Sub Duct Ion Zone. 23rd Nov,2011 (Lec#1 Onwards)

8/3/2019 Sub Duct Ion Zone. 23rd Nov,2011 (Lec#1 Onwards)

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Week # 9-

Subduction Zones


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Subduction Zones

When two tectonic plates converge often

one will get buried orsubductedbeneath the

other

The plate boundary regions where this

occurs are calledsubduction zones


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There are two types of lithosphere, oceanic and

continental, so there are three possibilities at a

convergent boundary:

oceanic and oceanic

oceanic and continental

continental and continental

In which of these cases can subduction occur ?


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Subduction zones only occur at convergentboundaries between oceans and continents, andoceans and oceans

When oceanic lithosphere converges withcontinental lithosphere it is the oceanic materialthat is always subducted beneath the continentalmaterial.

When the convergent boundary is between twooceans it the older (heavier) plate which usuallysubducts.


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Oceanic-Continental Subduction

Examples of an oceanic lithospheresubducting beneath a continentallithosphere:

South America subduction zone: Nazca plate(oceanic) subducting beneath South Americanplate (continental)

Aleutian subduction zone: Pacific plate(oceanic) subducting beneath North Americanplate (continental) in Alaska


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Subduction Zones

Examples of oceanic lithosphere subductingbeneath oceanic lithosphere ofanotherplate:

Marianas subduction zone: Pacific platesubducting beneath Phillipine Sea plate inwestern Pacific

Tonga subduction zone: Pacific platesubducting beneath Australian plate in westernPacific


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Ocean-oceanp Island Arc (IA)

Ocean-continentp Continental Arc or

Active Continental Margin (ACM)

Figure Principal subduction zones associated with orogenic volcanism and plutonism. Triangles are on the overriding

plate. PBS = Papuan-Bismarck-Solomon-New Hebrides arc. After Wilson (1989) Igneous Petrogenesis, Allen

Unwin/Kluwer.


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General Picture of Subduction


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General Picture of Ocean-Ocean

Convergence


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General Picture of Ocean-Continent Subduction


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Second General Example of

Ocean-Continent Subduction


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The Termination of a Subduction

Zone: Indian-Eurasian Boundary


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Subduction zones exist at convergent plate

boundaries where one plate of oceaniclithosphere converges with another plate.

The down-going slab -- the leading edge of

the subducting plateis overridden byleading edge of the other plate.

The down-going slab -- the leading edge of

the subducting plateis overridden byleading edge of the other plate.


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At a depth of approximately 80120 km, the

basalt of the oceanic slab is converted to ametamorphic rock called eclogite.

At this point, the density of the oceaniclithosphere increases and it is carried into themantle by the downwelling convectivecurrents.

It is at subduction zones that the Earth's

lithosphere, oceanic crust, sedimentary layers,and some trapped water are recycled into thedeep mantle


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Earth is the only planet where subduction is

known to occur. Without subduction, plate

tectonics could not exist

The subducting basalt and sediment are

normally rich in hydrous minerals and clays.

During the transition from basalt to eclogite,

these hydrous materials break down,

producing copious quantities of water,

which at such great pressure andtemperature exists as a supercritical fluid.


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The supercritical water, which is hot andmore buoyant than the surrounding rock, risesinto the overlying mantle where it lowers thepressure in (and thus the melting temperatureof) the mantle rock to the point of actual

melting, generating magma.T

hese magmas,in turn, rise, because they are less dense thanthe rocks of the mantle.

These mantle-derived magmas (which are

basaltic in composition) can continue to rise,ultimately to the Earth's surface, resulting in avolcanic eruption.


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Subduction results from convection in the

asthenosphere. The heat from the core of the

earth that is imparted to the mantle causes the

mantle to convect much the way boiling water

convects in a pan on the stove.

Hot mantle at the core-mantle boundary riseswhile cool mantle sinks, causing convection

cells to form.


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At points where two downward movingconvecting cells meet (cold mantle sinking),

convection can occur, forcing the oceanic

crust below either continents or other oceanic

crust.

Continental crust tends to override oceanic

crust because it consists of less dense granite

compared to the basalt of the oceanic crust.


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Oceanic plates are subducted creating oceanic trenches


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Subduction Zones

The dominant features associated with

subduction zones are:

deep earthquakes

volcanoes- mountain building


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Subduction Zones and

Deep Earthquakes It turns out the the deep earthquakes we

observe (depth > 200 km) are occurring in

lithosphere that has been subducted. Deep earthquakes do not occur in any place

except for subduction zones since this is the

only place where brittle material

(lithosphere) exists below its normal depth.


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Subduction Zones and

Deep Earthquakes Deep earthquakes occur in planar (2D)

arrangements called Wadati-BenioffZones

Seismologists use the locations of deep

earthquakes to map out the geometry of

subducting lithosphere.


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Sometimes Slab Geometry is Simple


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Often it is Complicated (South America)


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Often it is Complicated (Tonga)


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Subduction Zones and Volcanoes

Volcanic activity is associated with all

active subduction zones

We see dormant and fossil volcanoes atplaces where subduction used to occur

This type of volcanic activity is

fundamentally different than volcanoes at

mid-ocean ridges and hot-spots


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As oceanic crust ages and moves away from

the ridge where it was formed it

accumulates sediments which are rich iswater

Water also reacts with the newly formed

crust and becomes chemically boundto it


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Some sediment layers get scraped off the

oceanic crust when it subducts at a trench;

however a large amount of water is retainedin the subductiong slab of oceanic material.

Thus, some water gets transported into the

mantle while chemically bound to the rocks.


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At about a depth of 100 km the temperature

becomes hot enough that a chemical

reaction takes place and the water isliberated from the material which carried it

down into the mantle.

This is called a dehydration reaction.



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The free water that has just been liberated

immediately starts to percolate upwards and

begins topartially meltthe asthenosphereabove it.

This partially molten material, and water, is

much lighter than the surrounding material

and begins rising


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When the partially molten material nears the surface it

often becomes fully molten because of decreasing pressure

now we call it magma.

The outermost crust at the Earths surface is cold, brittle

and strong so it is difficult for the magma to break-through

Thus magma will oftenpondbeneath volcanoes in amagma chamber until the pressure becomes high enough

for it to break though the outermost crust and erupt


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Morphology of oceanic Subduction Zone

Numerous geologic environments are associated

with subduction.

In an idealized arc system, three zones arerecognized:

- the arc-trench gap

- the arc- the arc-rear area


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Island arc systems are formed when oceanic

lithosphere is subducted beneath oceanic

lithosphere

They are consequently typical of the

margins of shrinking oceans such as the

Pacific, where the majority of island arcs

are located.


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All of the components of island arc systems areusually convex to the underthrusting ocean.

Proceeding from the oceanward side of the systemthe following features would occur:

1 - Bulge

2 - Fore arc region

3 - Island arc

4 - Back arc region


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1- Bulge: Between 100 & 200 km from the

trench the surface of the lithosphere rises toabout 500 m to form a broad arch called theouter swell orperipheral bulge.

2- Fore arc region: It is infront ofIsland arc

towards the underthrusting ocenic plate &comprised of;

a- Trench

b- Accretionary wedge or prismc- Fore arc basin


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Trench

Trenches are linear or curvilinear troughsthat mark the boundary, at the Earthssurface, between the downgoing slab and the

accretionary prism of the overriding plate Trenches exist because the subducted

portion of the downgoing slab pulls the slab

downwards to a depth greater than it wouldbe if the lithospheric plate were isostaticallycompensated.


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The deepest locations in the oceans occur

at trenches Trench-floor depth reflects two factors:

(1) the age of the downgoing slab (the floor of

older oceanic lithosphere is deeper than thefloor of younger oceanic lithosphere)

(2) the sediment supply into the trench (if a

major river system from a continent spillsinto a trench, the trench fills with sediment)


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To see the effect of these parameters, lets

compare the geology of the Mariana Trench

and that of the Oregon-Washington Trench.

The great depth of the Mariana Trench is a

result of its location far from a continentalsupply of sediment and the fact that the plate

being subducted at the Mariana Trench is

relatively old (Mesozoic)


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In contrast, the trench along the Pacific

northwest margin of the United States hasfilled with sediments carried into the Pacific

by the Columbia River, and the downgoing

slab beneath the trench is quite young (Late

Cenozoic)

Even though the thickness of sediments in

trenches is variable, all trenches contain

some sediment, called trench fill.


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Typically, trench fill consists of flat-lying

turbidites and debris flows that decended into

the trench via submarine canyons

The sediment comes from the volcanic arc

and its basement, from the forearc basin, and

from older parts of the accretionary wedge.


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F I G U R E Trenches fill with turbidites. Much of this sediment

flows down submarine canyons and then accumulates in turbidite fans

on the floor of the trench.


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Accretionary wedge or prism

An accretionary wedge oraccretionaryprism is formed from sediments that are

accreted onto the non-subducting tectonic

plate at a convergent plate boundary.

During the process of subduction, the surface

of the downgoing plate shears against the

edge of the overriding plate forming

accretionary prism.


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The accretionary prism (or subduction

zone complex) consists of a series of

steeply-inclined, fault-bounded wedges of

sediment and volcanic rocks above a

descending slab.

These wedges represent oceanic crust and

trench sediments which have been accreted

to the front of the arc. Individual wedges in

the accretionary prism decrease in age as thetrench is approached


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This is a wedge consisting of deformed

pelagic sediment and oceanicnbasalt, which

were scraped off the downgoing plate, and

of deformed turbidite that had been

deposited in the trench.

Traditionally, the process of forming an

accretionary prism has been likened to theprocess of forming a sand pile in front of a

bulldozer


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The blade of the bulldozer can be called a

backstop, in the sense that it is a surfacethat blocks the movement of material that

had been moving with the downgoing plate.


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FIGURE (c) The bulldozer analogy for the formation of an

accretionary prism. The blade acts as the backstop.


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Compressional deformation in the

accretionary prism produces thrust faults,folds, and cleavage.

Accretionary prisms are intensely deformedproducing melange, which is a mappablebody of rock characterized by the lack ofcontinuous bedding and the inclusion offragments of rocks of all sizes (up to more

than a kilometre across) contained in a fine-grained, deformed matrix.


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Fore arc basin

Forearc basins are marine depositional

basins on the trench sides of arcs (Figure

3.16), and they vary in size and abundance

with the evolutionary stage of an arc.

This forearc basin contains flat-lying strata

derived by erosion of the arc


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The forearc basin is a region of tranquil, fl

at-bedded sedimentation between the

accretionary prism and island arc.

Typically, strata of the forearc basin overlie

older, subsided, portions of the prism.


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3 - Island arc

The island arc is made up of;

1-an outer sedimentary arc and

2-an inner magmatic arc. The sedimentary arc comprises coralline

and volcaniclastic sediments underlain by

volcanic rocks older than those found in themagmatic arc


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This volcanic substrate may represent theinitial site of volcanism as the relatively cool

oceanic plate began its descent.

T

he arc-trench gap, meaning the distancebetween the arc axis and the trench axis,

varies significantly among convergent

margins


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Two factors control the width of the arc-

trench gap at a given convergent margin:(1) Dip of the downgoing slab: Geometric

principles dictate that if the downgoing slab

dips very steeply, then the arc-trench gapmust be narrow, but if the downgoing slab

dips gently, then the arctrench gap must be

broad.


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(2) Width of the accretionary prism:

Where subduction has continued for a longtime, or where a large river fills the trench

with sediment, the accretionary prism grows

to be very large. When this happens, theprism acts as a weight that flexurally

depresses the downgoing slab, and as the

prism builds seaward, the trench location

migrates seaward.


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4 - Back arc region

The backarc region refers to the region on

the opposite side of the volcanic arc from

the forearc basin

Active back-arc basins occur over

descending slabs behind arc systems

The structural character of backarc regions

varies with tectonic setting


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Types of backarc regions

The three types of backarc regions are:

(1) contractional,

(2) extensional, and(3) stable


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In a contractional backarc region ( a

backarc basin does not form. Rather, crustal

shortening generates a fold-thrust belt

Contractional backarc is commonly called

an Andean-type backarc.


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(a) An Andean-type continental arc, with a compressional

backarc region.


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In extensional backarc regions, crustal

stretching takes place. This stretching

produces a backarc basin.

If the stretching produces only a continental

rift, then continental crust underlies the

backarc basin, but if seafloor spreading

takes place, then oceanic crust underlies thebackarc basin.


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Extensional backarcs are commonly called

Mariana-type back arcs.


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(b) A Mariana-type island arc, with an extensional backarc.


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In a stable backarc, no strain accumulates


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(c) A Japan-type volcanic arc, in which the island arc has continental

basement that had rifted off a continent when a backarc basin grew.

Here, the backarc spreading has ceased, and the backarc is stable

Structure of Island arc system from


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Structure ofIsland arc system from

earthquakes

Subduction zones exhibit intense seismicactivity.

A large number of events occur on a plane

that dips on average at an angle of about 45away from the underthrusting oceanic plate

The plane is known as a Benioff (or BenioffWadati) zone, afterits discoverer(s), and

earthquakes on it extend from near thesurface, beneath the forearc region, down to amaximum depth of about 670 km.


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The earthquake activity associated with the

downgoing slab occurs as a result of four

distinct processes

In region a earthquakes are generated in

response to the bending of the lithosphere as it

begins its descent.


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Figure Plate model of subduction zones; a, b, c, and d indicate

regions of distinctive focal mechanisms


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Bending, or downward fl exure of the

lithosphere, puts the upper surface of the

plate into tension, and the normal faulting

associated with this stress regime gives rise

to the observed earthquakes, which occur to

depths of up to 25 km


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Region b (Fig. 9.8) is characterized by

earthquakes generated from thrust faulting

along the contact between the overriding

and underthrusting plates.

Focal mechanism solutions for earthquakes

associated with regions a and b


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The earthquakes occurring in the Benioffzone in zone c, at depths greater than thethickness of the lithosphere at the surface,

are not generated by thrusting at the top ofthe descending plate

At these depths earthquakes occur as aresult of the internal deformation of therelatively cold and hence strong descendingslab of lithosphere


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Below 300 km (zone d in Fig. 9.8) theearthquake mechanism is believed to be a

result of the sudden phase change from

olivine to spinel structure, producing

transformational or anticrack faulting.

This takes place by rapid shearing of the

crystal lattice along planes on which minute

spinel crystals have grown


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At normal mantle temperatures this phasechange occurs at a depth of approximately

400 km

However, the anomalously low

temperatures in the core of a downgoing

slab enable olivine to exist metastably to

greater depths, potentially until it reaches

a temperature of about 700C


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In old, rapidly subducting slabs this may,

exceptionally, be at a depth of

approximately 670 km, explaining the

termination of subduction zone seismicity at

this depth

M hi i


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Metamorphism at convergent margins

The anomalous thermal & pressure conditions

associated with subduction zones gives rise to

distinctive suites of metamorphic rocks whose

deposition depends on the direction ofunderthrusting

The subduction zones contains paired

metamorphic belts.an outer high pressure/low

temperature metamorphic rock on oceanward side


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In subduction zone contain an outer high

pressure/low temperature metamorphic rock

on oceanward side while the parallel to it

the low pressure/high temperature belt of

similar age associated the island arc at about

100-259 km apart

Hi h /l t t


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High pressure/low temperature

metamorphic belt An abnormally low geothermal gradient of

about 10 C km-1 results from the rapid

descent of relatively cool oceanic

lithosphere at trenches to depth of about 30km.

The high pressure & low temperature in this

environment give rise to a metamorphiccomplex characterized by the presence of

glaucophane & jadeite which are indicatve

of blueschist facies

L /Hi h t t


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Low pressure/High temperature

metamorphic belt

The ascent of magma generated by

dewatering-induced mantle melting gives

rise to anomalously high geothermal

gradients of more than 25C

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Sub Duct Ion Zone. 23rd Nov,2011 (Lec#1 Onwards)