New field mapping and bulk-rock geochemical investigations in the southern Klamath Mountains and central Sierran Foothills combined with previous structural, petrochemical, and geochronologic studies allow the distinction between three Triassic–Jurassic basaltic arcs built along the continental edge versus two roughly coeval basaltic complexes that formed farther off the Californian margin. The three Klamath Mountains arcs are: (1) The Hayfork Summit–Salmon River segment of the southern North Fork terrane, formed offshore as a sequence of interlayered chert, volcaniclastic strata and shale, and ocean island basalt (OIB), deposited on a mélanged and serpentinized basement containing blocks of 310- to 265-Ma mid-ocean ridge basalt (MORB). (2) Northward, the Sawyers Bar sector of the central North Fork terrane formed closer to the continental margin; this mafic arc originated at ca. 200–170 Ma as a stack of interdigitated island-arc tholeiites (IAT) and minor OIBs interstratified with, and largely overlying, distal turbidites derived from eastern Klamath terranes. (3) The currently farther outboard Rattlesnake Creek terrane consists of continent-sourced, Lower Jurassic metasedimentary quartzose strata interbedded with island-arc volcanic rocks; this near-shore section was laid down on older ophiolitic basement consisting of tectonized serpentinite and MORB blocks. The remaining two arcs are in the Sierran Foothills: (4) The offshore Peñon Blanco arc consists of cherty and volcaniclastic sedimentary strata interlayered with 200-Ma mafic volcanic-plutonic arc rocks, all resting on a 300-Ma ophiolitic basement; suturing against the structurally higher Mariposa Formation took place after deformation of the latter at ca. 150 Ma. (5) The Slate Creek complex, and possibly the Lake Combie, Owl Gulch, and Sullivan Creek entities, formed along the margin of North America; superjacent units consist chiefly of 207- to 170-Ma volcaniclastic, sedimentary, and arc volcanic rocks deposited on an ophiolitic mélange basement. Metamorphic belts of the central Klamath Mountains and Sierran Foothills evidently contain both near-shore and offshore oceanic arcs.