Lecture Outlines Physical Geology, 12/e

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Lecture OutlinesPhysical Geology, 12/e

• Plummer Carlson

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Mountain Belts and the Continental
CrustPhysical Geology 12/e, Chapter 20
Introduction Characteristics of Major Mountain
Belts Evolution of Mountain Belts The Growth of
Continents
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Introduction Mountain Belts and Earths Systems

• Mountain range is a group of closely spaced
  mountains or parallel ridges
• Mountain belts are chains of mountain ranges that
  are 1000s of km long
• Commonly located at or near the edges of
  continental landmasses
• Mountain belts are part of the geosphere
• Form and grow by tectonic and volcanic processes
  over tens of millions of years

Major Mountain Belts
Andes
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Introduction Mountain Belts and Earths Systems

• As mountains grow higher and steeper, erosion
  rates (particularly from running water and ice -
  hydrosphere) increase
• Air (atmosphere) rising over mountain ranges
  results in precipitation and erosion

Major Mountain Belts
Andes
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Introduction Mountain Belts and Earths Systems

• Major controlling factors during a mountain
  belts history
• 1. Intense deformation
• Mainly compression
• Folds faults
• Foliation metamorphism
• Orogeny is an episode of intense deformation
• 2.Isostasy
• Vertical movement before after an orogeny
• Continental crust floats on mantle
• 3. Weathering erosion
• Depends on climate, rock type, elevation, etc.

Major Mountain Belts
Andes
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Mountain Belts and the Continental
CrustCharacteristics of Major Mountain Belts
Size Alignment Ages of Mountain Belts
Continents Thickness Characteristics of Rock
Layers Patterns of Folding Faulting Metamorphism
Plutonism Normal Faulting Thickness Density
of Rocks Features of Active Mountain Ranges
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Characteristics of Mountain Belts

• Mountain belts are very long compared to their
  width
• The North American Cordillera runs from
  southwestern Alaska down to Panama
• Mountain belts in North America tend to parallel
  coast lines. Others, e.g. Himalayas dont.
• Older mountain ranges (Appalachians) tend to be
  lower than younger ones (Himalayas) due to
  erosion
• Young mountain belts are tens of millions of
  years old, whereas older ones may be hundreds of
  millions of years old

The mountain belts and craton of North America
Schematic cross section through part of a
mountain belt (left) and part of the continental
interior (craton)
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Characteristics of Mountain Belts

• Ancient mountain belts (billions of years old)
  have eroded nearly flat to form the stable cores
  (cratons) of the continents
• Shields - areas of cratons laid bare by erosion

Schematic cross section through part of a
mountain belt (left) and part of the continental
interior (craton)
Satellite image of part of a craton in Western
Australia
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Rock Patterns in Mountain Belts

• Mountain belts typically contain thick sequences
  of folded and faulted sedimentary rocks, often of
  marine origin (passive margin)
• May also contain great thicknesses of volcanic
  rock (active margin)

Recumbent folds in the Andes
False-color satellite image of part of the Valley
and Ridge province of the Appalachian mountain
belt, near Harrisburg, Pennsylvania
Cross section of an Andean type mountain belt
(oceanic-continental convergence)
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Rock Patterns in Mountain Belts

• Fold and thrust belts (composed of many folds and
  reverse faults) indicate crustal shortening (and
  thickening) produced by compression
• Common at convergent boundaries
• Typically contain large amounts of metamorphic
  rock

Recumbent folds in the Andes
False-color satellite image of part of the Valley
and Ridge province of the Appalachian mountain
belt, near Harrisburg, Pennsylvania
Cross section of an Andean type mountain belt
(oceanic-continental convergence)
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Rock Patterns in Mountain Belts

• Erosion-resistant batholiths may be left behind
  as mountain ranges after long periods of erosion
• Localized tension in uplifting mountain belts can
  result in normal faulting as a result of vertical
  uplift or horizontal

Schematic cross section of a mountain belt in
which gravitational collapse and spreading are
taking place during plate convergence
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Rock Patterns in Mountain Belts

• Horsts and grabens can produce mountains and
  valleys

Fault-block mountains with movement along normal
faults
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Thickness Density of Rocks Features of Active
Mountain Ranges

• Rocks of continental crust (including mountain
  belts) are less dense than oceanic crust
• Seismic velocities suggest granite compostion
• Seismic models suggest crust thicker beneath
  mountain belts than cratons and thickest under
  younger mountain belts
• Earthquakes common along faults in mountain
  ranges
• Deep-ocean trenches parallel many young mountain
  belts, e.g. the Andes

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Mountain Belts and the Continental
CrustEvolution of Mountain Belts
Orogenies Plate Convergence Post-Orogenic
Uplift Block-Faulting
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Evolution of Mountain Belts Orogenies Plate
Convergence

• Rocks (sedimentary and volcanic) that will later
  be uplifted into mountains are deposited during
  accumulation stage
• Typically occurs in marine environment, at
  opening ocean basin or convergent plate boundary

Schematic cross section of a mountain belt in
which gravitational collapse and spreading are
taking place during plate convergence
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Evolution of Mountain Belts Orogenies Plate
Convergence

• Mountains are uplifted at convergent boundaries
  during the orogenic stage
• Result of ocean-continent, arc-continent, or
  continent-continent convergence
• Subsequent gravitational collapse and spreading
  may bring deep-seated rocks to the surface

Schematic cross section of a mountain belt in
which gravitational collapse and spreading are
taking place during plate convergence
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Evolution of Mountain BeltsOrogenies Plate
Convergence

• Orogenies Ocean-Continent Convergence
• Accretionary wedge
• Igneous and metamorphic processes
• Fold thrust belts on craton (backarc side)
• Gravitational collapse spreading

Cross section of an Andean type mountain belt
(oceanic-continental convergence)
Schematic cross section of a mountain belt in
which gravitational collapse and spreading are
taking place during plate convergence
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Evolution of Mountain BeltsOrogenies Plate
Convergence

• Arc-Continent Convergence
• Figure 20.11
• Arc too buoyant to subduct
• Ocean crust may break and a new trench forms
  seaward or a flip in subduction direction may
  occur, e.g. New Guinea
• Island arc added to Sierra Nevada in Mesozoic
• Appalachians in Paleozoic

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Evolution of Mountain BeltsOrogenies Plate
Convergence

• Orogenies and Continent-Continent Convergence
• Figure 20.13 (Alps)
• Figure 20.14 (Himalayas)
• Continent crust too buoyant to subduct
• Suture zone
• Appalachian mountains (Alleghenian Orogeny)
• Wilson Cycle is the opening closing of ocean
  basins and continental collisions

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Evolution of Mountain Belts Post-Orogenic Uplift
Block Faulting

• After convergence stops, a long period of
  erosion, uplift and block-faulting occurs
• As erosion removes overlying rock, the crustal
  root of a mountain range rises by isostatic
  adjustment

Isostasy in a mountain belt
Development of fault-block mountain ranges
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Evolution of Mountain Belts Post-Orogenic Uplift
Block Faulting

• Tension in uplifting and spreading crust results
  in normal faulting and fault-block mountain
  ranges
• Horizontal extensional strain
• Isostatic vertical adjustment
• Bounded on both sides by normal faults or tilted
  fault blocks

Development of fault-block mountain ranges
The Teton Range, Wyoming, a tilted fault-block
range
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Evolution of Mountain Belts Post-Orogenic Uplift
Block Faulting

• Basin-and-Range province of western North America
  may be the result of delamination
• Overthickened mantle lithosphere beneath old
  mountain belt may detach and sink into
  asthenosphere
• Resulting inflow of hot asthenosphere can stretch
  and thin overlying crust, producing normal faults

The Basin and Range and adjoining geological
provinces (block-faulting)
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Evolution of Mountain Belts Post-Orogenic Uplift
Block Faulting

• Basin-and-Range province of western North America
  may be the result of delamination
• Overthickened mantle lithosphere beneath old
  mountain belt may detach and sink into
  asthenosphere
• Resulting inflow of hot asthenosphere can stretch
  and thin overlying crust, producing normal faults

Upwelling, hot, buoyant mantle (asthenosphere)
causes extension, thinning, and block-faulting of
the overlying crust
Delamination and thinning of continental crust
following orogeny
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Growth of Continents

• Continents grow larger as mountain belts evolve
  along their margins
• Accumulation and igneous activity add new
  continental crust

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Growth of Continents

• Tectonostratigraphic terranes (terranes) are
  regions within which there is geologic continuity
  usually bounded by faults
• Suspect terranes are terranes that may not have
  formed at their present site
• Accreted terranes are known to have not formed at
  their present site
• Exotic terranes are terranes that can be shown to
  have traveled great distances

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Growth of Continents

• New accreted terranes can be added with each
  episode of convergence
• Western North America (especially Alaska)
  contains many such terranes
• Numerous terranes, of gradually decreasing age,
  surround older cratons that form the cores of the
  continents

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End of Chapter 20