THE NORTH FORK TERRANE, KLAMATH MOUNTAINS, CALIFORNIA: GEOLOGIC, GEOCHEMICAL AND GEOCHRONOLOGIC EVIDENCE FOR AN EARLY MESOZOIC FOREARC

Terranes of the Klamath Mountains are primarily the products of convergent margin processes that span the mid-Devonian to Jurassic time interval. One of the more enigmatic units of this orogenic belt is the early Mesozoic, NS-trending, medially situated North Fork terrane. We present a forearc basin model for the deposition of mafic metavolcanic units and associated metasedimentary strata based on the integration of new and previously published data. Our geologic mapping in the southern Klamath Mountains reveals that NW-SE-striking structures in the North Fork terrane are regionally consistent, ophiolitic rocks are variably deformed (one expression being a serpentinite matrix mélange), and the terrane is dominated by mafic volcanic rocks and very fine-grained clastic metasediments interbedded with chert. Bulk-rock geochemical data for metasedimentary strata in the study area indicate that chert and shale possess a component of continental material. Detrital zircon data for three clastic samples (one from the southern and two from the central North Fork terrane) show that (1) sediments of the southern and central North Fork terrane had a similar source of Mesozoic zircons, (2) sediments 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 terrane was derived primarily from an early Mesozoic source. We conclude that the North Fork forearc basin was located seaward from the early Mesozoic continental arc of the Eastern Klamaths, and that it lay further offshore in the southern portion of the terrane than the central portion.