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NARROW
GeoRef Subject
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all geography including DSDP/ODP Sites and Legs
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Black Rock Desert (1)
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El Paso Mountains (1)
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Sierra Nevada (1)
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United States
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California (1)
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Colorado Plateau (1)
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geologic age
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Mesozoic
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Jurassic
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Lower Jurassic
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Sinemurian (1)
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Toarcian (1)
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Middle Jurassic
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Bajocian (1)
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lower Mesozoic (1)
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Triassic
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Upper Triassic
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Carnian (1)
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Paleozoic
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Permian (1)
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igneous rocks
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igneous rocks
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volcanic rocks (1)
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minerals
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silicates
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orthosilicates
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nesosilicates
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zircon group
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zircon (1)
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Primary terms
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absolute age (1)
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igneous rocks
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volcanic rocks (1)
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Mesozoic
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Jurassic
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Lower Jurassic
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Sinemurian (1)
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Toarcian (1)
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Middle Jurassic
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Bajocian (1)
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lower Mesozoic (1)
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Triassic
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Upper Triassic
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Carnian (1)
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Paleozoic
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Permian (1)
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sedimentary rocks
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clastic rocks (1)
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sedimentation (1)
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United States
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California (1)
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Colorado Plateau (1)
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sedimentary rocks
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sedimentary rocks
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clastic rocks (1)
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ABSTRACT Lower Mesozoic clastic rocks that unconformably overlie Paleozoic rocks within the Northern Sierra terrane provide clues regarding the evolution of the terrane during a 60 m.y. interval spanning the late Carnian through Bajocian. New detrital-zircon data provide fresh insights into the ages and provenance of these clastic rocks, together with new inferences about the Mesozoic tectonic evolution of the terrane. Previous studies have shown that from the late Carnian to the Sinemurian (~40 m.y.), a 1-km-thick section of subaerial to shallow-marine clastic arc-derived sediment accumulated and shallow-marine carbonate was deposited. At the base of this section, detrital-zircon results suggest the Northern Sierra terrane was located near a source area, possibly the El Paso terrane, containing Permian igneous rocks ranging in age from 270 to 254 Ma. By the earliest Jurassic, the detrital-zircon data suggest the Northern Sierra terrane was located near a source containing latest Triassic–earliest Jurassic igneous rocks spanning 209–186 Ma. The source of this material may have been the Happy Creek volcanic complex of the Black Rock terrane. A deep-marine, anoxic basin developed within the Northern Sierra terrane ca. 187–168 Ma. Approximately 3.5 km of distal turbidites were deposited in this basin. Previously reported geochemical characteristics of these turbidites link the Northern Sierra terrane with arc rock of the Black Rock terrane during this interval, except for a short time in the late Toarcian, when the terrane received an influx of quartz-rich sediment, likely derived from Mesozoic erg deposits now exposed on the Colorado Plateau. Clastic deposition within the Northern Sierra terrane ended in the Bajocian. Eruption of proximal-facies, mafic volcanic rocks and intrusion of hypabyssal rock and 168–163 Ma plutons reflect development of a magmatic arc within the terrane. These igneous rocks represent the first unequivocal evidence that the Northern Sierra terrane was located within a convergent-margin arc during the Triassic and Jurassic. Because detrital-zircon data from Lower Mesozoic strata within the Northern Sierra terrane indicate that it was depositionally linked with differing source areas through time early in the Mesozoic, the terrane may have been mobile along the western margin of Laurentia. There is little evidence from sediment within the Lower Mesozoic section of the terrane that can clearly be tied to the craton or the continental-margin Triassic arc prior to the late Toarcian. The absence of Upper Triassic or Lower Jurassic plutonic rocks within the terrane prior to the mid-Bajocian is also consistent with some form of isolation from the continental-margin arc system. While new detrital-zircon results place the Northern Sierra terrane proximal to the western margin of Laurentia in the late Toarcian, the current location of the terrane likely reflects Early Cretaceous offset along the Mojave–Snow Lake fault.
Lower Mesozoic volcanic rocks exposed in an 8- to 15-km-wide belt between Westwood and Taylorsville, northern California, are the northernmost exposures of a volcanic sequence that originally spanned the eastern flank of the present-day Sierra Nevada batholith. The northwest-striking, southwest-dipping, west-facing, homoclinal volcanic section is only weakly metamorphosed and deformed. The more than 11-km-thick volcanic sequence records at least four episodes of subaerial, alkalic to calc-alkalic, andesitic to dacitic volcanism punctuated by intervals of clastic deposition. Rock types include monolithic intrusive breccia, porphyritic flow rock, tuff breccia, and hypabyssal intrusive rock. The interbedded clastic rocks consist of fluvial to shallow-water bedded volcanic arenites, conglomerates, laharic breccias, debris flows, siltstones, and shales. Major- and trace-element abundances for the volcanic rocks, including rare earth elements (REE), are similar to those in modern continental arc sequences. Ce/Y:Y, La:Ba, La:Th, K 2 O:Na 2 O, and Ba:K 2 O resemble those in volcanic rocks from high-K provinces. On the basis of K 2 O:SiO 2 relations, most of the volcanic rocks are classified as latites or toscanites. The morphology, stratigraphy, and major- and trace-element chemistry of the lower Mesozoic rocks exposed in northeastern California resemble those of other mildly alkalic, lower Mesozoic volcanic sequences along the eastern flank of the Sierra Nevada, California, in western Arizona, and western Nevada. It is proposed that this belt of high-K volcanic rock (the high-K magmatic province) represents postorogenic magmatic activity associated with a phase of long-lived extensional tectonics following the Permian-Triassic Sonoma orogeny.