Chapter 15 Deformation

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Chapter 15 - Deformation

How did it get this way?
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Deformation All Changes in the original rock
form, size, location, orientation, usually
happens at plate margins. Convergent and
transform margins show most deformation. Stress
Amount of Force Applied to a given area
Differential stress causes deformation. Types of
Strain Brittle and Ductile Brittle Deformation
usually occurs near the Earths surface, causing
fractures, joints, and faults. Ductile
Deformation usually occurs at depth, due to heat
and/or confining pressure, causing folding and
flowage. How a rock reacts to Stress is related
to Temperature, Confining Pressure, Rock Type,
and Time.
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Types of Strain - all changes in the original
form and size of a given rock body

• Compressional

Tensional
Shear
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Chapter 15 -

• Styles of Deformation
• Brittle Faults, Fractures, Joints
• Near surface, low temperature, low confining
  pressure.
• Ductile Anticlines, Synclines, Monoclines
• Deeper, higher temperature, higher confining
  pressure.

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Tensional strain Brittle deformation possible
setting continental rift crustal stretching
Hanging wall drops
Graben basin
High angle Normal fault (gravity fault)
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Compressional strain Brittle deformation
possible setting continental collision
vertical shift
Hanging wall rises
High angle Reverse fault
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Compressional strain Ductile deformation
Possible setting continental collision
Anticlines Limbs dip away from axial plane.
Axial planes
Anticline
Synclines Limbs dip toward axial plane.
Youngest
Symmetrical folds both limbs have the same
angle of inclination, see pg. 427.
Limbs
Limb
Syncline
Oldest
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Compressional strain Brittle deformation
Possible setting continental collision
Low angle Reverse fault Thrust fault One sheet
ramps up over another.
Examples Alps, Appalachians, Juarez Mts.,
Mexico
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• Mapping Geologic Structures
• During the geologic study of an area, geologists
  
• Identify the major rock types
• Identify contacts between major rock types
• Identify any significant geologic structures,
  i.e., are there any variations in the orientation
  of the rock, is it in its original state?
• Sedimentary rocks most easily mapped because of
  their original horizontal bedding planes are
  often preserved, i.e., the rock often splits
  along bedding planes.
• Foliated metamorphic rocks orientations may be
  determined by joints that parallel foliated
  layers.
• Non-foliated metamorphic rocks and igneous
  intrusions the edges may be the best indication
  of the orientation.

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Strike and Dip Measurements page 423
Dip angle of downward inclination at right
angle to strike orientation
300
Strike Compass orientation of horizontal line
super-imposed on tilted rock surface.
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Foliated metamorphic rocks look for joints
parallel to foliations
If quartz layers seem parallel with foliations,
these may be mapped
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Compressional strain Ductile deformation
Eroded Dome Beds dip away from center
Oldest
Youngest
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Compressional strain Ductile deformation
Eroded basin Beds dip toward center
Youngest
Oldest
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Brittle deformation joints and fractures
joints may be in parallel sets and flat,
fractures are more irregular.
Campus andesite UT El Paso
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Map views of eroded anticlines and synclines
Axial planes
Oldest in core of fold
Youngest in core of fold
Syncline limbs dip toward axial plane
Anticline limbs dip away from axial plane
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Strike slip fault Map view of horizontal
movement May be associated with transform fault
zone
Opposite side of fault moves to observers right
Right lateral
Observer
X
Opposite side of fault moves to observers left
Left lateral
X
Observer
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