<P> Igneous basement of a nonaccretionary forearc may be continuously exposed by subduction erosion . This transfers material from the forearc to the subducting plate and can be accomplished by frontal erosion or basal erosion . Frontal erosion is most active in the wake of seamounts being subducted beneath the forearc . Subduction of large edifices (seamount tunneling) oversteepens the forearc, causing mass failures that carry debris towards and ultimately into the trench . This debris may be deposited in graben of the downgoing plate and subducted with it . In contrast, structures resulting from subduction erosion of the base of the forearc are difficult to recognize from seismic reflection profiles, so the possibility of basal erosion is difficult to confirm . Subduction erosion may also diminish a once - robust accretionary prism if the flux of sediments to the trench diminishes . </P> <P> Nonaccretionary forearcs may also be the site of serpentine mud volcanoes . These form where fluids released from the downgoing plate percolate upwards and interact with cold mantle lithosphere of the forearc . Mantle peridotite is hydrated into serpentinite, which is much less dense than peridotite and so rises diapirically when it can . Some nonaccretionary forearcs are subjected to strong extensional stresses, for example the Marianas, and this allows buoyant serpentinite to rise to the seafloor where they form serpentinite mud volcanoes . Chemosynthetic communities are also found on non-accretionary margins such as the Marianas, where they thrive on vents associated with serpentinite mud volcanoes . </P> <P> 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 . This is called trench rollback or hinge retreat (also hinge rollback) and is one explanation for the existence of back - arc basins . </P> <P> Slab rollback occurs during the subduction of two tectonic plates, and results in seaward motion of the trench . Forces perpendicular to the slab at depth (the portion of the subducting plate within the mantle) are responsible for steepening of the slab in the mantle and ultimately the movement of the hinge and trench at the surface . The driving force for rollback is the negative buoyancy of the slab with respect to the underlying mantle modified by the geometry of the slab itself . Back - arc basins are often associated with slab rollback due to extension in the overriding plate as a response to the subsequent subhorizontal mantle flow from the displacement of the slab at depth . </P>

Where are the majority of ocean trenches located