Geologic Structures

1
Geologic Structures

• Prepared by Betsy Conklin for
• Dr. Isiorho

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Tectonic Forces at Work

• structural geology the branch of geology
  concerned with the shapes, arrangement, and
  interrelationships of bedrock units and the
  forces that cause them
• stress a force per unit area
• strain the change in size (volume) or shape, or
  both, while an object is undergoing stress
  

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Stress and Strain in the Earths Crust

• compressive stress a stress due to a force
  pushing together on a body
  

4
Stress and Strain in the Earths Crust

• tensional stress caused by forces pulling away
  from one another in opposite directions

5
Stress and Strain in the Earths Crust

• shear stress due to movement prallel to but in
  opposite directions along a fulat or other
  boundary

6
Behavior of Rocks to Stress and Strain

• elastic strain strain in which a deformed body
  recovers its original shape after the stress is
  released (ex rubber band)
  
• elastic limit the maximum amount of stress that
  can be applied to a body before it deforms in a
  permanent way by bending or breaking
  
• ductile capable of being molded and bent under
  stress
  
• brittle strain cracking or rupturing of a body
  under stress

7
Present Deformation of the Crust

• Geologists often say the crust of the earth is
  mobile or restless because bedrock is moving
  and being deformed in many parts of the world
  
• fault a fracture in bedrock along which movement
  has taken place
  

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Geologic Maps and Field Methods

• geologic map a map which uses standardized
  symbols and patterns to represent rock types and
  geologic structures that is typically produced
  from the field map for a given area
• geologic cross section represents a vertical
  slice through a portion of the earth

9
Strike and Dip

• strike the compass direction of a line formed by
  the intersection of an inclined plane with a
  horizontal plane
  
• angle of dip a measurement downward from the
  horizontal plane to the bedding plane
  
• direction of dip the compass direction in which
  the angle of dip is measured
  

10
Folds

• fold bends or wave-like features in layered
  rock
  
• anticline an upward arching fold
• hinge line the axis of the fold
• syncline a downward-arching counterpart of an
  anticline
  
• axial plane a plane containing all of the hinge
  lines of a fold

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Plunging Folds

• plunging folds folds in which the hinge lines
  are not horizontal
  

Plunging folds anticline on left and right,
syncline in center. The hinge lines are at an an
gle to the block diagram, penetrating the
surface and emerging from the front cross section
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Structural Domes and Structural Basins

• structural dome a structure in which the beds
  dip away from a central point
  
• structural basin a structure in which the beds
  dip toward a central point

Structural basin
Structural dome
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Interpreting folds

• open folds a fold with gently dipping limbs
• isoclinal fold a fold in which the limbs are
  parallel to one another
  
• overturned fold a fold in which both limbs dip
  in the same direction
  
• recumbent fold a fold overturned to such an
  extent that the limbs are essentially horizontal

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Interpreting folds Unconformities
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Fractures in Rock

• joint a fracture or crack in bedrock where
  essentially no displacement occurs
  
• joint set where joints are oriented
  approximately parallel to one another

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Faults

• dip-slip fault movement is parallel to the dip
  of the fault surface
  
• strike-slip fault horizontal motion parallel to
  the strike of the fault surface
  
• oblique-slip fault both strike-slip and dip-slip
  components

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Dip-Slip Faults

• footwall the underlying surface of an inclined
  fault plane
  
• hanging wall the overlying surface of an
  inclined fault plane

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Dip-Slip Faults (cont.)

• normal fault a fault where the hanging-wall
  block has moved downward relative to the footwall
  block
  
• graben when a block bounded by normal faults
  drops down
  
• horst when a block bounded by normal faults is
  uplifted
  

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Dip-Slip Faults (cont.)

• reverse fault when the hanging-wall block has
  moved upward relative to the footwall block
  
• thrust fault a reverse fault in which the dip of
  the fault plane is at a low angle to horizontal
  

A reverse fault. The fault is unaffected by
erosion.
Arrows indicate compressive stress.
Diagram shows area after erosion dashed lines
indicate portion eroded away
Thrust fault due to horizontal compression.
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Strike-slip Faults

• strike-slip fault a fault where the movement is
  predominantly horizontal and parallel to the
  strike of the fault
  
• right-lateral fault a strike-slip fault in which
  the block seen across the fault appears displaced
  to the right
  
• left-lateral fault a strike-slip fault in which
  the block seen across the fault appears displaced
  to the left

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Pictures

• All pictures used in this power point
  presentation were taken from the following
  
• Carlson, Diane H., David McGeary and Charles C.
  Plummer. Physical Geology Updated Eighth
  Edition. New York City, McGraw-Hill Higher
  Education, 2001.