TABLE OF CONTENT >Introduction >General Morphology of Subduction Zone >Ocean Trenches >Back Arc Basins >Accretionary Prism >Variation in Zones Characteristics >Structure of Zones from Earthquakes >Thermal Structure of Down-going Slab >Gravity Anomalies >Volcanic and Plutonic Activity >Metamorphism at convergent boundaries

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
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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
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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
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4. 4

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,https://commons.wikimedia.org/w/index.php?curid=49035989
By MagentaGreen, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>
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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: https://en.wikipedia.org/wiki/Oceanic_trench
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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.
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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
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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
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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
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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).
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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)
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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
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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).
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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).
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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.
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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)
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20. References
Kearey, P., Klepeis, K. A., & Vine, F. J. (2009). Global tectonics. John
Wiley & Sons.
https://openpress.usask.ca/physicalgeology/chapter/10-3-types-of-
metamorphism-and-where-they-occur-2/
Source: https://en.wikipedia.org/wiki/Oceanic_trench
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21. THANK YOU FOR YOUR TIME
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