Editing Forearc (section)

==Formation==
During [[subduction]], an [[oceanic crust|oceanic]] plate is thrust below another tectonic plate, which can be oceanic or [[continental crust|continental]]. Water and other volatiles in the subducting plate cause [[flux melting]] in the [[upper mantle (Earth)|upper mantle]], creating magma that rises and penetrates the overriding plate, forming a [[volcanic arc]]. The weight of the subducting slab flexes the overriding plate and creates an [[oceanic trench]]. This area between the trench and the arc is called the forearc region, with the area behind the arc and away from the trench known as the [[back-arc region]].

The mantle region between the overriding plate and the subducting slab forms a wedge shape. This wedge is open-ended on the [[back-arc]] side and along its edges. The downward motion of the subducting slab will no doubt tend to pull adjacent mantle into the wedge and create mantle flow patterns within the wedge however the exact characteristics of this flow is difficult to model and poorly constrained by data.<ref>{{cite journal |last1=Long |first1=Maureen D. |last2=Wirth |first2=Erin A. |title=Mantle flow in subduction systems: The mantle wedge flow field and implications for wedge processes |journal=Journal of Geophysical Research: Solid Earth |date=February 2013 |volume=118 |issue=2 |pages=583–606 |doi=10.1002/jgrb.50063|bibcode=2013JGRB..118..583L |doi-access=free }}</ref> At the same time we can assume the temperature of the mantle wedge closer to the trench will be dominated by the denser and colder subducting slab, resulting in a cold, stagnant portion of the mantle wedge along its bottom.<ref name=":0" /><ref>{{cite journal |last1=Uchida |first1=Naoki |last2=Nakajima |first2=Junichi |last3=Wang |first3=Kelin |last4=Takagi |first4=Ryota |last5=Yoshida |first5=Keisuke |last6=Nakayama |first6=Takashi |last7=Hino |first7=Ryota |last8=Okada |first8=Tomomi |last9=Asano |first9=Youichi |title=Stagnant forearc mantle wedge inferred from mapping of shear-wave anisotropy using S-net seafloor seismometers |journal=Nature Communications |date=10 November 2020 |volume=11 |issue=1 |pages=5676 |doi=10.1038/s41467-020-19541-y|pmid=33173070 |pmc=7655809 |bibcode=2020NatCo..11.5676U }}</ref>

Initial theories proposed that the oceanic trenches and magmatic arcs were the primary suppliers of the accretionary sedimentation wedges in the forearc regions. More recent discovery suggests that some of the accreted material in the forearc region is from a mantle source along with trench [[turbidites]] derived from continental material. This theory holds due to evidence of pelagic sediments and continental crust being subducted in processes known as sediment subduction and subduction erosion respectively.<ref name="Kearey et al. 2006" />

Over geological time there is constant recycling of the forearc deposits due to erosion, deformation and sedimentary subduction. The material in the forearc region (accretionary prism, forearc basin and trench) cycles downwards from the subduction of the subducting plate and back upwards from the high pressure flow of water saturated rocks from deep within the collision zone.<ref>{{Cite news |last=Metageologist |date=2012-10-24 |title=Eclogites: back to the surface - Metageologist |url=https://all-geo.org/metageologist/2012/10/eclogites-back-to-the-surface/#:~:text=One%20possibility%20is%20that%20eclogite,be%20squeezed%20towards%20the%20surface. |access-date=2025-02-05 |work=Metageologist - Geology: beautiful things, incredible ideas |language=en-US}}</ref> This generates a mixture of igneous, metamorphic and sedimentary rocks to form which geologists call a [[mélange]]. Note also that, while in general there is an increase in [[metamorphic grade]] from trench to arc, we surprisingly sometimes find low-grade, high-pressure facies such as [[blueschist]] and [[eclogite]] in accretion prisms at the tip of the mantle wedge. These rocks must have been carried to significant depth and kept cool by the cold subducting slab before (somehow) being uplifted. This gives us further clues as to the possible motion and mixing within the wedge. Forearc regions are also where [[ophiolites]] are emplaced should [[obduction]] occur, but such deposits are not continuous and can often be removed by erosion.<ref name="Kearey et al. 2006" /><ref name="Caset and Dewey 2013">{{cite journal |last1=Casey |first1=J. |last2=Dewey |first2=J. |title=Arc/Forearc Lengthening at Plate Triple junctions and the Formation of Ophiolitic Soles |journal=Geological Research Abstracts |year=2013 |volume=13 |pages=13430 |bibcode=2013EGUGA..1513430C}}</ref>

As tectonic plates converge, the closing of an ocean will result in the convergence of two landmasses, each of which is either an [[island arc]] or continental margin. When these two bodies collide, the result is [[orogenesis]], at which time the underthrusting oceanic crust slows down.<ref name="Kearey et al. 2006" /><ref name="Brown and Spadea 1999">{{cite journal |last1=Brown |first1=D. |last2=Spadea |first2=P |title=Processes of forearc and accretionary complex formation during arc-continent collision in the southern Ural Mountains |journal=Geology |pages=649–652 |year=2013 |volume=27 |issue=7 |doi=10.1130/0091-7613(1999)027<0649:pofaac>2.3.co;2}}</ref> In early stages of [[arc-continent]] collision, there is uplift and erosion of the accretionary prism and forearc basin. In the later stages of collision, the forearc region may be sutured, rotated and shortened which can form syn-collisional folds and thrust belts.