Geologic structures

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• Chapter 20
• Geologic structures

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• How rocks respond to tectonic forces!

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• Tectonic forces
• Forces generated inside the earth are called
  tectonic forces.
• Tectonic forces are expressed in terms of Stress
  and the resulting Strain.

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• Stress force applied per unit area.
• Strain resulting change in size or shape or
  both.

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• Stress 3 types
• Compressive when a body is pushed together or
  squeezed from opposite directions.
• Tensional when a body is pulled away in opposite
  directions
• Shear when two bodies slide past one another.

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• How rocks respond to stresses?
• Rock behavior depends on
• Amount and rate of applied stress
• Type of rock
• TP conditions

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• Rocks behavior is of 3 types
• Elastic deformed body recovers its original
  shape after the stress is released.
• Ductile deformed body does not return to its
  original position and its bends.
• Brittle deformed body will fracture at stresses
  higher than the elastic limit.

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• Geologic field methods
• Geologic maps
• What is a map?
• Representation of the 3D world on a 2D flat
  surface.
• Maps are designed to show where things are.
• A geologic map shows the distribution of geologic
  features, including different kinds of rocks and
  structures.

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• What is Strike and Dip?
• Strike is the compass direction of a linear or
  planar feature.
• N30E
• N45W
• Dip is the angle an inclined plane makes with the
  horizontal.
• 30NE

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• Geologic structures
• They may be
• Folds
• Faults
• Joints

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• Folds bends or curved features in layered rocks.
  Best displayed in sed. rocks.
• Folds are the result of ductile deformation.

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• Fold geometry
• Anticline arching upwards
• Syncline arching downwards
• Parts of a fold
• Limb side of a fold
• Hinge point highest point on a fold
• Hinge line line joining hinge points. Also
  called fold axis.
• Axial plane plane joining all hinge lines.

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• Dome and basin
• Dome a structure in which beds dip away from a
  central point.
• Basin beds dip towards a central point.

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• Types of folds
• Open
• Isoclinal
• Overturned
• Recumbent
• Open limbs have gentle dips indicating moderate
  stresses.

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• Isoclinal limbs parallel to one another
  indicating intense stresses.

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• Overturned axial plane inclined and the limbs
  dip in the same direction. Indicate unequal
  compressive stresses.

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• Recumbent the limbs and axial planes are
  essentially horizontal. Extreme stresses are
  indicated.

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• Geologic structures and OIL
• Oil and gas are concentrated in the core of an
  anticline where they are trapped by an
  impermeable layer.

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• Faults
• Fracture in rocks along which there has been some
  movement.
• Parts of a fault
• Hanging wall
• Footwall
• Fault plane

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• Types of faults
• 3 types
• Dip slip
• Strike slip
• Oblique slip

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• Dip-slip faults movement is parallel to the dip
  of the fault plane.

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• Two types of dip slip faults normal and reverse
• Normal fault H.W. moves downwards relative to
  the F.W.
• Reverse fault H.W. moves upwards relative to the
  F.W. A thrust fault is a low angle reverse fault.

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• Graben and Horst
• A graben is a block of rock which is moved
  downwards due to two normal faults.
• A horst is an uplifted block of rocks due to two
  normal faults.

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• Strike-slip faults movement is parallel to the
  strike of the fault surface.
• San Andeas fault in CA.

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• Joints
• Joints are regular fractures along which there
  has been negligible movement.

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• Joints
• Joint sets form where they are oriented parallel
  to one another.
• Joints indicate the direction of applied stresses.

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