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Terranes of the Klamath Mountains, California-Oregon, are primarily the products of transpressive-transtensional margin processes that span the mid-Devonian to Middle Jurassic time interval. One of the more enigmatic units of this orogen is the early Mesozoic, north-south–trending, medially situated North Fork terrane. Based on the integration of new and previously published data, ophiolitic rocks in the North Fork terrane are interpreted as the basement to superjacent mafic metavolcanic units and associated metasedimentary strata. We propose that these volcanic and sedimentary rocks accumulated in a forearc position relative to the eastern Klamath fringing arc. New mapping in the southern Klamath Mountains reveals that northwest-southeast–striking structures in the southern North Fork terrane are regionally consistent with those in the rest of the belt, ophiolitic rocks are variably deformed (one expression being a serpentinite-matrix mélange), and the terrane is dominated by mafic volcanic rocks and fine-grained clastic metasedimentary rocks interbedded with chert. Bulk-rock geochemical data for metasedimentary rocks indicate that chert and shale possess a continental component. Detrital zircon U-Pb ages for three sandstone samples (one from the southern and two from the central North Fork) show that (1) sedimentary rocks of both terrane segments had a similar source of Mesozoic zircons, (2) sedimentary rocks of the central North Fork terrane have ages consistent with a source in the nearby eastern Klamath Mountains, and (3) sediment of the southern North Fork was derived primarily from an early Mesozoic source. We conclude that the North Fork terrane represents tectonically disrupted remnants of a forearc basin and its ophiolitic basement that were located seaward of the early to middle Mesozoic fringing arc of the eastern Klamath terrane and landward from a coeval oceanic trench. Slightly coarser-grained sedimentary rocks are preserved in the central part of the terrane, whereas rocks in the south lack continent-derived material coarser than mud.

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