Implications of new detrital-zircon data for the depositional history, provenance, and paleogeography of Upper Triassic–Middle Jurassic rocks within the Northern Sierra terrane, California, USA
Geoff Christe, Todd A. LaMaskin, Richard A. Schweickert, "Implications of new detrital-zircon data for the depositional history, provenance, and paleogeography of Upper Triassic–Middle Jurassic rocks within the Northern Sierra terrane, California, USA", Tectonics, Sedimentary Basins, and Provenance: A Celebration of the Career of William R. Dickinson, Raymond V. Ingersoll, Timothy F. Lawton, Stephan A. Graham
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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.