Abstract

The Sonoma Range of north-central Nevada exposes lower Paleozoic rocks of the Roberts Mountains allochthon that were thrust from east to west over Upper Triassic shelf strata in Jurassic time. Also present are Roberts Mountains allochthon rocks that were autochthonous in the Jurassic. Structural analysis of Jurassic allochthonous vs. Jurassic autochthonous rocks of the Harmony Formation indicates that both packages have the same east-verging Paleozoic deformation history: recumbent isoclines (F1), shallowly inclined tight folds (F2), then upright folds (F3).

Three generations of Jurassic folds (F4, F5, F6) are present in the Jurassic allochthonous Harmony Formation, whereas the Jurassic autochthonous Harmony Formation has only the youngest Jurassic phase (F6). First Jurassic folds (F4) in the Jurassic allochthonous Harmony Formation are synemplacement major fault-bend folds. F4 folds F1-F3 and does not fold F5 and F6. F5 folds are markedly different from Paleozoic deformations and the younger Jurassic folds (F6) present in both the Jurassic allochthonous and Jurassic autochthonous Harmony Formation (major, upright, west-verging). Recognition of F5 folds in rocks of the Roberts Mountains allochthon north and east of the Sonoma Range may help define the geographic extent of Jurassic deformation in northern Nevada. These boundaries cannot be established by traditional means because of the absence of exposure of Upper Triassic to Middle Jurassic rocks in the region.

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