# Farallon Trench

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Tectonic formation of the Cenozoic era

The **[Farallon](/source/Farallon_plate) Trench** was a subduction related tectonic formation located off the coast of the western [California](/source/California) continental margin during the late to mid [Cenozoic](/source/Cenozoic) era, around 50 miles southeast of modern-day [Monterey Bay](/source/Monterey_Bay). The time duration of subduction began from around 165 Ma when the Farallon Plate replaced the Mezcalera promontory, until the [San Andreas Fault](/source/San_Andreas_Fault) straightening around 35 Ma.[1][2][3] As data accumulated over time, a common view developed that one large oceanic plate, the Farallon Plate, acted as a conveyor belt, conveying [accreted](/source/Accretion_(geology)) [terranes](/source/Terrane) onto the North American west coast. As the continent overran the subducting Farallon Plate, the denser plate became subducted into the mantle below the continent. When the plates converged, the dense oceanic plate sank into the mantle to form a [slab](/source/Slab_(geology)) below the lighter continent. Rapid [subduction](/source/Subduction) under the southwestern North America continent began 40 to 60 million years ago (Ma),[4] during the mid [Paleocene](/source/Paleocene) to mid [Eocene](/source/Eocene) epochs. This convergent subduction margin created a distinctive geomorphologic feature called an [oceanic trench](/source/Oceanic_trench), which occurs at a [convergent plate boundaries](/source/Convergent_boundary) as a heavy metal rich, [lithospheric plate](/source/Plate_tectonics) moves below a light silica rich [continental plate](/source/Plate_tectonics). The trench marks the position at which the flexed subducting slab begins to descend beneath and deform the continental plate margin. By 43 Ma, during the Eocene, worldwide plate motions changed and the Pacific Plate began to move away from North America and subduction of the Farallon Plate slowed dramatically.[4] By around 36 Ma, the easternmost part of the East Pacific Rise, located between the Pioneer and Murray fracture zones at that time, approached the trench and the young, hot, buoyant [lithosphere](/source/Lithosphere) appears to have clogged part of the subduction zone, resulting in widespread dramatic uplift on land.[4] The eventual complete subduction of this plate, consequential contact of the [Pacific Plate](/source/Pacific_Plate) with the California continental margin, and creation of the [Mendocino triple junction](/source/Mendocino_triple_junction) (MTJ), took place around 30 to 20 Ma.[5] The partial complete subduction and division of the Farallon Plate by the Pacific Plate, created the [Juan de Fuca Plate](/source/Juan_de_Fuca_Plate) to the north and the [Cocos Plate](/source/Cocos_Plate) to the south. The final stages of the evolution of California's continental margin was the growth of the [San Andreas transform fault](/source/San_Andreas_Fault) system, which formed as the Pacific Plate came into contact with the continental margin and the MTJ was formed.[5] As subduction of the Pacific Plate continued along this margin, and the contact zone grew, the San Andreas proportionally grew as well.

## Geologic evidence

Evidence of the existence of the Farallon Trench and past subduction of the Farallon Plate is evident in specific geologic units observed along paleo-coastlines of the west coast of the United States and California continental region. Late Cretaceous–Paleogene magma can be seen overlying subhorizontally subducted sediments from the Farallon Plate[6] as far inland as Utah and Arizona. The earliest record of subhorizontal subduction of the Farallon slab is the extinguishing of magmatism in the Sierra Nevada batholith of California roughly 85 Ma.[7] As the Farallon Plate subducted below the California continental margin an accretionary wedge was formed in the trench, which yielded unique rock types as a result of [regional metamorphism](/source/Metamorphism). The formation of [Franciscan Melange](/source/Franciscan_Assemblage) and [blueschist](/source/Blueschist) units along paleo-coastlines resulted from this subduction and are direct evidence of the Farallon Plate's past existence. Other forms of evidence include the Farallon Islands, Catalina Islands, and uplift of the Diablo Mountain Range as a result of the clogged subduction zone mentioned above. These observations can be explained by a model for the weakening and ultimate falling apart of the uppermost part of the subducted oceanic plate in the 20–30 m.y. after the end of rapid subduction.[4] As the plate falls apart, not only is compressional stress relieved, but significant back-slip along the old subduction zone is also possible, perhaps bringing blueschist rapidly upward from 20- to 30-km depths,[4] where it can be observed along the California coast to this day.

## Recent research

To understand the subduction of the Farallon Plate, the creation of the Farallon Trench, and the present location of the subducted plate, detailed [seismic tomography](/source/Seismic_tomography) was used to render images of the existing submerged remnants.[8] The plate can now be seen at depths of around 200 km below the central continental United States. Since the North American coast shows an extremely complicated geologic structure, intensive work has been required to understand the complexity of this system. In 2013 a new explanation emerged from recent research, proposing two additional now fully subducted plates, accounting for some of the complexity of this coast line. As of 2013, it is generally accepted that the western quarter of North America consists of accreted [terrane](/source/Terrane) accumulated over roughly the past 200 m.y as the remnant Farallon Plate (the Juan De Fuca and Cocos plates) continues to convey oceanic terrane onto the [continental margin](/source/Continental_margin). This model, however, was unable to explain many terrane complexities, and is inconsistent with [seismic tomographic images](/source/Seismic_tomography) of subducting slabs which penetrate the [lower-mantle](/source/Mantle_(geology)). Further study will be needed to understand this inconsistency in data and will, with all luck, provide a solid and concrete understanding of the western continental margin of North America and its complexities upon completion.[8]

## See also

- [Farallon Plate](/source/Farallon_Plate)

- [Juan de Fuca Plate](/source/Juan_de_Fuca_Plate)

- [List of tectonic plates](/source/List_of_tectonic_plates)

- [North American Plate](/source/North_American_Plate)

## References

1. **[^](#cite_ref-1)** Michaud, F. (2006). Oceanic-ridge subduction vs. slab break off: Plate tectonic evolution along the Baja California Sur continental margin since 15 Ma. *Geology.,* *34*(1), 13.

1. **[^](#cite_ref-2)** Schellart, W. P.; Stegman, D. R.; Farrington, R. J.; Freeman, J.; Moresi, L. (2010-07-16). "Cenozoic Tectonics of Western North America Controlled by Evolving Width of Farallon Slab". *Science*. **329** (5989): 316–319. [Bibcode](/source/Bibcode_(identifier)):[2010Sci...329..316S](https://ui.adsabs.harvard.edu/abs/2010Sci...329..316S). [doi](/source/Doi_(identifier)):[10.1126/science.1190366](https://doi.org/10.1126%2Fscience.1190366). [ISSN](/source/ISSN_(identifier)) [0036-8075](https://search.worldcat.org/issn/0036-8075). [PMID](/source/PMID_(identifier)) [20647465](https://pubmed.ncbi.nlm.nih.gov/20647465). [S2CID](/source/S2CID_(identifier)) [12044269](https://api.semanticscholar.org/CorpusID:12044269).

1. **[^](#cite_ref-3)** Lonsdale, Peter (2005-08-01). "Creation of the Cocos and Nazca plates by fission of the Farallon plate". *Tectonophysics*. **404** (3–4): 237–264. [Bibcode](/source/Bibcode_(identifier)):[2005Tectp.404..237L](https://ui.adsabs.harvard.edu/abs/2005Tectp.404..237L). [doi](/source/Doi_(identifier)):[10.1016/j.tecto.2005.05.011](https://doi.org/10.1016%2Fj.tecto.2005.05.011).

1. ^ [***a***](#cite_ref-Ward_4-0) [***b***](#cite_ref-Ward_4-1) [***c***](#cite_ref-Ward_4-2) [***d***](#cite_ref-Ward_4-3) [***e***](#cite_ref-Ward_4-4) Ward, Peter L. (1991). "On plate tectonics and the geologic evolution of southwestern North America". *Journal of Geophysical Research*. **96** (B7): 12479. [Bibcode](/source/Bibcode_(identifier)):[1991JGR....9612479W](https://ui.adsabs.harvard.edu/abs/1991JGR....9612479W). [CiteSeerX](/source/CiteSeerX_(identifier)) [10.1.1.522.2461](https://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.522.2461). [doi](/source/Doi_(identifier)):[10.1029/91jb00606](https://doi.org/10.1029%2F91jb00606). [ISSN](/source/ISSN_(identifier)) [0148-0227](https://search.worldcat.org/issn/0148-0227).

1. ^ [***a***](#cite_ref-Ernst_5-0) [***b***](#cite_ref-Ernst_5-1) *The Geotectonic development of California*. Ernst, W. G. (Wallace Gary), 1931–, Rubey, William Walden, 1898–1974. Englewood Cliffs, N.J.: Prentice-Hall. 1981. [ISBN](/source/ISBN_(identifier)) [978-0133539387](https://en.wikipedia.org/wiki/Special:BookSources/978-0133539387). [OCLC](/source/OCLC_(identifier)) [6627256](https://search.worldcat.org/oclc/6627256).{{[cite book](https://en.wikipedia.org/wiki/Template:Cite_book)}}: CS1 maint: others ([link](https://en.wikipedia.org/wiki/Category:CS1_maint:_others))

1. **[^](#cite_ref-6)** Chapin, C. E. (2012-01-12). ["Origin of the Colorado Mineral Belt"](https://doi.org/10.1130%2Fges00694.1). *Geosphere*. **8** (1): 28–43. [Bibcode](/source/Bibcode_(identifier)):[2012Geosp...8...28C](https://ui.adsabs.harvard.edu/abs/2012Geosp...8...28C). [doi](/source/Doi_(identifier)):[10.1130/ges00694.1](https://doi.org/10.1130%2Fges00694.1). [ISSN](/source/ISSN_(identifier)) [1553-040X](https://search.worldcat.org/issn/1553-040X).

1. **[^](#cite_ref-7)** Saleeby, Jason (2003-06-01). ["Segmentation of the Laramide Slab—evidence from the southern Sierra Nevada region"](https://web.archive.org/web/20230602183605/https://authors.library.caltech.edu/38297/). *GSA Bulletin*. **115** (6): 655–668. [Bibcode](/source/Bibcode_(identifier)):[2003GSAB..115..655S](https://ui.adsabs.harvard.edu/abs/2003GSAB..115..655S). [doi](/source/Doi_(identifier)):[10.1130/0016-7606(2003)115<0655:sotlsf>2.0.co;2](https://doi.org/10.1130%2F0016-7606%282003%29115%3C0655%3Asotlsf%3E2.0.co%3B2). [ISSN](/source/ISSN_(identifier)) [0016-7606](https://search.worldcat.org/issn/0016-7606). Archived from [the original](https://authors.library.caltech.edu/38297/) on 2023-06-02. Retrieved 2023-01-21.

1. ^ [***a***](#cite_ref-Sigloch_50–56_8-0) [***b***](#cite_ref-Sigloch_50–56_8-1) Sigloch, Karin; Mihalynuk, Mitchell G. (April 2013). ["Intra-oceanic subduction shaped the assembly of Cordilleran North America"](https://hal.archives-ouvertes.fr/hal-03425593/file/SiglochMihalynuk2013_IntraOceanicSubduction_2013-02-28.pdf) (PDF). *Nature*. **496** (7443): 50–56. [Bibcode](/source/Bibcode_(identifier)):[2013Natur.496...50S](https://ui.adsabs.harvard.edu/abs/2013Natur.496...50S). [doi](/source/Doi_(identifier)):[10.1038/nature12019](https://doi.org/10.1038%2Fnature12019). [ISSN](/source/ISSN_(identifier)) [0028-0836](https://search.worldcat.org/issn/0028-0836). [PMID](/source/PMID_(identifier)) [23552944](https://pubmed.ncbi.nlm.nih.gov/23552944). [S2CID](/source/S2CID_(identifier)) [205233259](https://api.semanticscholar.org/CorpusID:205233259).

v t e Major seismically active faults of North America North America (crosses national borders) Aleutian Trench (US–RU) Basin and Range Province (US–MX) Canadian Arctic Rift System (CA–GL) Cascadia subduction zone (US–CA) Clarendon-Linden fault system (US–CA) Denali Fault (US–CA) Northern Cordilleran Volcanic Province (CA–US) Puget Sound faults (US–CA) Rio Grande rift (US–MX) San Andreas Fault (US–MX) Southern Great Lakes seismic zone (US–CA) Tintina Fault (US–CA) Canada Charlevoix seismic zone Laurentian Slope seismic zone Leech River Fault Queen Charlotte Fault Saguenay Graben Saint Lawrence rift system Timiskaming Graben Western Quebec seismic zone United States Washington and Oregon Brothers Fault Zone Olympic–Wallowa lineament Puget Sound (Seattle Fault, Tacoma Fault) Walker Lane California Calaveras Fault Garlock Fault Hayward Fault Zone Mendocino fracture zone Newport–Inglewood Fault Puente Hills Fault Rose Canyon Fault San Jacinto Fault Zone (Elsinore Fault Zone) Great Basin Independence Valley fault system Moab Fault Teton Fault Wasatch Fault Great Plains Humboldt Fault Long Point–Eureka Heights fault system Meers Fault Nemaha Fault Zone Midwest Marianna Fault New Madrid seismic zone Sandwich Fault Zone Wabash Valley seismic zone Appalachian Mountains and Atlantic Coast Charleston seismic zone Eastern Tennessee seismic zone Northern Appalachians seismic zone Ramapo Fault Virginia seismic zones Alaska Salcha seismic zone Caribbean and Mexico Cayman Trough Chixoy-Polochic Fault Enriquillo–Plantain Garden fault zone (Cuba) Gulf of California Rift Zone Lesser Antilles subduction zone Middle America Trench (Central America) Motagua Fault (Central America) Pedro Miguel Fault Puerto Rico Trench

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Adapted from the Wikipedia article [Farallon Trench](https://en.wikipedia.org/wiki/Farallon_Trench) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Farallon_Trench?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.