Editing Oceanic trench (section)

== Morphology ==
[[File:Subduction Trench Schematic.jpg|thumb|upright=2.0|Cross section of an oceanic trench formed along an oceanic-oceanic [[convergent boundary]]]]
[[File:Peru-Chile trench.jpg|thumb|160px| The [[Peru–Chile Trench]] is located just left of the sharp line between the blue deep ocean (on the left) and the light blue continental shelf, along the west coast of South America. It runs along an oceanic-continental boundary, where the oceanic [[Nazca plate]] subducts beneath the continental [[South American plate]]]]

Oceanic trenches are {{convert|50 to 100|km|sigfig=1|sp=us}} wide and have an asymmetric V-shape, with the steeper slope (8 to 20 degrees) on the inner (overriding) side of the trench and the gentler slope (around 5 degrees) on the outer (subducting) side of the trench.{{sfn|Kearey|Klepeis|Vine|2009|pp=250–251}}{{sfn|Geersen|Voelker|Behrmann|2018|p=420}} The bottom of the trench marks the boundary between the subducting and overriding plates, known as the basal plate boundary shear{{sfn|Geersen|Voelker|Behrmann|2018|pp=411–412}} or the subduction [[décollement]].{{sfn|Stern|2005}} The depth of the trench depends on the starting depth of the oceanic lithosphere as it begins its plunge into the trench, the angle at which the slab plunges, and the amount of sedimentation in the trench. Both starting depth and subduction angle are greater for older oceanic lithosphere, which is reflected in the deep trenches of the western Pacific. Here the bottoms of the Marianas and the Tonga–Kermadec trenches are up to {{convert|10–11|km||sp=us}} below sea level. In the eastern Pacific, where the subducting oceanic lithosphere is much younger, the depth of the Peru-Chile trench is around {{convert|7 to 8|km||sp=us}}.{{sfn|Kearey|Klepeis|Vine|2009|pp=250–251}}

Though narrow, oceanic trenches are remarkably long and continuous, forming the largest linear depressions on earth. An individual trench can be thousands of kilometers long.{{sfn|Kearey|Klepeis|Vine|2009|p=250}} Most trenches are convex towards the subducting slab, which is attributed to the spherical geometry of the Earth.{{sfn|Kearey|Klepeis|Vine|2009|p=251}}

The trench asymmetry reflects the different physical mechanisms that determine the inner and outer slope angle. The outer slope angle of the trench is determined by the bending radius of the subducting slab, as determined by its elastic thickness. Since oceanic lithosphere thickens with age, the outer slope angle is ultimately determined by the age of the subducting slab.{{sfn|Bodine|Watts|1979}}{{sfn|Geersen|Voelker|Behrmann|2018|pp=411–412}} The inner slope angle is determined by the [[angle of repose]] of the overriding plate edge.{{sfn|Geersen|Voelker|Behrmann|2018|pp=411–412}} This reflects frequent earthquakes along the trench that prevent oversteepening of the inner slope.{{sfn|Stern|2005}}

As the subducting plate approaches the trench, it bends slightly upwards before beginning its plunge into the depths. As a result, the outer trench slope is bounded by an ''outer trench high''. This is subtle, often only tens of meters high, and is typically located a few tens of kilometers from the trench axis. On the outer slope itself, where the plate begins to bend downwards into the trench, the upper part of the subducting slab is broken by bending faults that give the outer trench slope a [[horst and graben]] topography. The formation of these bending faults is suppressed where oceanic ridges or large seamounts are subducting into the trench, but the bending faults cut right across smaller seamounts. Where the subducting slab is only thinly veneered with sediments, the outer slope will often show [[seafloor spreading]] ridges oblique to the horst and graben ridges.{{sfn|Geersen|Voelker|Behrmann|2018|pp=411–412}}

=== Sedimentation ===
Trench morphology is strongly modified by the amount of sedimentation in the trench. This varies from practically no sedimentation, as in the Tonga-Kermadec trench, to completely filled with sediments, as with the Cascadia subduction zone. Sedimentation is largely controlled by whether the trench is near a continental sediment source.{{sfn|Kearey|Klepeis|Vine|2009|p=251}} The range of sedimentation is well illustrated by the Chilean trench. The north Chile portion of the trench, which lies along the [[Atacama Desert]] with its very slow rate of weathering, is sediment-starved, with from 20 to a few hundred meters of sediments on the trench floor. The tectonic morphology of this trench segment is fully exposed on the ocean bottom. The central Chile segment of the trench is moderately sedimented, with sediments onlapping onto pelagic sediments or ocean basement of the subducting slab, but the trench morphology is still clearly discernible. The southern Chile segment of the trench is fully sedimented, to the point where the outer rise and slope are no longer discernible. Other fully sedimented trenches include the Makran Trough, where sediments are up to {{convert|7.5|km||sp=us}} thick; the Cascadia subduction zone, which is completed buried by {{convert|3 to 4|km||sp=us}} of sediments; and the northernmost Sumatra subduction zone, which is buried under {{convert|6|km||sp=us}} of sediments.{{sfn|Geersen|Voelker|Behrmann|2018|pp=412–416}}

Sediments are sometimes transported along the axis of an oceanic trench. The central Chile trench experiences transport of sediments from source fans along an axial channel.{{sfn|Völker|Geersen|Contreras-Reyes|Reichert|2013}} Similar transport of sediments has been documented in the Aleutian trench.{{sfn|Stern|2005}}

In addition to sedimentation from rivers draining into a trench, sedimentation also takes place from landslides on the tectonically steepened inner slope, often driven by [[megathrust earthquake]]s. The Reloca Slide of the central Chile trench is an example of this process.{{sfn|Völker|Weinrebe|Behrmann|Bialas|2009}}

=== Erosive versus accretionary margins ===
Convergent margins are classified as erosive or accretionary, and this has a strong influence on the morphology of the inner slope of the trench. Erosive margins, such as the northern Peru-Chile, Tonga-Kermadec, and Mariana trenches, correspond to sediment-starved trenches.{{sfn|Kearey|Klepeis|Vine|2009|p=250}} The subducting slab erodes material from the lower part of the overriding slab, reducing its volume. The edge of the slab experiences subsidence and steepening, with normal faulting. The slope is underlain by relative strong igneous and metamorphic rock, which maintains a high angle of repose.{{sfn|Geersen|Voelker|Behrmann|2018|p=416}} Over half of all convergent margins are erosive margins.{{sfn|Stern|2005}}

Accretionary margins, such as the southern Peru-Chile, Cascadia, and Aleutians, are associated with moderately to heavily sedimented trenches. As the slab subducts, sediments are "bulldozed" onto the edge of the overriding plate, producing an ''[[accretionary wedge]]'' or ''accretionary prism''. This builds the overriding plate outwards. Because the sediments lack strength, their angle of repose is gentler than the rock making up the inner slope of erosive margin trenches. The inner slope is underlain by [[Imbrication (sedimentology)|imbricated]] [[thrust sheets]] of sediments. The inner slope topography is roughened by localized [[mass wasting]].{{sfn|Geersen|Voelker|Behrmann|2018|p=416}} Cascadia has practically no bathymetric expression of the outer rise and trench, due to complete sediment filling, but the inner trench slope is complex, with many thrust ridges. These compete with canyon formation by rivers draining into the trench.  Inner trench slopes of erosive margins rarely show thrust ridges.{{sfn|Geersen|Voelker|Behrmann|2018|p=420}}

Accretionary prisms grow in two ways. The first is by frontal accretion, in which sediments are scraped off the downgoing plate and emplaced at the front of the accretionary prism.{{sfn|Stern|2005}} As the accretionary wedge grows, older sediments further from the trench become increasingly [[lithification|lithified]], and faults and other structural features are steepened by rotation towards the trench.{{sfn|Kearey|Klepeis|Vine|2009|pp=264–266}} The other mechanism for accretionary prism growth is underplating<!-- not the same as magmatic underplating!-->{{sfn|Stern|2005}} (also known as basal accretion{{sfn|Bangs|Morgan|Tréhu|Contreras-Reyes|2020}}) of subducted sediments, together with some [[oceanic crust]], along the shallow parts of the subduction decollement. The [[Franciscan Group]] of [[California]] is interpreted as an ancient accretionary prism in which underplating is recorded as tectonic mélanges and duplex structures.{{sfn|Stern|2005}}

[[File:Oceanic-oceanic convergence Fig21oceanocean.gif|thumb|upright=1.1|Oceanic trench formed along an oceanic-oceanic [[convergent boundary]]]]
[[File:Cross section of mariana trench.svg|right|upright=1.7|thumb| The [[Mariana Trench]] contains the deepest part of the world's oceans, and runs along an oceanic-oceanic convergent boundary. It is the result of the oceanic [[Pacific plate]] [[subduction|subducting]] beneath the oceanic [[Mariana plate]].]]

=== Earthquakes ===
Frequent [[megathrust earthquake]]s modify the inner slope of the trench by triggering massive landslides. These leave semicircular landslide scarps with slopes of up to 20 degrees on the headwalls and sidewalls.{{sfn|Völker |Geersen |Contreras-Reyes |Sellanes |2014}}

Subduction of seamounts and [[aseismic ridges]] into the trench may increase [[aseismic creep]] and reduce the severity of earthquakes. Contrariwise, subduction of large amounts of sediments may allow ruptures along the subduction décollement to propagate for great distances to produce megathrust earthquakes.{{sfn|Geersen|Voelker|Behrmann|2018|p=421}}