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