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Abstract
Recent studies indicate an Early Cretaceous strike-slip fault system, the Mojave-Snow-Nevada-Idaho, extended from southeast California through northwest Nevada intowestern Idaho and accommodated 400-500 km of dextral offset. Younger cover andplutonic intrusions have obscured the fault. Therefore, its location and displacement arebased on the juxtaposition of unrelated geologic units. To investigate the westernboundary of the proposed fault, the Fox Range in northwest Nevada was extensivelymapped. The pre-Cenozoic rocks of the range are divided into four metasedimentaryunits: a graphitic argillite unit (Smith Canyon argillite), a massive quartzite unit(Cottonwood Canyon quartzite), a quartz-muscovite-schist (Smoke Creek schist), and aquartz-biotite paragneiss (Fox Canyon gneiss), all intruded by Cretaceous plutons anddikes. Two phases of deformation were identified in all units: 1) bedding deformed bytight to isoclinal D1 folds verging west, accompanied by an axial planar foliation (S1); 2)tight to open D2 folds deforming S1 and D1 folds with S2 present locally. Thesedeformational events are not found to the east of the proposed fault in the adjacent BlackRock and Late Triassic back-arc terranes. U-Pb detrital zircon ages were obtained fromseveral samples of quartzite in the Fox Range. The distribution of zircon ages areessentially identical to those of Jurassic erg deposits of the Colorado plateau and their redeposited equivalents in the southwest Cordillera. This indicates the Fox Range isunrelated to neighboring terranes and was translated northwards along the fault systempossibly from an origin as far as southwestern Arizona.