Ocean Trenches

Oceanic trenches are the direct manifestation of under-thrusting oceanic lithosphere, and are developed on the oceanward side of both the island arcs and Andean-type orogens that form above subduction zones (Fig. 9.1). They represent the largest linear depressed features of the Earth's surface, and are remarkable for their depth and continuity. The Peru-Chile Trench is 4500 km long and reaches depths of 2-4 km below the surrounding ocean floor so that its base is 7-8 km below sea level. The trenches in the western Pacific are typically deeper than those of the eastern Pacific margin, the greatest trench depths, of 10-11 km, occurring in the Mariana and Tonga-Kermadec trenches. The main control on the maximum depth of a particular trench would appear to be the age of the oceanic lithosphere being subducted, as this determines the depth to the oceanic crust entering the trench (Section 6.4). The striking contrast between trench depths in the east and west Pacific is largely explained therefore by the systematic difference in the age of the ocean floor in these areas (Plate 4.1 between pp. 244 and 245). Trenches are generally 50100 km in width and in section form an asymmetric

Figure 9.1 The location of convergent plate margins (thin solid lines with barbs). Accretionary margins are indicated by solid barbs, and erosive margins by open barbs (Sections 9.6,9.7). The thick solid lines are active spreading centers and include those in backarc basins (Section 9.10) (modified from Stern, 2002, and from Clift & Vanucchi, 2004, by permission of the American Geophysical Union. Copyright © 2002 and 2004 American Geophysical Union).

Figure 9.1 The location of convergent plate margins (thin solid lines with barbs). Accretionary margins are indicated by solid barbs, and erosive margins by open barbs (Sections 9.6,9.7). The thick solid lines are active spreading centers and include those in backarc basins (Section 9.10) (modified from Stern, 2002, and from Clift & Vanucchi, 2004, by permission of the American Geophysical Union. Copyright © 2002 and 2004 American Geophysical Union).

V-shape with the steepest slope, of 8-20°, on the side opposite the underthrusting ocean floor. The sediment fill of trenches can vary greatly, from virtually nothing, as in the Tonga-Kermadec trench, to almost complete, as in the Lesser Antilles and Alaskan trenches because of the supply of sediment from adjacent continental areas. Trench depth is also reduced by the subduction of aseismic ridges (Section 10.2.2).

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