Transcript EDS_21_Convergent_sm

Eric H. Christiansen
Brigham Young University
Major Concepts
1.Convergent plate boundaries are zones where
lithospheric plates collide and include (a)
convergence of two oceanic plates, (b)
convergence of an oceanic and a continental
plate, and (c) collision of two continental
plates. The first two involve subduction of
oceanic lithosphere into the mantle.
2. Plate temperatures, convergence rates, and convergence directions play important
roles in determining the final character of a convergent plate boundary.
3. Most subduction zones have an outer swell, a
trench and forearc, a magmatic arc, and a
backarc basin. In contrast, continental
collision produces a wide belt of folded and
faulted mountains in the middle of a new
continent.
4. Subduction of oceanic lithosphere produces a
narrow, inclined zone of earthquakes that
extends to more than 600 km depth, but
broad belts of shallow earthquakes form where
two continents collide.
5. Crustal deformation at subduction zones
produces melange in the forearc and extension
or compression in the volcanic arc and backarc
areas. Continental collision is always marked
by strong compression
6. Magma is generated at subduction zones
because dehydration of oceanic crust causes
partial melting of the overlying mantle.
Andesite and other silicic magmas that
commonly erupt explosively are distinctive
products . Plutons range from diorite to
granite. In continental collision zones, magma
is less voluminous, dominantly granitic, and
probably derived by melting of preexisting
continental crust.
7. Metamorphism at subduction zones produces
low-temperature–high-pressure facies near the
trench and higher-temperature facies near the
magmatic arc. Broad belts of highly deformed
metamorphic rocks mark the sites of past
continental collision.
8. Continents grow larger as low-density silicarich rock is added to the crust at convergent
plate boundaries and by terrane accretion.
Convergent Boundaries
 Zones where
lithospheric plates
collide
 Three major types



Ocean - Ocean
Ocean - Continent
Continent - Continent
 Direction and rate of
plate motion
influence final
character
Convergent Boundaries
Ocean-Ocean Convergence
 One plate thrust
under to form
subduction zone
 Subducted plate is
heated, magma
generated
 Andesitic
volcanism forms
island arc
 Broad belts of
crustal
deformation and
metamorphism
form
Ocean-Ocean Convergence
 Outer swell,
trench & forearc
wedge, magmatic
arc, and backarc
basin
 Associated
earthquakes range
from shallow to
deep
Ocean-Ocean Subduction Zones
 Associated
earthquakes range
from shallow to
deep
Earthquakes - Subduction Zones
 Subducting slab forms
inclined seismic zone
 Angle of plunge
between 40-60o
 Reaches depths of >
600 km
 Shallow quakes in
broad zone from
shearing of two plates
 Deeper quakes occur
within slab
Thermal Structure of Subduction
 Cold slab
 Cold subducting plate
heats very slowly
 Temperature at 150 km


Cold slab ~ 400oC
Surrounding mantle ~
1200oC
 T variation influences
slab behavior


More brittle & stronger
Moves downward as
coherent slab
Thermal Structure of Subduction
 Hot Arc
 Heat flow is elevated
beneath volcanic arc
 Ascending magma
carries heat from
mantle
 Subducting plate may
cause mixing in the
asthenosphere
beneath the arc
Fig. 21.6. Thermal structure of subduction zone
Thermal Structure of Subduction
 Cold slab
 Cold subducting plate heats very slowly
 Temperature at 150 km depth


Cold slab ~ 400oC
Surrounding mantle ~ 1200oC
• Hot Arc
– Heat flow is elevated beneath volcanic arc
– Ascending magma carries heat from mantle
Ocean-Continent
Convergence
 Oceanic plate thrust
under to form
subduction zone
 Subducted plate is
heated, magma
generated
 Andesitic volcanism
forms continental arc
with more silicic
magma
 Broad belts of crustal
warping occur
including folded
mountain belts
Ocean-Continent
Convergence
Accretionary Wedge
Continent-Continent Convergence
 One continent thrust
over the other
 No active subduction
zone
 Folded mountain belt
forms at suture of two
continental masses
 Crust becomes very
thick
 Orogenic
metamorphism occurs
with generation of
granitic magmas
Deformation at Convergent Boundaries
 Crustal deformation
is common
 Melange produced in
accretionay wedge at
subduction zone
 Extension &
compression in
backarc
 Continental
collisions involve
strong horizontal
compression
Accretionary Wedge at Subduction Zones
 Unconsolidated
sediments form
accretionary wedge
 Sediments scraped off of
subducting plate
 Folds of various sizes
formed

Fold axes parallel to trench
 Thrust faulting &
metamorphism occur
 Growing mass tends to
collapse
Accretionary Wedge at Subduction Zones
 Melange is a complex
mixture of rock types
 Includes
metamorphosed
sediments and
fragments of
seamounts & oceanic
crust
 Not all sediment is
scraped off
 20-60% carried down
with subducting slab
Orogenic Belts at Subduction Zones
 Compression
creates at ocean continent margins
 Pronounced folding
and thrust faulting
 Granitic plutons
develop, add to
deformation
 Rapid uplift creates
abundant erosion
Continental Margin Orogenic Belt
Fig. 21.13. Mesozoic Structure of western United States
Compression in Continent Collisions
 Accretionary wedge and
magmatic arc remnants
included in orogenic belt
 Continental collision
thickens crust
 Tight folds and thrust
faulting
 Possible intrusion of
granitic plutons
 Substantial uplift
associated with erosion
Himalaya Mountains
Extension at Convergent Boundaries
 Extension may be
common at convergent
boundaries
 Warping of crust creates
extensional stress
 Extreme extension creates
rifting and formation of
new oceanic crust
 Influenced by angle of
subduction & absolute
motion of overriding plate
Extension
Extension at Convergent Boundaries
 Creates rifting and formation of new oceanic crust
 Influenced by angle
of subduction &
absolute motion
of overriding
plate
Metamorphism at convergent margins
• Driven by changes in environment
• Tectonic & magmatic processes at
convergent margins create changes
in P & T
 Occurs in wide linear belts
 Associated horizontal compression
 High temperature metamorphism
may occur in association with
magmas
 Marks the roots of folded mountain
belts
• Paired metamorphic belts are
commonly associated with
subduction zones
Paired Metamorphic Belts

Outer metamorphic belt forms
in accretionary wedge

Blueschist facies metamorphism

High P - low T

Metamorphosed rocks brought
back to surface by faulting
Include chunks of oceanic
crust and serpentine
Inner metamorphic
belt forms near magmatic arc






Range from Low T and P to High
T and P conditions
Contact metamorphism occurs
near magma bodies
Orogenic metamorphism occurs
in broader area
Greenschist to amphibolite
grade
Paired Metamorphic Belts

Outer metamorphic belt forms
in accretionary wedge

Blueschist facies metamorphism

High P - low T

Metamorphosed rocks brought
back to surface by faulting
Include chunks of oceanic
crust and serpentine
Inner metamorphic
belt forms near magmatic arc






Range from Low T and P to High
T and P conditions
Contact metamorphism occurs
near magma bodies
Orogenic metamorphism occurs
in broader area
Greenschist to amphibolite
grade
Paired
Metamorphic Belts
Major Concepts
1.Convergent plate boundaries are zones where
lithospheric plates collide and include (a)
convergence of two oceanic plates, (b)
convergence of an oceanic and a continental
plate, and (c) collision of two continental
plates. The first two involve subduction of
oceanic lithosphere into the mantle.
2. Plate temperatures, convergence rates, and convergence directions play important
roles in determining the final character of a convergent plate boundary.
3. Most subduction zones have an outer swell, a
trench and forearc, a magmatic arc, and a
backarc basin. In contrast, continental
collision produces a wide belt of folded and
faulted mountains in the middle of a new
continent.
4. Subduction of oceanic lithosphere produces a
narrow, inclined zone of earthquakes that
extends to more than 600 km depth, but
broad belts of shallow earthquakes form where
two continents collide.
5. Crustal deformation at subduction zones
produces melange in the forearc and extension
or compression in the volcanic arc and backarc
areas. Continental collision is marked by
strong compression
6. Metamorphism at subduction zones produces
low-temperature–high-pressure facies near the
trench and higher-temperature facies near the
magmatic arc. Broad belts of highly deformed
metamorphic rocks mark the sites of past
continental collision.
7. Magma is generated at subduction zones
because dehydration of oceanic crust causes
partial melting of the overlying mantle.
Andesite and other silicic magmas that
commonly erupt explosively are distinctive
products . Plutons range from diorite to
granite. In continental collision zones, magma
is less voluminous, dominantly granitic, and
probably derived by melting of preexisting
continental crust.
8. Continents grow larger as low-density silicarich rock is added to the crust at convergent
plate boundaries and by terrane accretion.
Magmatism at Convergent Boundaries
 Continental collision
produces silicic magmas
from melting of lower
portions of thickened
continental crust
 Subduction produces
basaltic, andesitic, and
rhyolitic magma
Magma Generation: Continental Collision
 Smaller volumes of
granitic magma are
produced at continental
collisions
 Melting is induced by deep
burial of crust
 Melt forms from partial
melting of metamorphic
rocks
 Granites have distinct
compositions and include
several rare minerals
Magma Generation at Subduction Zones
 Water in slab is




released by
metamorphism, rises
and induces melting
of overlying mantle
Water lowers mineral
melting points
Characteristically
andesite in
composition
Contains more water
and gases than basalt
and is more silicic
Results in more
violent volcanism
Magma Generation at Subduction Zones
Magma Generation at Subduction Zones
 Hybrid magma
rises & interacts
with crust
 Magma may have
components from
oceanic crust,
sediment, mantle,
and overlying crust
 Fractional
crystallization
enriches the
magma is silica
Fig. 21.21. Intrusion at convergent margins
Island Arc Magmatism
 Volcanic islands form
arcuate chain
 ~ 100 km from trench
 High heat flow & magma
production
 Build large composite
volcanoes

Basalt , Andesite with little
rhyolite
 Volcanoes built on oceanic
crust & metamorphic
rocks
 Volcanoes tend to be
evenly spaced
Continental Arc Magmatism
 Volcanoes form chains
 ~ 100 - 200 km from
trench
 Build large composite
volcanoes

Andesite with more
abundant rhyolite
 Plutons of granite &
diorite
 Volcanoes built on older
igneous & metamorphic
rocks
 Volcanoes tend to be
evenly spaced
Volcanic Eruptions at Subduction Zones
 Mt St Helens 1980 and
beyond
1982
Earthquakes
29 August 2004
Welt
February 2005
August 2009
Stopped growing in January 2008
August 2009
Continental Growth at Convergent
Boundaries
 Continents grow
by accretion
Formation of Continental Crust
 Continental crust
grows by accretion
 New material
introduced by arc
magmatism
 Older crust is strongly
deformed
 New crust is enriched
in silica & is less dense
 No longer subject to
subduction
How to Build a Continent
 Continental crust grows by
accretion
 New material introduced
by arc magmatism
 Old crust is deformed
 New crust is enriched in
silica
 Cannot subduct
Accreted Terranes
 Continental margins
contain fragments of
other crustal blocks
 Each block is a
distinctive terrane with
its own geologic
history

Formation may be
unrelated to current
associated continent
 Blocks are separated by
faults

Mostly strike-slip
Fig. 21.28.
Accreted terranes
along convergent
margin
Continental Growth Rates
 Basement ages in continents
form “concentric rings” of
outward decreasing age
 Each province represents of
series of mountain building
events
 Rate varies over geologic time
 Slow rate during early history
- some crust may have been
swept back into mantle
 Rapid growth between 3.5
and 1.5 bya
 Subsequent growth slower
Growth of
Continents
Major Concepts
1.Convergent plate boundaries are zones where
lithospheric plates collide and include (a)
convergence of two oceanic plates, (b)
convergence of an oceanic and a continental
plate, and (c) collision of two continental
plates. The first two involve subduction of
oceanic lithosphere into the mantle.
2. Plate temperatures, convergence rates, and convergence directions play important
roles in determining the final character of a convergent plate boundary.
3. Most subduction zones have an outer swell, a
trench and forearc, a magmatic arc, and a
backarc basin. In contrast, continental
collision produces a wide belt of folded and
faulted mountains in the middle of a new
continent.
4. Subduction of oceanic lithosphere produces a
narrow, inclined zone of earthquakes that
extends to more than 600 km depth, but
broad belts of shallow earthquakes form where
two continents collide.
5. Crustal deformation at subduction zones
produces melange in the forearc and extension
or compression in the volcanic arc and backarc
areas. Continental collision is always marked
by strong compression
6. Metamorphism at subduction zones produces
low-temperature–high-pressure facies near the
trench and higher-temperature facies near the
magmatic arc. Broad belts of highly deformed
metamorphic rocks mark the sites of past
continental collision.
7. Magma is generated at subduction zones
because dehydration of oceanic crust causes
partial melting of the overlying mantle.
Andesite and other silicic magmas that
commonly erupt explosively are distinctive
products . Plutons range from diorite to
granite. In continental collision zones, magma
is less voluminous, dominantly granitic, and
probably derived by melting of preexisting
continental crust
8. Continents grow larger as low-density silicarich rock is added to the crust at convergent
plate boundaries and by terrane accretion.