the presence of western-facies and overlap-assemblage Paleozoic rocks thrust over eastern-facies and overlap-assemblage rocks in parts of northern Elko County in post-Antler time.

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