Mesozoic assimilation of oceanic crust and island arc into the North American continental margin in California and Nevada: Insights from geophysical data

Abstract

We present an updated, crustal-scale, geologic transect across northern California and Nevada that includes constraints from geologic data and geophysical models. We use this new model to explore the tectonic history of the region and conclude that prior to 175 Ma, oppositely dipping subduction zones were active along the North American continental margin at Sierra Nevada latitudes. An ensimatic arc terrane composed of the Jurassic Smartville Complex and its equivalents in the Sierra Nevada foothills developed on and across a disrupted ophiolitic basement in the upper plate of a west-dipping (in the present coordinate system) subduction zone. The east-dipping subduction zone was responsible for the formation of a Jurassic continental-margin arc that evolved on previously accreted terranes and a Permian-Triassic magmatic arc. As subduction continued at both convergent margins, the consumption of the intervening oceanic lithosphere resulted in the juxtaposition of the ensimatic arc terrane with the North American continental-margin arc at ca. 170 Ma. A short period of extension then followed during which the Coast Range ophiolite formed by backarc spreading. Almost immediately after the formation of the Coast Range ophiolite, tectonic underplating of the North American continental margin beneath the ensimatic arc terrane caused eastward transport of the ophiolites and the culmination of the Late Jurassic Nevadan orogeny. A new east-dipping subduction zone (Franciscan subduction) was then initiated west of the Coast Range ophiolite, following this collisional event. Franciscan subduction and Sierra Nevada magmatism continued along with the development of the Great Valley forearc basin until the Late Cretaceous. Intracontinental shearing within and/or along the eastern edge of the Sierra Nevada batholith (the Mojave-Snow Lake and Walker Lane shear zones) at ca. 140 Ma (Early Cretaceous) resulted in up to 400 km of dextral translation of a crustal block, causing a doubling of tectonic components along our east-west transect. In our interpretation, the Coast Range and (subsurface) Great Valley ophiolites, the Smartville arc complex and its equivalents in the Sierra Nevada foothills, and the late Paleozoic-early Mesozoic disrupted ophiolitic basement collectively form an intraoceanic terrane consisting of island arcs, intra-arc or backarc basins, and a multiply deformed oceanic substratum on which the island arc(s) were built. The Great Valley ophiolite is a major spatial and temporal link between the Coast Range ophiolite on the west and the Sierra Nevada foothills ophiolite belt on the east and is likely to represent the oceanic basement of this ensimatic Jurassic arc terrane. The accretion of the oceanic terrane to the North American continental margin in the Middle Jurassic marks a major orogenic event in the tectonic history of the North American Cordillera. We suggest that one of the most critical terrane boundaries in the western U.S. Cordillera occurs in the Sierra Nevada Foothills metamorphic belt, where the Jurassic intraoceanic terrane is juxtaposed with the Jurassic continental-margin arc and its basement.