Farallon Trench

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The Farallon Trench was a subduction related tectonic formation located off the coast of the western California continental margin during the late to mid Cenozoic era, around 50 miles southeast of modern-day 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 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 terranes 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 below the lighter continent. Rapid subduction under the southwestern North America continent began 40 to 60 million years ago (Ma), [4] during the mid Paleocene to mid Eocene epochs. This convergent subduction margin created a distinctive geomorphologic feature called an oceanic trench, which occurs at a convergent plate boundaries as a heavy metal rich, lithospheric plate moves below a light silica rich continental plate. 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 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 with the California continental margin, and creation of the 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 to the north and the 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 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.

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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. The formation of Franciscan Melange and 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 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 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. This model, however, was unable to explain many terrane complexities, and is inconsistent with seismic tomographic images of subducting slabs which penetrate the lower-mantle. 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

Related Research Articles

<span class="mw-page-title-main">Subduction</span> A geological process at convergent tectonic plate boundaries where one plate moves under the other

Subduction is a geological process in which the oceanic lithosphere and some continental lithosphere is recycled into the Earth's mantle at convergent boundaries. Where the oceanic lithosphere of a tectonic plate converges with the less dense lithosphere of a second plate, the heavier plate dives beneath the second plate and sinks into the mantle. A region where this process occurs is known as a subduction zone, and its surface expression is known as an arc-trench complex. The process of subduction has created most of the Earth's continental crust. Rates of subduction are typically measured in centimeters per year, with rates of convergence as high as 11 cm/year.

Obduction is a geological process whereby denser oceanic crust 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, normally the denser oceanic crust sinks under the continental crust in the process of subduction. Obduction, which is less common, normally occurs in plate collisions at orogenic belts or back-arc basins.

<span class="mw-page-title-main">North American Plate</span> Large tectonic plate including most of North America, Greenland and part of Siberia

The North American Plate is a tectonic plate covering most of North America, Cuba, the Bahamas, extreme northeastern Asia, and parts of Iceland and the Azores. With an area of 76 million km2 (29 million sq mi), it is the Earth's second largest tectonic plate, behind the Pacific Plate.

<span class="mw-page-title-main">Juan de Fuca Plate</span> Tectonic plate in the eastern North Pacific

The Juan de Fuca Plate is a small tectonic plate (microplate) generated from the Juan de Fuca Ridge that is subducting beneath the northerly portion of the western side of the North American Plate at the Cascadia subduction zone. It is named after the explorer of the same name. One of the smallest of Earth's tectonic plates, the Juan de Fuca Plate is a remnant part of the once-vast Farallon Plate, which is now largely subducted underneath the North American Plate.

<span class="mw-page-title-main">Farallon Plate</span> Ancient oceanic plate that has mostly subducted under the North American Plate

The Farallon Plate was an ancient oceanic plate. It formed one of the three main plates of Panthalassa, alongside the Phoenix Plate and Izanagi Plate, which were connected by a triple junction. The Farallon Plate began subducting under the west coast of the North American Plate—then located in modern Utah—as Pangaea broke apart and after the formation of the Pacific Plate at the centre of the triple junction during the Early Jurassic. It is named for the Farallon Islands, which are located just west of San Francisco, California.

<span class="mw-page-title-main">Nazca Plate</span> Oceanic tectonic plate in the eastern Pacific Ocean basin

The Nazca Plate or Nasca Plate, named after the Nazca region of southern Peru, is an oceanic tectonic plate in the eastern Pacific Ocean basin off the west coast of South America. The ongoing subduction, along the Peru–Chile Trench, of the Nazca Plate under the South American Plate is largely responsible for the Andean orogeny. The Nazca Plate is bounded on the west by the Pacific Plate and to the south by the Antarctic Plate through the East Pacific Rise and the Chile Rise respectively. The movement of the Nazca Plate over several hotspots has created some volcanic islands as well as east–west running seamount chains that subduct under South America. Nazca is a relatively young plate both in terms of the age of its rocks and its existence as an independent plate having been formed from the break-up of the Farallon Plate about 23 million years ago. The oldest rocks of the plate are about 50 million years old.

<span class="mw-page-title-main">Pacific Plate</span> Oceanic tectonic plate under the Pacific Ocean

The Pacific Plate is an oceanic tectonic plate that lies beneath the Pacific Ocean. At 103 million km2 (40 million sq mi), it is the largest tectonic plate.

<span class="mw-page-title-main">Forearc</span> The region between an oceanic trench and the associated volcanic arc

Forearc is a plate tectonic term referring to a region in a subduction zone between an oceanic trench and the associated volcanic arc. Forearc regions are present along convergent margins and eponymously form 'in front of' the volcanic arcs that are characteristic of convergent plate margins. A back-arc region is the companion region behind the volcanic arc.

<span class="mw-page-title-main">Trans-Mexican Volcanic Belt</span> Active volcanic belt that covers central-southern Mexico

The Trans-Mexican Volcanic Belt, also known as the Transvolcanic Belt and locally as the Sierra Nevada, is an active volcanic belt that covers central-southern Mexico. Several of its highest peaks have snow all year long, and during clear weather, they are visible to a large percentage of those who live on the many high plateaus from which these volcanoes rise.

<span class="mw-page-title-main">Kula Plate</span> Former oceanic tectonic plate

The Kula Plate was an oceanic tectonic plate under the northern Pacific Ocean south of the Near Islands segment of the Aleutian Islands. It has been subducted under the North American Plate at the Aleutian Trench, being replaced by the Pacific Plate.

<span class="mw-page-title-main">Mendocino Triple Junction</span> Point where the Gorda plate, the North American plate, and the Pacific plate meet

The Mendocino Triple Junction (MTJ) is the point where the Gorda plate, the North American plate, and the Pacific plate meet, in the Pacific Ocean near Cape Mendocino in northern California. This triple junction is the location of a change in the broad plate motions which dominate the west coast of North America, linking convergence of the northern Cascadia subduction zone and translation of the southern San Andreas Fault system. This region is can be characterized by transform fault movement, the San Andreas also by transform strike slip movement, and the Cascadia subduction zone by a convergent plate boundary subduction movement. The Gorda plate is subducting, towards N50ºE, under the North American plate at 2.5 – 3 cm/yr, and is simultaneously converging obliquely against the Pacific plate at a rate of 5 cm/yr in the direction N115ºE. The accommodation of this plate configuration results in a transform boundary along the Mendocino Fracture Zone, and a divergent boundary at the Gorda Ridge. This area is tectonically active historically and today. The Cascadia subduction zone is known to be capable of producing megathrust earthquakes on the order of MW 9.0.

<span class="mw-page-title-main">Carnegie Ridge</span> Aseismic ridge on the Nazca Plate that is being subducted beneath the South American Plate

<span class="mw-page-title-main">Rivera Plate</span> Small tectonic plate off the west coast of Mexico

The Rivera Plate is a small tectonic plate located off the west coast of Mexico, just south of the Baja California Peninsula. It is bounded on the northwest by the East Pacific Rise, on the southwest by the Rivera Transform Fault, on the southeast by a deformation zone, and on the northeast by the Middle America Trench and another deformation zone.

<span class="mw-page-title-main">Pacific-Farallon Ridge</span> Spreading ridge during the Late Cretaceous

The Pacific-Farallon Ridge was a spreading ridge during the Late Cretaceous that extended 10,000 km in length and separated the Pacific Plate to the west and the Farallon Plate to the east. It ran south from the Pacific-Farallon-Kula triple junction at 51°N to the Pacific-Farallon-Antarctic triple junction at 43°S. As the Farallon Plate subducted obliquely under the North American Plate, the Pacific-Farallon Ridge approached and eventually made contact with the North American Plate about 30 million years ago. On average, this ridge had an equatorial spreading rate of 13.5 cm per year until its eventual collision with the North American Plate. In present day, the Pacific-Farallon Ridge no longer formally exists since the Farallon Plate has been broken up or subducted beneath the North American Plate, and the ridge has segmented, having been mostly subducted as well. The most notable remnant of the Pacific-Farallon Ridge is the 4000 km Pacific-Nazca segment of the East Pacific Rise.

This is a list of articles related to plate tectonics and tectonic plates.

<span class="mw-page-title-main">Slab window</span> Type of gap in a subducted oceanic plate

In geology, a slab window is a gap that forms in a subducted oceanic plate when a mid-ocean ridge meets with a subduction zone and plate divergence at the ridge and convergence at the subduction zone continue, causing the ridge to be subducted. Formation of a slab window produces an area where the crust of the over-riding plate is lacking a rigid lithospheric mantle component and thus is exposed to hot asthenospheric mantle. This produces anomalous thermal, chemical and physical effects in the mantle that can dramatically change the over-riding plate by interrupting the established tectonic and magmatic regimes. In general, the data used to identify possible slab windows comes from seismic tomography and heat flow studies.

Slab pull is a geophysical mechanism whereby the cooling and subsequent densifying of a subducting tectonic plate produces a downward force along the rest of the plate. In 1975 Forsyth and Uyeda used the inverse theory method to show that, of the many forces likely to be driving plate motion, slab pull was the strongest. Plate motion is partly driven by the weight of cold, dense plates sinking into the mantle at oceanic trenches. This force and slab suction account for almost all of the force driving plate tectonics. The ridge push at rifts contributes only 5 to 10%.

<span class="mw-page-title-main">Siletzia</span> Rock formation that forms the basement rock of the southern Pacific Northwest coast

Siletzia is a massive formation of early to middle Eocene epoch marine basalts and interbedded sediments in the forearc of the Cascadia subduction zone, on the west coast of North America. It forms the basement rock under western Oregon and Washington and the southern tip of Vancouver Island. It is now fragmented into the Siletz and Crescent terranes.

<span class="mw-page-title-main">Geology of the Pacific Ocean</span> Overview about the geology of the Pacific Ocean

The Pacific Ocean evolved in the Mesozoic from the Panthalassic Ocean, which had formed when Rodinia rifted apart around 750 Ma. The first ocean floor which is part of the current Pacific Plate began 160 Ma to the west of the central Pacific and subsequently developed into the largest oceanic plate on Earth.

References

  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.
  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:2010Sci...329..316S. doi:10.1126/science.1190366. ISSN   0036-8075. PMID   20647465. S2CID   12044269.
  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:2005Tectp.404..237L. doi:10.1016/j.tecto.2005.05.011.
  4. 1 2 3 4 5 Ward, Peter L. (1991). "On plate tectonics and the geologic evolution of southwestern North America". Journal of Geophysical Research. 96 (B7): 12479. Bibcode:1991JGR....9612479W. CiteSeerX   10.1.1.522.2461 . doi:10.1029/91jb00606. ISSN   0148-0227.
  5. 1 2 The Geotectonic development of California. Ernst, W. G. (Wallace Gary), 1931–, Rubey, William Walden, 1898–1974. Englewood Cliffs, N.J.: Prentice-Hall. 1981. ISBN   978-0133539387. OCLC   6627256.{{cite book}}: CS1 maint: others (link)
  6. ↑ Chapin, C. E. (2012-01-12). "Origin of the Colorado Mineral Belt". Geosphere. 8 (1): 28–43. Bibcode:2012Geosp...8...28C. doi: 10.1130/ges00694.1 . ISSN   1553-040X.
  7. ↑ Saleeby, Jason (2003-06-01). "Segmentation of the Laramide Slab—evidence from the southern Sierra Nevada region". GSA Bulletin. 115 (6): 655–668. Bibcode:2003GSAB..115..655S. doi:10.1130/0016-7606(2003)115<0655:sotlsf>2.0.co;2. ISSN   0016-7606.
  8. 1 2 Sigloch, Karin; Mihalynuk, Mitchell G. (April 2013). "Intra-oceanic subduction shaped the assembly of Cordilleran North America" (PDF). Nature. 496 (7443): 50–56. Bibcode:2013Natur.496...50S. doi:10.1038/nature12019. ISSN   0028-0836. PMID   23552944. S2CID   205233259.
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