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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