Mesozoic structure, stratigraphy, and magmatism in the eastern Pueblo Mountains, southeast Oregon and northwest Nevada
Wolak, Chad Edward
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Mesozoic rocks in the eastern Pueblo Mountains, along the Oregon-Nevada border, represent a Middle Jurassic stratovolcano complex. New mapping, stratigraphic, magmatic, and structural studies indicate these rocks constitute a distinct terrane (Pueblo terrane), whose relationship to other Mesozoic terranes, in the U.S. Cordillera, is uncertain. The Pueblo terrane contains a thick sequence of volcanogenic strata intruded by shallow crustal-level comagmatic plutons (~179-176 Ma). Volcanogenic strata consist of dacitic-andesitic lava, rhyolitic-andesitic volcaniclastics, and rhyolitic-dacitic tuffs. These rocks reflect an arc constructed on transitional, but perhaps continental, crust and deposition in a subaerial environment. A major ductile shear zone (Pueblo Mountains shear zone) deforms the southeastern Pueblo terrane, and is characterized by greenschist grade mylonites with northeast-striking, moderately southeast-dipping foliations and down-dip mineralstretching lineations. Along its northwest boundary, strain quickly fades, and the northwestern Pueblo terrane is undeformed by the shear zone. In it, shear sense indicators include abundant S-C fabrics and mantled porphyroclasts that indicate top-to-thenorthwest directed shear and reverse-sense compressive tectonism. Preliminary 40Ar/39Ar geochronologic data suggest regional metamorphism and the shear zone are Albian (mid- Cretaceous).