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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 A triple junction is the point where the boundaries of three tectonic plates meet. At the triple junction each of the three boundaries will be one of three types – a ridge (R), trench (T) or transform fault (F) – and triple junctions can b ...
. The Farallon Plate began subducting under the west coast of the
North American Plate 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 , it is the Earth's second largest tectonic plate, behind the Paci ...
—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 San Francisco (; Spanish for " Saint Francis"), officially the City and County of San Francisco, is the commercial, financial, and cultural center of Northern California. The city proper is the fourth most populous in California and 17t ...
. Over time, the central part of the Farallon Plate was completely subducted under the southwestern part of the North American Plate. The remains of the Farallon Plate are the Juan de Fuca, Explorer and Gorda Plates, subducting under the northern part of the
North American Plate 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 , it is the Earth's second largest tectonic plate, behind the Paci ...
; the
Cocos Plate The Cocos Plate is a young oceanic tectonic plate beneath the Pacific Ocean off the west coast of Central America, named for Cocos Island, which rides upon it. The Cocos Plate was created approximately 23 million years ago when the Farallon Plat ...
subducting under Central America; and the Nazca Plate subducting under the South American Plate. The Farallon Plate is also responsible for transporting old island arcs and various fragments of continental crustal material rifted off from other distant plates and accreting them to the North American Plate. These fragments from elsewhere are called terranes (sometimes, "exotic" terranes). Much of western North America is composed of these accreted terranes.


Current state

The understanding of the Farallon Plate is rapidly evolving as details from seismic tomography provide improved details of the submerged remnants. Since the North American west coast shows a convoluted structure, significant work has been required to resolve the complexity. In 2013 a new and more nuanced explanation emerged, proposing two additional now-subducted plates which would account for some of the complexity.


Historic view

As data accumulated, a common view developed that one large oceanic plate, the Farallon plate, acted as a conveyor belt, conveying terranes to North America's west coast, where they accreted. 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.


Farallon Plate subduction forms North American Cordillera

, it is generally accepted that the western quarter of North America consists of accreted terrane accumulated over the past 200 million years as a result of the oceanic Farallon plate moving terranes onto the
continental margin A continental margin is the outer edge of continental crust abutting oceanic crust under coastal waters. It is one of the three major zones of the ocean floor, the other two being deep-ocean basins and mid-ocean ridges. The continental margin ...
as it subducts under the continent. However this simple model was unable to explain many terrane complexities, and is inconsistent with seismic tomographic images of subducting slabs penetrating the lower-mantle. In April 2013 Sigloch and Mihalynuk noted that under North America these subducting slabs formed massive, essentially vertical walls of 800 km to 2,000 km deep and 400–600 km wide, forming "slab walls". One such large "slab wall" runs from north-west Canada to the eastern U.S. and extends to Central America; this "slab wall" had traditionally been associated with the subducting Farallon plate. Sigloch and Mihalynuk proposed that the Farallon should be partitioned into Northern Farallon, Angayucham, Mezcalera and Southern Farallon segments based on recent tomographic models. Under this model, the North American continent overrides a series of subduction trenches and incorporates microcontinents (similar to those in the modern-day Indonesian Archipelago) as it moves west in the following sequence: * 165–155 Myr ago the Mezcalera promontory (the leading terrane to strike North America) strikes land and begins to be overridden. The overridden segment is replaced by an incipient South Farallon trench. * 160–155 Myr ago the Rocky Mountain deformation begins, recorded by a synorogenic (formed contemporaneously with the orogen) clastic wedge. The Franciscan subduction complex on the South Farallon plate begins. * 125 Myr ago the collision of the North America margin with an archipelago of terranes (Mezcalera / Angayucham /Southern Farallon island arcs) begins. This broad expanse causes strong deformations and creates the
Sevier Mountains The Sevier orogeny was a mountain-building event that affected western North America from northern Canada to the north to Mexico to the south. The Sevier orogeny was the result of convergent boundary tectonic activity, and deformation occurred fr ...
and the
Canadian Rocky Mountains The Canadian Rockies (french: Rocheuses canadiennes) or Canadian Rocky Mountains, comprising both the Alberta Rockies and the British Columbian Rockies, is the Canadian segment of the North American Rocky Mountains. It is the easternmost part ...
. * 124–90 Myr ago the Omineca magmatic belts are formed in the Pacific Northwest along with a gradual override of the Mezcalera promontory by the Pacific Northwest. * 85 Myr ago the South Farallon trench moves westward after accretion of the Shatsky Rise Conjugate plateau. Sonora volcanism results from the slab sinking. The Tarahumara ignimbrite province is formed. * 85–55 Myr ago Strong transpressive coupling of Farallon plate to terranes produces the buoyant Shatsky Rise. The
Laramide orogeny The Laramide orogeny was a time period of mountain building in western North America, which started in the Late Cretaceous, 70 to 80 million years ago, and ended 35 to 55 million years ago. The exact duration and ages of beginning and end of the ...
results from basement uplift more than 1,000 km inland. * 72–69 Myr ago the Angayucham arc, is overridden by North America and Carmacks volcanic episode results. * 85–55 Myr ago Conjugate subducts. Northward shuffle of Insular terrane, Intermontane terrane, and Angayucham terranes along margin. * 55–50 Myr ago saw the override of the Cascadia Root arc by the Pacific Northwest along with accretion of the Siletzia and Pacific Rim terranes. * 55–50 Myr ago Final override of westernmost Angayucham occurred, with an explosive end of Coast Mountain arc volcanism When the final archipelago, the Siletzia archipelago, lodged as a terrane, the associated trench stepped west as the terrane accreted, converting an intra-oceanic subduction trench into the current
Cascadia subduction zone The Cascadia subduction zone is a convergent plate boundary that stretches from northern Vancouver Island in Canada to Northern California in the United States. It is a very long, sloping subduction zone where the Explorer, Juan de Fuc ...
and creating a
slab window 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. ...
.; .


See also

* Izanagi Plate *
Kula 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 ...
* Kula–Farallon Ridge *
Pacific-Farallon Ridge 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 tripl ...
* San Andreas Fault


References

Notes Bibliography * * * * ---- * *


External links


USGS Professional Paper 1515: images of Farallon Plate


California Institute of Technology
The Farallon Plate
Goddard Space Flight Center {{Geological history, p, m Tectonic plates Geology of North America Geology of the Pacific Ocean Historical tectonic plates Historical geology Cenozoic geology Mesozoic geology Cenozoic North America Mesozoic North America Cenozoic California Mesozoic California Geology of Marin County, California Geology of Mendocino County, California Geology of Sonoma County, California