Abstract

Devonian–Middle Jurassic terrane assemblies in the Klamath Mountains and Sierra Nevada Foothills consist chiefly of ophiolite-chert-argillite sequences. Mafic-ultramafic complexes are oceanic, whereas associated fine-grained deep-water terrigenous sediments were derived mainly from adjacent, previously docked Klamath-Sierran terranes. Coeval calc-alkaline arc rocks are volumetrically rare. Geologic and petrochemical relations suggest a rifted arc origin for Klamath mafic metavolcanic units inter-layered with distal turbidites in the 170–200 Ma North Fork terrane; detrital zircon U-Pb ages indicate that the clastic debris had a regional eastern Klamath source. The Eastern Hayfork cherty mélange contains ophiolitic scraps and distinctive olistostromal sandstone blocks evidently derived from the nearby Eastern Klamath Antelope Mountain Quartzite. The seaward 200 Ma Rattlesnake Creek terrane is an ophiolitic mélange with North Fork petrotectonic affinities. The North Fork–Eastern Hayfork–Rattlesnake Creek amalgam correlates with the Calaveras Complex and the outboard Jura-Triassic arc belt in the Sierran Foothills. Geochemical bulk-rock and zircon U-Pb age data support interpretation of the 200 Ma Jura-Triassic arc as an adjacent offshore mafic belt overlying a 300 Ma ophiolitic basement. These oceanic complexes were sutured against the Central Metamorphic Belt–Eastern Klamath–Feather River–Northern Sierra terrane backstop before deposition and deformation of the outboard Upper Jurassic Galice and Mariposa formations. Klamath-Sierran terrane assemblies reflect ∼230 m.y. of transpression-transtension involving only minor episodes of subduction, producing ubiquitous ophiolite-chert-argillite lithologies and rare felsic arc rocks.

In contrast, the Late Jurassic to largely Cretaceous Klamath–Sierra Nevada quartzo-feldspathic volcanic-plutonic arc attests to massive calc-alkaline magmatism attending a strong eastward component of underflow by the Farallon plate. The coeval Galice-Mariposa formations, followed by the Cretaceous Great Valley forearc and Franciscan trench deposits, are first-cycle felsic debris shed mainly from the Klamath-Sierran arc. These units record ∼70 m.y. of rapid sialic crustal growth attending major periods of approximately margin-normal convergence. This profound transition in northern California included Devonian–Middle Jurassic rifting, drifting, and stranding of ophiolite-chert-argillite terranes along an adjacent curvilinear continental margin, then nearly head-on Cretaceous subduction that resulted in massive calc-alkaline igneous activity, the erosion of which generated the felsic Great Valley Group forearc basin and Franciscan Complex trench clastic sedimentary units.

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