Models for origin and emplacement of Jurassic ophiolites of Northern California

Abstract

Four Jurassic ophiolite complexes in northern California have crystallization ages of 170-160 Ma; the Coast Range (including Great Valley) ophiolite is oldest (170-165 Ma), the Smartville ophiolite, intermediate (164-160 Ma), and the Josephine ophiolite, youngest (162 Ma). The Smartville ophiolite was obducted during the Sierran phase of the Nevadan orogeny (162-155 Ma), and the Josephine ophiolite was obducted during the Klamath phase of the Nevadan orogeny (153-150 Ma). The Great Valley ophiolite was not highly deformed during the Nevadan orogeny and became oceanic basement for the post-Nevadan forearc basin. Three conflicting models for origin of the Coast Range (including Great Valley) ophiolite have been proposed: (1) Formation by intra-arc and backarc spreading related to an east-facing intraoceanic arc, which collided with a west-facing continental-margin arc during the Nevadan orogeny (Sierran phase). (2) Formation by openocean seafloor spreading and incorporation into the continental margin during trench initiation outboard of an existing continental-margin trench. (3) Formation by forearc oblique rifting along the continental margin, followed by partial closure.Lithospheric rifting is favored where there is either thick continental crust or thin mantle lithosphere because continental crust is weaker than oceanic crust and both are weaker than mantle lithosphere. Favorable sites for rifting are extant rifts, intra-arc settings, suture belts, and areas where mantle lithosphere has been delaminated from overlying crust. Upon cessation of magmatism related to rifting or subduction, mantle lithosphere rapidly strengthens, so that by 20 m.y., lithosphere strongly resists extensional stresses. Forearcs cool and strengthen even faster because of refrigeration by subducted slabs. Mature forearcs are strong and unlikely to yield to stresses, especially when weaker intra-arc and backarc settings are nearby. Trench initiation is favored by (1) the presence of a preexisting lithospheric boundary, (2) the presence of thin oceanic lithosphere, (3) attempted subduction of buoyant crust, and (4) plate reorganization. Trench initiation in intact oceaniclithosphere older than 20 m.y. is implausible. The following facts render models for Jurassic trench initiation (model 2) or rifting (model 3) in forearc settings implausible: (1) Subduction began along the California margin in the Triassic, so that forearc lithosphere was considerably older than 20 m.y. by the Late Jurassic. (2) The Triassic-Jurassic magmatic arc related to this earlier subduction zone died in eastern California at the same time that the Jurassic-Cretaceous magmatic arc was initiated in the westernmost Sierra Nevada. (3) The colliding buoyant crust that forced this plate reorganization is found in the Sierran foothills. The most plausible model for formation of the Great Valley, Smartville, and Josephine ophiolites is by lithospheric rifting within and/or behind magmatic arcs. The Great Valley and Smartville ophiolites formed behind an east-facing intraoc...

Accretionary Mesozoic–Cenozoic expansion of the Cordilleran continental margin in California and adjacent Oregon

“…On the other hand, if the older of the more western plutons are viewed as the record of igneous activity within an offshore intraoceanic arc complex, then accretion of the exotic arc terranes was delayed until Late Jurassic time. In either case, however, arc crust of intraoceanic origin was incorporated into the Sierra Nevada foothills by arc-continent collision (Moores and Day, 1984;Godfrey and Dilek, 2000;Ingersoll, 2000).…”

“…An inferred extension of the ophiolite into the subsurface of the forearc basin to the east has been thought to form obducted basement beneath the Great Valley. The ophiolitic assemblage as a whole has long been regarded as a dismembered slab of oceanic crust and lithosphere overlain depositionally by basal horizons of the Great Valley succession (Bailey et al, 1970;Dickinson and Seely, 1979;Ingersoll, 1982Ingersoll, , 2000. Recent evaluation of the ultramafi c components of the ophiolitic assemblage has shown, however, that the bulk of the ultramafi c belt is serpentinite mélange (Tehama-Colusa serpentinite mélange of Hopson and Pessagno, 2004), viewed here as an eastern component of the Franciscan subduction complex analogous to the hydrated mantle described from modern arctrench systems as forearc serpentinite (Bostock et al, 2002;Blakely et al, 2005).…”

Subduction initiation along transform faults: The proto-Franciscan subduction zone

“…It is one of the most extensive ophiolite terranes in North America and has long been central to our understanding of Cordilleran tectonics. Nonetheless, its origin is controversial and three primary hypotheses have been advanced: (1) formation at a mid-ocean-ridge spreading center at low paleolatitudes, and subsequent rapid drift northward to collide with North America (Hopson et al, 1981Pessagno et al, 2000); (2) formation as a backarc basin behind an east-facing volcanic arc that collided with North America during the Late Jurassic Nevadan orogeny (Godfrey and Klemperer, 1998;Ingersoll, 2000); and (3) formation by forearc or intra-arc rifting along the western margin of North America, in response to nascent or renewed subduction of oceanic plates beneath North America (Shervais and Kimbrough, 1985;Shervais, 1990Shervais, , 2001Stern and Bloomer, 1992). The consensus of most recent studies support the suprasubduction-zone model (Stern and Bloomer, 1992;Shervais et al, 2004Shervais et al, , 2005aShervais, 2008 Subduction initiation along transform faults: The proto-Franciscan subduction zone | RESEARCH Choi et al, 2008a;Jean et al, 2010), but models based on backarc basins and oceanic spreading centers still persist (e.g., Hopson et al, 2008).…”

Facies architecture and provenance of a boulder-conglomerate submarine channel system, Panoche Formation, Great Valley Group: A forearc basin response to middle Cretaceous tectonism in the California convergent margin

“…2; Ernst, 1970;Dickinson, 1971;Hopson et al, 1981Hopson et al, , 2008Saleeby et al, 1982;Saleeby, 1983;Ingersoll et al, 1999;Shervais et al, 2004Shervais et al, , 2005. CRO has island-arc geochemistry (Shervais and Kimbrough, 1985;Shervais, 1990;Giaramita et al, 1998;Evarts et al, 1999), but tectonic origin interpretations vary (see Dickinson et al [1996] for a summary) from supra-subduction zone (Shervais and Kimbrough, 1985;Robertson, 1989;Shervais, 1990;Stern and Bloomer, 1992;Shervais et al, 2004Shervais et al, , 2005, mid-ocean ridge (Hopson et al, , 2008, and backarc oceanic crust ( Schweickert et al, 1984;Schweickert, 1997;Godfrey and Klemperer, 1998;Ingersoll, 2000). The CRO most likely served as basement for the western forearc basin and was periodically affected by accretionary wedge tectonism (Shervais et al, 2004;Mitchell et al, 2010;Wakabayashi, 2015).…”

“…Limited available outcrops, borehole penetrations, and geophysical data suggest the Late Jurassic-Early Cretaceous phase of arc magmatism was prevalent in the western Sierra near the SLR area ( Fig. 15A; May and Hewitt, 1948;California Division of Oil and Gas, 1964;Bateman, 1983;Harwood and Helley, 1987;Saleeby et al, 1989a;Ingersoll, 2000;Cecil et al, 2012). According to Cecil et al (2012), the western boundary of the Late Jurassic-Early Cretaceous arc lies under the San Joaquin subbasin as close as 35 km to the SLR deposits (Figs.…”