Trenches define one of the most important natural boundaries on the Earth’s solid surface, that between two lithospheric plates. There are three types of lithospheric plate boundaries: divergent (where lithosphere and oceanic crust is created at mid-ocean ridges), convergent (where one lithospheric plate sinks beneath another and returns to the mantle), and transform (where two lithospheric plates slide past each other). Trenches are the spectacular and distinctive morphological features of plate boundaries. Plates move together along convergent plate boundaries at convergence rates that vary from a few millimeters to ten or more centimeters per year. A trench marks the position at which the flexed, subducting slab begins to descend beneath another lithospheric slab. Trenches are generally parallel to a volcanic island arc, and about 200 km from a volcanic arc. Oceanic trenches typically extend 3 to 4 km (1.9 to 2.5 mi) below the level of the surrounding oceanic floor. The deepest ocean depth to be sounded is in the Challenger Deep of the Mariana Trench at a depth of 10,911 m (35,798 ft) below sea level. Oceanic lithosphere disappears into trenches at a global rate of about a tenth of a square meter per second.
There are about 50,000 km of convergent plate margins, mostly around the Pacific Ocean – the reason for the reference “Pacific-type” margin - but they are also in the eastern Indian Ocean, with relatively short convergent margin segments in the Atlantic Ocean and in the Mediterranean Sea. Trenches are sometimes buried and lack bathymetric expression, but the fundamental structures that these represent mean that the great name should also be applied here. This applies to Cascadia, Makran, southern Lesser Antilles, and Calabrian trenches. Trenches along with volcanic arcs and zones of earthquakes that dip under the volcanic arc as deeply as 700 km are diagnostic of convergent plate boundaries and their deeper manifestations, subduction zones. Trenches are related to but distinguished from continental collision zones (like that between India and Asia to form the Himalaya), where continental crust enters the subduction zone. When buoyant continental crust enters a trench, subduction eventually stops and the convergent plate margin becomes a collision zone. Features analogous to trenches are associated with collisions zones; these are sediment-filled foredeeps referred to as peripheral foreland basins, such as that which the Ganges River and Tigris-Euphrates rivers flow along.
During the 1920’s and 1930’s, Felix Andries Vening Meinesz developed a unique gravimeter that could measure gravity in the stable environment of a submarine and used it to measure gravity over trenches. His measurements revealed that trenches are sites of downwelling in the solid Earth. The concept of downwelling at trenches was characterized by Griggs in 1939 as the tectogene hypothesis, for which he developed an analogue model using a pair of rotating drums. World War II in the Pacific led to great improvements of bathymetry in especially the western and northern Pacific, and the linear nature of these deeps became clear. The rapid growth of deep sea research efforts, especially the widespread use of echosounders in the 1950’s and 1960’s confirmed the morphological utility of the term. The important trenches were identified, sampled, and their greatest depths sonically plumbed. The heroic phase of trench exploration culminated in the 1960 descent of the Bathyscaphe "Trieste", which set an unbeatable world record by diving to the bottom of the Challenger Deep. Following Robert S. Dietz’ and Harry Hess’ articulation of the seafloor spreading hypothesis in the early 1960’s and the plate tectonic revolution in the late 1960’s the term ‘trench’ has been redefined with plate tectonic as well as bathymetric connotations!
Trenches are centerpieces of the distinctive physiography of a convergent plate margin. Transects across trenches yield asymmetric profiles, with relatively gentle (~5°) outer (seaward) slope and a steeper (~10-16°) inner (landward) slope. This asymmetry is due to the fact that the outer slope is defined by the top of the downgoing plate, which must bend as it starts its descent. The great thickness of the lithosphere requires that this bending be gentle. As the subducting plate approaches the trench, it is first bent upwards to form the outer trench swell, then descends to form the outer trench slope. The outer trench slope is disrupted by a set of subparallel normal faults which staircase the seafloor down to the trench. The plate boundary is defined by the trench axis itself. Beneath the inner trench wall, the two plates slide past each other along the subduction decollement, the seafloor intersection of which defines the trench location. The overriding plate contains volcanic arc (generally) and a forearc. The volcanic arc is caused by physical and chemical interactions between the subducted plate at depth and asthenospheric mantle associated with the overriding plate. The forearc lies between the trench and the volcanic arc. Forearcs have the lowest heatflow from the interior Earth because there is no asthenosphere (convecting mantle) between the forearc lithosphere and the cold subducting plate.
The inner trench wall marks the edge of the overriding plate and the outermost forearc. The forearc consists of igneous and metamorphic crust, and this crust acts as buttress to a growing accretionary prism (sediments scraped off the downgoing plate onto the inner trench wall, depending on how much sediment is supplied to the trench). If the flux of sediments is high, material will be transferred from the subducting plate to the overriding plate. In this case an accretionary prism grows and the location of the trench migrates progressively away from the volcanic arc over the life of the convergent margin. Convergent margins with growing accretionary prisms are called accretionary convergent margins and make up nearly half of all convergent margins. If the sediment flux is low, material will be transferred from the overriding plate to the subducting plate by a process of tectonic ablation known as subduction erosion and carried down the subduction zone. Forearcs undergoing subduction erosion typically expose igneous rocks. In this case, the location of the trench will migrate towards the magmatic arc over the life of the convergent margin. Convergent margins experiencing subduction erosion are called nonaccretionary convergent margins and comprise more than half of convergent plate boundaries. This is an oversimplification, because different parts of a convergent margin can experience sediment accretion and subduction erosion over its life.
The asymmetric profile across a trench reflects fundamental differences in materials and tectonic evolution. The outer trench wall and outer swell comprise seafloor that takes a few million years to move from where subduction-related deformation begins near the outer trench swell until sinking beneath the trench. In contrast, the inner trench wall is deformed by plate interactions for the entire life of the convergent margin. The forearc is continuously subjected to subduction-related earthquakes. This protracted deformation and shaking ensures that the inner trench slope is controlled by the angle of repose of whatever material it is composed of. Because they are composed of igneous rocks instead of deformed sediments, non-accretionary trenches have steeper inner walls than accretionary trenches.
There an evolution in trench morphology can be expected as oceans close and continents converge. While the ocean is wide, the trench may be far away from continental sources of sediment and so may be deep. As the continents approach each other, the trench may become filled with continental sediments and become shallower. A simple way to approximate when the transition from subduction to collision has occurred is when the plate boundary previously marked by a trench is filled enough to rise above sealevel.
The slope of the inner trench slope of an accretionary convergent margin reflects continuous adjustments to the thickness and width of the accretionary prism. The prism maintains a ‘critical taper’, established in conformance with Mohr-Coulomb Theory for the pertinent materials. A package of sediments scraped off the downgoing lithospheric plate will deform until it and the accretionary prism that it has been added to attain a critical taper (constant slope) geometry. Once critical taper is attained, the wedge slides stably along its basal decollement. Strain rate and hydrologic properties strongly influence the strength of the accretionary prism and thus the angle of critical taper. Fluid pore pressures modify rock strength and are important controls of critical taper angle. Low permeability and rapid convergence may result in pore pressures that exceed lithostatic pressure and a relatively weak accretionary prism with a shallowly tapered geometry, whereas high permeability and slow convergence result in lower pore pressure, stronger prisms, and steeper geometry.
The Hellenic trench system is unusual because this convergent margin subducts evaporites. The slope of the surface of the southern flank of the Mediterranean Ridge (its accretionary prism) is low, about 1°, which indicates very low shear stress on the decollement at the base of the wedge. Evaporites influence the critical taper of the accretionary complex, as their mechanical properties differ from those of siliciclastic sediments, and because of their effect upon fluid flow and fluid pressure, which control effective stress. In the 1970s, the linear deeps of the Hellenic trench south of Crete were interpreted to be similar to trenches at other subduction zones, but with the realization that the Mediterranean Ridge is an accretionary complex, it became apparent that the Hellenic trench is actually a starved forearc basin, and that the plate boundary lies south of the Mediterranean Ridge.
Chemosynthetic communities thrive where cold fluids seep out of the forearc. Cold seep communities have been discovered in inner trench slopes down to depths of 6000 m in the western Pacific, especially around Japan, in the Eastern Pacific along North, Central and South America coasts from the Aleutian to the Peru-Chile trenches, on the Barbados prism, in the Mediterranean, and in the Indian Ocean along the Makran and Sunda convergent margins. These communities receive much less attention than the chemosynthetic communities associated with hydrothermal vents. Chemosynthetic communities are located in a variety of geological settings: above over-pressured sediments in accretionary prisms where fluids are expelled through mud volcanoes or ridges (Barbados, Nankai and Cascadia); along active erosive margins with faults; and along escarpments caused by debris slides (Japan trench, Peruvian margin). Surface seeps may be linked to massive hydrate deposits and destabilization (e.g. Cascadia margin). High concentrations of methane and sulfide in the fluids escaping from the seafloor are the principal energy sources for chemosynthesis.
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.
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 will rise diapirically when there is an opportunity to do so. 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.
There are several factors that control the depth of trenches. The most important control is the supply of sediment, which fills the trench so that there is no bathymetric expression. It is therefore not surprising that the deepest trenches (deeper than 8,000 m) are all nonaccretionary. In contrast, all trenches with growing accretionary prisms are shallower than 8000 m. A second order control on trench depth is the age of the lithosphere at the time of subduction. Because oceanic lithosphere cools and thickens as it ages, it subsides. The older the seafloor, the deeper it lies and this determines a minimum depth from which seafloor begins its descent. This obvious correlation can be removed by looking at the relative depth, the difference between regional seafloor depth and maximum trench depth. Relative depth may be controlled by the age of the lithosphere at the trench, the convergence rate, and the dip of the subducted slab at intermediate depths. Finally, narrow slabs can sink and roll back more rapidly than broad plates, because it is easier for underlying asthenosphere to flow around the edges of the sinking plate. Such slabs may have steep dips at relatively shallow depths and so may be associated with unusually deep trenches, such as the Challenger Deep.
|Mariana Trench||Pacific Ocean||10,911m (32,733)|
|Tonga Trench||Pacific Ocean||10,882m (32,646)|
|Kuril Trench||Pacific Ocean||10,542m (31,626)|
|Philippine Trench||Pacific Ocean||10,540m|
|Kermadec Trench||Pacific Ocean||10,047m|
|Izu-Bonin Trench (Izu-Ogasawara Trench)||Pacific Ocean||9,780 m|
|Japan Trench||Pacific Ocean||9,000m|
|Puerto Rico Trench||Atlantic Ocean||8,605 m|
|Peru-Chile Trench or Atacama Trench||Pacific Ocean||8,065 m|
|Aleutian Trench||West of Alaska|
|Bougainville Trench||South of New Guinea|
|Cayman Trench||Western Caribbean Sea|
|Cedros Trench (inactive)||Pacific coast of Baja California|
|Hikurangi Trench||East of New Zealand|
|Japan Trench||Northeast Japan|
|Kuril-Kamchatka Trench||Near Kuril islands|
|Mariana Trench (deepest known part of the oceans)||Western Pacific ocean; east of Mariana Islands|
|Middle America Trench|
|New Hebrides Trench||West of New Caledonia|
|Puerto Rico Trench (deepest known part of the Atlantic Ocean)||Boundary of Caribbean Sea and Atlantic ocean|
|Peru-Chile Trench||Eastern Pacific ocean; off coast of Peru & Chile|
|Philippine Trench||East of Philippine Islands|
|Ryukyu Trench||Eastern edge of Japan's Ryukyu Islands|
|South Sandwich Trench|
|Sunda Arc and Java Trench|
|Tonga Trench||North-east of Australia|
|Yap Trench||Western Pacific ocean; between Palau Islands and Mariana Trench|
|Intermontane Trench||Western North America; between Intermontane Islands and North America|
|Insular Trench||Western North America; between Insular Islands and Intermontane Islands|
|Farallon Trench||Western North America|
|Tethyan Trench||South of Turkey, Iran, Tibet and Southeast Asia|