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Subduction Zones and their Associated Features

  1. SUBDUCTION ZONES AND THEIR ASSOCIATED FEATURES Presenter : Saad Wani (Masters Student) Geology, Mineralogy and Geophysics Institute R U HR U N IV ER S ITAT BOC HU M 1
  2. T A B L E O F C O N T E N T I. Introd u c tion II. Gen eral Morp h olog y of S u b d u c tion Zon e a ) O c e a n Tr e n c h e s b ) B a c k A r c B a s i n s c ) A c c r e t i o n a r y P r i s m III. Variation in Zon es Ch arac teristic s IV . S tru c tu re of Zon es from Earth q u akes V . Th ermal S tru c tu re of Down -goin g S lab VI. Gravity Anomalies V II. Volcan ic an d Plu ton ic A c tivity V III.Metamorp h ism at convergent b ou n d aries 2
  3. I N T R O D U C T I O N • What is Plate Tectonics? • Types of Plate Tectonics • Convergent Boundaries • Types of Convergence • Subduction 3
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  5. General Morphology of Island Arcs • Island Arcs form when one Tectonic plate subducts beneath another. • Island arcs are divided into those in which the overriding plate is continental (Andean- type arcs) and those in which the overriding plate is oceanic (Primitive arcs). • Mostly the Island Arcs are in curved shape as seen from above. • Some well-known examples of island arcs are Japan, Aleutian Islands of Alaska, Mariana Islands, all of which are in the Pacific, and the Lesser Antilles in the Caribbean. By KDS4444 - Own work, CC BY-SA 4.0, By MagentaGreen, CC BY-SA 3.0 <> 5
  6. OCEANIC TRENCHES • Oceanic Trenches are linear depressed features of Earths crust remarkable for their continuity and depth. • Trenches are generally 50-100km in width, V-shaped and slope of 8°-20° • Trenches can be find filled with sediments and empty both. Trench Mariana Trench Tonga Trench Philippin e Trench Kuril–Kam chatka Trench Kermadec Trench Izu–Bonin Trench (Izu–Ogas awara Trench) New Britain Trench Puerto Rico Trench South Sandwich Trench Peru–Chile Trench or A tacama Trench Japan Trench Ocean Pacific Ocean Pacific Ocean Pacific Ocean Pacific Ocean Pacific Ocean Pacific Ocean Pacific Ocean (Solomon Sea) Atlantic Ocean Atlantic Ocean Pacific Ocean Pacific Ocean Lowest Point Challenger Deep Horizon Deep Emden Deep Planet Deep Brownson Deep Meteor Deep Richards Deep Maximum Depth 10,920 m (35,830 ft) 10,820 m (35,500 ft) 10,540 m (34,580 ft) 10,542 m (34,587 ft) 10,047 m (32,963 ft) 9,810 m (32,190 ft) 9,140 m (29,990 ft) 8,380 m (27,490 ft) 8,265 m (27,116 ft) 8,055 m (26,427 ft) 8,412 m (27,598 ft) Source: 6
  7. BACKARC BASINS • Small Basin form behind the Island Arc on over riding Tectonic Plate. • Firstly Karig (1970) introduced that backarc basins are formed by the rifting of an existing island arc based on research work at Lau Basin • The result were based on Basin’s (Lau Basin) Topographic feature, Sediment thickness difference between basin and sides, Asymmetric cross-section and continuation of arc-basin-ridge system. • To explain formation of backarc basins, common thought is the extension and crustal accretion • All continental backarcs are not associated with extension like modern Andean Margin where shortening of crust and orogeny continues. Modified from Zheng YF, Chen YX, Dai LQ and Zhao ZF (2015) Developing plate tectonics theory from oceanic subduction zones to collisional orogens. Science China: Earth Sciences 58: 1045–1069. 7
  8. ACCRETIONARY PRISM • Accretionary prisms are wedge-shaped accumulations of oceanic sediment and some volcanic rock that were layered over the trench slope after being scraped off the top of the subducting oceanic crust. • Accretionary prism is subjected to available sediments is the oceanic front. Redrawn by Stern, 2002, by permission of the American Geophysical Union. Copyright © 2002 American Geophysical Union Accretionary Non-Accretionary 8
  9. ACCRETIONARY PRISM FEATURES Generalized cross-section of Nankai Accretionary Prism modified from Moore et al., 2005, by permission of the American Geophysical Union. Copyright © 2005 American Geophysical Union 9
  10. VARIATIONS IN SUBDUCTION ZONE CHARACTERISTICS • The age and convergence rate of the under thrusting plate affects the characteristics of Subduction zone. • Typically angle of the Benioff zone is assumed 45 ° but in real there is huge variation e.g. 90° in Mariana and 10° in Peru. • Buoyancy and the push from asthenosphere are responsible for the dip angle. • Accretionary or erosive? Modified from Uyeda & Kanamori, 1979, and Stern, 2002, by permission of the American Geophysical Union. Copyright © 1979 and 2002 American Geophysical Union 10
  11. STRUCTURE OF ZONES FROM EARTHQUAKES • Extreme to low seismicity can be found on Subduction zones. • Mostly generated from Benioff zone. • Focus or Hypocenter extend from shallow to hundred of km (~670km). • Amplitude difference of recorded seismic waves (leads to Q-system implication). Redrawn from Isacks et al., 1969, with permission from the Geological Society of America Tonga Arc Ο : Foci South Projection (0-150km) Δ : Foci North Projection (0-150km) Hypothetical section across the Tonga arc based on the attenuation of seismic waves (redrawn from Oliver & Isacks, 1967, by permission of the American Geophysical Union. Copyright © 1967 American Geophysical Union). 11
  12. • Point a(<25km): Earthquakes due to bending of lithosphere • Point b(25-150km):Thrust faulting b/w overriding and under thrusting Plate • Point c(150-300km):Internal deformation, dehydration embrittlement (Serpentine dehydration leads to increase pore pressure and then activating faulting) • Point d (>300km): Sudden phase change tends to produce transformational/anti- cracking faulting( Olivine to spinel structure). Distribution of earthquakes beneath the northeastern Japan arc. Shaded line is probably the top of the descending lithosphere (redrawn from Hasegawa et al., 1978, with permission from Blackwell Publishing). Observed and theoretical profiles of lithosphere bending at a trench: (a) Mariana Trench, with an elastic lithosphere 29 km thick; (b) Tonga Trench, better modeled by an elastic-perfectly plastic plate 32 km thick (redrawn from Turcotte et al., 1978, with permission from Elsevier) 12
  13. THERMAL STRUCTURE OF DOWN-GOING SLAB • Relatively low temperature of down-going slab compared to normal mantle material at these depths is responsible for the strength, high negative buoyancy, and the ability to suddenly fail in earthquakes. • The Down-going slab can retain its characteristics only up to certain depth, but after considerable depth the heat transformation takes place and that depends on : The rate of subduction The age and thickness of oceanic lithosphere Frictional heating Conduction ability The adiabatic heating • Heat from Radioactive minerals • The latent heat associated with phase changes (like olivine-spinel at 400 km is exothermic and spinel- oxides at 670 km is endothermic) 13
  14. GRAVITY ANOMALIES ON SUBDUCTION ZONE • Typical of most subduction zones, the Aleutian arc's free air gravity anomaly profile is shown in this fig. • A positive gravity anomaly on flexural bulge of the descending lithosphere to seaward of the trench. • Trench and accretionary prism are marked by a large negative anomaly due to displacement of material • Island arc is also marked by a large positive anomaly indicated high thickness and density. After Grow, 1973, with permission from the Geological Society of 14
  15. VOLCANIC AND PLUTONIC ACTIVITY • The magmatic activity starts when the subducting plate reaches at 65-130km and start forming Island Arc. • Thin Arc crust represent young age, young crust and the extensional environments and vice versa for thick Arc crust. • The low potassium tholeiitic series, calc-alkaline series and alkaline series of volcanic rocks are mostly found on subduction zones. • Dacites and Rhyolites are abundant in case of Continental Arc. • Sometime the volcanic series exabits an a spatial distribution like along the Arc or across the Arc but not always e.g. Japanese island arc (tholeiite/calc-alkaline/alkaline from the trench) Type Rock Type Origin Abundance Tholeiitic Series Basalt (low Potassium) > iron rich Basaltic Andesite/Andesites Shallow mantle(65– 100 km) On young Island Arcs Calc-alkaline series Andesites (moderate Potassium) with low percentage of other materials and rare earth elements. >100 km Abundant in volcanic series Alkaline series Alkaline basalts and the rare, very high Potassium-bearing >100 km More abundant in Continental rifts 15
  16. Potassium-silica diagram for the mean composition of 62 volcanoes collected along the Izu-Bonin–Mariana arc system (modified from Stern et al., 2003, by permission of the American Geophysical Union. Copyright © 2003 American Geophysical Union). 16
  17. • Massive, linear belts of plutonic rock referred as batholiths are usually encountered in mature arc systems, especially continental arcs, and used to identify the extinct convergent margins (gabbro, tonalite and diorite to granodiorite and granite). • Where the Magma comes from? Role of Benioff zone! • Initially it was assumed that the magma comes from the melting of subducting plate. • Although It is a great controversy up to now but scientist come to a point that the magma come from partial melting and contain a very low amount of crustal melts and sediments from oceanic crust. • The Melts then derived to the surface or trapped (dykes and sills) through/in cracks/faults. Idealized section through an island arc illustrating the numerous processes involved in its construction. Similar processes may operate beneath Andean-type arcs (redrawn from Stern, 2002, by permission of the American Geophysical Union. Copyright © 2002 American Geophysical Union). 17
  18. METAMORPHISM AT CONVERGENT BOUNDARIES • Oceanic basalt may have low pressure (0.6 GPa) and low temperature (350°C) before subduction, resulting in assemblages of the zeolite and prehnite-pumpellyite facies (Green Schist facies in some cases). • When the Plate is subducted under the other Plate it undergoes several events of temperature and pressure caused by different processes (e.g. chemical reaction). • As basalt descends into a subduction zone, it travels through the blueschist facies pressure-temperature field, which is distinguished by the presence of Glaucophane (a sodic blue amphibole) & Jadeite • Dehydration and densification caused by the transition from the blueschist facies to the eclogite facies raise the negative buoyancy, increasing slab pull. 18
  19. • Some convergent margins also exhibit high temperatures (>500°C) and low to moderate pressures environment (high geothermal gradients). • Andalusite and Sillimanite provides the evidence of high temperatures. • The most common groups of rocks associated with regional metamorphism belong to the greenschist, amphibolite and the granulite facies. • Regional metamorphism refers to large-scale metamorphism, the Himalaya range is an example of where regional metamorphism is happening because two continents are colliding. Regional metamorphism of oceanic crust at a subduction zone occurs at high pressure but relatively low temperatures. Source: Steven Earle (2015) Regional metamorphism beneath a mountain range resulting from continent-continent collision. Arrows show the forces due to the collision. Source: Karla Panchuk (2018) CC BY 4.0, modified after Steven Earle (2015) 19
  20. References Kearey, P., Klepeis, K. A., & Vine, F. J. (2009). Global tectonics. John Wiley & Sons.  metamorphism-and-where-they-occur-2/ Source: 20