Океан шұңқыры мен жанартау доғасы аралық аймағы

The region between an oceanic trench and the associated volcanic arc

Forearc is a plate tectonic term referring to a region in a subduction zone between an oceanic trench and the associated volcanic arc. Forearc regions are present along convergent margins and eponymously form 'in front of' the volcanic arcs that are characteristic of convergent plate margins. A back arc region is the companion region behind the volcanic arc. Many forearcs have an accretionary wedge which may form a topographic ridge known as an outer arc ridge that parallels the volcanic arc. A forearc basin between the accretionary wedge and the volcanic arc can accumulate thick deposits of sediment, sometimes referred to as an outer arc trough. Due to collisional stresses as one tectonic plate subducts under another, forearc regions are sources for powerful earthquakes.

During subduction, an oceanic plate is thrust below another tectonic plate, which can be oceanic or continental. Water and other volatiles in the subducting plate cause flux melting in the 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 experiences corner flow near the back arc driven by the down dip motion of the subducting slab. At the same time, the temperature of the mantle wedge closer to the trench is dominated by the denser and colder subducting slab, resulting in a cold, stagnant portion of the mantle wedge. 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. 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. 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.

At the surface, the forearc region can include a forearc basin(s), outer arc high, accretionary prism and the trench itself. The accretionary prism is located at the slope of the trench break where there is significantly decreased slope angle. Between the break and the magmatic arc, a sedimentary basin filled with erosive material from the volcanic arc and substrate can accumulate into a forearc basin which overlays the oldest thrust slices in the wedge of the forearc region.