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In the northern Ruby Mountains, Nevada, the transition from high-grade, migmatitic infrastructure to the nonmetamorphosed but allochthonous rocks of the suprastructure is a zone of intense ductile deformation and high strain. This zone, which is disharmonic to the terranes both structurally above and below, is characterized by amphibolite-facies metamorphism, polyphase deformation, mylonitic textures, a west-northwest lineation, and tabular masses of orthogneiss. Major map-scale folds are overturned westward opposite the common direction of overturning in the infrastructure. Many of the large folds in the transition zone (Abscherungszone after the concepts of Haller, 1956Abscherungszone after the concepts of Haller, 1971) have evolved into ductile faults (tectonic slides) and form complex braided systems. Tectonic slices of low-grade metasedimentary rocks (greenschist facies?) mark the boundary between the top of the Abscherungszone and an overlying low-angle fault complex of unmetamorphosed upper Paleozoic and locally Tertiary rocks. These upper Paleozoic rocks represent attenuated fragments of the suprastructure which was structurally above the metamorphic complex during middle Mesozoic regional metamorphism and deformation. The Tertiary sedimentary and volcanic rocks are Miocene (K-Ar age on sanidine from a rhyolite flow), and conglomeratic facies contain clasts derived from the underlying metamorphic complex. Therefore, the low-angle fault complex is a Neogene brittle phenomenon apparently superimposed on an older Mesozoic “stockwerk” terrane. Despite these relations, a tectonic continuum involving a diachronistic transition from ductile to brittle deformation is an alternative explanation; however, such a tectonic regime must have spanned an interval from middle Mesozoic to Miocene.

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