Australia: The Land Where Time Began

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Subduction Zones - Variation of Characteristics

A number of characteristics of subduction zones are affected by the age and convergence rate of oceanic lithosphere that is being subducted. These include the thermal structure of the descending plate, seismic zone length, as well as a number of other subduction zone characteristics. The dip of the Benioff Zone is often of about 45o, but this can vary greatly in different places. Beneath the Marianas the dip is 90o, while beneath Peru it is 10o. It is believed the dip at a particular place is mostly the result of a combination of the subducting slab negative buoyancy, tending to cause sinking, which is opposed by the asthenosphere flow, induced by lithosphere that is underthrusting, tending to raise the slab. The greater the degree of underthrusting, the greater the uplift. Younger lithosphere is hotter and relatively thin, and therefore more buoyant, than older, colder oceanic lithosphere that is heavier. This leads to the prediction that the shallowest dips will result from young subducting lithosphere that is underthrusting at a high rate, as is seen in the case of Peru and Chile. It has been suggested that the absolute motion of the overriding plate probably also influences the dip of the Benioff zone (Cross & Pilger, 1982).

A stronger coupling with the overriding plate is present in subduction zones of shallow dip (Uyeda & Kanamori, 1979), which produces earthquakes of larger magnitude in the region where the subducting plate bends. In the mantle wedge above the subduction zone the flow of aesthenosphere is restricted, with the result in extreme cases of complete suppression of supra subduction zone magmatism (section 10.2.2, Source 1), in all cases resulting in backarc compression and not extension. Subduction zones of 2 types have been recognised, referred to as Chilean type , arc under compression, and Mariana type, arc under extension (Uyeda & Kanamori, 1979).

Subduction zones can be accretionary or erosive. Magmatic arcs and oceanic trenches have been considered to be situations where material, derived from continental and oceanic crust, is accreted to the overriding plate margin. This accretion is in the form of a sediment wedge in the region of the forearc, and in the magmatic arc, an igneous material edifice. It has since come to be believed that most of the pelagic sediments, as well as the oceanic crust, are actually subducted into the mantle. As well as this, it is now believed that in about half of the convergent margins at least some of the overriding plate is also eroded and subducted. Sediment subduction is the term used for the subduction of peagic sediments on subducting plates. Subduction erosion refers to the process of subduction of rock or sediment derived from the overriding plate. The material involved in subduction erosion can originate either from the base of the landward slope of the trench or the under side of the overriding plate. It has also been suggested that most of the material accreted in the magmatic arc is derived from the mantle, and not from subducted crust (Section 9.8, Source 1). As a result of this process, subduction zones have been characterised as accretionary or erosive (Figs. 9.1, 9.19, Source 1). The Nankai Trough and the Barbados Prism are examples of accretionary margins (Section 9.7, Source 1) (Saffer & Bekins, 2006). Erosive prisms are found off the coast of Costa Rica (Morris & Villinger, 2006) and Chile (Section 10.2.2.3, Source 1).

It is believed the thickness of sediment on a subducting oceanic plate needs to be greater than about 400-1000 m for sedimentary material to be added to the accretionary prism, implying the the proportion of pelagic sediment in the trench that is being subducted is about 80 %, trench turbidites derived from continental material comprise most of the sediment accreted in the forearc region (von Huene & Scholl, 1991). The supply of oceanic plate sediments, and clastic material derived from  a continent, to the trench is therefore believed to determine whether the subduction zone is accretionary or non-accretionary. The causes of subduction erosion is not well understood (Huene et al., 2004). At accretionary margins the thickness of sediments is typically grater than 1 km (Saffer & Bekins, 2006). Orthogonal convergence rates of <76 mm/year and forearcs bathymetric slopes of <3o are other parameters correlating with accretionary margins. At erosive margins subsidence characterises the forearc region, reflecting  upper plate thinning along its base, and the steeper slope of the forearc region is steeper. When drill cores are available from sedimentary sequences in the region, the amount of subsidence can be measured. This allows an estimation of the erosion rate at the forearc crust base (Clift & Vannucchi, 2004).

See Source 1 for more detailed information on all aspects of plate tectonics

Sources & Further reading

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

Links

  1. Japan's Giant Shock Rattles Ideas about Earthquake Behaviour
Author: M. H. Monroe
Email:  admin@austhrutime.com
Last updated 13/03/2011

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                                                                                           Author: M.H.Monroe  Email: admin@austhrutime.com     Sources & Further reading