ESC 101: 7.4 Faults

• A fault is a planar surface within the Earth where rocks have broken and slid.
• Faults originate from elastic strain that leads to brittle failure.
• Rocks on each side of a fault shift in opposite, offset directions.
• If the fault is not vertical, there exists rocks above and beneath the fault.

Hanging Wall and Footwall

• The rocks above a fault are known as the hanging wall.
• Rocks under the fault are known as the footwall.

Normal and Detachment Faults

• In a normal fault, the hanging wall moves down relative to the footwall.
• A detachment fault is a low-angle normal fault separating deep, ductile crustal rocks (granite and gneiss) from brittle upper crustal rocks (sedimentary or volcanic).
• Detachment faults are located along the boundaries of metamorphic core complexes.
• Normal and detachment faults are located in areas of crustal tension.
• A divergent plate boundary represents a zone of large normal faults.
• Other areas of crustal tension with normal faults include the Basin and Range region of the western United States.

Reverse and Thrust Faults

• In a reverse or thrust fault, the hanging wall moves up relative to the footwall.
• The difference between a reverse fault and thrust fault is dip angle; reverse faults have dips steeper than 30°.
• Reverse and thrust faults occur in areas undergoing compression.
• Convergent plate boundaries represent zones of major reverse and thrust faults.
• Subduction zones can also be referred to as megathrust faults.
• Reverse and thrust faults occur in compressional settings such as the Transverse Mountain Ranges, north of Los Angeles.

Strike-Slip Faults

• Strike-slip faults are steep or vertical, with rocks moving horizontally in opposite directions.
• Transform plate boundaries are zones of large strike-slip faults.
• The San Andreas fault is a major strike-slip fault at a transform boundary.