Folds, Faults, and Mountain Building

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Folds, Faults, and Mountain Building

• Chapter 9

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Stress!!!

• Stressa force exerted on an object
• Rocks react to stress by
• Deforming plastically
• Deforming elastically
• Breaking by brittle fracture
• Question of the hour (1)
• How will a particular rock react to a particular
  stress?

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Its all in the Nature

• Rock composition
• Ex Hitting a chunk of concrete with a
  sledgehammer VS. hitting a giant rubber tire with
  a sledgehammer

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I Cant Take the Pressure!!

• Pressure and Temp. increase with depth
• Temp vs. pressure
• Burial promotes plastic deformation

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If you cant stand the heat

• Higher temperatures favor plastic deformation
• Ex. Bending a steel nail
• Need to be super strong
• Or just heat the thing up

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Its About Time

• Stress applied slowly favors plastic behavior
• Stress applied suddenly favors brittle behavior

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May the Tectonic Force be With You.

• Plate tectonic forces
• Tensional
• Stretch and pull formations apart
• Compressive
• Squeeze and shorten rock formations
• Shearing
• Two sides of a formation are pushed in opposite
  directions (sliding past one another)

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Geologic Structures

• Plate tectonicsprimary stressor
• 3 main types of structures
• Foldsductile deformation (primarily)
• Faultsbrittle deformation
• Jointsbrittle deformation

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Folds

• A fold is a bend in the rock
• Plastic deformation
• Three characteristics
• Usually results from compression
• Folding always shortens the horizontal distances
  in rock
• A fold usually occurs as part of a group of folds

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Anticline Vs. Syncline
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Plunging Folds
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Overturned and Asymmetric
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Circular Features

• Domes
• Circular anticlinal structure (both pics?)
• Beds dip away from center
• Basin
• Circular synclinal structure
• Beds dip towards center

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All My Faults are Stress Related!!!

• Brittle deformation
• Faults
• Cracks in rock along which motion has occurred
• Movement may be gradual or sudden (earthquake)
• Usually associated with other faults in a fault
  zone
• Question of the hour (2) Why do rocks move
  repeatedly along faults and fault zones?

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Nature is Lazy

• Rock moves repeatedly along faults and fault
  zones because
• Tectonic forces often affect one location for a
  long time
• Its easier
• Three types of faults
• Depend on tectonic forces

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Normal Faulting
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Reverse Fault (part 1)
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Reverse Fault (part 2)
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Strike-slip Fault
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Ah, How My Joints Ache

• A joint is a fracture in rock along which no
  motion has occurred
• Planes of weaknesslike mineral cleavage
• Good for mining

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Styles of Continental Deformation

• Relating Small Structures to Big Picture

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Question of the Hour (3)

• Where do mountains come from?
• In a subducting zone
• Magmas/lavasstall/erupt
• High temprocks expand when heated
• Continent/continent collisionunderthrusting
• Crust is shoved under other crust
• Compression squeezes crustthickness increases

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Tensional Tectonics

• Plates can split apart
• Rift valleys long narrow troughs formed by a
  block that has dropped down between two big
  normal faults

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Rifting and Normal Faulting
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Compressive Tectonics

• Fold and thrust belts
• When two continental plates collide, crust
  compressed
• Crust thickened by underthrustingthicken crust
  2x

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Transverse Margins