The Coast Range Ophiolite is an

ophiolite An ophiolite is a section of Earth's oceanic crust and the underlying upper mantle that has been uplifted and exposed above sea level and often emplaced onto continental crustal rocks. The Greek word ὄφις, ''ophis'' (''snake'') is found ...

of Middle to Late

Jurassic The Jurassic ( ) is a geologic period and stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately Mya. The Jurassic constitutes the middle period of ...

age located in the

California Coast Ranges The Coast Ranges of California span from Del Norte or Humboldt County, California, south to Santa Barbara County. The other three coastal California mountain ranges are the Transverse Ranges, Peninsular Ranges and the Klamath Mountains. P ...

. They form the basement of the extreme western margin of central and northern California. Exposures straddle the coast from

Santa Barbara County Santa Barbara County, California, officially the County of Santa Barbara, is located in Southern California. As of the 2020 census, the population was 448,229. The county seat is Santa Barbara, and the largest city is Santa Maria. Santa Barba ...

up to San Francisco. The formation then trends inland up to the southern end of the

Klamath Mountains The Klamath Mountains are a rugged and lightly populated mountain range in northwestern California and southwestern Oregon in the western United States. As a mountain system within both the greater Pacific Coast Ranges and the California Coast ...

.Bailey, et al. It is arguably the most extensive ophiolite terrane in the United States, and is one of the most studied ophiolites in the North America.

Description

As indicated by the name, the ophiolite is defined by the presence of

ultramafic Ultramafic rocks (also referred to as ultrabasic rocks, although the terms are not wholly equivalent) are igneous and meta-igneous rocks with a very low silica content (less than 45%), generally >18% MgO, high FeO, low potassium, and are composed ...

rocks in the California Coast Ranges.

Pillow lava Pillow lavas are lavas that contain characteristic pillow-shaped structures that are attributed to the extrusion of the lava underwater, or ''subaqueous extrusion''. Pillow lavas in volcanic rock are characterized by thick sequences of discont ...

s and oceanic

basalt Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90 ...

s are among the most common rocks found within the formation. There are a number of exposures that straddle the ancient Mohorovičić discontinuity. Metamorphosed peridotite, in the form of a serpentine rock, is an indicator of the formation.

Distribution

In general, the ophiolite is exposed near the boundary between the sequences of rocks associated with the Coast Ranges, and rocks associated with the

Great Valley Sequence The Great Valley Sequence of California is a -thick group of related geologic formations that are Late Jurassic through Cretaceous in age (150–65 Ma) on the geologic time scale. These sedimentary rocks were deposited during the late Mesozoic ...

. Where it is exposed, it generally underlies the various sedimentary rocks of the Great Valley Sequence, and may be coextensive with the contemporaneous (but slightly younger on average)

Franciscan Assemblage The Franciscan Complex or Franciscan Assemblage is a geologic term for a late Mesozoic terrane of heterogeneous rocks found throughout the California Coast Ranges, and particularly on the San Francisco Peninsula. It was named by geologist Andr ...

, as would be expected by an observational application of the

law of superposition The law of superposition is an axiom that forms one of the bases of the sciences of geology, archaeology, and other fields pertaining to geological stratigraphy. In its plainest form, it states that in undeformed stratigraphic sequences, the ...

. However, in many localities, the ultramafic rocks of the ophiolite can be found intruding or on top of the local country rock.

Origin and Tectonics

The Coast Range Ophiolite is characterized by

obduction Obduction is a geological process whereby denser oceanic crust (and even upper mantle) is scraped off a descending ocean plate at a convergent plate boundary and thrust on top of an adjacent plate. When oceanic and continental plates converge ...

of oceanic crust onto land. Most of the rocks in the ophiolite were part of an accretionary wedge on the continental margin of Laurasia that was thrust onto land during the Jurassic through the influence of subduction. The exact origins of the rocks found in the ophiolite are a matter of debate; some geologists hypothesize that the rock primarily consists of mid-ocean lithosphere, while others assert different ideas related to

island arc Island arcs are long chains of active volcanoes with intense seismic activity found along convergent tectonic plate boundaries. Most island arcs originate on oceanic crust and have resulted from the descent of the lithosphere into the mantle alon ...

terranes associated with the

Nevadan orogeny The Nevadan orogeny occurred along the western margin of North America during the Middle Jurassic to Early Cretaceous time which is approximately from 155 Ma to 145 Ma. Throughout the duration of this orogeny there were at least two different kin ...

.Dickinson et al.

References

Sources

* *{{cite Q, Q97990697, author-mask1=Dickinson, W. R., , author-mask2=Hopson, C. A., , author-mask3=Saleeby, J. B., , author-mask4=Schweickert, R. A., , author-mask5=Ingersoll, R. V., , author-mask6=Pessagno Jr, E. A., , author-mask7=Mattinson, J. M., , author-mask8=Luyendyk, B. P., , author-mask9=Beebe, W., , author-mask10=Hull, D. M., , author-mask11=Muñoz, I. M. & , author-mask12=Blome, C. D. *Shervais, J. (n.d.). John W. Shervais: Research Projects - Ophiolites and Oceanic Crust. Retrieved September 6, 2014. Ophiolites Natural history of the California Coast Ranges Ophiolite, Coast Range Jurassic California Middle Jurassic North America Late Jurassic North America Geologic formations of California Geology of Monterey County, California Geology of San Francisco Geology of San Luis Obispo County, California Geology of Santa Barbara County, California Geology of California

Description

Distribution

Origin and Tectonics

See also

References

Sources