Structural Geology: Deformation and Mountain Building

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Structural Geology Deformation and Mountain
Building
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Tectonic Stresses ? Large Scale Strain of the
Crust - Geologic Structures
Crust Rigid, Thin

• Inner core Solid iron
• Outer core Liquid iron, convecting (magnetic
  field)
• Mantle (Asthenosphere) Solid iron-magnesium
  silicate, plastic, convecting
• Crust (Lithosphere) Rigid, thin 5-30km

Mantle Plastic, Convecting
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Tectonics and Structural Geology

• Tectonic Stresses resulting from Internal Energy
  (heat driving convection) Strains (deforms) the
  Mantle and Crust
• Bends Rocks
• ductile strain (Folds)
• Breaks Rock
• brittle strain (Joints)
• Moves large blocks
• Faults
• Releases energy Earthquakes

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Folds and Faults (Palmdale, Ca)
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Kaynasli, Turkey
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Stresses at Plate Boundaries

• Divergent
• (Tensional)
• ??
• Convergent
• (Compressional)
• ??
• Transform
• (Shear)
• e.g., Pacific NW

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Geological Structures

• Different stresses result in various forms of
  strain (geologic structures)
• Folds (compressive stresses may cause ductile
  strain)
• Faults (Any type of stress may cause brittle
  strain. The type of fault depends on the type of
  stress)

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Geological Structures

• Fault a discontinuity surface across which there
  has been shear displacement
• Hangingwall the wall and body of rock above an
  inclined fault
• Footwall the wall and body of rock beneath an
  inclined fault

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Geological Structures

• Normal Fault hanging wall moved downwards
  compared to the footwall
• Reverse Fault hanging wall moved upwards
  compared to the footwall
• Thrust fault low angle reverse fault

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Geological Structures

• Strike Slip Fault displacement parallel to the
  fault plane.
• Can be left or right handed!

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Stikes and Dips are used to identify geologic
structures
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Strike and Dip

• Define and map the orientation of planar features
• Bedding planes (sedimentary rocks)?
• Foliation
• Joints
• Faults
• Dikes
• Sills
• Ore Veins

Fig. 10-4, p. 221
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Strike and Dip
Strike and Dip Map Symbol

• Strike The line of intersection between the
  plane and a horizontal surface
• Dip Angle that the plane makes with that
  horizontal plane

Fig. 10-4, p. 221
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Anticline (fold)
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Syncline (fold)
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Plunging Anticline
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Fold Terminology

• Hinge the greatest curvature of a folded surface
• Axial plane a planar surface defined by the
  successive positions of fold hinges
• Plunge

Axis Axis
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Plunging Anticline, Colorado
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Domes and Basins
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Brittle Strain ? Joints

• When shallow crust is strained rocks tend to
  exhibit brittle strain

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Sheet Joints
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Defining Fault Orientation

• Strike of fault plane parallels the
• fault trace
• fault scarp
• Direction of Dip of the fault plane indicates the
  Hanging wall block

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Fault

• Movement occurring along a discontinuity
• Brittle strain and subsequent movement as a
  result of stress
• Fault terminology?

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Faults

• Fault When movement occurs along a discontinuity
• Fault type depends on the type of stress

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Normal Faults
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Normal Faults, Horsts and Grabens
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Horsts and Grabens

• Older Rocks are exposed along the ridges formed
  by the horsts
• Younger rocks lie beneath the grabens
• Sediment fills in the linear valleys

Horst
Horst
Graben
Graben
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Nevada

• Washboard topography is the result of Horsts
  and Grabens
• A.k.a, Basin and Range
• E.g., Humbolt Range
• E.g., Death Valley (Graben)

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Horst and Graben, Nevada
Horst
Graben
Humboldt Range, Northern Nevada
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Horst and Graben, Nevada
Horst
Graben
Humboldt Range, Northern Nevada
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Reverse and Thrust Faults

• Compressive stress causes the hanging wall to
  move upward relative to the foot wall ? Reverse
  Fault
• At convergent plate boundaries ancient rocks can
  be thrust over younger rocks ? Thrust Fault

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Thrust Fault Glacier NP, Montana
Old
Younger
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Strike Slip Faults

• Physiographic Features