TheFarallon plate was an ancientoceanictectonic plate. It formed one of the three main plates ofPanthalassa, alongside theIzanagi plate and thePhoenix plate, which were connected by atriple junction. The Farallon plate begansubducting under the west coast of theNorth American plate—then located in modernUtah—asPangaea broke apart and after the formation of thePacific plate at the center of the triple junction during theEarly Jurassic. It is named for theFarallon Islands, which are located just west ofSan 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 theExplorer,Gorda, andJuan de Fuca plates, which are subducting under the northern part of theNorth American plate; theCocos plate subducting underCentral America; and theNazca plate subducting under theSouth American plate.[1]
The Farallon plate is also responsible for transporting oldisland arcs and various fragments ofcontinental crust, which hadrifted off of other distant plates. These fragments from elsewhere are calledterranes (sometimes, "exotic" terranes). During the subduction of the Farallon plate, itaccreted these island arcs and terranes to theNorth 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 fromseismic 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.[2]
Seismic tomography can be used toimage the remainder of the subducted plate because it is still "cold," meaning that it has not reachedthermal equilibrium with the mantle. This is important for the use of tomography becauseseismic waves have different velocities in materials of different temperatures, so the Farallon slab appears as a velocity anomaly on the tomography model.[3][4]
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).[3][5][6][7] This phenomenon accounts for the far-inlandorogenesis of theRocky Mountains and other ranges in North America which are much farther from theconvergent plate boundary than is typical of a subduction-generatedorogeny.[5][6]
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.[3][4][7] In other words, more of the slab should be in the lower mantle, but the deformation has caused it to remain shallower, in the upper mantle.[3]
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 inslab rollback.[3] Another cause of flat slab subduction may be slabbuoyancy, a characteristic influenced by the presence ofoceanic plateaus (or oceanic flood basalts).[3][8] In addition to influencing slab buoyancy, some oceanic plateaus may have also become accreted to North America.[7][8]
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 is supported by tomography studies and provides some more explanation of the formation ofLaramide structures that are further inland from the edge.[4][7][9]
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.[2][10] Under this model, the North American continent overrode a series of subduction trenches, and severalmicrocontinents (similar to those in the modern-dayIndonesian 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, the plate moved west, causing the following geologic events to occur:[10]
When the finalarchipelago, 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 currentCascadia subduction zone. This created aslab window.[11]
Other models have been proposed for the Farallon's influence on the Laramide orogeny, including thedewatering of the slab which led to intense uplift andmagmatism.[6]
Notes
 | This article includes a list ofgeneral references, butit lacks sufficient correspondinginline citations. Please help toimprove this article byintroducing more precise citations.(June 2015) (Learn how and when to remove this message) |
Bibliography
