<P> The Lesser Antilles convergent margin demonstrates the importance of proximity to sediment sources for trench morphology . In the south, near the mouth of the Orinoco River, there is no morphological trench and the forearc (including the accretionary prism) is almost 500 km (310 mi) wide . The large accretionary prism reaches above sea level to form the islands of Barbados and Trinidad . Northward, the forearc narrows, the accretionary prism disappears, and, north of ~ 17 ° N, the morphology of a trench dominates . Further north, far from major sediment sources, the Puerto Rico Trench is over 8,600 m (28,200 ft) deep and there is no active accretionary prism . </P> <P> A similar relationship between proximity to rivers, forearc width, and trench morphology can be observed from east to west along the Alaskan - Aleutian convergent margin . The convergent plate boundary offshore Alaska changes along its strike from a filled trench with broad forearc in the east (near the coastal rivers of Alaska) to a deep trench with narrow forearc in the west (offshore the Aleutian islands). Another example is the Makran convergent margin offshore Pakistan and Iran, which is a trench filled by sediments from the Tigris - Euphrates and Indus rivers . Thick accumulations of turbidites along a trench can be supplied by down - axis transport of sediments that enter the trench 1,000--2,000 km (620--1,240 mi) away, as is found for the Peru--Chile Trench south of Valparaíso and for the Aleutian Trench . </P> <P> Convergence rate can also be important for controlling trench depth--especially for trenches near continents--because slow convergence makes the convergent margin capacity insufficient to dispose of sediment . An evolution in trench morphology can be expected, as oceans close and continents converge . While the ocean is wide, the trench may be far from continental sources of sediment and so may be deep . As continents approach each other, the trench can fill 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 sea level . </P> <P> Accretionary prisms grow in two ways: by frontal accretion, whereby sediments are scraped off the downgoing plate, bulldozer - fashion, near the trench, and by underplating of subducted sediments (and sometimes oceanic crust) along the shallow parts of the subduction decollement . Frontal accretion over the life of a convergent margin results in younger sediments defining the outermost part of the accretionary prism and the oldest sediments defining the innermost portion . Older (inner) parts of the accretionary prism are more lithified and have steeper structures than the younger (outer) parts . Underplating is difficult to detect in modern subduction zones but may be recorded in ancient accretionary prisms such as the Franciscan Group of California in the form of tectonic mélanges and duplex structures . </P>

Why are trenches located at the edges of oceans