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43-726: The plate first came into existence as a microplate 190 million years ago, at the triple junction between the Farallon , Phoenix , and Izanagi plates . The Pacific plate subsequently grew to where it underlies most of the Pacific Ocean basin. This reduced the Farallon plate to a few remnants along the west coast of the Americas and the Phoenix plate to a small remnant near the Drake Passage , and destroyed

86-472: A ridge (R), trench (T) or transform fault (F) – and triple junctions can be described according to the types of plate margin that meet at them (e.g. fault–fault–trench, ridge–ridge–ridge, or abbreviated F-F-T, R-R-R). Of the ten possible types of triple junctions only a few are stable through time ( stable in this context means that the geometrical configuration of the triple junction will not change through geologic time). The meeting of four or more plates

129-463: 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 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 –

172-403: 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 a tear in the slab, where a piece of the subducted Farallon plate has broken off, creating multiple slab remnants. This

215-454: A single point, for the triple junction to exist stably. These lines necessarily are parallel to the plate boundaries as to remain on the plate boundaries the observer must either move along the plate boundary or remain stationary on it. The point at which these lines meet, J, gives the overall motion of the triple junction with respect to the Earth. Using these criteria it can easily be shown why

258-423: Is also theoretically possible, but junctions will only exist instantaneously. The first scientific paper detailing the triple-junction concept was published in 1969 by Dan McKenzie and W. Jason Morgan . The term had traditionally been used for the intersection of three divergent boundaries or spreading ridges. These three divergent boundaries ideally meet at near 120° angles. In plate tectonics theory during

301-456: Is believed to have caused the formation of the Pacific plate about 190 million years ago. By assuming that plates are rigid and that the Earth is spherical, Leonhard Euler 's theorem of motion on a sphere can be used to reduce the stability assessment to determining boundaries and relative motions of the interacting plates. The rigid assumption holds very well in the case of oceanic crust , and

344-514: Is demonstrated below – as the perpendicular bisectors of the sides of a triangle always meet at a single point, the lines ab, bc and ca can always be made to meet regardless of relative velocities. RTF junctions are less common, an unstable junction of this type (an RTF(a)) is thought to have existed at roughly 12 Ma at the mouth of the Gulf of California where the East Pacific Rise currently meets

387-405: Is retained with time as the plates involved move. This places restrictions on relative velocities and plate boundary orientation. An unstable triple junction will change with time, either to become another form of triple junction (RRF junctions easily evolve to FFR junctions), will change geometry or are simply not feasible (as in the case of FFF junctions). The inherent instability of an FFF junction

430-487: 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 the Farallon should be partitioned into Northern Farallon, Angayucham , Mezcalera and Southern Farallon segments based on recent tomographic models. Under this model,

473-685: The Drake Passage , and destroyed the Izanagi plate by subduction under Asia. Farallon plate 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

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516-803: The Japan Trench . The plate forms a convergent boundary by subducting under the Philippine Sea plate creating the Mariana Trench , has a transform boundary with the Caroline plate , and has a collision boundary with the North Bismarck plate . In the south-west, the Pacific plate has a complex but generally convergent boundary with the Indo-Australian plate , subducting under it north of New Zealand forming

559-592: The Mid-Atlantic Ridge , and an associated aulacogen , the Benue Trough , in the Niger Delta region of Africa. RRR junctions are also common as rifting along three fractures at 120° is the best way to relieve stresses from uplift at the surface of a sphere; on Earth, stresses similar to these are believed to be caused by the mantle hotspots thought to initiate rifting in continents. The stability of RRR junctions

602-791: The North American plate along the San Andreas Fault , and a boundary with the Cocos plate . The south-eastern side is a divergent boundary with the Nazca plate forming the East Pacific Rise . The southern side is a divergent boundary with the Antarctic plate forming the Pacific–Antarctic Ridge . The western side is bounded by the Okhotsk microplate at the Kuril–Kamchatka Trench and

645-441: The San Andreas Fault zone. The Guadeloupe and Farallon microplates were previously being subducted under the North American plate and the northern end of this boundary met the San Andreas Fault . Material for this subduction was provided by a ridge equivalent to the modern East Pacific Rise slightly displaced to the west of the trench. As the ridge itself was subducted an RTF triple junction momentarily existed but subduction of

688-717: The Tonga Trench and the Kermadec Trench . The Alpine Fault marks a transform boundary between the two plates, and further south the Indo-Australian plate subducts under the Pacific plate forming the Puysegur Trench . The southern part of Zealandia , which is to the east of this boundary, is the plate's largest block of continental crust. Hillis and Müller are reported to consider the Bird's Head plate to be moving in unison with

731-451: The convergent plate boundary than is typical of a subduction-generated orogeny . Significant deformation of 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

774-515: The Asian oceanic trenches. The oldest part disappearing by way of the plate tectonics cycle is early- Cretaceous (145 to 137 million years ago). The Pacific plate originated at the triple junction of the three main oceanic plates of Panthalassa , the Farallon , Phoenix , and Izanagi plates , around 190 million years ago. The plate formed because the triple junction had converted to an unstable form surrounded on all sides by transform faults , due to

817-488: The Euler poles are distant from the triple junction concerned. The definitions they used for R, T and F are as follows: For a triple junction between the plates A, B and C to exist, the following condition must be satisfied: where A v B is the relative motion of B with respect to A. This condition can be represented in velocity space by constructing a velocity triangle ABC where the lengths AB, BC and CA are proportional to

860-637: The FFF triple junction is not stable: the only case in which three lines lying along the sides of a triangle can meet at a point is the trivial case in which the triangle has sides lengths zero, corresponding to zero relative motion between the plates. As faults are required to be active for the purpose of this assessment, an FFF junction can never be stable. McKenzie and Morgan determined that there were 16 types of triple junction theoretically possible, though several of these are speculative and have not necessarily been seen on Earth. These junctions were classified firstly by

903-592: The Farallon slab appears as a velocity anomaly on the tomography model. Multiple studies show that the subduction of the Farallon plate 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

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946-718: The Izanagi plate by subduction under Asia. The Pacific plate contains an interior hot spot forming the Hawaiian Islands . The north-eastern side is a divergent boundary with the Explorer plate , the Juan de Fuca plate and the Gorda plate forming respectively the Explorer Ridge , the Juan de Fuca Ridge and the Gorda Ridge . In the middle of the eastern side is a transform boundary with

989-475: 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 a different perspective on the history of collision. Based on this model,

1032-544: The Pacific plate, but Bird considers them to be unconnected. The northern side is a convergent boundary subducting under the North American plate forming the Aleutian Trench and the corresponding Aleutian Islands (see also: Aleutian Arc ). The Pacific plate is almost entirely oceanic crust , but it contains some continental crust in New Zealand, Baja California , and coastal California . The Pacific plate has

1075-535: The RRF configuration could be stable under certain conditions. An RRR junction is always stable using these definitions and therefore very common on Earth, though in a geological sense ridge spreading is usually discontinued in one direction leaving a failed rift zone . There are many examples of these present both now and in the geological past such as the South Atlantic opening with ridges spreading North and South to form

1118-418: The area around a triple junction is small enough (relative to the size of the sphere) and (usually) far enough from the pole of rotation, that the relative motion across a boundary can be assumed to be constant along that boundary. Thus, analysis of triple junctions can usually be done on a flat surface with motions defined by vectors. Triple junctions may be described and their stability assessed without use of

1161-495: The breakup of a continent, three divergent boundaries form, radiating out from a central point (the triple junction). One of these divergent plate boundaries fails (see aulacogen ) and the other two continue spreading to form an ocean. The opening of the south Atlantic Ocean started at the south of the South American and African continents, reaching a triple junction in the present Gulf of Guinea , from where it continued to

1204-414: The development of a kink in one of the plate boundaries. The "Pacific Triangle", the oldest part of the Pacific plate, created during the initial stages of plate formation, is located just east of the Mariana Trench . The growth of the Pacific plate reduced the Farallon plate to a few remnants along the west coast of the Americas (such as the Juan de Fuca plate ) and the Phoenix plate to a small remnant near

1247-415: The distinction of showing one of the largest areal sections of the oldest members of seabed geology being entrenched into eastern Asian oceanic trenches . A geologic map of the Pacific Ocean seabed shows not only the geologic sequences, and associated Ring of Fire zones on the ocean's perimeters, but the various ages of the seafloor in a stairstep fashion, youngest to oldest, the oldest being consumed into

1290-558: 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 . 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

1333-410: The geological details but simply by defining the properties of the ridges , trenches and transform faults involved, making some simplifying assumptions and applying simple velocity calculations. This assessment can generalise to most actual triple junction settings provided the assumptions and definitions broadly apply to the real Earth. A stable junction is one at which the geometry of the junction

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1376-420: The geological details of the crust are then needed. Another useful simplification is that the kinematics of triple junctions on a flat Earth are essentially the same as those on the surface of a sphere. On a sphere, plate motions are described as relative rotations about Euler poles (see Plate reconstruction ), and the relative motion at every point along a plate boundary can be calculated from this rotation. But

1419-433: 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 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

1462-452: 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 the current Cascadia subduction zone . This created

1505-446: The radius of the Earth at the equator and poles only varies by a factor of roughly one part in 300 so the Earth approximates very well to a sphere. McKenzie and Morgan first analysed the stability of triple junctions using these assumptions with the additional assumption that the Euler poles describing the motions of the plates were such that they approximated to straight line motion on a flat surface. This simplification applies when

1548-522: The ridge caused the subducted lithosphere to weaken and 'tear' from the point of the triple junction. The loss of slab pull caused by the detachment of this lithosphere ended the RTF junction giving the present day ridge – fault system. An RTF(a) is stable if ab goes through the point in velocity space C, or if ac and bc are colinear. A TTT(a) junction can be found in central Japan where the Eurasian plate overrides

1591-436: The same velocity space diagrams in the following way. The lines ab, bc and ca join points in velocity space which will leave the geometry of AB, BC and CA unchanged. These lines are the same as those that join points in velocity space at which an observer could move at the given velocity and still remain on the plate boundary. When these are drawn onto the diagram containing the velocity triangle these lines must be able to meet at

1634-420: The slab should be in the lower mantle, but the deformation has caused it to remain shallower, in 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 ,

1677-445: 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, 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

1720-461: 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 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

1763-499: The types of plate boundaries meeting – for example RRR, TTR, RRT, FFT etc. – and secondly by the relative motion directions of the plates involved. Some configurations such as RRR can only have one set of relative motions whereas TTT junctions may be classified into TTT(a) and TTT(b). These differences in motion direction affect the stability criteria. McKenzie and Morgan claimed that of these 16 types, 14 were stable with FFF and RRF configurations unstable, however, York later showed that

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1806-433: The velocities A v B , B v C and C v A respectively. Further conditions must also be met for the triple junction to exist stably – the plates must move in a way that leaves their individual geometries unchanged. Alternatively the triple junction must move in such a way that it remains on all three of the plate boundaries involved. McKenzie and Morgan demonstrated that these criteria can be represented on

1849-402: The west. The NE-trending Benue Trough is the failed arm of this junction. In the years since, the term triple-junction has come to refer to any point where three tectonic plates meet. The properties of triple junctions are most easily understood from the purely kinematic point of view where the plates are rigid and moving over the surface of the Earth. No knowledge of the Earth's interior or

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