Types of Faults
Because a fault moves formerly continuous rock layers
apart, the careful mapping of different rock masses can
define discontinuities. These discontinuities are the surface expression of faults.
When regional forces create a large enough stress
differential in rock on either side of a fracture, then
movement occurs and the fracture becomes a fault.
Accumulated movements of rock along faults range from
millimetres to hundreds of kilometres. To describe the
location in three-dimensional (3-D) space of a deformed
rock layer, a fault surface, or any other planar feature,
geologists make measurements known as dip and strike.
Dip is the angle of inclination of the tilted rock layer
measured from the horizontal (Figure 4.40). Strike is the
compass bearing of the rock layer where it intersects a
horizontal plane.
(a)
Stri
ke
Dip
DIP-SLIP FAULTS
Many ore veins were formed in ancient fault zones. The
classification of faults (Table 4.7), therefore, uses some
terminology of early miners working in excavated fault
zones. They called the floor beneath their feet the footwall and the rock above their heads the hangingwall
(Figure 4.41). This terminology is used to define the two
major types of faults dominated by vertical movements—
the dip-slip faults. Dip-slip faults have the major amounts
of their offset in the dip direction and are caused by either
tension or compression.
Dip-slip faults where tension is dominant are recognized by the separation of the pulled-apart rock layers
in a zone of omission (Figure 4.42); this type of fault is
referred to as a normal fault. A normal fault occurs when
the hangingwall moves down relative to the footwall.
Normal-style faults are typical of the faults at oceanic
spreading centres and continental rift zones.
If the dominant force that creates a fault movement is compressional, then the rock layers are pushed
together, or repeated, when viewed in a cross-section
Hor
i
inte zontal
rsec
li
tion ne of
Water surface
Dip
n
ctio
dire
Dip
angle
(b)
Figure 4.40
(a) A 75-million-year-old sandstone layer at La Jolla Bay,
California, exposed at a moderately high tide. The sea surface
forms a horizontal plane against the inclined sandstone bed.
(b) The strike of a rock layer is the compass bearing of the
“shoreline.” The dip is the angle of inclination below a horizontal plane, represented here by the water surface.
Photo: © Pat Abbott.
Table 4.7
Classification of Faults
Fault Type
Deformation Force
Normal
Tension
Reverse
Compression
Dip-slip
Strike-slip
Shear
Description
Vertical
movement
Tectonic Environment
Hangingwall moves
Divergence zone
down relative to footwall
Hangingwall moves up
relative to footwall
Horizontal movement
Convergence zone
Transform fault
Source: © Claire Samson.
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Mine
Ground surface
Hangingwall
Footwall
Figure 4.41
Schematic cross-section of miners excavating ore that precipitated in broken rock within an old fault zone. Notice that the rock layers in
the footwall and hangingwall are no longer continuous; this gives evidence of the movements that occurred along the fault in the past.
Zone
of
omission
Figure 4.42
Schematic cross-section of a normal fault; that is, the hangingwall has moved downward (in a relative sense). Extensional forces are
documented by the zone of omission, where the originally continuous rock layers are missing. The small arrows indicate movement;
the larger arrows show force.
(Figure 4.43). With compressional forces, the hangingwall moves upward relative to the footwall; this type of
fault is referred to as a reverse fault. The compressional
motions of reverse faults are commonly found in areas of
plate convergence where subduction or continental collision occurs.
Types of Faults.indd 2
STRIKE-SLIP FAULTS
When most of the movement along a fault is horizontal
(parallel to the strike direction), the fault is referred to as
a strike-slip fault. Strike-slip faults are further classified
on the basis of the relative movement directions of the
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Zone
of
repetition
Figure 4.43
Schematic cross-section of a reverse fault; that is, the hangingwall has moved upward (in a relative sense). Compressional forces are
documented by the zone of repetition, where the originally continuous rock layers have been split, shoved together, and stacked
above each other. The small arrows indicate movement; the larger arrows show force.
Right-lateral fault
Straddle the fault; right-hand
side moves toward you.
rock masses on either side of the fault (Figure 4.44). If
you straddle a fault and the rock mass on your right-hand
side has moved relatively toward you, then it is called a
right-lateral, or dextral, fault. Similarly, if features on
the left-hand side of the fault have moved closer to you,
then it is a left-lateral, or sinistral, fault.
Strike-slip faults are common at transform plate
boundaries. The transform faults described by Tuzo
Wilson are strike-slip faults. The Alpine (New Zealand),
North Anatolian (Turkey), Queen Charlotte (Canada) and
San Andreas (United States) faults are right-lateral strikeslip faults. The Dead Sea (Middle East) and EnriquilloPlantain Garden (Haiti) faults are left-lateral strike-slip
faults.
Figure 4.44
Map of a right-lateral, strike-slip fault. As the man straddles
the fault, the right-hand side of the fault has moved relatively
closer to him. If he turns around, will the right-hand side of the
fault still have moved closer to him?
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