Tectonic implications of detrital zircon data from Paleozoic and Triassic strata in western Nevada and Northern California

Abstract

U-Pb analyses of detrital zircons from various allochthonous assemblages of Paleozoic and early Mesozoic age in western Nevada and northern California yield new constraints on the sediment dispersal patterns and tectonic evolution of western North America. During early Paleozoic time, a large submarine fan system formed in slope, rise, basinal, and perhaps trench settings near the continental margin, west of continental shelf deposits of the Cordilleran miogeocline. Our detrital zircon data suggest that most of the detritus in this fan system along the western U.S. segment of the margin was derived from the Peace River Arch region of northwestern Canada, and some detritus was shed from basement rocks of the southwestern United States or western Mexico. In most cases, the detritus in the allochthonous assemblages was recycled through platformal and/or miogeoclinal sedimentary units prior to accumulating in offshelf environments. Lower Paleozoic rocks of the Roberts Mountains allochthon, Shoo Fly Complex, and Yreka terrane are interpreted to have been parts of this fan complex that accumulated along the central U.S. segment of the continental margin, probably within 1000 km of the miogeocline. During the mid-Paleozoic Antler orogeny, parts of the lower Paleozoic fan complex were deformed and uplifted, and strata of the Roberts Mountains allochthon were tectonically emplaced onto the continental margin. This orogeny was apparently driven at least in part by convergence of the Sierra-Klamath arc with the continental margin, as has been proposed by many previous workers, because these arc terranes are overlain by Mississippian clastic strata derived from the Roberts Mountains allochthon. Our data are not sufficient, however, to determine the polarity of the arc, or whether the arc formed along the continental margin or was exotic to western North America. Detrital zircon data indicate that following the Antler orogeny, clastic sediments derived from the Roberts Mountains allochthon were deposited both on the continental margin to the east and within intra-arc and backarc basins to the west. The occurrence of this detritus in terranes of western Nevada and northern California indicates that they were proximal to each other and to the continental margin during late Paleozoic time. The presence of upper Paleozoic volcanic and plutonic rocks and arc-derived detrital zircons in strata of the northern Sierra, eastern Klamath, and Black Rock terranes records the existence of a west-facing magmatic arc near the continental margin during late Paleozoic time. Our data are not supportive of scenarios in which these arc terranes were located farther north or thousands of kilometers offshore of the Nevada continental margin during late Paleozoic time. Following a second phase of uplift, erosion, and allochthon emplacement during the Permian-Early Triassic Sonoma orogeny, Middle and Upper Triassic strata now preserved in west-central Nevada accumulated in a backarc basin. Our data indicate that the basinal assemblages contain detritus from arc terranes to the west as well as the craton to the east.