Oceanic trench

Much of the fluid trapped in sediments of the subducting slab returns to the surface at the oceanic trench, producing mud volcanoes and cold seeps.

Troughs are elongated depressions of the sea floor with steep sides and flat bottoms, while trenches are characterized by a V-shaped profile.

[8] Also not a trench is the New Caledonia trough, which is an extensional sedimentary basin related to the Tonga-Kermadec subduction zone.

[9] Additionally, the Cayman Trough, which is a pull-apart basin within a transform fault zone,[10] is not an oceanic trench.

The laying of transatlantic telegraph cables on the seafloor between the continents during the late 19th and early 20th centuries provided further motivation for improved bathymetry.

[15] The term trench, in its modern sense of a prominent elongated depression of the sea bottom, was first used by Johnstone in his 1923 textbook An Introduction to Oceanography.

[11] He proposed the tectogene hypothesis to explain the belts of negative gravity anomalies that were found near island arcs.

According to this hypothesis, the belts were zones of downwelling of light crustal rock arising from subcrustal convection currents.

The early phase of trench exploration reached its peak with the 1960 descent of the Bathyscaphe Trieste to the bottom of the Challenger Deep.

Both starting depth and subduction angle are greater for older oceanic lithosphere, which is reflected in the deep trenches of the western Pacific.

In the eastern Pacific, where the subducting oceanic lithosphere is much younger, the depth of the Peru-Chile trench is around 7 to 8 kilometers (4.3 to 5.0 mi).

[18] Though narrow, oceanic trenches are remarkably long and continuous, forming the largest linear depressions on earth.

[21] The trench asymmetry reflects the different physical mechanisms that determine the inner and outer slope angle.

[2] As the subducting plate approaches the trench, it bends slightly upwards before beginning its plunge into the depths.

The southern Chile segment of the trench is fully sedimented, to the point where the outer rise and slope are no longer discernible.

The central Chile trench experiences transport of sediments from source fans along an axial channel.

[25] Convergent margins are classified as erosive or accretionary, and this has a strong influence on the morphology of the inner slope of the trench.

The slope is underlain by relative strong igneous and metamorphic rock, which maintains a high angle of repose.

[2] Accretionary margins, such as the southern Peru-Chile, Cascadia, and Aleutians, are associated with moderately to heavily sedimented trenches.

Because the sediments lack strength, their angle of repose is gentler than the rock making up the inner slope of erosive margin trenches.

The first is by frontal accretion, in which sediments are scraped off the downgoing plate and emplaced at the front of the accretionary prism.

The Franciscan Group of California is interpreted as an ancient accretionary prism in which underplating is recorded as tectonic mélanges and duplex structures.

[35] 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.

The subducting slab undergoes backward sinking due to the negative buoyancy forces causing a retrogradation of the trench hinge along the surface.

[36] In the area of the Southeast Pacific, there have been several rollback events resulting in the formation of numerous back-arc basins.

Stagnation at the 660-km discontinuity causes retrograde slab motion due to the suction forces acting at the surface.

[35] Slab rollback induces mantle return flow, which causes extension from the shear stresses at the base of the overriding plate.

Slabs can either penetrate directly into the lower mantle, or can be retarded due to the phase transition at 660 km depth creating a difference in buoyancy.

Methane clathrates and gas hydrates also accumulate in the inner slope, and there is concern that their breakdown could contribute to global warming.

[2] The fluids released at mud volcanoes and cold seeps are rich in methane and hydrogen sulfide, providing chemical energy for chemotrophic microorganisms that form the base of a unique trench biome.