Australia: The Land Where Time Began

A biography of the Australian continent 

Subduction Zones

Places where the lithospheric plates are colliding and one is diving under the other. Associated with these places are deep oceanic trenches, volcanoes and deep earthquakes caused by the friction generated as the plunging plate grinds against the underside of the plate is it  plunging beneath. When a continent is on the leading edge of a plate on one or both sides of a subduction zone volcanism and mountain building occurs. Examples are the Andes Mountains and the Himalayas. The rise of the Andes Mountains of South America results from oceanic lithosphere being subducted beneath the continental margin along the west coast of South America. The Himalayas are rising as a result of India slamming into the southern part of the Asian continent as the lithospheric plate that carried it from its connection with Gondwana north to Asia where it was being subducted.

Subduction Zones - Stern3

The author3 describes subduction zones as the exterior expression of the Earth's convergent tectonic plate margins, totalling 55,000 km and the geodynamic system that builds island arcs. According to the author3 the power required to move the tectonic plates and induce convection in the overlying mantle wedge is provided by the excess density of mantle lithosphere in subduction zones. As the slab sinks it sucks asthenospheric mantle to the trench the interaction between the slab and water, as well as with incompatible elements that rise from the plunging plate, with the result that the mantle melts, the melts rising vertically, passing through the downwelling mantle and erupting at arc volcanoes. According to the author3 "Subduction zones are thus interior Earth systems of unparalleled scale and complexity." Igneous activity at these zones was responsible for the formation of most ore deposits and continental crust, and they are the sites of earthquakes caused by the descent of the subducting plate, a process that the author3 suggests is a growing hazard to society. According to the author3 this overview summarizes the state of understanding of subduction zones that includes perspectives of the incoming plate, the descending plate, mantle wedge and arc-trench complex at the time of publishing, 31 December 2002.

Subduction zones are the dominant physical and chemical system of the interior of the Earth. They are the descending limb of mantle convection cells. Most of the force required to drive the movement of tectonic plates is provided by the subduction of oceanic lithosphere at subduction zones, and the cause of spreading at mid-ocean ridges, therefore subduction zones and plate tectonics are the dominant tectonic mode of the Earth, both surficial and interior. The largest recycling systems of the Earth are subduction zones, "delivering raw materials to the subduction factory, where oceanic lithosphere, sediments and seawater reequilibrate with ambient mantle, triggering melting and incidentally continental crust". Of the subducted material any that is not recycled in the uppermost 300 km of the subduction zone continues to the boundary of the core and mantle, where reheating could continue for about 1 billion years after which it makes it way back towards the surface as a mantle plume (Hoffmann, 1997). From subduction of normal oceanic lithosphere, to collision with continental crust or oceanic lithosphere with thickened crust, there is a continuum. The smooth operation of a subduction zone is disrupted to a spectacular degree if continental crust is dragged down in the subduction process, the result may be the uplift of a mountain range on the scale of the Himalayas (O'Brien, 2001). In some cases continental crust has reached depths of 100 km or more before it resurfaces (Ernst, 1999).

In our solar system Earth appears to be the only planet with plate tectonics and subduction zones. Mercury and the Moon are dead, tectonically and magmatically, and on Venus there is a thick crust and mantle plumes (Phillips & Hansen, 1998). On Mars, in the ancient southern highlands there are linear magnetic anomalies (Connerney et al., 1999) and rocks similar to andesite (Rieder et al., 1997), suggesting that plate tectonics and subduction zones may have operated in the distant past, though they are no longer present. According to the author3 Mars is now a planet with a single giant plate that is dominated by extremely large volcanoes in the Tharsis region, the largest "hot spot" volcanoes in the solar system.

Subduction zones profoundly impact our society, according to the author3, having a central role in plate tectonics, the generation of melt, and the  evolution of crust, causing great destruction by earthquakes and volcanic activity, but also provide great benefits in the form of mineral deposits as what the author3 calls "specialty" distillates of subduction zones. The author3 suggests that it can be speculated that the continents would not exist without the presence of subduction zones, the solid surface of the earth being completely flooded, the obvious consequence would be that terrestrial life could not have evolved. Though subduction zones descend deep into the mantle they are not completely camouflaged, being detectable by geophysics and geochemistry, the shallowest parts of them being known best. For the first several hundred kilometres they are strongly asymmetrical, their dimensions being defined by deep trenches, and parallel to these trenches lines of volcanoes, and planar arrays of deep earthquakes dipping away from the trench beneath the volcanoes, extending as far as the discontinuity at 660 Km. The earthquakes that occur at subduction zones occur at much greater depths than occur elsewhere on the Earth, these earthquakes that occur away from subduction zones being limited to the top 20 km of the mantle, requiring that either in subduction zones brittle conditions extend much deeper or that seismicity in subduction zones is caused by unusual conditions. Additional details are gained by the use of seismic tomography, the use of seismic velocity data gathered to form a 3D model of relative velocity from many paths taken by seismic waves as they travel through the interior of the Earth from sources such as earthquakes or explosions. Below regions in which the seismic waves travel anomalously slowly are regions of mantle through which seismic waves travel anomalously fast, corresponding to the plane of the earthquake, define the descending lithospheric slab. By the use of this method it is sometimes possible to trace the subducted lithosphere into the lower mantle beneath the discontinuity at 660 km.

Sometimes the term convergent (destructive) plate margin is used interchangeably with the term subduction zone, and the term Island arc is also sometimes used. These 3 terms are not synonymous, though they are intimately related. The 3D manifestation of convective downwelling is termed subduction zone. The surficial manifestation of downwelling is referred to by the term convergent plate margins. Surficial and crustal manifestations of a subduction zone, that operates beneath it, are termed arcs (arc-trench complexes). An historical review of the concept of subduction zone has been provided (White et al., 1970).

As plate margins are required by the processes of plate tectonics, the movement of rigid spherical shells around the Euler pole of rotation, subduction zones are formally distinct from convergent plate margins as once lithosphere has descended below the surface the geometric rules of plate tectonics no longer apply to the descending lithosphere. The "Wadati-Benioff Zone", the inclined array of earthquakes, define subduction zones. Arcs are the results of the processes of subduction zones that have been preserved in the surface crust, geological evidence of the deformation and chemical recycling that have resulted from the processes of subduction. Though not all arcs are really island arcs, the term fits the description of island arcs as it "captures the sense of a curved array of discrete volcanoes engendered by subduction".

Arcs are important, though in terms of mass they are relatively minor components of subduction zones, in the study of subduction zones as they allow the easy collection of samples of the rocks produced by the interaction of mantle and subducted materials and provide evidence of the operation of subduction zones in the distant past. The formation and further processing of continental crust, in the form of thickened welts of low density rocks, takes place in arcs, remaining there until they are accreted to other blocks of buoyant crust.

When the typical representations or arcs and subduction zones are considered together the relationship between them and convergent plate margins is appreciated better. It is usual to display convergent plate margins in map view and the display of subduction zones is in cross-section view. When these 2 perspectives are combined the result is a more realistic display that shows the nature of the subduction zone system to be globe-encircling and mantle-permeating. 3D representations of subduction zones have been displayed (Chiu et al., 1991).

At more the 55,000 km the total length of convergent plate margins (Lallemand, 1999), and the total length of the mid-ocean ridges is almost equal at 60,000 km (Kearey & Vine, 1990). Lithosphere that has been subducted at subduction zones may descend to varying depths, from ponding at the base of the transition zone at a depth of 660 km, or may continue descending to the core-mantle boundary at a depth of 2,900 km, the subduction zone system being, according to the author3, the most extensive, pervasive feature of the entire Earth. According to the author3 the consensus of opinion among geodynamicists is that sinking lithosphere at subduction zones is the main driving force in the movement of the tectonic plates (Forsyth & Uyeda, 1975; Davies and Richards, 1992), the scale and role of subduction zones indicating that they are the most important tectonic feature on Earth.

Components of Subduction Zones

Sources & Further reading

  1. Kearey, Philip, Klepeis, Keith A. & Vine, Frederick J., 2009, Global Tectonics, 3rd Edition, Wiley-Blackwell.
  2. Emery, William J., Pickard, George L., Tally, Lynne D., & Swift, James H., 2011, Descriptive Physical Oceanography, an Introduction, Academic Press.
  3. Stern, Robert J. "Subduction Zones." Reviews of Geophysics 40, no. 4 (2002): 3-1-3-38.

Links

  1. Seismic Tomography
  2. Seismology: the Hunt for Plumes
  3. IRIS videos
  4. Elastic Rebound on a Strike-slip Fault
  5. Seismology: The hunt for plumes 
  6. The Izu-Bonin-Mariana subduction factory
Author: M. H. Monroe
Email:  admin@austhrutime.com
Last Updated 16/01/2013
 

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