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Subduction Zones • http://www.geo.utep.edu/class_notes/PT99/Lectures/subduction.html Subduction zones • also termed convergent or consuming plate margins • occur where adjacent plates move toward each other and relative motion is accommodated by one plate over-riding the other. • These zones are classified as either oceanic or subcontinental, depending on the overriding plate. • If the "subducting" plate is continental, subduction will cease and a mountain belt will form within a collision zone. Where do subduction zones occur? • along the "Ring of Fire" around the Pacific Ocean. • Two short subduction zones occur at the Lesser Antilles, at the eastern side of the Carribean plate and the South Sandwich Island plate. • Three short segments of the Alpine Himalayan system involve subduction of oceanic lithosphere. – the Calabrian and Aegean boundaries in the Mediterranean Sea – Makran boundary along the SW boundary of the Iran plate. Physiography • Outer Swell • Outer Trench Wall • Trench • Forearc (Arc-Trench Gap) • Volcanic Arc • Back-Arc Physiography 2 • Outer swell – Low topographic bulge (a few hundred meters of relief) – develops just outboard of where subducting plate bends down into the mantle. Outer Trench Wall – Slope on ocean floor between the outer swell and the trench floor. – Slope dip is typically -5 degrees • Trench – Deep valley that develops at the plate boundary. – Continuous for 1000s of km – typically 10 - 15 km deep (5 - 10 km below surrounding ocean floor.) Forearc (Arc-Trench Gap) – Consists of region between trench and the arc. – steep inner trench wall (lower trench slope) • dips of - 10 deg – flattens into a gentle slope termed the forearc basin (upper trench slope). – The inner trench wall is usually separated from the forearc by the outer ridge. – The accretionary prism underlies the inner trench wall, the outer ridge and part of the forearc basin. Volcanic Arc • Active arc built on a topographically high region of older rocks, the arc basement • may be a shallow marine platform or an emergent region of older rocks. • In continental arcs, the basement is continental crust standing a few kms above sea level. • Volcanoes in island arcs are usually 1 - 2 km above sea level. Volcano elevation in continental arcs is strongly influenced by continental crust thickness. Back-Arc • Area behind the volcanic arc. • In island arcs this region consists of basins with oceanic crustal structure and abyssal water depths. • Sometimes remnant arcs are preserved behind the island arcs. • On continents this region is the continental platform which may be subaerially exposed, or the site of a shallow marine basin. Gravity • Typically, similar free-air gravity profiles – 50 mGal gravity high associated with the outer bulge – 200 mGal low associated with the trench and accretionary prism – 200 mGal high associated with the arc. • Isostatic anomalies have the same polarity as the free-air gravity • Suggests that the gravity anomalies are caused by the dynamic equilibrium imposed by the system by compression. • Compressional forces cause the trench to be deeper and the arc to have less of a root than they would be if only isostatic forces were at work. Structure from Earthquakes • Subduction zones are characterized by dipping seismic zones termed Benioff zones or Wadati-Benioff zones • Result from deformation of the downgoing lithospheric slab. The zones have dips ranging from 40 to 60 deg • Because, the slab is colder and more dense than surrounding asthenosphere, it's position can be mapped seismically as high velocity anomalies and as high "Q" (little attenuation of seismic waves) zones in the mantle. High Q, and high velocity are thought to correspond to relatively high density, cold material earthquake hypocenters related to their position within the slab • Shallow depths • predominantly thrust faults within the upper part of the downgoing plate or in the adjacent overriding plate. • Down to depths of 400 km, down-dip extension. – In some slabs, down-dip extension is found in the upper part of the slab, accompanied by down-dip compression at the base of the slab. The extension probably results from the lithosphere being pulled into the mantle by the weight of the downgoing portion. • Deep slabs usually show down-dip compression – may result from increased viscous resistance at depth. – deeper part of the slab will feel a push from the weight of the shallower portion of the slab. • Between 70 - 300 km, faulting may occur due to dehydration of serpentinite. • From 300 - 700 krn may also be due to the sudden phase change of olivine to spinel which may be accommodated by rapid shearing of the crystal lattice along planes on which minute spinel crystals have grown. Structural Geology- Trenches • Trenches normally contain flat-lying turbidites deposited by currents flowing down into the trench from the overriding plate or along the axis of the trench. The outer swell is probably caused by elastic bending of the subducting plate. Forearc • may be underlain either by the accretionary prism or arc basement rocks covered by a thin veneer of sediments or both. • Where there is little sediment accumulation on the subducting plate, island arc or continental basement may extend all the way to the lower trench slope and little or no accretionary prism may occur. • Forearc basement may draped by a thin veneer of sediment, and is commonly cut by normal faults toward the trench. Accretionary Prism • wedge of deformed sedimentary rocks • the main locus of crustal deformation • Rocks are typically cut by numerous imbricate thrust faults that dip in the same direction as the subduction zone. • As more material is added to the toe of the wedge, the thrusts are moved upwards and rotate arcwards. • Rocks within the accretionary prism are derived from the downgoing and/or overriding plates. Accretionary Prism • At the toe of the wedge, sediments are added thru offscraping • propagation of the basal thrust into undeformed sediments on the subducting plate. • This process results in progressive widening of the wedge, and eventually a decrease in dip on the subduction zone. Accretionary Prism • When sediments on the downgoing plate are subducted without being disturbed they can still be added to the prism thru underplating • propagation of the basal thrust into the downgoing undeformed sediments to form a duplex beneath the main part of the prism. Subduction Erosion • erosion and subsequent subduction of rocks from the toe of the prism. • Sediment on the subducting plate that is not added to the overriding plate thru these processes may descend into the mantle and contribute to the generation of arc magmas. Forearc Basin • Wide sedimentary basin – develops above irregular basement on the upper part of the arc-trench gap. – Sediments from the active arc or arc basement rocks • deposited by turbidity currents traveling along the basin axis or perpendicular to the arc. • asymmetric basin – inner part of the upper slope basin subsides – outer edges rises due to accretion at the toe of the wedge. • high-P, low-T metamorphism – increases in grade toward the inner forearc region – in the direction of subduction Arc • Arc basement – older more deformed and metamorphosed rocks in platform on which the modem arc is built. – oceanic rocks – On the continents, complex continental basement. • Volcanic arc – a chain of largely andesitic stratovolcanoes spaced at fairly regular intervals of 70 km. – The structural environment of these arcs is commonly extensional (numerous normal faults) – volcanoes in grabens termed volcanic depressions. – underlain by large plutonic bodies (e. g. the Sierra Nevada). Arcs • Metamorphism – common and suggest a high geothermal gradient. – Much of the lower crust may be at the melting temperature of granite. • Sediments – debris from active volcanoes. – deposited as turbidites. – In tropics, settings these volcanogenic sediments may interfinger with carbonate reefs. – In continental arcs, sediments are often deposited subaerially. Back-arc • extensional tectonics and subsidence. • In oceans arc-derived sediments are deposited in an ocean basin behind the arc termed the back-arc basin. • In continents, sediments are deposited into basins on the continental platform and are termed foreland basins or retro-arc basins. Foreland Fold and Thrust Belts • Relation between foreland fold and thrust belts and subduction not understood • not all continental arcs display these features. • Possible explanations if there is a relation – Thrust belt caused by compression at margin of overriding plate due to subduction of hot, buoyant lithosphere. – Thrust belt associated with shallow dip of a downgoing slab. – Thrust belt associated with subduction of an aseismic ridge. Models of thermal processes in subduction zones • Rate of Subduction – The faster the descent of the slab, the less time it has to absorb heat from the mantle. • Slab Thickness – The thicker the descending slab, the more time it takes to come into equilibrium with the surrounding lithosphere. • Frictional Heating – occurs at top of slab due to friction as slab descends and is resisted by the lithosphere. • Conduction – heat into slab from the asthenosphere • Adiabatic Heating – associated with compression of slab with increased pressure at depth. • Heat of Radioactive Decay – decay of radioactive minerals in the oceanic crust (minor) • Latent Heat of Mineral Phase Transitions – olivine-spinel transition at 400 km is exothermic. Spinel- oxide transition at 670 km could be either exothermic or endothermic. • All thermal models show that the downgoing slab maintains its thermal identity to great depths (e. g. contrasts of 700 deg C can still exist at 700 krn depth). If the slab is so cold, how do we get enough heating to cause arc magmatism? • Melting of Slab in Presence of Water – Partial melting may take place at lower temperatures due to presence of water as slab dehydrates. Water is released by transition of amphibolite to ecologite, and dehydration of serpentinite at depths of - 100 km. • Corner Flow and Melting of Mantle – Downgoing slab may cause flow of hot mantle into the comer of the overriding mantle where it impinges on the downgoing slab. This may provide enough heat to cause melting. Origins of back-arc basins • Entrapment of previous oceanic crust – Change of plate motion may lead to abandonment of a fragment of oceanic crust behind the arc. (e.g., Aleutian Basin and West Philippines Basin ) • Formation of new crust - behind the arc. 3 models o Spreading caused by forceable injection of a diapir rising from the downgoing slab. o Spreading induced in the overriding plate by the viscous drag in the mantle wedge caused by the motion of the downgoing plate (comer flow). o Spreading induced by the relative drift of the overriding plate away from the downgoing slab (slab fixed with respect to mantle). This is also termed roll- back.
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