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Prepared by Mark R' Noll

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Associated with outer swell, trench & forearc, magmatic arc, and ... Volcanic islands form arcuate chain ~ 100 km from trench. High heat flow & magma production ... – PowerPoint PPT presentation

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Title: Prepared by Mark R' Noll


1
  • Prepared by Mark R. Noll
  • SUNY College at Brockport

2
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

3
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

4
Convergent Boundaries
  • Crustal deformation is common
  • Melange produced at subduction zone
  • Extension compression in backarc basin
  • Continental collisions involve strong horizontal
    compression

5
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

6
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

7
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

8
Fig. 21.2. Ocean-Ocean convergence
9
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

10
Fig. 21.3. Ocean-Continent convergence
11
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

12
Fig. 21.4. Continent-Continent convergence
13
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

14
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

15
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

16
Fig. 21.6. Thermal structure of subduction zone
17
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

18
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

19
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

20
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

21
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

22
Fig. 21.13. Structure of western NA
23
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

24
Fig. 21.15. Formation of Himalaya Mtns
25
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

26
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

27
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

28
Fig. 21.19. Magma production at subduction zones
29
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

30
Fig. 21.20. Magma generation mechanisms
31
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

32
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

33
Fig. 21.21. Intrusion at convergent margins
34
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

35
Fig. 21.26. Paired metamorphic belts
36
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

37
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

38
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

39
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

40
Fig. 21.28. Accreted terranes along
convergent margin
41
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

42
Figs. 21.30-31. Growth of continents
43
End of Chapter 21
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