sub duct ion zone. 23rd nov,2011 (lec#1 onwards)
Post on 06-Apr-2018
220 Views
Preview:
TRANSCRIPT
-
8/3/2019 Sub Duct Ion Zone. 23rd Nov,2011 (Lec#1 Onwards)
1/80
Lecture-10 1
Week # 9-
Subduction Zones
-
8/3/2019 Sub Duct Ion Zone. 23rd Nov,2011 (Lec#1 Onwards)
2/80
Lecture-10 2
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
-
8/3/2019 Sub Duct Ion Zone. 23rd Nov,2011 (Lec#1 Onwards)
3/80
Lecture-10 3
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 ?
-
8/3/2019 Sub Duct Ion Zone. 23rd Nov,2011 (Lec#1 Onwards)
4/80
Lecture-10 4
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.
-
8/3/2019 Sub Duct Ion Zone. 23rd Nov,2011 (Lec#1 Onwards)
5/80
Lecture-10 5
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
-
8/3/2019 Sub Duct Ion Zone. 23rd Nov,2011 (Lec#1 Onwards)
6/80
Lecture-10 6
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
-
8/3/2019 Sub Duct Ion Zone. 23rd Nov,2011 (Lec#1 Onwards)
7/80
Lecture-10 7
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.
-
8/3/2019 Sub Duct Ion Zone. 23rd Nov,2011 (Lec#1 Onwards)
8/80
Lecture-10 8
General Picture of Subduction
-
8/3/2019 Sub Duct Ion Zone. 23rd Nov,2011 (Lec#1 Onwards)
9/80
Lecture-10 9
General Picture of Ocean-Ocean
Convergence
-
8/3/2019 Sub Duct Ion Zone. 23rd Nov,2011 (Lec#1 Onwards)
10/80
Lecture-10 10
General Picture of Ocean-Continent Subduction
-
8/3/2019 Sub Duct Ion Zone. 23rd Nov,2011 (Lec#1 Onwards)
11/80
Lecture-10 11
Second General Example of
Ocean-Continent Subduction
-
8/3/2019 Sub Duct Ion Zone. 23rd Nov,2011 (Lec#1 Onwards)
12/80
Lecture-10 12
The Termination of a Subduction
Zone: Indian-Eurasian Boundary
-
8/3/2019 Sub Duct Ion Zone. 23rd Nov,2011 (Lec#1 Onwards)
13/80
Lecture-10 13
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.
-
8/3/2019 Sub Duct Ion Zone. 23rd Nov,2011 (Lec#1 Onwards)
14/80
Lecture-10 14
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
-
8/3/2019 Sub Duct Ion Zone. 23rd Nov,2011 (Lec#1 Onwards)
15/80
Lecture-10 15
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.
-
8/3/2019 Sub Duct Ion Zone. 23rd Nov,2011 (Lec#1 Onwards)
16/80
Lecture-10 16
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.
-
8/3/2019 Sub Duct Ion Zone. 23rd Nov,2011 (Lec#1 Onwards)
17/80
Lecture-10 17
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.
-
8/3/2019 Sub Duct Ion Zone. 23rd Nov,2011 (Lec#1 Onwards)
18/80
Lecture-10 18
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.
-
8/3/2019 Sub Duct Ion Zone. 23rd Nov,2011 (Lec#1 Onwards)
19/80
Lecture-10 19
Oceanic plates are subducted creating oceanic trenches
-
8/3/2019 Sub Duct Ion Zone. 23rd Nov,2011 (Lec#1 Onwards)
20/80
Lecture-10 20
Subduction Zones
The dominant features associated with
subduction zones are:
deep earthquakes
volcanoes- mountain building
-
8/3/2019 Sub Duct Ion Zone. 23rd Nov,2011 (Lec#1 Onwards)
21/80
-
8/3/2019 Sub Duct Ion Zone. 23rd Nov,2011 (Lec#1 Onwards)
22/80
Lecture-10 22
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.
-
8/3/2019 Sub Duct Ion Zone. 23rd Nov,2011 (Lec#1 Onwards)
23/80
Lecture-10 23
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.
-
8/3/2019 Sub Duct Ion Zone. 23rd Nov,2011 (Lec#1 Onwards)
24/80
Lecture-10 24
Sometimes Slab Geometry is Simple
-
8/3/2019 Sub Duct Ion Zone. 23rd Nov,2011 (Lec#1 Onwards)
25/80
Lecture-10 25
Often it is Complicated (South America)
-
8/3/2019 Sub Duct Ion Zone. 23rd Nov,2011 (Lec#1 Onwards)
26/80
Lecture-10 26
Often it is Complicated (Tonga)
-
8/3/2019 Sub Duct Ion Zone. 23rd Nov,2011 (Lec#1 Onwards)
27/80
Lecture-10 27
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
-
8/3/2019 Sub Duct Ion Zone. 23rd Nov,2011 (Lec#1 Onwards)
28/80
Lecture-10 28
Subduction Zones and Volcanoes
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
-
8/3/2019 Sub Duct Ion Zone. 23rd Nov,2011 (Lec#1 Onwards)
29/80
Lecture-10 29
Subduction Zones and Volcanoes
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.
-
8/3/2019 Sub Duct Ion Zone. 23rd Nov,2011 (Lec#1 Onwards)
30/80
Lecture-10 30
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.
Subduction Zones and Volcanoes
-
8/3/2019 Sub Duct Ion Zone. 23rd Nov,2011 (Lec#1 Onwards)
31/80
Lecture-10 31
Subduction Zones and Volcanoes
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
-
8/3/2019 Sub Duct Ion Zone. 23rd Nov,2011 (Lec#1 Onwards)
32/80
Lecture-10 32
Subduction Zones and Volcanoes
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
-
8/3/2019 Sub Duct Ion Zone. 23rd Nov,2011 (Lec#1 Onwards)
33/80
Lecture-10 33
-
8/3/2019 Sub Duct Ion Zone. 23rd Nov,2011 (Lec#1 Onwards)
34/80
Lecture-10 34
-
8/3/2019 Sub Duct Ion Zone. 23rd Nov,2011 (Lec#1 Onwards)
35/80
Lecture-10 35
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
-
8/3/2019 Sub Duct Ion Zone. 23rd Nov,2011 (Lec#1 Onwards)
36/80
Lecture-10 36
-
8/3/2019 Sub Duct Ion Zone. 23rd Nov,2011 (Lec#1 Onwards)
37/80
Lecture-10 37
-
8/3/2019 Sub Duct Ion Zone. 23rd Nov,2011 (Lec#1 Onwards)
38/80
Lecture-10 38
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.
-
8/3/2019 Sub Duct Ion Zone. 23rd Nov,2011 (Lec#1 Onwards)
39/80
Lecture-10 39
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
-
8/3/2019 Sub Duct Ion Zone. 23rd Nov,2011 (Lec#1 Onwards)
40/80
Lecture-10 40
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
-
8/3/2019 Sub Duct Ion Zone. 23rd Nov,2011 (Lec#1 Onwards)
41/80
Lecture-10 41
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.
-
8/3/2019 Sub Duct Ion Zone. 23rd Nov,2011 (Lec#1 Onwards)
42/80
Lecture-10 42
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)
-
8/3/2019 Sub Duct Ion Zone. 23rd Nov,2011 (Lec#1 Onwards)
43/80
Lecture-10 43
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)
-
8/3/2019 Sub Duct Ion Zone. 23rd Nov,2011 (Lec#1 Onwards)
44/80
Lecture-10 44
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.
-
8/3/2019 Sub Duct Ion Zone. 23rd Nov,2011 (Lec#1 Onwards)
45/80
Lecture-10 45
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.
-
8/3/2019 Sub Duct Ion Zone. 23rd Nov,2011 (Lec#1 Onwards)
46/80
Lecture-10 46
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.
-
8/3/2019 Sub Duct Ion Zone. 23rd Nov,2011 (Lec#1 Onwards)
47/80
Lecture-10 47
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.
-
8/3/2019 Sub Duct Ion Zone. 23rd Nov,2011 (Lec#1 Onwards)
48/80
Lecture-10 48
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
-
8/3/2019 Sub Duct Ion Zone. 23rd Nov,2011 (Lec#1 Onwards)
49/80
Lecture-10 49
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
-
8/3/2019 Sub Duct Ion Zone. 23rd Nov,2011 (Lec#1 Onwards)
50/80
Lecture-10 50
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.
-
8/3/2019 Sub Duct Ion Zone. 23rd Nov,2011 (Lec#1 Onwards)
51/80
Lecture-10 51
FIGURE (c) The bulldozer analogy for the formation of an
accretionary prism. The blade acts as the backstop.
-
8/3/2019 Sub Duct Ion Zone. 23rd Nov,2011 (Lec#1 Onwards)
52/80
Lecture-10 52
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.
-
8/3/2019 Sub Duct Ion Zone. 23rd Nov,2011 (Lec#1 Onwards)
53/80
Lecture-10 53
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
-
8/3/2019 Sub Duct Ion Zone. 23rd Nov,2011 (Lec#1 Onwards)
54/80
Lecture-10 54
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.
-
8/3/2019 Sub Duct Ion Zone. 23rd Nov,2011 (Lec#1 Onwards)
55/80
Lecture-10 55
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
-
8/3/2019 Sub Duct Ion Zone. 23rd Nov,2011 (Lec#1 Onwards)
56/80
Lecture-10 56
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
-
8/3/2019 Sub Duct Ion Zone. 23rd Nov,2011 (Lec#1 Onwards)
57/80
Lecture-10 57
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.
-
8/3/2019 Sub Duct Ion Zone. 23rd Nov,2011 (Lec#1 Onwards)
58/80
Lecture-10 58
(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.
-
8/3/2019 Sub Duct Ion Zone. 23rd Nov,2011 (Lec#1 Onwards)
59/80
Lecture-10 59
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
-
8/3/2019 Sub Duct Ion Zone. 23rd Nov,2011 (Lec#1 Onwards)
60/80
Lecture-10 60
Types of backarc regions
The three types of backarc regions are:
(1) contractional,
(2) extensional, and(3) stable
-
8/3/2019 Sub Duct Ion Zone. 23rd Nov,2011 (Lec#1 Onwards)
61/80
Lecture-10 61
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.
-
8/3/2019 Sub Duct Ion Zone. 23rd Nov,2011 (Lec#1 Onwards)
62/80
Lecture-10 62
(a) An Andean-type continental arc, with a compressional
backarc region.
-
8/3/2019 Sub Duct Ion Zone. 23rd Nov,2011 (Lec#1 Onwards)
63/80
Lecture-10 63
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.
-
8/3/2019 Sub Duct Ion Zone. 23rd Nov,2011 (Lec#1 Onwards)
64/80
Lecture-10 64
Extensional backarcs are commonly called
Mariana-type back arcs.
-
8/3/2019 Sub Duct Ion Zone. 23rd Nov,2011 (Lec#1 Onwards)
65/80
Lecture-10 65
(b) A Mariana-type island arc, with an extensional backarc.
-
8/3/2019 Sub Duct Ion Zone. 23rd Nov,2011 (Lec#1 Onwards)
66/80
Lecture-10 66
In a stable backarc, no strain accumulates
-
8/3/2019 Sub Duct Ion Zone. 23rd Nov,2011 (Lec#1 Onwards)
67/80
Lecture-10 67
(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
-
8/3/2019 Sub Duct Ion Zone. 23rd Nov,2011 (Lec#1 Onwards)
68/80
Lecture-10 68
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.
-
8/3/2019 Sub Duct Ion Zone. 23rd Nov,2011 (Lec#1 Onwards)
69/80
Lecture-10 69
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.
-
8/3/2019 Sub Duct Ion Zone. 23rd Nov,2011 (Lec#1 Onwards)
70/80
Lecture-10 70
Figure Plate model of subduction zones; a, b, c, and d indicate
regions of distinctive focal mechanisms
-
8/3/2019 Sub Duct Ion Zone. 23rd Nov,2011 (Lec#1 Onwards)
71/80
Lecture-10 71
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
-
8/3/2019 Sub Duct Ion Zone. 23rd Nov,2011 (Lec#1 Onwards)
72/80
Lecture-10 72
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
-
8/3/2019 Sub Duct Ion Zone. 23rd Nov,2011 (Lec#1 Onwards)
73/80
Lecture-10 73
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
-
8/3/2019 Sub Duct Ion Zone. 23rd Nov,2011 (Lec#1 Onwards)
74/80
Lecture-10 74
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
-
8/3/2019 Sub Duct Ion Zone. 23rd Nov,2011 (Lec#1 Onwards)
75/80
Lecture-10 75
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
-
8/3/2019 Sub Duct Ion Zone. 23rd Nov,2011 (Lec#1 Onwards)
76/80
Lecture-10 76
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
-
8/3/2019 Sub Duct Ion Zone. 23rd Nov,2011 (Lec#1 Onwards)
77/80
Lecture-10 77
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
-
8/3/2019 Sub Duct Ion Zone. 23rd Nov,2011 (Lec#1 Onwards)
78/80
Lecture-10 78
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
-
8/3/2019 Sub Duct Ion Zone. 23rd Nov,2011 (Lec#1 Onwards)
79/80
Lecture-10 79
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
-
8/3/2019 Sub Duct Ion Zone. 23rd Nov,2011 (Lec#1 Onwards)
80/80
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
top related