The Farallon plate was an ancient oceanic tectonic plate . It formed one of the three main plates of Panthalassa , alongside the Izanagi plate and the Phoenix 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 center of the triple junction during the Early Jurassic . It is named for the Farallon Islands , which are located just west of San Francisco , California .
26-831: Over time, the central part of the Farallon plate was subducted under the southwestern part of the North American plate. The remains of the Farallon plate are the Explorer , Gorda , and Juan de Fuca plates, which are 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
52-477: A different perspective on the history of collision. Based on this model, the plate moved west, causing the following geologic events to occur: When the final archipelago , the Siletzia archipelago, lodged as a terrane, the associated trench stepped west. When this happened, the trench that had been characterized as an oceanic-oceanic subduction environment approached the North American margin and eventually became
78-675: A highly variable basin between 1,400 metres (4,600 ft) and 2,200 metres (7,200 ft) in depth. The eastern boundary of the Explorer plate is being subducted under the North American plate. The southern boundary is a collection of transform faults , the Sovanco Fracture Zone , separating the Explorer plate from the Pacific plate . To the southeast is another transform boundary, the Nootka Fault , which separates
104-432: A tear in the slab, where a piece of the subducted Farallon plate has broken off, creating multiple slab remnants. This is supported by tomography studies and provides some more explanation of the formation of Laramide structures that are further inland from the edge. A 2013 study proposed two additional now-subducted plates that would account for some of the unexplained complexities of the accreted terranes, suggesting that
130-457: Is also responsible for transporting old island arcs and various fragments of continental crust , which have rifted off of other distant plates. These fragments from elsewhere are called terranes (sometimes, "exotic" terranes). During the subduction of the Farallon plate, it accreted these island arcs and terranes to the North American plate . Much of western North America is composed of these accreted terranes. As an ancient tectonic plate,
156-536: Is considered a microcontinent, though not a continental fragment. Other hotspot islands such as the Hawaiian Islands and Iceland are considered neither microcontinents nor continental fragments. Not all islands can be considered microcontinents: Borneo , the British Isles , Newfoundland , and Sri Lanka , for example, are each within the continental shelf of an adjacent continent, separated from
182-572: Is partially subducted under the North American plate . Along with the Juan de Fuca plate and Gorda plate , the Explorer plate is a remnant of the ancient Farallon plate , which has been subducted under the North American plate. The Explorer plate separated from the Juan de Fuca plate roughly 4 million years ago. In its smoother, southern half, the average depth of the Explorer plate is roughly 2,400 metres (7,900 ft) and rises up in its northern half to
208-426: Is the most seismically active area of Canada, but is anomalous as a subduction zone since most of the seismic activity occurs around the plate's perimeter rather than at the subduction interface. Events are generally centered around the southern and north-western areas where the borders of the plate are in contact with other plates; however, the newer ocean crust created at Explorer ridge and Juan de Fuca ridge reduces
234-425: Is very similar to that of typical continental crust. Strike-slip fault zones cause the fragmentation of microcontinents. The zones link the extensional zones where continental pieces are already isolated through the remaining continental bridges. Additionally, they facilitate quick crustal thinning across narrow zones and near-vertical strike-slip-dominated faults . They develop fault-block patterns that slice
260-469: The Caribbean Sea , are composed largely of granitic rock as well, but all continents contain both granitic and basaltic crust, and there is no clear dividing line between islands and microcontinents under such a definition. The Kerguelen Plateau is a large igneous province formed by a volcanic hotspot ; however, it was associated with the breakup of Gondwana and was for a time above water, so it
286-588: The Explorer plate from the Juan de Fuca plate and forms a triple junction with the North American plate. To the northwest is a divergent boundary with the Pacific plate forming the Explorer Ridge , and the Winona Basin located within the northwest boundaries and the Pacific continental shelf. The Queen Charlotte triple junction is located where the Pacific plate and North American plate meets with
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#1732764922649312-415: The Explorer plate. The subducted portion of the plate extends downward to more than 300 km (186 mi) depth, and laterally as far as mainland Canada. The relative buoyancy of the subducting plate and the underlying mantle may be inhibiting the Explorer plate's ability to descend further into the mantle. There is an ongoing debate regarding the process of subduction of the Explorer plate and how
338-461: The Explorer plate. Upon breaking apart 4 million years ago, the Juan De Fuca plate continued moving northeast at 26 mm/year (1 in/year) while the Explorer plate's velocity changed, stalling or moving slowly north up to 20 mm/year. The Nootka Fault boundary between the Juan De Fuca plate and the Explorer plate has varied in length and direction since their separation. The formation of
364-474: The Farallon plate must be studied using methods that allow researchers to see deep beneath the Earth's surface. The understanding of the Farallon plate has evolved as details from seismic tomography provide improved details of the submerged remnants. Since the North American west coast has a convoluted structure, significant work has been required to resolve the complexity. Seismic tomography can be used to image
390-514: 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 overrode a series of subduction trenches, and several microcontinents (similar to those in the modern-day Indonesian Archipelago ) were added to it. These microcontinents must have had adjacent oceanic plates that are not represented in previous models of Farallon subduction, so this interpretation brings forth
416-564: The Nookta Fault and the shearing of plate boundaries has caused a clockwise rotation, reorienting the Sovanco Fracture Zone northwards along the North American plate and slowing the Explorer plate's subduction. The Sovanco Fracture Zone originated as a spreading center offset more than 7 million years ago which shows southward movement from the influence of the Explorer ridge and results in uneven spreading eastward unto
442-569: The boundary between the Explorer plate and the North American plate are defined: As a part of the Pacific Ring of Fire , the Explorer plate has a high level of seismic activity. However, the activity consists of low-magnitude events; no earthquake above magnitude 6.5 has been recorded in the region, though a swarm of several dozen magnitude 5–6 earthquakes occurred just north of the Seminole Seamount in 2008. The Explorer plate
468-560: The continental fragments to form most likely impacts their layers and overall thickness along with the addition of mafic intrusions to the crust. Studies have determined that the average crustal thickness of continental fragments is approximately 24.8 ± 5.7 kilometres (15.4 ± 3.5 mi). The sedimentary layer of continental fragments can be up to 5 kilometres (3.1 mi) thick and can overlay two to three crustal layers. Continental fragments have an average crustal density of 2.81 g/cm (0.102 lb/cu in) which
494-549: The current Cascadia subduction zone . This created a slab window . Other models have been proposed for the Farallon's influence on the Laramide orogeny, including the dewatering of the slab which led to intense uplift and magmatism . Notes Bibliography Explorer plate The Explorer plate is an oceanic tectonic plate beneath the Pacific Ocean off the west coast of Vancouver Island , Canada, which
520-462: The mainland by inland seas flooding its margins. Several islands in the eastern Indonesian Archipelago are considered continental fragments, although this designation is controversial. The archipelago is home to numerous microcontinents with complex geology and tectonics. This makes it complicated to classify landmasses and determine causation for the formation of the landmass. These include southern Bacan , Banggai - Sulu Islands ( Sulawesi ),
546-481: The portion of continent into detachable slivers. The continental fragments are located at various angles from their transform faults . Some microcontinents are fragments of Gondwana or other ancient cratonic continents; examples include Madagascar ; the northern Mascarene Plateau , which includes the Seychelles Microcontinent ; and the island of Timor . Other islands, such as several in
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#1732764922649572-405: The remainder of the subducted plate because it is still "cold," as in, it has not reached thermal equilibrium with the mantle. This is important for the use of tomography because seismic waves have different velocities in materials of different temperatures, so the Farallon slab appears as a velocity anomaly on the tomography model. Multiple studies show that the subduction of the Farallon plate
598-520: The rigidity of the region and contributes to the low magnitude of events in the region. 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. Continental fragments and microcontinent crustal compositions are very similar to those of regular continental crust . The rifting process that caused
624-430: The slab also occurred due to this flat subduction phenomenon, which has been imaged by seismic tomography. There is a concentration of velocity anomalies in the tomography that is thicker than the slab itself should be, indicating that folding and deformation occurred beneath the surface during subduction. In other words, more of the slab should be in the lower mantle, but the deformation has caused it to remain shallower, in
650-600: The upper mantle. Multiple hypotheses have been proposed to explain this shallow subduction angle and resulting deformation. Some studies suggest that the faster movement of the North American plate caused the slab to flatten, resulting in slab rollback . Another cause of flat slab subduction may be slab buoyancy , a characteristic influenced by the presence of oceanic plateaus (or oceanic flood basalts). In addition to influencing slab buoyancy, some oceanic plateaus may have also become accreted to North America. It has been suggested that this deformation may go so far as to include
676-575: Was characterized by a period of " flat-slab subduction ," which is the subduction of a plate at a relatively shallow angle to the overriding crust (in this case, North America). This phenomenon is one that accounts for the far-inland orogenisis of the Rocky Mountains and other ranges in North America which are much farther from the convergent plate boundary than is typical of a subduction-generated orogeny . Significant deformation of
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