Faults Part-I
Goals: To understand and use the basic terminology for describing faults,
and to recognize faults in the field and on a map.
1. Basic Terminology
 Hanging wall and footwall: Come from 18th century English coal
mines
 Dip-slip faults: Slip up or down the dip.
a) Normal fault: Hanging wall down — indicate extension
b) Reverse fault: Hanging wall up — indicate shortening
 Strike-slip faults: Slip parallel with earth’s surface. Parallel with strike.
Typically have subvertical dip. Transcurrent motion.
 Oblique-slip faults: Slip is oblique to dip and strike.
a) Slip can be resolved into strike-slip and dip-slip components
b) Most faults are oblique-slip in nature, but are often dominantly
strike-slip or dip-slip
 Slip: Total amount of movement along a fault surface.
a) Vector quantity with vector lying in fault surface
b) Direction of vector (slip-line) can be expressed as trend and
plunge or rake of slip-line in fault plane
 Separation: Total amount of apparent offset along fault when viewed
in 2-D (either map or cross section).
 To determine slip, you need a piercing point
a) Piercing point: Place where a line intersects fault surface and is
off-set by fault
b) Match hanging-wall cutoff with footwall cutoff to determine slip
direction and magnitude
 Slip direction can also be determined by measuring slickenlines
 Character of faults
a) Discrete, single plane
b) Zone of anastomosing, closely spaced faults (fault zone)
c) Wide zone of penetrative, plastic deformation
2. Fault rocks: Two main categories, frictional/brittle and plastic
 Frictional/brittle fault rocks: Record mechanical disaggregation and
“grinding” of original rocks.
a) Fault gouge: Clay-sized particles ground from original rocks on
either side of fault.
b) Fault breccia: Angular chunks of protolith from either side of
fault surrounded by gouge and/or vein material precipitated
from solution.
c) Cataclasite: Indurated version of fault gouge. Forms deeper in
the crust.
d) Pseudotachylyte: Glass formed from frictionally generated melt
at fault surface.
 Plastic fault rocks: Record plastic flow of minerals at the molecular
scale in response to differential stress. Deformation is “penetrative”
and commonly results in grain-size reduction due to deformationdriven (dynamic) recrystallization.
a) Protomylonite: Up to 10% dynamically recrystallized material
b) Mylonite: 10–90% dynamically recrystallized material
c) Ultramylonite: 90–100% dynamically recrystallized material
3. Recognizing faults in the field and on maps
 Truncation of rock units
 Visible off-set of rock units
 Omitted or repeated stratigraphy or biostratigraphy
 Juxtaposition of seemingly unrelated rock units