University of Colorado GEOLOGY 1010

Class Note 16

Geologic Structures

Stress and Strain

Stress is the applied force. Strain is the resultant deformation.

Stress can be compressional, tensional, shear, or isostatic.

• Compressional stress pushes matter together.
• Tensional stress pulls matter apart.
• Shear stress is rotational.
• Isostatic stress is even pressure as with burial.

All applied stresses cause rock (or any other solid) to deform (strain).

Strain can be elastic or plastic.

• Elastic strain is recovered (disappears) on release of the stress.
• Plastic strain is not recovered (permanent) on release of stress.

If a material undergoes continuous plastic deformation, it is said to be ductile. If it fractures, it is said to be brittle.

Rock Folds

Fig 16.1. Split Mountain Anticline, Dinosaur National Monument, Colorado. This is an example of a plunging, anticlinal fold in Paleozoic and Mesozoic sedimentary rocks. The river is the Green just below its confluence with the Yampa. There are also some pre-Cambrian sediments exposed in the core of this classic anticline, upstream of this photo.

Sedimentary rocks that deform plastically are said to form folds.

• A syncline is a concave-upward fold.
• An anticline is a concave-downward fold.
• A monocline is a fold connecting two vertically offset sections of sedimentary rocks.

The orientation ofthe strata (beds) at any point in a fold can be described by the strike and dip.

• The strike is the compasss direction of the intersection of the stratum with the horizontal plane.
• The dip is angle measured from the horizontal to the stratum perpendicular to strike.

Rock Fractures

If no movement has occurred on a fracture it is called a joint.

If movement has occurred on a fracture it is called a fault.

Movement on a fault is described as dip-slip, strike-slip, or oblique-slip.

• Dip-slipmovement is movement of one block up or down the dip of the fault plane.
  • A reverse fault is upward dip-slip movement of the upper block relative to the lower block.
  • A thrust fault is a low-angle reverse fault.
  • Reverse faults are the result of compressional stress.
  • A normal is downward dip-slip movement of the upper block.
  • Normal faults are the reult of tensional stress.

• A strike-slip fault is the horizontal movement of one block relative to the other.
  • A left-lateral strike-slip is one in which the opposite block has moved to the left.
  • A right-lateral strike-slip is one in which the opposite block has moved to the right.

• An oblique-slip fault has a combination of strike-slip and dip-slip movements.

Unconformities

• A disconformity is a break in sedimentation with erosion with no deformation so that the upper beds are parallel to the lower.

  

• An angular unconformity is a break in sedimentation with tilting and erosion of the surface before sedimentation resumes (without metamorphism of the lower beds). This is the "great unconformity" of the Grand Canyon. Here the Paleozoic sediments lie on top of the slightly tilted late Proterozoic sediments at the bottom right.

• A non-conformity is an angular unconformity in which the older beds are metamorphosed and deformed.

Class Note 17

Class Note 15

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