An expanded view of Jurassic orogenesis in the western United States Cordillera: Middle Jurassic (pre-Nevadan) regional metamorphism and thrust faulting within an active arc environment, Klamath Mountains, California

Abstract

Basaltic to basaltic andesitic volcaniclastic rocks and their contemporaneous mafic-ultramafic intrusive complexes delineate a Middle Jurassic arc terrane within the Klamath Mountain province of northern California. Exposures of the supracrustal volcaniclastic rocks are restricted to a single fault-bounded terrane, but the deeper level intrusive complexes were emplaced into most, if not all, the pre-Late Jurassic terranes of the Klamath Mountain region. The pre-Late Jurassic terranes thus constitute the basement of the Middle Jurassic arc. U-Pb zircon analyses of 39 zircon fractions from 12 intrusive complexes plus K-Ar dating of the volcaniclastic strata demonstrate magmatic activity over the interval of ∼177-159 Ma. The active arc and its basement were imbricated by a compressive deformational event, the signature of which included thrust faulting, isoclinal folding, and regional metamorphism. Several diverse lines of evidence, including K-Ar dating of metamorphic rocks, crosscutting relations of dated intrusive complexes to thrust faults, and U-Pb dating of synmetamorphic intrusive complexes, establish a distinctly pre-Nevadan Middle Jurassic age (ongoing at ∼169 Ma and over by at least 161 Ma) for this compressive deformational episode. Outboard and structurally beneath the Middle Jurassic arc and its basement are several terranes that collectively comprise the western Jurassic belt. These terranes were deformed and regionally metamorphosed during the Late Jurassic Nevadan orogeny, which occurred within the time interval of ∼157-150 Ma, as Upper Jurassic plutons with 150- to 142-m.y.-old zircon ages have contact aureoles that overprint the Nevadan fabric, and the ∼157-m.y.-old Rogue Formation was deformed in the Nevadan event. The Middle and Late Jurassic compressive deformational events were thus distinct and separated by as much as 15-20 m.y. The relation between Middle and Late Jurassic magmatism and deformation suggests that the Klamath Mountain province records the evolution of a considerably long-lived arc system that evolved above an east-dipping subduction zone. In addition, we suggest that this are system may represent an oceanic continuation of the long-recognized early Mesozoic arc terrane of the western U.S. Cordillera.