ABSTRACT This field guide describes stops in the Oregon Klamath Mountains that visit near-complete ophiolite sections, pre- and post-accretion arc plutons, greenschist- to amphibolite-grade metamorphosed wallrocks, arc volcanic rocks, and interbedded chert, argillite, and olistostromal deposits. Structural features at these stops include local- and regional-scale folds and faults, as well as penetrative metamorphic fabrics such as slaty cleavage, gneissic layering, and mineral lineations. The geologic history here reveals a period of Late Triassic and Jurassic ophiolite and oceanic-arc formation followed by Middle Jurassic terrane accretion, tectonic mélange formation, and continued oceanic arc magmatism. Rifting from ca. 165 to 160 Ma produced the Rogue-Chetco arc, Josephine ophiolite, and remnant arc comprised of older Klamath Mountains terranes. Deformation and magmatism during the Late Jurassic Neva-dan orogeny accreted this active arc–inter-arc basin–remnant arc triad to western North America, producing the lithotectonic belts observed today. The Oregon Klam-ath Mountains therefore provide an exceptional opportunity to examine the deep to shallow levels of multi-phase oceanic lithosphere and deformational features related to the accretion of these terranes to the continental margin.

Upper Paleozoic rocks of the northern Sierra and eastern Klamath terranes provide detailed stratigraphic records of ensimatic arc-related sedimentation and magmatism. Comparison of Paleozoic stratigraphic relations between the two terranes, however, suggests certain contrasts in depositional environments and the nature, volume, and timing of volcanism for given time intervals. Some lithologic and provenance ties indicate a paleogeographic relation. Variations in stratigraphy between the two terranes and within terranes imply differences in geodynamic setting. These and regional geologic relations indicate an early and persistent paleogeographic tie between the two areas, and they further suggest that the eastern Klamath terrane may have lain outboard and trenchward of the northern Sierra terrane during much of their late Paleozoic evolution. Stratigraphic ties for mid-Permian and lower Mesozoic rocks imply a closer relation and more similar geodynamic setting during subsequent evolution.