Crustal Deformation

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Crustal Deformation
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Take-Away Points

• Structures in the earth have practical
  implications
• Basic terms for earth structures
• Small structures provide clues to much larger
  structures
• The earths crust is floating on the denser
  mantle beneath
• Gentle uplifts and downwarps affect the interiors
  of continents

3
Economic Consequences of Geologic Structures

• Tracing Coal Seams, Aquifers, etc.
• Ore Deposits are often localized along faults and
  folds
• Petroleum Traps

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The Big Picture
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Anticlines and Synclines
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Monoclines and Homoclines
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Axial Plane
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How Fast Can Folds Form?
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Which Way was Up?

• Sedimentary Structures are governed by
• Gravity
• Exposure to the Surface
• They all have a right way up

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Foliation

• Foliation is a sheetlike structure that forms
  when rocks are deformed.

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Foliation

• Flattening
• Grain Rotation
• Solution
• New Minerals
• Partial Melting
• Shearing

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Foliation

• In every case, the foliation is
• In the direction of least resistance
• at right angles to the direction of greatest
  compression.

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Folds and Foliation

• On a small scale (microscopic to centimeters),
  foliation forms by a variety of mechanisms, but
  always at right angles to the direction of
  greatest compression
• On a large scale (centimeters to kilometers),
  rocks fold. The axial plane of the fold is also
  at right angles to the direction of greatest
  compression

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Folds and Foliation
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The Importance of Minor Folds
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How Geologists Use These Clues

• Here's an outcrop that might be seen in the
  field.

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How Geologists Use These Clues

• Picture the axial plane of the fold as parallel
  to the foliation.
• The other side of the fold is roughly a mirror
  image of the side we can see.

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How Geologists Use These Clues

• We can mentally fill out the sketch to get an
  idea of the shape of the fold.
• Note that we still have no idea how big the fold
  is, only what kind it is.

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What if we did it the other way?
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Isostasy
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The Airy Model
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The Pratt Model
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Both Ideas are Correct at Times
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How Isostasy Works
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Isostatic Rebound in Canada
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Isostatic Rebound in Scandinavia
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Domes and Basins
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Domes and Basins
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Domes and Basins
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Take-Away Points

• Structures in the earth have practical
  implications
• Basic terms for earth structures
• Small structures provide clues to much larger
  structures
• The earths crust is floating on the denser
  mantle beneath
• Gentle uplifts and downwarps affect the interiors
  of continents