Variations In Subduction Zone Characteristics

The age and convergence rate of the subducting oceanic lithosphere affect not only the thermal structure of the downgoing slab, and the length of the seismic zone, but a number of other characteristics of subduction zones. It can be seen from Fig. 9.15 that, although the dip of the Benioff zone is often approximately 45°, as typically illustrated, there is a great variation in dips, from 90° beneath the Marianas to 10° beneath Peru. It appears that the dip is largely determined by a combination of the negative buoyancy of the subducting slab, causing it to sink, and the forces exerted on it by flow in the asthenosphere, induced by the underthrusting lithosphere, which tend to uplift the slab. A higher rate of underthrusting produces a greater degree of uplift. Young oceanic lithosphere is relatively thin and hot; consequently it is more buoyant than older oceanic lithosphere. One would predict, therefore, that young subducting lithosphere, underthrusting at a high rate, will give rise to the shallowest dips, as in the case of Peru and Chile. It seems probable that the absolute motion of the overriding plate is also a contributing factor in determining the dip of the Benioff zone (Cross & Pilger, 1982).

Subduction zones with shallow dips have a stronger coupling with the overriding plate (Uyeda & Kanamori, 1979), giving rise to larger magnitude earthquakes in region "b" of Fig. 9.8. Shallow dips also restrict the flow of asthenosphere in the mantle wedge above the subduction zone, in extreme cases suppressing all supra subduction zone magmatism (Section 10.2.2), and in all cases giving rise to backarc compression rather than extension. Thus, Uyeda & Kanamori (1979) recognized two end-member types of subduction zone, which they referred to as Chilean and Mariana types (Fig. 9.18).

compression

Shallow trench

compression

Shallow trench

Earthquake Fossil
(Arc under compression)
Characteristics Earthquakes

Figure 9.18 End-member types of subduction zone based on the age of the underthrusting lithosphere and the absolute motion of the overriding plate (modified from Uyeda & Kanamori, 1979, and Stern, 2002, by permission of the American Geophysical Union. Copyright © 1979 and 2002 American Geophysical Union).

Figure 9.18 End-member types of subduction zone based on the age of the underthrusting lithosphere and the absolute motion of the overriding plate (modified from Uyeda & Kanamori, 1979, and Stern, 2002, by permission of the American Geophysical Union. Copyright © 1979 and 2002 American Geophysical Union).

Another first order variation in the nature of subduction zones is whether they are accretionary or erosive. Historically, oceanic trenches and magmatic arcs were considered to be the settings where material derived from the continental and oceanic crust is accreted to the margin of the overriding plate in the form of a wedge of sediments in the forearc region, and an edifice of igneous material in the magmatic arc. Increasingly however it has been realized that most of the oceanic crust and pelagic sediments is subducted into the mantle, and that, in approximately half of the convergent margins, some of the overriding plate is eroded and subducted. The process by which pelagic sediments on the downgoing plate are subducted is known as sediment subduction and the process whereby rock or sediment from the upper plate is subducted is termed subduction erosion. The latter may be derived from the base of the landward slope of the trench or from the underside of the upper plate. Moreover, the majority of the material accreted in the magmatic arc is thought to be derived from the mantle rather than subducted crust (Section 9.8). Thus, subduction zones have also been characterized as accretionary or erosive (Figs 9.1, 9.19). Examples of accretionary margins include the Nankai Trough and Barbados prisms (Section 9.7) (Saffer & Bekins, 2006); erosive prisms occur offshore of Costa Rica (Morris & Villinger, 2006) and Chile (Section 10.2.3).

On the basis of seismic reflection profiling data, it appears that the thickness of sediment on the oceanic plate entering a trench must exceed 400-1000 m for sediment to be scraped off and added to the accretion-ary prism. This implies that perhaps 80% of the pelagic sediments entering trenches is subducted, and that most of the sediment accreted in the forearc region is trench turbidites derived from continental material (von Huene & Scholl, 1991). The accretionary or nonaccretionary nature of a subduction zone will depend in part, therefore, on the supply of oceanic plate sediments and continentally derived clastic material to the trench. However, the causes of subduction erosion are very poorly understood (von Huene et al., 2004). Typically the thickness of trench sediments at accretionary margins exceeds 1 km (Saffer & Bekins, 2006). Other parameters that correlate with accreting margins are: orthogonal convergence rates of less than 76 mm a-1 and forearc bathymetric slopes of less than 3°. In addition to the steeper slope of the forearc region at erosive margins, the forearc is characterized by subsidence, which reflects the thinning of the upper plate along its base. The amount of subsidence can be measured if drill cores are available from sedimentary sequences in this region. It is then possible to estimate the rate of erosion at the base of the forearc crust (Clift & Vannucchi, 2004).

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