Download Chapter 21 Convergent Plate Boundaries

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 ContinentContinent 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