Forearc
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.[1][2]
Formation
During
The mantle region between the overriding plate and the subducting slab experiences corner flow near the
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
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, forearc 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.[2][6]
As tectonic plates converge, the closing of an ocean will result in the convergence of two landmasses, each of which is either an
Structure
At the surface, the forearc region can include a forearc basin(s), outer-arc high, accretionary prism and the trench itself.[2] The forearc subduction interface can include a seismogenic zone, where megathrust earthquakes can occur, a decoupled zone, and a viscously coupled zone.[4][8]
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.[2]
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.
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 represents a forearc basin formed with little to no sediment supply. Conversely, the second model represents a basin with a healthy sediment supply. Basin depth depends 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 a heat flow of ~30–40 mW/m2, which indicates cold, strong mantle.[9]
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.[2] Other good examples are:
- Central Andean Forearc
- Banda Forearc
- Savu-Wetar Forearc
- Luzon arc-forearc
- Tohoku Forearc
- Between Western Cordilleraand Peru-Chile Trench
See also
References
- Einsele, Gerhard (2000) Sedimentary Basins : Evolution, Facies, and Sediment Budget 2nd ed., Ch. 12, Springer ISBN 3-540-66193-X
- USGS definition
- Forearc Basin Architecture, abstract
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- Bibcode:2013EGUGA..1513430C.
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