The Coast Range ophiolite of California is one of the most extensive ophiolite terranes in North America, extending over 700 km from the northernmost Sacramento Valley to the southern Transverse Ranges in central California. This ophiolite, and other ophiolite remnants with similar mid-Jurassic ages, represent a major but short-lived episode of oceanic crust formation that affected much of western North America. The history of this ophiolite is important for models of the tectonic evolution of western North America during the Mesozoic, and a range of conflicting interpretations have arisen. Current petrologic, geochemical, stratigraphic, and radiometric age data all favor the interpretation that the Coast Range ophiolite formed to a large extent by rapid extension in the forearc region of a nascent subduction zone. Closer inspection of these data, however, along with detailed studies of field relationships at several locales, show that formation of the ophiolite was more complex, and requires several stages of formation.Our work shows that exposures of the Coast Range ophiolite preserve evidence for four stages of magmatic development. The first three stages represent formation of the ophiolite above a nascent subduction zone. Rocks associated with the first stage include ophiolite layered gabbros, a sheeted complex, and volcanic rocks with arc tholeiitic or (more rarely) low-K calc-alkaline affinities. The second stage is characterized by intrusive wehrlite-clinopyroxenite complexes, intrusive gabbros, Cr-rich diorites, and volcanic rocks with high-Ca boninitic or tholeiitic ankaramite affinities. The third stage includes diorite and quartz diorite plutons, felsic dike and sill complexes, and 1 Corresponding author; 290 SHERVAIS ET AL.calc-alkaline volcanic rocks. The first three stages of ophiolite formation were terminated by the intrusion of mid-ocean ridge basalt dikes, and the eruption of mid-ocean ridge basalt or ocean-island basalt volcanic suites. We interpret this final magmatic event (MORB dikes) to represent the collision of an active spreading ridge. Subsequent reorganization of relative plate motions led to sinistral transpression, along with renewed subduction and accretion of the Franciscan Complex. The latter event resulted in uplift and exhumation of the ophiolite by the process of accretionary uplift.

Devonian-Middle Jurassic terrane assemblies in the Klamath Mountains and SierraNevada Foothills consist chiefl y of ophiolitechert-argillite sequences. Mafi c-ultramafi c complexes are oceanic, whereas associated fi ne-grained deep-water terrigenous sediments were derived mainly from adjacent, previously docked Klamath-Sierran terranes. Coeval calc-alkaline arc rocks are volumetrically rare. Geologic and petrochemical relations suggest a rifted arc origin for Klamath mafi c metavolcanic units interlayered with distal turbidites in the 170-200 Ma North Fork terrane; detrital zircon U-Pb ages indicate that the clastic debris had a regional eastern Klamath source. The Eastern Hayfork cherty mélange contains ophiolitic scraps and distinctive olistostromal sandstone blocks evidently derived from the nearby Eastern Klamath Antelope Mountain Quartzite. The seaward 200 Ma Rattlesnake Creek terrane is an ophiolitic mélange with North Fork petrotectonic affi nities. The North Fork-Eastern Hayfork-Rattlesnake Creek amalgam correlates with the Calaveras Complex and the outboard Jura-Triassic arc belt in the Sierran Foothills. Geochemical bulk-rock and zircon U-Pb age data support interpretation of the 200 Ma Jura-Triassic arc as an adjacent offshore mafi c belt overlying a 300 Ma ophiolitic basement. These oceanic complexes were sutured against the Central Metamorphic Belt-Eastern Klamath-Feather River-Northern Sierra terrane backstop before deposition and deformation of the outboard Upper Jurassic Galice and Mariposa formations. KlamathSierran terrane assemblies refl ect ~230 m.y. of transpression-transtension involving only minor episodes of subduction, producing ubiquitous ophiolite-chert-argillite lithologies and rare felsic arc rocks. In contrast, the Late Jurassic to largely Cretaceous Klamath-Sierra Nevada quartzofeldspathic volcanic-plutonic arc attests to massive calc-alkaline magmatism attending a strong eastward component of underfl ow by the Farallon plate. The coeval GaliceMariposa formations, followed by the Cretaceous Great Valley forearc and Franciscan trench deposits, are fi rst-cycle felsic debris shed mainly from the Klamath-Sierran arc. These units record ~70 m.y. of rapid sialic crustal growth attending major periods of approximately margin-normal convergence. This profound transition in northern California included Devonian-Middle Jurassic rifting, drifting, and stranding of ophiolitechert-argillite terranes along an adjacent curvilinear continental margin, then nearly head-on Cretaceous subduction that resulted in massive calc-alkaline igneous activity, the erosion of which generated the felsic Great Valley Group forearc basin and Franciscan Complex trench clastic sedimentary units.

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We present new U/Pb ages for detrital zircons separated from six quartzose metagraywackes collected from different Franciscan Complex imbricate nappes around San Francisco Bay. All six rocks contain a broad spread of Late Jurassic-Cretaceous grains originating from the Klamath-Sierra Nevada volcanic-plutonic arc. Units young structurally downward, consistent with models of progressive underplating and offscraping within a subduction complex. The youngest specimen is from the structurally lowest San Bruno Mountain sheet; at 52 Ma, it evidently was deposited during the Eocene. None of the other metagraywackes yielded zircon ages younger than 83 Ma. Zircons from both El Cerrito units are dominated by ca. 100-160 Ma grains; the upper El Cerrito also contains several grains in the 1200-1800 Ma interval. These samples are nearly identical to 97 Ma metasedimentary rock from the Hunters Point shear zone. Zircon ages from this mélange block exhibit a broad distribution, ranging from 97 to 200 Ma, with only a single pre-Mesozoic age. The Albany Hill specimen has a distribution of pre-Mesozoic grains from 1300 to 1800 Ma, generally similar to that of the upper El Cerrito sheet; however, it contains zircons as young as 83 Ma, suggesting that it is signifi cantly younger than the upper El Cerrito unit. The Skaggs Spring Schist is the oldest studied unit; its youngest analyzed grains were ca. 144 Ma, and it is the only investigated specimen to display a signifi cant Paleozoic detrital component.Sedimentation and subduction-accretion of this tract of the trench complex took place along the continental margin during Early to early-Late Cretaceous time, and perhaps into Eocene time. Franciscan and Great Valley deposition attests to erosion of an Andean arc that was active over the entire span from ca. 145 to 80 Ma, with an associated accretionary prism built by progressive underthrusting. We use these new data to demonstrate that the eastern Franciscan Complex in the northern and central Coast Ranges is a classic accretionary prism, where younger, structurally lower allochthons are exposed on the west, and older, structurally higher allochthons occur to the east, in the heavily studied San Francisco Bay area.