Prepared by Mark R' Noll

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• Prepared by Mark R. Noll
• SUNY College at Brockport

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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

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Convergent Boundaries

• Convergent boundaries may form subduction zones
• Occurs in oceanic crust
• Associated with outer swell, trench forearc,
  magmatic arc, and backarc basin
• Associated earthquakes range from shallow to deep

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Convergent Boundaries

• Crustal deformation is common
• Melange produced at subduction zone
• Extension compression in backarc basin
• Continental collisions involve strong horizontal
  compression

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Convergent Boundaries

• Magma is generated
• Subduction and partial melting of oceanic crust,
  sediments and surrounding mantle
• Produces andesitic magma
• Continental convergence produces silicic magmas
  from melting of lower portions of thickened
  continental crust

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Convergent Boundaries

• Metamorphism occurs in broad belts
• Metamorphism is associated with high pressure
  from horizontal compression
• High temperature metamorphism may occur in
  association with magmas
• Continents grow by addition

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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 warping occur

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Fig. 21.2. Ocean-Ocean convergence
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Ocean-Continent Convergence

• Oceanic plate thrust under to form subduction
  zone
• Subducted plate is heated, magma generated
• Andesitic volcanism forms continental arc
• Broad belts of crustal warping occur including
  folded mountain belts

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Fig. 21.3. Ocean-Continent convergence
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Continent-Continent Convergence

• One plate thrust over the other
• No subduction zone associate warping occurs
• Folded mountain belt forms at suture of two
  continental masses
• Orogenic metamorphism occurs with generation of
  granitic magmas

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Fig. 21.4. Continent-Continent convergence
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Plate Buoyancy

• Processes at convergent margins influenced by
  plate density
• Sharp contrast in density of oceanic and
  continental crust
• Differences in thickness change density
• Thick oceanic crust forms less dense lithospheric
  plate
• Temperature age also affect density

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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

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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

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Fig. 21.6. Thermal structure of subduction zone
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Plate Motion

• Direction rate of plate motion are important
  factors in plate dynamics
• Head on collisions form large subduction zones
  with intense compression and igneous activity
• Oblique angle collisions are less energetic and
  have smaller subduction zones

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Earthquakes - Subduction Zones

• Subducting slab forms inclined seismic zone
• Angle of plunge between 40-60o
• Reaches depths of gt 600 km
• Shallow quakes in broad zone from shearing of two
  plates
• Deeper quakes occur within slab

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Compression 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

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Compression 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

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Compression at Subduction Zones

• Orogenic belts are created at ocean - continent
  margins
• Pronounced folding and thrust faulting
• Granitic plutons develop, add to deformation
• Rapid uplift creates abundant erosion

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Fig. 21.13. Structure of western NA
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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

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Fig. 21.15. Formation of Himalaya Mtns
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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

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Island Arc Magmatism

• Volcanic islands form arcuate chain
• 100 km from trench
• High heat flow magma production
• Build large composite volcanoes
• Andesite with some rhyolite
• Volcanoes built on oceanic crust metamorphic
  rocks
• Volcanoes tend to be evenly spaced

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Continental Arc Magmatism

• Volcanic islands form arcuate chain
• 100 - 200 km from trench
• Build large composite volcanoes
• Andesite with some rhyolite
• Plutons of granite diorite
• Volcanoes built on older igneous metamorphic
  rocks
• Volcanoes tend to be evenly spaced

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Fig. 21.19. Magma production at subduction zones
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Magma Generation

• Characteristically andesite in composition
• Contain more water and gases than basalt
• Results in more violent volcanism
• Water in slab is released, induces melting of
  overlying mantle
• Water lowers mineral melting points

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Fig. 21.20. Magma generation mechanisms
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Magma Generation

• Hybrid magma rises interacts with crust
• Magma has oceanic crust, sediment mantle and
  overlying crust components
• Fractional crystallization enriches the magma is
  silica

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Magma Generation

• 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 geochemistry and include
  several rare minerals

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Fig. 21.21. Intrusion at convergent margins
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Metamorphism

• Metamorphism driven by changes in environment
• Tectonic magmatic processes at convergent
  margins create changes in P T
• Paired metamorphic belts are commonly associated
  with subduction zones

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Fig. 21.26. Paired metamorphic belts
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Metamorphism

• 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

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Metamorphism

• Inner metamorphic belt forms near magmatic arc
• High T and varying P conditions
• Contact metamorphism occurs near magma bodies
• Orogenic metamorphism occurs in broader area
• Greenschist and amphibolite grade

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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

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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

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Fig. 21.28. Accreted terranes along
convergent margin
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Continental Growth Rates

• Basement ages in NA 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

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Figs. 21.30-31. Growth of continents
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End of Chapter 21