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. StrikeSlip 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