The Farallon Plate was an ancient
oceanic plate
Oceanic crust is the uppermost layer of the oceanic portion of the tectonic plates. It is composed of the upper oceanic crust, with pillow lavas and a dike complex, and the lower oceanic crust, composed of troctolite, gabbro and ultramafic c ...
. It formed one of the three main plates of
Panthalassa
Panthalassa, also known as the Panthalassic Ocean or Panthalassan Ocean (from Greek "all" and "sea"), was the superocean that surrounded the supercontinent Pangaea, the latest in a series of supercontinents in the history of Earth. During th ...
, alongside the
Phoenix Plate and
Izanagi Plate, which were connected by a
triple junction. The Farallon Plate began
subduct
Subduction is a geological process in which the oceanic 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 ...
ing under the west coast of the
North American Plate—then located in modern
Utah—as
Pangaea
Pangaea or Pangea () was a supercontinent that existed during the late Paleozoic and early Mesozoic eras. It assembled from the earlier continental units of Gondwana, Euramerica and Siberia during the Carboniferous approximately 335 million y ...
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.
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
Juan de Fuca (10 June 1536, Cefalonia 23 July 1602, Cefalonia)Greek Consulate of Vancouver,Greek Pioneers: Juan de Fuca. was a Greek pilot who served PhilipII of Spain. He is best known for his claim to have explored the Strait of Aniánnow k ...
,
Explorer
Exploration refers to the historical practice of discovering remote lands. It is studied by geographers and historians.
Two major eras of exploration occurred in human history: one of convergence, and one of divergence. The first, covering most ...
and
Gorda Plates, subducting under the northern part of the
North American Plate; the
Cocos Plate subducting under
Central America; and the
Nazca Plate subducting under the
South American Plate.
The Farallon Plate is also responsible for transporting old
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 ...
s and various fragments of
continental crust
Continental crust is the layer of igneous, sedimentary, and metamorphic rocks that forms the geological continents and the areas of shallow seabed close to their shores, known as continental shelves. This layer is sometimes called '' sial'' be ...
al 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
Slab or SLAB may refer to:
Physical materials
* Concrete slab, a flat concrete plate used in construction
* Stone slab, a flat stone used in construction
* Slab (casting), a length of metal
* Slab (geology), that portion of a tectonic plate tha ...
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 as it
subduct
Subduction is a geological process in which the oceanic 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 ...
s 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
microcontinent
Continental crustal fragments, partly synonymous with microcontinents, are pieces of continents that have broken off from main continental masses to form distinct islands that are often several hundred kilometers from their place of origin.
Caus ...
s (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
An orogenic belt, or orogen, is a zone of Earth's crust affected by orogeny. An orogenic belt develops when a continental plate crumples and is uplifted to form one or more mountain ranges; this involves a series of geological processes collecti ...
) 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 and the
Canadian Rocky Mountains.
* 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 Shatsky Rise is Earth's third largest oceanic plateau, (after Ontong Java and Kerguelen) located in the north-west Pacific Ocean east of Japan. It is one of a series of Pacific Cretaceous large igneous provinces (LIPs) together with Hess R ...
. The
Laramide orogeny 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 and creating a
slab window.
[; .]
See also
*
Izanagi Plate
*
Kula Plate
*
Kula–Farallon Ridge
*
Pacific-Farallon Ridge
*
San Andreas Fault
References
Notes
Bibliography
*
*
*
*
----
*
*
External links
USGS Professional Paper 1515: images of Farallon Plate–
California Institute of Technology
The California Institute of Technology (branded as Caltech or CIT)The university itself only spells its short form as "Caltech"; the institution considers other spellings such a"Cal Tech" and "CalTech" incorrect. The institute is also occasional ...
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