Editing Oceanic trench (section)

== Trench rollback ==
Trenches seem positionally stable over time, but scientists believe that some trenches—particularly those associated with subduction zones where two oceanic plates converge—move backward into the subducting plate.{{sfn|Dvorkin|Nur|Mavko|Ben-Avraham|1993}}{{sfn|Garfunkel|Anderson|Schubert|1986}} This is called '''trench rollback''' or '''retreat''', '''hinge rollback''' or '''retreat''', '''slab rollback''' or '''retreat''' and is one explanation for the existence of [[back-arc basin]]s.
 
Forces perpendicular to the slab (the portion of the subducting plate within the mantle) are responsible for steepening of the slab and, ultimately, the movement of the hinge and trench at the surface.{{sfn|Schellart|Moresi|2013}} These forces arise from the negative buoyancy of the slab with respect to the mantle{{sfn|Schellart|Lister|Toy|2006}} modified by the geometry of the slab itself.{{sfn|Nakakuki|Mura|2013}} The extension in the overriding plate, in response to the subsequent subhorizontal mantle flow from the displacement of the slab, can result in formation of a back-arc basin.{{sfn|Flower|Dilek|2003}}

===Processes involved===

Several forces are involved in the process of slab rollback. Two forces acting against each other at the interface of the two subducting plates exert forces against one another. The subducting plate exerts a bending force (FPB) that supplies pressure during subduction, while the overriding plate exerts a force against the subducting plate (FTS). The slab pull force (FSP) is caused by the negative buoyancy of the plate driving the plate to greater depths. The resisting force from the surrounding mantle opposes the slab pull forces. Interactions with the 660-km discontinuity cause a deflection due to the buoyancy at the [[phase transition]] (F660).{{sfn|Nakakuki|Mura|2013}}  The unique interplay of these forces is what generates slab rollback. When the deep slab section obstructs the down-going motion of the shallow slab section, slab rollback occurs. The subducting slab undergoes backward sinking due to the negative buoyancy forces causing a retrogradation of the trench hinge along the surface. Upwelling of the mantle around the slab can create favorable conditions for the formation of a back-arc basin.{{sfn|Flower|Dilek|2003}}

[[Seismic tomography]] provides evidence for slab rollback. Results demonstrate high temperature anomalies within the mantle suggesting subducted material is present in the mantle.{{sfn|Hall|Spakman|2002}} Ophiolites are viewed as evidence for such mechanisms as high pressure and temperature rocks are rapidly brought to the surface through the processes of slab rollback, which provides space for the exhumation of [[ophiolites]].

Slab rollback is not always a continuous process suggesting an episodic nature.{{sfn|Schellart|Lister|Toy|2006}} The episodic nature of the rollback is explained by a change in the density of the subducting plate, such as the arrival of buoyant lithosphere (a continent, arc, ridge, or plateau), a change in the subduction dynamics, or a change in the plate kinematics. The age of the subducting plates does not have any effect on slab rollback.{{sfn|Nakakuki|Mura|2013}} Nearby continental collisions have an effect on slab rollback. Continental collisions induce mantle flow and extrusion of mantle material, which causes stretching and arc-trench rollback.{{sfn|Flower|Dilek|2003}} In the area of the Southeast Pacific, there have been several rollback events resulting in the formation of numerous back-arc basins.{{sfn|Schellart|Lister|Toy|2006}}

===Mantle interactions===

Interactions with the [[mantle (geology)|mantle]] discontinuities play a significant role in slab rollback. Stagnation at the 660-km discontinuity causes retrograde slab motion due to the suction forces acting at the surface.{{sfn|Nakakuki|Mura|2013}} Slab rollback induces mantle return flow, which causes extension from the [[shear stress]]es at the base of the overriding plate. As slab rollback velocities increase, circular mantle flow velocities also increase, accelerating extension rates.{{sfn|Schellart|Moresi|2013}} Extension rates are altered when the slab interacts with the discontinuities within the mantle at 410&nbsp;km and 660&nbsp;km depth. Slabs can either penetrate directly into the [[lower mantle (Earth)|lower mantle]], or can be retarded due to the phase transition at 660&nbsp;km depth creating a difference in buoyancy. An increase in retrograde trench migration (slab rollback) (2–4&nbsp;cm/yr) is a result of flattened slabs at the 660-km discontinuity where the slab does not penetrate into the lower mantle.{{sfn|Christensen|1996}} This is the case for the Japan, Java and Izu–Bonin trenches. These flattened slabs are only temporarily arrested in the transition zone. The subsequent displacement into the lower mantle is caused by slab pull forces, or the destabilization of the slab from warming and broadening due to thermal diffusion. Slabs that penetrate directly into the lower mantle result in slower slab rollback rates (~1–3&nbsp;cm/yr) such as the Mariana arc, Tonga arcs.{{sfn|Christensen|1996}}

[[Image:Atlantic-trench.JPG|thumb|upright=1.4|The [[Puerto Rico Trench]]]]