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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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