Tectonic evolution of a coherent Late Triassic subduction complex, Stuart Fork terrane, Klamath Mountains, northern California

The Stuart Fork (SF) terrane in the central Klamath Mountains of northern California is a coherent Late Triassic blueschist belt that documents early plate convergence in western North America. Complex deformation and polyphase metamorphism of the terrane directly reflect evolving convergent-margin processes. The origin and subsequent petrotectonic evolution of the SF terrane provide strong evidence that the early to middle Mesozoic plate margin of North America may be characterized as a convergent ocean-continent boundary with a relatively simple architecture. SF metabasaltic and conformably overlying siliceous metasedimentary rocks originated as submarine volcanic oceanic crust and pelagic marine sediments of probable latest Paleozoic age. Major-, trace-, and rare-earth-element geochemical data show the metabasaltic rocks to be slightly enriched mid-ocean tholeiites with compositions similar to those of modern mid-ocean-ridge or back-arc-basin basalts. Two major periods of deformation are recorded in rocks of the SF terrane: D ₁ , an early (Late Triassic) phase of deformation that produced complex folds and imbricate west-directed thrusts, and D ₂ , a late-stage (Middle Jurassic) deformation that formed simple, open folds during westward thrusting of the entire terrane over neighboring terranes during regional shortening. Each of these episodes of shortening is related to convergence, yet D ₁ occurred in an accretionary complex and accompanied high-pressure blueschist-facies metamorphism during subduction, whereas D ₂ took place as a result of final thrusting (or accretion) of the entire assemblage to the continental margin. Petrologic evidence indicates that SF rocks experienced three distinct periods of metamorphic recrystallization: M ₁ , a prehniteactinolite to greenschist-facies hydrothermal event; M ₂ , a blueschistfacies regional event; and M ₃ , a greenschist- to amphibolite-facies contact event. M ₁ greenschist assemblages, reflecting ocean-ridge-type metamorphism, are preserved locally within metabasaltic rocks containing relict igneous augite and are incipiently overprinted by blueschist assemblages. M ₂ blueschist assemblages formed pervasively during the dominant recrystallization event, which was associated with complex fold-and-thrust deformation of a subduction-zone accretionary system (D ₁ ). M ₃ assemblages form contact aureoles around approximately Middle Jurassic arc-related granodiorite plutons. The petrotectonic evolution of the Stuart Fork terrane may be explained by a model in which petrologic and structural episodes are related to latest Paleozoic-earliest Mesozoic(?) back-arc-basin spreading, Late Triassic subduction-zone, and Middle Jurassic postaccretion, magmatic-arc tectonic environments.