Structure: Faults

A. Certain terms help us describe faults

1. In a cross-section view of a fault, the rock above the fault zone is called the hanging wall.

2. The rock below the fault zone is called the foot wall.

3. Faults are often planar in orientation, which allows geologists to measure their strike and dip. (see lecture 21 on stratigraphy)

B. What are the different types of faults?

1. When motion on a fault is vertical, it is called a dip-slip fault. Two types of dip-slip faults can be distinguished by the relative movement of the hanging wall and foot wall of a fault.

a. If the hanging wall moves down relative to the foot wall, then it is a normal fault.

b. If the foot wall moves down relative to the hanging wall, then it is a reverse fault.

2. When the motion on a fault is more horizontal, then it is called a strike-slip or lateral fault. Two types of strike slip faults can be distinguished by relative lateral motion of the fault. Pretend that you are standing on one sied of a strike slip fault:

a. If the opposite side has moved to the right relative to your position, that is called a right lateral fault. If it moves to the left relative to your position, then it is a left lateral fault. What kind of fault is the San Andreas?

b. A transform fault refers to larger strike-slip faults associated with plate boundaries and spreading ridges.

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