1. Eric H. Christiansen Brigham Young University

2. 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 silica-rich rock is added to the crust at convergent plate boundaries and by terrane accretion.

3. 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

4. Convergent Boundaries

5. 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

6. Ocean-Ocean Convergence
Outer swell, trench & forearc wedge, magmatic arc, and backarc basin
Associated earthquakes range from shallow to deep

7. Ocean-Ocean Subduction Zones
Associated earthquakes range from shallow to deep

8. 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

9. 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

10. 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

11. Fig. 21.6. Thermal structure of subduction zone

12. 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

13. 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

14. Ocean-Continent Convergence

15. Accretionary Wedge

17. 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

18. 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

19. 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

20. 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

21. 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

22. Continental Margin Orogenic Belt
Fig Mesozoic Structure of western United States

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. Himalaya Mountains

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. Extension at Convergent Boundaries
Creates rifting and formation of new oceanic crust
Influenced by angle
of subduction &
absolute motion
of overriding
plate

27. 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

28. 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

29. 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

30. Paired Metamorphic Belts

31. 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 silica-rich rock is added to the crust at convergent plate boundaries and by terrane accretion.

33. 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

34. 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

35. 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

36. Magma Generation at Subduction Zones

37. 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

38. Fig. 21.21. Intrusion at convergent margins

39. 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

40. Continental Arc Magmatism
Volcanoes form chains
~ 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

41. Volcanic Eruptions at Subduction Zones
Mt St Helens 1980 and beyond

55. 1982

56. Earthquakes

58. 29 August 2004

61. Welt

63. February 2005

64. August 2009
Stopped growing in January 2008

65. August 2009

66. Continental Growth at Convergent Boundaries
Continents grow by accretion

67. 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

68. 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

69. 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

70. Fig
Accreted terranes
along convergent
margin

71. 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

72. Growth of Continents

73. 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 silica-rich rock is added to the crust at convergent plate boundaries and by terrane accretion.