Faults and Folds

D Coombs San Tan Junior High School 2015

Normal Faults

In a normal fault, the hanging wall block above the fault moves down relative to the footwall block below the fault. This fault motion is caused by tensional forces and results in extension. Normal faults extend the crust in a direction perpendicular to the fault trace. Because the hanging wall moves downward, normal faults place younger rocks over older rocks.[Other names: normal-slip fault, tensional fault or gravity fault] EX., Sierra Nevada/Owens Valley; Basin & Range faults.

Conjugate Normal faults, Canyonlands National Park, Utah:

Reverse Faults

In a reverse fault, the hanging wall block above the fault moves up relative to the footwall block below the fault. This fault motion is caused by compressional forces and results in shortening. [Other names: thrust fault, reverse-slip fault or compressional fault] EX., Rocky Mountains, Himalayas

Reverse Fault located near Durango, Colorado

Strike-Slip Faults

In a Strike-Slip Fault, the movement of blocks along a fault is horizontal. If the block on the far side of the fault moves to the left, as shown in this animation, the fault is called left-lateral. If the block on the far side moves to the right, the fault is called right-lateral. The fault motion of a strike-slip fault is caused by shearing forces. Strike-Slip Faults typically have near-vertical fault surfaces. Examples: San Andreas Fault, California; Anatolian Fault, Turkey [Other names: transcurrent fault, lateral fault, tear fault or wrench fault.]

Strike-slip Fault near Las Vegas, Nevada

Reverse faults and associated fold. Near Klamath Falls, Oregon

Folds:  make Anticlines and Synclines

 Anticlines and synclines are the up and down folds that usually occur together and are caused by compressional stress.

Anticlines are folds in which each half of the fold dips away from the crest.  Synclines are folds in which each half of the fold dips toward the trough (bottom)of the fold.  You can remember the difference by noting that anticlines form an “A” shape, and synclines form the bottom of an “S.” 

Ramp Anticline, southern British Columbia, Canada

Syncline and anticline in Tertiary rock near Palmdale, California.  This roadcut is adjacent to the San Andreas fault.

Anticline-Syncline pair in Devonian Old Red Sandstone, SW Wales, UK