Mechanics of Folding and Faulting

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Heat escaping from the earth’s interior drives the motion of the earth’s tectonic plates.  As the plates move, forces act on the rocks making up the crust and upper mantle.  The analysis of these forces and their effect on earth materials is called structural geology.

 At an advanced level mathematical analysis plays a critical role in understanding structural geology.  For a student in a beginning geology course, however, it is sufficient to identify some basic geologic structures, and to try to visualize how they were formed within the context of plate tectonics.

Stress and Strain

Stress is a word used to describe the forces that act on earth materials (engineers use the word in the same way).  There are three types of stress that can be applied to rocks.  Compressional stress applies a squeezing force to the rocks.  Tensional stress is a stretching force.  A shear stress is applied to a material when forces act in opposite directions on opposite sides of the material (to illustrated shear stress place a deck of cards between your hands, then slowly move your hands in opposite directions).

Strain describes the change in shape of a material in response to a stress.  If the change in shape is permanent, geologists are able to reconstruct the ancient stresses which acted on a rock by analyzing the preserved strain.

Types of Deformation

There are three ways in which rocks will deform (strain) when stresses are applied to them.  If a rock undergoes elastic deformation it will change shape in response to a stress, then snap back into its original shape if the stress is removed (in other words the strain is recoverable).  Elastic deformation is important as a storage reservoir for the energy released by an earthquake (due to elastic rebound).  Brittle deformation occurs when stresses overcome the strength of the material and it simply breaks.  Plastic deformation will occur where pressures are relatively high, and the strain rate relatively low.  During plastic deformation the material flows, and changes in shape are permanent.

Types of Geological Structures

Review your notes and make sure you know how to identify each of the following structures.  You should also be able to what type of deformation produced each structure, and what it tells you about the stresses (forces) acting on the earth.

I.        Folds

A.     Terminology

1.                  Axis

2.                  Limb

B.     Types of Folds

1.                  Anticline

2.                  Syncline

3.                  Horizontal fold

4.                  Plunging fold

5.                  Overturned fold

II.     Joints

III.   Faults

A.     Terminology

1.                  Strike

2.                  Dip

3.                  Slip

4.                  Hanging wall

5.                  Footwall

6.                  Scarp

B.     Types of Faults

1.                  Dip-slip

a)                  Normal

b)                  Reverse

c)                  Thrust

2.                  Strike-slip

a)                  Right-lateral

b)                  Left-lateral

3.                  Oblique

Click here to practice identifying geologic structures

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