Coast Range Ophiolite

Coast Range Ophiolite
Name	Coast Range Ophiolite
Type	Ophiolite complex
Age	Mesozoic (Jurassic–Cretaceous)
Primary lithology	Peridotite, gabbro, basalt, chert
Other lithology	Sheared mélange, pelagic shale
Named for	California Coast Ranges
Region	California, Oregon
Country	United States

Coast Range Ophiolite is a Mesozoic ophiolitic assemblage exposed in the California and Oregon Coast Ranges that provides a classic onshore record of oceanic lithosphere and suprasubduction processes. It has been studied across multiple scales by investigators from institutions and agencies including United States Geological Survey, Stanford University, University of California, Berkeley, University of California, Santa Cruz, and California Geological Survey. The complex is integral to regional frameworks involving the Franciscan Complex, Great Valley Sequence, Sierra Nevada, Gorda Plate, and paleotectonic reconstructions linking the Farallon Plate and North America.

Geologic Setting and Extent

The ophiolite crops out discontinuously from coastal Mendocino County, California through San Francisco Bay Area exposures to southern San Luis Obispo County, California and fragments in Oregon Coast Range, interfingered with terranes like the Franciscan Complex and the Great Valley Sequence. Regional mapping by the United States Geological Survey and state surveys situates it within Mesozoic convergent margin assemblages related to the subduction of the Farallon Plate beneath the North American Plate and interactions with the Insular Superterrane and Klamath Mountains. Tectonostratigraphic correlations link ophiolitic slices to oceanic domains addressed in syntheses by researchers at California Institute of Technology, Massachusetts Institute of Technology, Geological Society of America, and American Geophysical Union.

Lithology and Stratigraphy

Stratigraphic sections document a classic ophiolite sequence: tectonized mantle peridotite, layered and isotropic gabbro, sheeted dike complexes, pillowed basalt flows, and overlying pelagic chert, radiolarian-rich shale, and limestone correlated with Jurassic–Cretaceous biozones studied by paleontologists at Smithsonian Institution and chronostratigraphers at USGS Pacific Region. Interbeds of mélange, olistostromes, and tectonic breccia associate with structural contacts mapped by teams from University of California, Davis and California State University, Long Beach. Exhumed mantle peridotite, including harzburgite and lherzolite, has been described in fieldwork led by faculty from University of Oregon and Oregon State University.

Petrology and Geochemistry

Petrographic and geochemical analyses reveal depleted to moderately enriched mantle signatures in peridotites, with high- and low-Ti gabbros and MORB- to arc-like basalts characterized by trace-element ratios interpreted by geochemists from University of California, Santa Barbara, University of Washington, University of British Columbia, and Scripps Institution of Oceanography. Isotopic studies using Sr–Nd–Pb–Hf systems by laboratories at Lamont–Doherty Earth Observatory, WHOI, and Caltech constrain mantle source heterogeneity and possible slab-derived fluid inputs, invoking models tested against datasets from International Ocean Discovery Program cores and analogue ophiolites such as the Troodos Ophiolite and Semail Ophiolite. Geochemical fingerprints have been published in journals of the Geological Society of America, Nature Geoscience, and Earth and Planetary Science Letters.

Age and Tectonic Evolution

Radiometric ages (U–Pb zircon, Ar–Ar on plagioclase and amphibole, and radiolarian biostratigraphy) place ophiolite formation broadly in the Late Jurassic to Early Cretaceous, constrained by work at University of California, Los Angeles and chronologists at USGS Menlo Park. Tectonic models invoke initial creation at a spreading center or back-arc basin followed by supra-subduction modification during obduction or accretion to the continental margin during episodes related to the migration of the Farallon Plate and interactions with the Kula Plate. Regional tectonic synthesis by scholars affiliated with Stanford University, Caltech, and the Geological Society of America integrates paleomagnetic results and structural mapping with plate reconstructions from groups at NOAA and NASA.

Paleogeography and Formation Models

Competing models propose formation in a mid-ocean ridge, back-arc basin, or forearc setting; proponents cite parallels with the Alpujarride complex and the Peninsular Ranges Batholith arc–continent interactions investigated by researchers at University of Arizona and University of New Mexico. Paleogeographic reconstructions developed with contributions from British Geological Survey, Geological Survey of Canada, and international collaborators use stratigraphic ties to the Great Valley Sequence and faunal correlations with Mediterranean and Pacific basins to infer the ophiolite’s original location relative to the Farallon Plate spreading ridges and island-arc systems cataloged by the International Union of Geological Sciences.

Economic Resources and Mineralization

The ophiolitic sequence hosts ultramafic-associated chromite, podiform chromitite, and localized sulfide mineralization, with nickel, chromium, cobalt, and platinum-group element potential evaluated by mineralogists at USGS Mineral Resources Program and state geological surveys. Serpentinized peridotite exposures have been assessed for asbestos, talc, and construction aggregate by regulatory bodies including the California Department of Toxic Substances Control and California Department of Conservation. Groundwater influences and lateritic alteration have been studied for environmental implications by teams at University of California, Irvine and California State University, Chico.

History of Investigation and Nomenclature

Early descriptions arose from 19th-century geological surveys by figures associated with California Geological Survey and publications by geologists working with the United States Geological Survey; later 20th-century syntheses and refinements were produced by investigators at Stanford University, UC Berkeley, and USGS Menlo Park. Seminal monographs and field guides have been issued under auspices of the Geological Society of America, American Association of Petroleum Geologists, and state geological societies, with ongoing research supported by grants from National Science Foundation and collaborations with international programs such as IODP. Contemporary nomenclature reflects integration of field mapping, petrochronology, and tectonostratigraphic frameworks developed by leaders in ophiolite research associated with California Academy of Sciences and university departments across North America.

Category:Ophiolites Category:Geology of California Category:Geology of Oregon