The Cuesta Ridge ophiolite is a well-preserved remnant of the Middle Jurassic Coast Range ophiolite tectonically overlying rocks of the Franciscan complex. It is a nearly complete ophiolite section, consisting of over 1 km of serpentinized harz-burgite and dunite, sills of wehrlite, pyroxenite, and lherzolite, isotropic gabbro, a sheeted complex of quartz-hornblende diorite, an ∼1200-m-thick volcanic section, late-stage mafic dikes, and 5–10 m of tuffaceous radiolarian chert. The volcanic section at Cuesta Ridge has two chemically distinct volcanic groups. The lower volcanic section is characterized by low Ti/V ratios (11–21), enriched large ion lithophile element (LILE) concentrations, and depleted high field strength elements (HFSEs). Boninitic lavas with high MgO, Cr, and Ni abundances are present in this suite, along with arc tholeiites (basaltic andesites to dacites). Basalts of the upper volcanic section, which conformably overlie the lower volcanic section, and late-stage basaltic dikes that crosscut the hornblende–quartz diorite plutonic section are characterized by higher Ti/V ratios (20–27) and HFSE abundances and lower LILE abundances than the underlying section. These late-stage volcanic rocks have mid-ocean-ridge basalt–like chemistry. The field and geochemical data indicate formation in a suprasubduction-zone setting above an east-dipping proto-Franciscan subduction zone due to the onset of subduction and subsequent slab rollback. Multiple stages of magmatism ensued, until the emplacement of the late-stage dikes and uppermost flows. These late-stage dikes, which are present in several Coast Range ophiolite remnants, signify the end of ophio-lite formation and are interpreted to represent a Late Jurassic ridge collision.

New single-grain detrital zircon U-Pb age data from sandstone lenses in the Upper Jurassic Mariposa Formation of the Sierra Nevada foothills metamorphic belt indicate that: (1) the earliest phase of clastic sedimentation mainly involved material derived from the Bragdon and Baird Formations of the Eastern Klamaths and the Paleozoic miogeocline of Nevada ± sources farther to the east, with modest input from the Sierra Nevada arc; (2) the arc became the dominant sediment source for the upper turbidite interval in the Mariposa Formation; and (3) the youngest zircon ages constrain the onset of clastic deposition at 152 ± 1 Ma. Zircon age data also suggest that the local drainage divide migrated westward, resulting in a higher proportion of detritus derived from the Sierra Nevada arc over time. New geologic mapping in the central Sierra Nevada foothills shows that the Mariposa Formation thickens eastward, and that the number of coarse-grained sandstone bodies increases up section. These observations indicate that a topographically low Sierran volcanic arc gradually began to rise, providing increasing amounts of clastic debris to the Mariposa depositional basin. The Mariposa Formation was deposited in a volcanically active deep-water forearc basin and was subsequently disrupted by Nevadan orogenesis during the Late Jurassic. Inasmuch as it was located in the forearc inboard from the Middle Jurassic Coast Range ophiolite, Nevadan deformation cannot have resulted from arc-continent collision in the Sierra Nevada foothills but instead must have been related to tectonism along the plate margin.

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.