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Chapter 9Continental Tectonics and Mountain Chains

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Guiding Questions• How does continental rifting begin and what environments of deposition does it

produce? • How do rocks become folded?• How does a mountain chain form when a continental margin encounters a subduction

zone? • Why does a foreland basin accumulate large volumes of sediment on the continent?• Why is continental crust not subducted?• What is the significance of ophiolites?• What is the zonation of a typical mountain chain?• How have the Andes formed? • How did the Pyrenees form? • What is an exotic terrane?• What broad features does rock deformation create in continents far from their margins?

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Terranes• Geologically

distinctive regions of Earth’s crust, each of which has behaved as a coherent crustal block

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San Andreas Fault• 5.5 cm/year

– (~2 inches)

• Moving L.A. (on the Pacific plate) closer to San Francisco (on the North American plate)

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Rifting• Triple junction

– Three-armed grabens at plate boundaries

– Associated with doming

• Hot spot

– May have multiple types of plate boundaries

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Rifting• Formation of Atlantic

Ocean– Red Sea provides

modern analogue

– Three-armed rift forms

– One may die out• Failed rift

– Mississippi River

– Amazon

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Rifting• Rift valleys

– Extension breaks continental crust into fault blocks

• Blocks subside rapidly

– Accumulate sediments, lakes

• Rift Valley, East Africa

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Rifting• When rifting

continues, continents separate along ridge axis

• Margins cool, sink

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Rifting• Passive margin

– Tectonically inactive areas of continental crust that accumulate sediment along shallow shelves

• Eastern U.S.

• Active margin– Zones of tectonic deformation

and igneous activity

• Western U.S.

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Bending and Flowing of Rocks• Rocks can bend and

flow under stress– Metamorphosis at

high pressures and temperatures

– Compressive forces• Folding

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Bending and Flowing of Rocks• Syncline

– Rocks folded concave up

– Vertices at bottom

• Anticline– Rocks folded

concave down– Vertices at top

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Bending and Flowing of Rocks• Dip

– Angle that the bed forms with the horizontal plane

• Strike– Compass direction that lies

at right angles to the dip– Always horizontal– Regional strike

• Overall trend of fold axes

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Bending and Flowing of Rocks

• Axial plane– Imaginary plane

that cuts through fold and divides it symmetrically

• Overturned fold– If either limb is

rotated more than 90° from its original position

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Bending and Flowing of Rocks

• Axis of a fold– Line of intersection

between axial plane and beds of folded rock

• Plunging fold– Axis lies at an angle

to the horizontal

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Bending and Flowing of Rocks• Plunging fold

– When eroded, produces a curved outcrop pattern

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Mountain Building• Orogenesis

– Process of mountain building

– Orogenies• Mountain

building events

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Mountain Building

• Continental Collision– Continental crust cannot

be subducted– Suturing

• Unification of two continents along a subduction zone

– Ophiolite• Remnant of seafloor

pinched up along suture

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Mountain Building• Magma rises into overlying

continental slab– Volcanoes form, elevate

crust• Mountain peaks

– Plutons cool to form igneous core

• Metamorphic Belt– Rocks on either side of core

are deformed by core’s heat and other processes

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• Fold and Thrust belt– On continental edge– Turned over away from

core• Brittle deformation

• Thrust sheets– Large slices of crust

formed by thrust faulting– Slide along basal surface

Mountain Building

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Mountain Building• Folds and faulting

– Increase folding– Develop overturned

fold– Overturned fold can

break

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Mountain Building• Cross-section of Rocky Mountains

• Thrust faults slice through previously folded rocks

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Deformation Processes• Deformation caused by

– Pressure applied by subducted plate

• Pushes mountain chain toward interior of continent

– Folding near igneous arc and inland

– Gravity spreading• Rock deforms under its own

weight, spreads out• Deformation along folds and

thrusts

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• Foreland basin– Downwarping of

lithosphere beneath actively forming mountain chain beyond fold and thrust belt

– Axis is parallel to mountain chain

– Rapid formation– Deep, often flooded

Deformation Processes

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• Foreland basin sediments– Flysch

• Shales, turbidites– Floods rapidly– Turbidites accumulate

Deformation Processes– Molasse

• Nonmarine sediments– Mountain evolves– Fold and thrust moves inland– Chokes basin, folds flysch– Alluvial fans, floodplains, etc.

• Clastic wedge

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Andes Mountain Building• Igneous rocks added since Mesozoic• Continuing to build up

– Bobs isostatically– Mountain chain is migrating inland

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Andes Mountain Building• Magma shifting

inland as subduction angle is reduced

• Change in angle means change in plate movement

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Andes Mountain Building• 10 M years ago

– Foreland basin connected to Atlantic along thin seaway

• Infilling of foreland basin led to formation of Amazon River from seaway– Stranded marine animals

that adapted to freshwater

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The Pyrenees• Formed when Iberia

collided with Eurasia– Cretaceous and Paleogene– Iberia originally part of

Eurasia– Subduction began,

reattached toward north– Ophiolites in northern

Pyrenees mark suturing– Foreland basin received

flysch then molasse

• Exotic terrane

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Continental Interiors• Structural basin

– Circular or oval depression of stratified rock

• Structural dome– Circular or oval uplift

of stratified rock

• Erosion leads to circular pattern

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Continental Interiors• Black hills of South Dakota

– Oblong dome

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Continental Interiors• Domes and basins of North America

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Continental Interiors• Michigan

– Structural basin

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