Forearc Region

A forearc is the region between an oceanic trench and the associated volcanic arc. As such, forearc regions are found at convergent margins, and include any accretionary wedge and forearc basin that may be present. Due to tectonic stresses as on tectonic plate rides over another, forearc regions are sources for great thrust earthquakes.

Formation

During subduction, an oceanic plate is thrust below another tectonic plate, which may be oceanic or continental. Water and other volatiles in the down-going 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 down-going slab flexes the down-going plate creating an oceanic trench. The area between the trench and the arc is the forearc region, and the area behind the arc (i.e., on the side away from the trench) is the back-arc region.

Initial theories purposed 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.

Over geological time there is constant recycling of the forearc deposits due to erosion, deformation and sedimentary subduction. The constant circulation of material in the forearc region (accretionary prism, forarc basin and trench) generates a mixture of igneous, metamorphic and sedimentary sequences. In general, there is an increase in metamorphic grade from trench to arc where highest grade (blueschist to eclogite) is structurally uplifted (in the prisms) compared to the younger deposits (basins). Forearc regions are also where ophiolites are emplaced should obduction occur, but such deposits are not continuous and can often be removed by erosion.

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.

Structure

The forearc region includes any forearc basin, 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.

In general, the forearc topography (specifically in the trench region) is trying to achieve an equilibrium between buoyancy and tectonic forces caused by subduction. Upward motion of the forearc is related to buoyancy forces and the downward motion is associated with the tectonic forcing which causes the oceanic lithosphere to descend. The relationship between surface slope and subduction thrust also plays a huge role in the variation of forearc structure and deformation. A subduction wedge can be classified as either stable with little deformation or unstable with pervasive internal deformation (see section on Models). Some common deformation in forearc sediments are synsedimentary deformation and olistostromes, which is seen in the Magnitogorsk forearc region.

Models

There are two models which characterize a forearc basin formation and deformation and are dependent on sediment deposition and subsidence (see figure). The first model is associated with a forearc basin formed with little to no sediment supply. Conversely, the second model is associated with sediment supply. Topographic depressions which are accertionary and nonaccretionary in nature will depend on the supply of oceanic plate sediments, continentally derived clastic material and orthogonal convergence rates.[1][2] The accretionary flux (sediment supply in and out) also determines the rate at which the sedimentation wedges grow within the forearc [1]

The age of the oceanic crust along with the convergent velocity controls the coupling across the converging interface of the continental and oceanic crust. The strength of this coupling controls the deformation associated with the event and can be seen in the forearc region deformation signatures.[2]
Seismicity

The intense interaction between the overriding and underthrusting plates in the forearc regions have shown to evolve strong coupling mechanisms which result in megathrust earthquakes such as the Tohoku-oki Earthquake which occurred off the Pacific coast of Northeast Japan (Tian and Liu. 2013). These mega thrust earthquakes may be correlated with low values of heat flow generally associated with forearc regions. Geothermal data shows heat flow of ~30-40 mW/m^2 which represent cold, strong mantle.
Examples

One good example is the Mariana forearc, where scientists have done extensive research. In this setting there is an erosive margin and forearc slope which consists of 2 km high and 30 km diameter serpentine- mud volcanoes. The erosive properties of these volcanoes are consistent with the metamorphic grades (blueschists) expected for this region in the forearc. There is evidence from geothermal data and models which show the slab-mantle interface, levels of friction and the cool oceanic lithosphere at the trench. Other good examples are: Central Andean Forearc; Banda Forearc; Savu-Wetar Forearc; Luzon arc-forearc; Tohoku Forearc.
Between Western Cordillera and Peru-Chile Trench

http://en.wikipedia.org/wiki/Back-arc_region

Back-arc region

The back-arc region is the area behind a volcanic arc. In island volcanic arcs it consists of back-arc basins of oceanic crust with abyssal depths, which may be separated by remnant arcs, similar to island arcs. In continental arcs the back-arc region is part of continental platform, either dry land (subaerial) or forming shallow marine basins.

Formation

Back-arc deformation is a product of subduction at convergent plate tectonic boundaries. It initiates and evolves behind the volcanic arc on the overriding plate of a subduction zone. The stresses responsible for the deformation in this region of a subduction zone result from a combination of processes. The absolute motion of the upper plate as it moves towards or away from the trench strongly contributes to deformation in the back-arc region. Since the downgoing slab is partly anchored in the viscous layers of the mantle, and therefore its lateral movement is significantly slower than the surface plate, then any motion of the overriding plate will cause extensional or compressional stress in the back-arc region depending on the direction of motion. In addition, mantle convection in the upper mantle wedge caused by the downward movement of the subducted slab causes stress in the upper plate and the high heat flow that characterizes back-arcs. The pulling effect of the slab as it goes down into the mantle causes a rollback motion of the trench, which also applies stress on the back-arc region of the upper plate. However, this last process has less of an impact on deformation compared to upper plate motion.

Back-arcs can form on either oceanic crust or continental crust. In the case of oceanic crust, most back-arc regions are subjected to tensional stresses and thus develop a spreading centre where new oceanic crust is formed. The composition of this new crust is similar to MORB, although it contains higher amounts of water.
Back-arc extension vs. compression

The back-arc deformation may be either extensional or compressional. The overriding plate will shorten when its motion is directed towards the trench, resulting in a compression of the back-arc region. This type of deformation is associated with a shallow dipping subducted slab. Inversely, an overriding plate moving away from the trench will result in extension, and a back-arc basin will form. This extensional deformation is associated with a steeply dipping slab.

The extreme cases of these two types of back-arc deformation can be found in Chile and at the Marianas arc. The shallow dipping slab subducting beneath Chile at an angle of about 10-15° causes a compressional stress on the back-arc region behind the Andes. On the other extreme, the slab going down into the mantle at the Marianas subduction zone is so steep it is nearly vertical. This is the perfect example of an oceanic back-arc basin experiencing extensional forces. The Oriente in Ecuador (the eastern part of the country covered by rainforest) is also a good example of an extensional back-arc basin, this time in a continental setting. The continental crust in this area east of the Andes has been stretched out and covered by layers of sediments.