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SUBDUCTION ZONE

Subduction: At convergent boundary when two plates moves towards each other and leading

edge of one bent downward and slides beneath other. Subduction occurs because density of

descending plate is greater than the density of over-riding plate and density of underlying

asthenoshpere. In general, Oceanic lithosphere is more dense than the continental lithosphere,

so continental lithosphere resist during subduction.

Subduction angle: Slabs of oceanic crust descends into mantle at varying angle from few

degrees to nearly vertical (angle depends largely upon the density and distance of spreading

centre).

For e.g. Peru-Chile in which spreading centre is located near-subducting lithosphere. The

crust is young-warm and buoyant due to which angle is small. Consequently the region along

Peru-Chile trench experiences great earthquakes (including Chilean earthquake –one of the 10

largest earthquakes).

Conversely, if the spreading centre is farther from the trench area then it cools gradually the

crust would be thicker with more density. Western Pacific oceanic lithosphere is of 180 Ma

(oldest and most dense oceanic crust). So, Very dense slabs in this region typically plunge into

mantle at high angles. This also explains the fact that most trenches in the western Pacific are

deeper than eastern Pacific.

Types of convergent boundaries: Although all convergent zones have some basic

characteristics, they have highly variable features. Each is controlled by the type of crustal

material involved.

Oceanic-Continental convergence: The buoyant continental block remains floating while

denser oceanic slab sink into mantle. When descending slab reaches a depth of about nearly

100 km melting is triggered within the edge of hot asthenoshpere that lies above it.

Descending cold slab contain water in the descending plate which is dehydrated as pressure

increases. At higher pressure melting point of the rocks is reduced. This causes partial melting

in the mantle. These partial melts rise up to from volcanic eruptions. The mountains that are

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produced by volcanics actively associated subduction of oceanic plate under continental

lithosphere is k/a Continental Volcanic Arcs.

Oceanic-Oceanic convergence: Many features are common as above and one oceanic slab

descends beneath the other and initiate volcanic activity by the same mechanism.

Which one will descend? Denser crust

Volcanic islands derived by such convergence k/a Island arcs. In general, Island arcs are

nearly 100-300 km from trench. e.g. Aleutian island and Tonga island

Continent-Continent convergence: The in-between oceanic crust between two continental

plates is completely subducted and the continents collide to form an orogenic belt. It is also

termed as orogeny or mountain building activity. Multiple generation of folding, meta-

sediments and highly deformed rocks along with many thrust faults are quite common

features that are found in an orogenic belt.

Examples: Himalayas, Alps, Appalachians, Urals.

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OCEANIC TRENCHES

Oceanic trenches are direct manifestation of underthrusting one lithosphere beneath another

(generally oceanic beneath continental). They represent the largest linear depressed feature of the

Earth’s surface. Trenches are developed nearly parallel to continental volcanic arcs or island

arcs.

Example: Peru-Chile trench is 4500 km long and reaches the depth of 2-4 km below surrounding

ocean floor.

Western Pacific trenches are deeper than the eastern Pacific trenches

Morphology of Trenches: Trenches are thousands of Kms long and 50-100 km in width. In

cross-section trenches are symmetric V-shaped. The sediment fill of trenches can vary greatly to

virtually nothing as in Tonga-Kermadec trench to almost complete as in Lesser Antilles and

Alaskan trenches because of variation in supply of sediments from adjacent continental areas.

Subduction in Pacific Ocean:

Ring of Fire, also called Circum-Pacific Belt or Pacific Ring of Fire is a long horseshoe-

shaped seismically active belt of earthquake epicentres, volcanoes, and tectonic plate boundaries

in the periphery of the Pacific basin. For much of its 40,000-km (24,900-mile) length, the belt

follows chains of island arcs such as Tonga and New Hebrides, the Indonesian archipelago,

the Philippines, Japan, the Kuril Islands, and the Aleutians, as well as other arc-shaped

geomorphic features, such as the western coast of North America and the Andes Mountains.

Volcanoes are associated with the belt throughout its length, for this reason it is called the “Ring

of Fire.” It is marked as the most active region in terms of earthquakes and approximately 75

percent of the world’s volcanoes occur within the Ring of Fire.

Mariana trench: The Challenger deep in the Mariana trench (11,022 m) below sea level marked

as deepest known part in the World Ocean.

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MORPHOLOGY OF ISLAND ARC SYSTEM

Island arc system is formed when one plate will be subducted beneath overriding plate.

Although not all the components are present in every system. Proceeding from oceanward side

of the system, a flexural bulge of about 500 m high occur b/w 100-200 km from trench.

Fore arc region comprises of trench itself, the accretionary prism and the fore arc basin. The

accretionary prism is consists of thrust slices of trench fill turbidites and some pelagic

sediments that has been scraped off from the downgoing slab by the leading edge the

overriding plate. Fore arc basin is the region of calm conditions flat bedded sedimentation b/w

accretionary prism and island arc. The sedimentary arc comprises of coralline and volcano-

clastic sediments underlain by volcanic rocks.

Back arc basins are marginal basin behind island arc/magmatic arc. However, as shown in

figure all back-arc basins are not formed from spreading of an active subduction zone. In

general these basins are formed on either side of volcanic/ island arc on the overriding plate at

subduction zone.

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STRUCTURE DECIPHERED FROM EARTHQUAKE

Subduction zone exhibit intense seismic activity and large number of events occur on a plane that

dips on average at an angle of about 45°

In region (a) the earthquake activity associated with the downgoing slab occurs as a result of

distinct processes:

Response of bending of lithosphere as it begins and descends.

Bending and downward flexure of the lithosphere put the upper surface of the plate into tension,

and normal faulting is also associated with this stress regime which give rise to earthquakes

(Chistensen and Ruff 1988).

In region (b) The earthquake is generated from the thrust faulting along the contact b/w over-

riding plate and under-thrusting plate.

In region (c) In the Benioff Zone Earthquakes at the depths greater than the thickness of

lithosphere at the surface are not generated by thrusting at the top of descending plate.

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The earthquakes can be produced by slip along the subduction thrust fault or by slip on faults

within the downgoing plate, as a result of bending and extension as the plate is pulled into the

mantle. The zones have dips typically ranging from 40 to 60 degrees. The zones are also known

as the Wadati-Benioff zone.

At such depth the earthquake occur as a result of the internal deformation of the relatively cold

descending slab of lithosphere.

In region (d): Below 300 km (zone d)- the earthquake mechanism is believed to be a result of

sudden phase change from olivine to spinel producing transformational faulting. This take place

shearing of crystal lattice along planes on which minute spinel crystals have grown. At normal

mantle this is at 400 km approx.

VOLCANIC AND PLUTONIC ACTIVITY

In general, when subducting oceanic crust reaches the depth of 65-130 km give rise to an island

arc or an Andean type continental arc approximately at 150-200 km from trench. The types of

volcanic rocks that occur in the subduction zone event generally belong to 3 series: Low

potassium Tholeiitic series, Calc-Alkaline series and Alkaline series (Shoshonite).

In general, tholeiitic magma series is well represented above young subduction zones. These

rocks are derived from the fractional crystallization of olivine from primary magma originally at

shallow depth of the mantle.

The calc-alkaline and alkaline series are encountered in more mature subduction zones and

reflects that the magmas are generated at greater depth than the tholeiitic rocks. Calc alkaline

magmas are represented by the andesite and basaltic andesite.

Some island arcs exhibit spatial patterns in the distribution of volcanic series with increasing

depth from trench.

Tholeiite-----Calc-Alkaline-----Alkaline

This trend reflects that the magma derived from increasingly greater depth and/or differences in

the degree of partial melting. A low degree of partial melting tends to concentrate alkalis and

other incompatible elements into the small melt fraction and could lead to an increase in

alkalinity away from trench due to greater depth of melting. This great spectrum of rocks

composition reflects the diversity of processes involved in arc magmatism, including variations

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in the depth and degree of partial melting, magma mixing, fractionation and assimilation. These

plutonic rocks range in composition from Gabbro, Tonalite, Diorite and Granodiorite and

Granite.

HOW MELT GENERATED at Subduction Zones?

Earlier model of Ringwood (1975) suggested that magma were derived from melting of top of

descending oceanic slab. But, it was rejected because thermal models indicate that subducted

lithosphere rarely becomes hot enough to melt (Peacock, 1991). On the basis of petrological and

mineralogical evidence (Arculus and Curram, 1972) and Helium isotope ratios (Hilton and Craig,

1989) indicate that parental magmas originate by the partial melting of asthenoshperic mantle

immediately overlying descending plate not from the melting of subducting slab. The partial

melting takes place at relatively low temperature because of high water vapour pressure resulting

from the dehydration of various mineral phases in the downgoing slab. Greater the amount of

water present more will be the melt produced. Thus, water acts as a primary agent that derives

partial melting beneath arcs.

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METAMORPHISM AT SUBDUCTION ZONE

Metamorphism refers to the changes in the pre-existing rock’s mineralogy, texture and/ or

chemical composition that occur predominantly in the solid state below the zone of weathering.

When oceanic basalt is subducted at convergent margin, it undergoes a series of chemical

reactions and cause release of water into upper mantle wedge and increase density of

subducting slab. Metamorphic transformations reflect the abnormally low geothermal gradient

(10°C/km) and high pressure. Prior to subduction, oceanic basalt exhibit low T/low P

metamorphic minerals (Zeolite and Prehenite-Pumpellyite facies and Greenschist facies). This

basalt is altered because of the hydrothermal activities near oceanic ridges. When this altered

basalt descends into the subduction zone, it passes through temp-press field of blueschist

facies characterized by the presence of pressure sensitive minerals glaucophane (Amp) and

Jadeite (pyx). Then, Blueschist facies changes to Eclogite (Garnet+ Omp). Dehydration and

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densification of subducted oceanic crust takes place and transformation to eclogite enhances

the negative buoyancy of the descending lithosphere and contributes to slab pull forces acting

on the subducting plate. As a result, Subduction zone exhibit High pressure and low

temperature type of metamorphism.

PAIRED METAMORPHIC BELT

The concept of paired metamorphic belt was introduced by Miyashiro (1961) in Japan.

Subduction zone- High P/T

Island arc- Low P/T

These two remains parallel to each other.

Refer: Spears (Metamorphic phase equilibria and Pr temp time paths)

GRAVITY ANOMALY AT SUBDUCTION ZONE

Free air gravity anomaly profile across the Aleutian arc that is typical of most of the subduction

zone.

Bulge: (+)ve

Trench and accretionary prism: (-)ve Due to low density sediments

Island arcs: Large (+)ve anomaly