Folds, Faults, and Geologic Maps

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Folds, Faults, and Geologic Maps

• Chapter 9
• Geology Today
• Barbara W. Murck Brian J. Skinner

Folds, South Georgia Island
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Stress

• Stress - the force acting on a surface, per unit
  area

Fig. 9.2, p. 243
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Stress and Strain

• Strain - the change in shape or volume of a rock
  in response to stress.
  (Fig. 9.3, p. 244)

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Types of Deformation

• Elastic - nonpermanent rock returns to original
  shape when stress is removed
• Ductile (plastic) - permanent rock flows or
  bends when stressed beyond its elastic limit
• Brittle - rock breaks or cracks

Fig. 9.5 A, p. 245
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• Factors that influence
• how rocks deform
• Temperature - increase in T makes solids more
  ductile
• Confining Pressure - increase in P inhibits
  formation of fractures solids resist breaking

Controlling Factors
(Fig. 9.6, p. 246)
Undeformed Ductile
Brittle
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• Factors that influence
• how rocks deform
• Rate of deformation - stress applied quickly
  enhances brittle behavior. Stress applied slowly
  over a long period of time promotes ductile
  behavior.

Controlling Factors
(Fig. 9.6, p. 246)
Undeformed Ductile
Brittle
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• Factors that influence
• how rocks deform
• Composition - some minerals are very brittle
  (quartz, garnet, olivine) as are rocks that
  contain them.
• Minerals that behave ductilely mica, clay,
  calcite, gypsum
• Rocks that behave ductilely shale, slate,
  limestone, marble

Controlling Factors
(Fig. 9.5, p. 245)
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Strike and Dip

• Strike - orientation of a horizontal line in a
  plane. (Expressed as a compass direction)
• Dip - angle between a tilted surface and a
  horizontal plane.
• (Expressed as an angle and direction)

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Faults

• Fault - a fracture in
• the crust along which
• movement has occurred.
• Normal Fault (Fig. 9.4, p. 245)
• Faults are classified by how steeply they dip and
  relative movement of the blocks
• Hanging wall - block on top of the fault
• Footwall - block below the fault

Hanging Wall
Footwall
Footwall
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Faults

• Normal Fault -
• hanging wall down relative to footwall.
• Tensional stress
• (Fig. 9.9, p. 250)

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Faults

• Horst - two normal faults dip away from each
  other, block between them is elevated.
• Graben - two normal faults dip towards each
  other, block between them drops down.
• Tensional stress
• (Fig. 9.10, p. 250)

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Faults

• Reverse Fault -
• hanging wall moves up
• relative to footwall
• Dip is steep
• Compressional stress
• (Fig. 9.11, p. 251)

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Faults
Reverse fault for comparison

• Thrust Fault - hanging wall moves up relative to
  footwall
• Dip is lt 30 degrees
• (Fig. 9.12, p. 251)

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Faults

• Strike-slip fault - movement is mostly horizontal
  and parallel to strike of fault.
• Left-lateral - block opposite you moves left.
• Right-lateral - block opposite you moves right.
• (Fig. 9.13, p. 251)

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How do you describe something as complex as a
fold?
Folds

• Fold axis - the line along which the axial plane
  intersects a single rock layer
• Axial plane (axial surface) - the plane that
  divides the fold in half as symmetrically as
  possible

Fig. 9.17 B, p. 255
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How do you describe something as complex as a
fold?
Folds

• Fold limbs - the two sides of the fold that are
  separated by the axial plane
• Anticline - limbs point down
• Syncline - limbs point up

Fig. 9.17 B, p. 255
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How do you describe something as complex as a
fold?
Folds

• Orientation of both axial plane and fold axis are
  used.
• Horizontal fold axis with vertical axial plane
  upright fold
• Plunging fold -
• fold axis is not horizontal

Fig. 9.17 B, p. 255
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• Five common types of fold geometries
• (Fig. 9.20, p. 257)

Folds
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To describe a fold, first determine orientation
of axial planes, limbs, axes
Folds

• Here, axial planes are not quite vertical, axes
  point straight out, limbs dip at different
  angles
• open, asymmetrical, anticline - syncline

Fig. 9.17 A, p. 255
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To describe a fold, first determine orientation
of axial planes, limbs, axes
Folds

• Here, axial planes are not quite vertical, axes
  point straight out, limbs dip at different
  angles
• open, asymmetrical, anticline - syncline

Fig. 9.17 A, p. 255
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Folds

• Here, both limbs dip in the same direction so
  that one limb is actually upside down
• overturned, symmetrical folds

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Fig. 9.1, p. 242
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Folds

• Here, both limbs dip in the same direction so
  that one limb is actually upside down
• overturned, symmetrical folds

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Fig. 9.1, p. 242
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• Open, asymmetrical folds
• (Fig. 9.4 A, p. 245)

Folds
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• Open, asymmetrical folds
• (Fig. 9.4 A, p. 245)

Folds
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Special fold types
Folds

• Domes and Basins (Fig. 9.18, p. 256)

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Folds
Special fold types

• Monocline - upper and lower limbs are horizontal,
  only the central limb is inclined (Fig.
  9.16, p. 254)

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Showing strata on maps(Fig. 9.21, p. 258)
Geologic Maps

• Contacts - boundaries between distinct rock types
• Block diagram shows eroded strata, cross section
• Geologic map shows contacts with strike and dip
  symbols.

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Showing folds on maps(Fig. 9.19, p. 256)
Folds

• Plunging folds make horseshoe patterns on surface
• Block diagram shows pattern projected to
  horizontal surface, as if strata were eroded flat
• Map shows how folds are depicted on geologic maps.

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Cross Sections

• Cross sections show how structures and rock
  layers behave at depth.
• Appalachian Mtns.
• (Fig. 9.26, p. 266)

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Geologic maps - show topograpic contours, major
folds and faults, contacts between rock units,
age and type of rock.(Fig. 9.22, p. 259)
Geologic Maps
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Topographic Maps

• Topographic maps use contour lines to depict
  topography. (Fig. 9.23, p. 263)

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