Multi-Stage Origin of the Coast Range Ophiolite, California: Implications for the Life Cycle of Supra-Subduction Zone Ophiolites

Abstract

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.

Mesozoic transpression, transtension, subduction and metallogenesis in northern and central California

“…The Siskiyou orogeny (Coleman et al, 1988;Wright and Fahan, 1988; has been attributed to arc-margin suturing at c. 175-185 Ma (Saleeby et al, 1989a). The Nevadan orogeny occurred during Late Jurassic time, reflecting either arc collision (Schweickert and Cowan, 1975;Schweickert et al, 1984Schweickert et al, , 1985 or flatslab underflow following ridge-trench annihilation (Shervais et al, 2004).…”

“…Crustal growth of the early to midMesozoic Cordilleran margin of northern and central California involved several accretionary events, reflecting arc-margin collision (Moores, 1970;Schweickert and Bogen, 1983;Schweickert et al, 1984), margin-parallel transpression (Saleeby, 1982), shallow subduction (Shervais et al, 2004), intra-arc spreading (Dilek et al, 1991) or a combination of these processes. Two contractional events are recognized, but deformation probably was semicontinuous in the Sierran Foothills (Sharp, 1988).…”

Tectonic underplating of trench sediments beneath magmatic arcs: the central California example

“…The Coast Range Ophiolite (Shervais et al 2004) crops out as dismembered structural klippe in the Santa Lucia and San Rafael mountains, and west of the town of Santa Maria. The Coast Range Ophiolite comprises the bulk of a nappe sitting structurally above the Franciscan units in the Nacimiento block.…”

First 40Ar-39Ar Ages from Low-T Mafic Blueschist Blocks in a Franciscan Mélange near San Simeon: Implications for Initiation of Subduction

“…Dated material abbreviations are as follows: A, actinolite; B, biotite; F, feldspar; G, glaucophane; Gr, garnet; H, hornblende; L, lawsonite; M, white mica; Pl, plagioclase; R, whole rock; S, sphene. Map is after Jennings (1966), Cloos (1986), Shervais et al (2004), Ernst et al (2009), Wakabayashi andDumitru (2007), and Jacobson et al (2011). The following sources are referenced (see inset labeled "Sources"): Lee et al (1964), Suppe (1969), Firsov and Dobretsov (1970), Coleman and Lanphere (1971), Suppe and Armstrong (1972), Lanphere et al (1978), Mattinson andEcheverria (1980), McDowell et al (1984), Mattinson (1986), Ross and Sharp (1988), Grove and Bebout (1995), Catlos and Sorensen (2003), Grove et al (2003Grove et al ( , 2008, Anczkiewicz et al (2004), Wakabayashi and Dumitru (2007), and Mulcahy et al (2009). geochronological results for mafic blueschists from the Nacimiento Block (Ernst 1980;Page 1981), located to the west of the San Andreas Fault ( fig.…”