Convergent Boundaries Zones where lithospheric plates collide Three major types Ocean - Ocean Ocean - Continent Continent Continent
Convergent Boundaries Convergent boundaries may form subduction zones Occurs in oceanic crust Associated with trench & forearc, magmatic arc, and backarc basin Associated earthquakes range from shallow to deep
Convergent Boundaries Crustal deformation is common Melange (chaotic mixture of rocks of different type, age, and origin) produced at subduction zone Extension & compression in backarc basin Continental collisions involve strong horizontal compression
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
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 (continental crust) grow by addition; Oceanic crust is destroyed
Ocean-Ocean Convergence One plate thrust under to form subduction zone (the colder, denser, older oceanic crust is subducted) Subducted plate is heated, magma generated Andesitic volcanism forms island arc Broad belts of crustal warping occur Ex.: Tonga Islands in western Pacific
Island Arc Magmatism Volcanic islands form arcuate chain 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
Fig. 21.2. Ocean-Ocean convergence
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 Ex.: Andes of S. Am; Cascade Range of OR & WA; older ex. is Rocky Mt. 50-150 mya
Continental Arc Magmatism Volcanic islands form arcuate chain 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
Fig. 21.3. Ocean-Continent convergence
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 Regional metamorphism occurs with generation of granitic magmas Ex.: Indian & Eurasian Plate produced Himalaya Mts. & Tibetan Plateau
Fig. 21.4. Continent-Continent convergence
Earthquakes - Subduction Zones Subducting slab forms inclined seismic zone Reaches depths of > 600 km (deep) Shallow quakes in broad zone from shearing of two plates Intermediate quakes also occur within slab
Compression at Subduction Zones Unconsolidated ocean sediments form accretionary wedge Sediments scraped off of subducting plate sometimes forming a melange Folds of various sizes formed Fold axes parallel to trench Thrust faulting & metamorphism occur
Compression at Subduction Zones Orogenic belts (mountain belts) are created at ocean - continent margins Pronounced folding and thrust faulting Granitic plutons develop, add to deformation Rapid uplift creates abundant erosion
Fig. 21.13. Structure of western NA
Compression in Continent- 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
Fig. 21.15. Formation of Himalaya Mtns
Extension at Convergent Boundaries Extension may be common at convergent boundaries Warping of crust can create tensional stress Extreme extension can create rifting and formation of new oceanic crust Influenced by angle of subduction & motion of overriding plate
Cascade Range of OR & WA
End of Chapter 21