Mountain Building

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Mountain Building
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CONTINENTS
Continental Crust Scum that floats buoyant
silicate rocks.
29 of Earths surface.
Upper portion of lithosphere, 30-60 km thick.
Contains oldest rocks (and minerals) on Earth.
Records many cycles of deformation, mountain
building (orogeny), erosion and glaciation.
GROWTH OF CONTINENTS Accretion of mountain belts
and island arcs.
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Shields old, highly deformed areas eroded to low
relief.
Represent continental nuclei that were later
added to.
Ages 0.6-4.1 Ga.
Platform
Very stable few earthquakes.
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Platform Areas of thin cover (lt 7,000 meters) of
younger, more-or-less undeformed sedimentary
rocks on top of shields.
Platform
An apron of younger rock on shield much of the
sediment was derived from the shield or mountain
ranges.
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Mountain Belts Areas of deformed crust.
Platform
Features of Active Mountain Belts earthquakes,
volcanoes (island arcs) and trenches nearby in
young mountain ranges.
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Fig. 20.01
Mountain Belt Chains thousands of km long
composed of numerous mountain ranges. Belts are
long and thin. Typically along continental
margins.
Mountain Range Group of closely spaced mountains
or parallel ridges. Contains deformed,
metamorphosed rocks.
HIGHER ONES ARE YOUNGER Himalayas 45 Ma and
still rising. Often find marine fossils at top
of mountains uplift! Appalachians stopped 250
Ma. Mountains eventually eroded.
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Major Mountain Belts of the World
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Fig. 20.03
Platform
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Fig. 20.04
CRATON region of a continent that has been
stable for a considerable time period.
Sedimentary rock on CRATONS not deformed.
Sedimentary rock in mountain belts folded,
faulted etc. Marine origin. Volcanics also.
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Satellite image of the Pilbara CratonWestern
Australia.
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METAMORPHISM PLUTONISM
Most intensely deformed areas complex
metamorphic and plutonic rocks.
MIGMATITES - interlayered granitic-metamorphic
rocks.
Melting granitic plutons which rise and now
eroded and exposed.
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PATTERNS OF FOLDING FAULTING
Crustal shortening and thickening
Open folds in areas of less intense compression
towards closed folds and then overturned and
recumbent folds
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Fig. 20.7a
Crustal Shortening
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Fig. 20.08
Recumbent Folding!
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PATTERNS OF FOLDING FAULTING
Reverse faults and thrust faults common in the
most intensely deformed areas.
Thrust faults may be stacked, with the lowermost
one called the detachment fault along which the
upper rocks have been transported and the rocks
beneath remain in place.
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Faulting due to Compression
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NORMAL FAULTING
Occurs after intense deformation
Result of vertical uplift or extension
Extension can occur simultaneously in the center
while compression occurs on the outer portions.
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EVOLUTION OF A MOUNTAIN BELT
Takes 100s of millions of years.
3 stages with no sharp boundaries between them
Accumulation Stage Orogenic Stage Uplift
Block-Faulting Stage
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EVOLUTION OF A MOUNTAIN BELT
Accumulation Stage
Sedimentary/volcanic rocks accumulate mostly in
marine environment.
Sediment source nearby continent or volcanic
island arc.
Can accumulate along a passive continental margin
limestones, shales, sandstones - volcanics are
rare or absent.
Can accumulate along convergent boundary shales
and sandstones, graywackes, pyroclastics and
eroded volcanics.
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EVOLUTION OF A MOUNTAIN BELT
Orogenic Stage
Episode of intense deformation accompanied by
metamorphism and igneous activity (volcanism and
plutonism).
Folding and reverse (thrust) faulting common
(compression).
Deeply buried rocks schists and gneisses.
Can occur concurrently with the Accumulation
Stage.
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EVOLUTION OF A MOUNTAIN BELT
Orogenic Stage
Ocean-Continent Convergence
Accretionary Wedge above trench.
Fold thrust belts may develop on the craton
side.
Thrusting is predominantly toward the craton.
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EVOLUTION OF A MOUNTAIN BELT
Orogenic Stage
Ocean-Continent Convergence
Magmatic arc is hotter, so it is higher.
Rising may produce normal faults in this region.
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EVOLUTION OF A MOUNTAIN BELT
Orogenic Stage
Ocean-Continent Convergence Gravitational
Collapse Spreading
Mountain belt gets too high and gravitationally
unstable.
Explains fold thrust belts simultaneous
normal faulting (compression extension) how
once deep-seated metamorphic rocks rise to the
surface.
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EVLN OF A MTN BELT
Orogenic Stage
Arc-Continent Convergence
Arc too light to subduct accreted continental
growth.
Can cause reversal of subduction (e.g., Solomon
Islands).
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EVOLUTION OF A MOUNTAIN BELT
Orogenic Stage
Continent-Continent Convergence e.g.,
Appalachians, Himalayas
Wilson Cycle cycle of splitting of a
supercontinent, opening of an ocean basin and
collision of continents (birth death of an
ocean).
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EVLN OF A MTN BELT
Uplift Block Faulting Stage
After plate convergence stops, relaxing the
compressive force, there is a long period of
uplift and erosion.
Isostasy. Concept lighter, less dense
continental crust floats higher on the mantle
than the denser oceanic crust.
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Fig. 17.11
Principle of Isostasy
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EVLN OF A MTN BELT
Uplift Block Faulting Stage
Craton equilibrium. Mtns thicker, so float
higher.
As material removed by erosions, the range floats
upward to regain its isostatic balance. Not
instantaneous time lag.
Isostatic adjustment occurs during orogeny, but
effects over shadowed by tectonic compression.
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EVLN OF A MTN BELT
Uplift Block Faulting Stage
Normal Faulting is characteristic of this stage.
Crust breaks into blocks if upthrown block is
large enough Fault-Block Mountain Range.
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Crust breaks into blocks if upthrown block is
large enough Fault-Block Mountain Range.
Caused by extension. Sometimes blocks are tilted.
May have isolated volcanic activity along faults
extending deep into the Earth.
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EVOLUTION OF A MOUNTAIN BELT
Uplift Block Faulting Stage
Block faulting may also occur as below.
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EVLN OF A MTN BELT
Delamination detachment of the mantle portion of
the lithosphere beneath a mtn belt.
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EVLN OF A MTN BELT
This is colder and denser and sinks
Hot asthenosphere flows into replace the cold
lithopshere, heats lower crust and cause it to
flow, thinning it.
Results in extension.
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TERRANES
Regions within which there is geological
continuity. Geology is markedly different in
neighboring terranes.
Mountain belts divided into terranes in an effort
to understand them.
Terrane boundaries are usually faults.
Alaska and W. Canada subdivided into gt 50
terranes
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TERRANES
Suspect Terranes
Terranes not formed at their present site.
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TERRANES
Accreted Terranes
A terrane did not form at its present site on a
continent (isotopic comparisons with neighboring
terranes).
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TERRANES
Exotic Terrane
Terrane that has traveled a great distance to its
present position (paleomagnetic data isotopic
comparisons).
This Figure shows a tentative reconstruction of
how parts of Alaska might have migrated in time
based on paleomagnetic data.
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Where are future exotic terranes?
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Cross-Section of the Alps
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