Australia: The Land Where Time Began

A biography of the Australian continent 

Ocean trenches develop on the oceanward side of the island arcs and orogens of Andean type, forming above subduction zones, direct manifestation of oceanic lithosphere that is underthrusting. They are very deep, long features, that are the largest linear depressed features on the surface of the Earth. The Peru-Chile Trench, 4500 km long, reaching depths of 2-4 km below the surface of the ocean floor, and 7-8 km below the surface of the ocean. The deepest trenches in the Pacific are typically on the western margin of that ocean, in the Mariana Trench and the Tonga-Kermadec Trench reaching depths of 10-11 km. The age of the subducting lithosphere is believed to to be the main controlling factor in any particular trench, determining the depth reached by the trench (see Source 1)..It is the age of the oceanic crust that determines the thickness of that crust entering the trench. The systematic difference in age of the oceanic floor of the east and wast Pacific results in the trenches in the western Pacific being much deeper than those in the east.

Trenches are generally of an asymmetric V-shape in section form, and the steeper slope of from 8-20^o is on the side opposite the ocean floor that is underthrusting. There is a wide range of sediment fill depth of trenches, depending on the distance from eroding continental surfaces and the rate at which that erosion is taking place. In the Tonga-Kermadec Trench there is very little fill, it is a great distance from the nearest large continental landmass, while the Lesser Antilles Trench and the Alaskan Trench, with a large sediment supply from adjacent landmasses, the trenches are nearly full of sediment. When features such as seismic ridges arrive at a trench, the depth of the trench is reduced as they are being subducted (see Source 1).

Sources & Further reading

1. Kearey, Philip, Klepeis, Keith A. & Vine, Frederick J., 2009, Global Tectonics, 3rd Edition, Wiley-Blackwell.