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CHAPTER 8
FAULTS AND FAULTING
Dr. Masdouq Al-Taj
FAULTS
• A fault is any surface or zone in the Earth across which
measurable slip (shear displacement) develops.
• Faults are fractures on which slip develops primarily by
brittle deformation processes.
• Fault zone is a brittle structure in which loss of cohesion
and slip occurs on several faults within a band of
definable width.
• Shear zone: occurs at depth without definable displacement
on the surface
We used four relative scales of
observations
- Micro: optical scale (microscope or even
electron microscope).
- Meso: single outcrop (personal scale).
- Macro: regional scale (mountain range).
- Mega: continental scale (plate dimensions).
Fractured feldspar grain in photomicrograph
Mesoscopic faults in outcrop
Fault trace in aerial photo
Fault components
• Rocks adjacent to the fault surface is the wall of the
fault, and the body of rocks that moved as
consequence of slip on the fault is a fault block.
• If the fault is not vertical, we can distinguish
between the hanging-wall block, which is the rock
body above the fault plane, and the footwall block,
which is the rock body below the fault plane.
Foot wall and Hanging Wall
HOW TO DESCRIBE THE
ATTITUDE OF FAULT
We need to measure:
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Strike
Dip angle, dip direction and hade
Net slip vector (rake)
Strike-slip component
Dip-slip component (Heave and throw)
Foot wall and Hanging Wall
•Note that the rake angle is
measured from the horizontal
to the direction of net-slip on
the fault plane
Fault types
The most common types of faults are:
1. Dip-slip faults
• Normal (Listric)
• Reverse or Thrust (if dip angle <45º)
2. Strike-slip faults
3. Oblique-slip faults
Other faults: Scissors (Rotational).
listric fault
Misleading Scarps
(Fault-line scarp)
• If the fault moves rock of much different strength
together, differential erosion may create a fault
scarp.
• Such scarps may have dips opposite to that of the
underlying fault.
Concepts of extensional and contractional
faults
Reverse fault
Thrust Sheet Diagram
• Window (fenster) shows of the autochthon
through the eroded allochthon
• Klippe is a piece of allochthon surrounded by
autochthon
Window (Fenster)
• Thrust faults are often thin sheets, and erosion
may open holes in them
• A hole through a thrust sheet is called a fenster, or
window
• Fenster: An eroded area of a thrust sheet that
displays the rocks beneath the thrust sheet
• Triangular teeth point outward fenster are used on
a map
Klippe
• If erosion leaves an isolated remnant of
thrust sheet, surrounded by exposed
footwall, the remnant is called a klippe
(German for cliff)
• Klippe are indicated on a map by inward
pointing teeth
Definitions
• Autochthon: A body of rocks that remains at its
site of origin, where it is rooted to its basement.
Although not moved from their original site.
• Allochthon: A mass of rock that has been moved
from its place of origin by tectonic processes, as in
a thrust sheet
• Many allochthonous rocks have been moved so far
from their original sites that they differ greatly in
facies and structure from those on which they now
lie
DESCRIPTION OF FAULT DIP
0° →horizontal fault
0 -10 →sub horizontal fault=Detachment:A
regional, low-angle, listric normal fault formed during crustal extension
10-30→shallowly dipping faults
30-60→modertly dipping fault
60-80→steeply dipping faults
80-90→sub vertical fault
90 →vertical fault
SEPARATION
• The distance between the separated parts of
the marker horizon is the separation, which
is not the same as the net slip unless the line
along which separation is measured happens
to parallel the net-slip vector.
Components of Separation
• Separation can be divided into seven
components:
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Stratigraphic separation
Heave
Throw
Strike separation
Vertical Separation
Horizontal separation
Dip separation
Stratigraphic Separation
• Offset measured perpendicular to bedding
Horizontal Separation
 Horizontal separation (H) Offset measured in a
horizontal direction along
a line perpendicular to the
offset surface.
Vertical Separation
 Vertical separation (V) Distance between two
points on the offset bed as
measured in a vertical
direction
 If borehole data is used, it
is vertical separation that is
measured between two
parts of an offset marker
horizon.
Dip Separation
 Dip separation (D): The
distance between the offset
horizons measured in the
dip direction
 Strike separation (S):
Distance between the offset
horizons measured along the
strike direction.
Components of Separation
 Dip separation has two
components;
 1. Heave: Horizontal
component of the dip
separation
 2. Throw: Vertical component
of the dip separation
 Strike separation: Distance
between the offset horizons
measured along the strike
direction.
Change in Fault attitude
(Fault bends)
• Fault bends or steps along strike-slip faults cause
abrupt changes in the strike of the fault and in the
associated structural features.
• Where movement across a segment of a strike-slip fault
results in some compression, we say that transpression
(restraining bends) is occurring across the fault (form
Pressure ridges); But where movement results in some
extension, we say that transtension (releasing bends) is
occurring across the fault (form sag pond (local area) or
pull-apart basin (regional scale, example is the Dead
Sea).
rele
San Andreas
Fault Ridge
• Ridge created by transpression along the fault
• Striped white and gray rocks are basement
rocks pushed up relative to dark sedimentary
cover.
Sag Pond
• Prominent scarps with sag
ponds are found along the
Denali fault trace.
• The ground is weakened on the
fault trace and has the tendancy
to sag and erode more easily
than surrounding land.
Change in attitude vertically
• Fault segments may parallel bedding in either the
footwall or hanging wall, but cut across bedding
in the opposite block.
Bedding and Fault Plane
Orientation
• Fault segments may parallel bedding in either the
footwall of the hanging wall, but cut across bedding in the
opposite block.
Pressure and
Temperature
Influence
Faulting
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Changes due to burial depth
Shallow faults, < 5 km
Intermediate, between 3-5 km and 10-15 km
Brittle faults end at 15 km depth
Age relationship between
different faults and their
termination relation
• A fault may terminate
where it has been cut
by a younger structure,
such as another fault (C
& D), an unconformity
(E), or an intrusion (B),
or at the ground surface
(A)
• Like joints, fault must
terminate, and can do so in
several different ways
• The Principle of CrossCutting Relationships can
be used to determine the
relative ages.
Representation of Faults on map and
cross section.
Cutoffs
• Faults which cross geologic contacts will displace the
contact, unless the net-slip vector is exactly parallel to
the fault-contact intersection
• The point of intersection on either a map or crosssection is called a cutoff.
Death of a Fault
• Faults can also split, to form an anastamosing array, which
may merge and diverge several times along its length
• A fault splay may develop, with the fault splitting and dying
out – these are called horsetails (B)
• A fault dies when its displacement becomes less and less,
finally reaching zero near the tip, in a zone of plastic
deformation (C).
Emergent Fault
• Faults can also
terminate at the
ground surface, or
appear to
• The San Andreas fault
does terminate at the
ground surface, and is
called an emergent
fault.
Exhumed Faults
• Other faults, blind
when they formed,
may be exposed by
erosion to become
exhumed faults.
• It may ACTIVE or
INACTIVE.
Blind Faults
• Blind faults are faults that terminated before
reaching the surface.
Blind Fault Effects
• Blind faults, by
definition, do not
directly affect the
surface
• Nevertheless,
surface elevations
can be changes, as
monoclinal folds
Fault Length and Displacement
• This is a general relationship,
supported by research within the last
two decades.
• The longer the fault, the
greater the displacement
• The best fit to the data is
D = C • Ln, with C =0.03,
and n = 1.06
Where D=displacement, L=
fault length, C is a constant,
and n is called the fractal
dimension.
Prediction of Fault Length or
Displacement
• In (a) the offset of XX’ is small.
• As the fault grows with time (from t1 to t2), the offset of XX’
increases.
Slickensides and slip lineation
• Slickensides are the fault surfaces
features that have been polished and
scratch (groove lineation, striations) by
the process of frictional sliding.
Fault Breccia
Indurated Breccia
Photomicrograph
• Note the angular fragments
(fr) of quartz sandstone in a
matrix of fine-grained iron
oxide cement (ic)
• Field of View 4 x 2.7 mm,
Cross Polarized Light
• Photomicrograph of fault
breccia in the Antietam
Formation, Blue Ridge
province
• Breccias form when
rocks are extensively
fractured in fault zones
and are cemented
together when minerals
precipitate in the cracks
and fractures
Fault Gouge
Photo
• Continued movement along
the fault may form gouge.
Pseudotachylyte Photo
• Newer pseudotachylyte
injection vein cuts the older
one.
• Silicate rocks are excellent
insulators, and heat generated
by friction does not escape
• Temperatures in excess of
1000ºC are possible
• Tachylyte is a type of
volcanic glass, and the prefix
pseudo means false, so the
name literally means false
volcanic glass
Argille
Scagliose
Photos
• Argille scagliose
melange associated
with obducted
ophiolite.
Cataclasite photo
• Foliated cataclasite in the core of the San
Gabriel fault, San Andreas System,
California.
Slip Fibers
• Slip fibers on fault
surface
• Note Brunton
compass for scale
• Steps indicate sense
of shear.
Quartz Fibers
• Quartz fibers in ductile shear zone.
Formation of Pits
• Any step on the fault surface
subjected to pressure solution
experiences more pressure than
the areas around them.
Slickolites
• Restraining steps become pitted by pressure solution, orming styolites.
• Releasing steps become the locus of grain growth.
San Andreas and Subsidiary Faults
• San Andreas Fault to left; Hayward Fault to right of SF Bay
Clay Experiment
• We can model the situation by placing a clay
layer over two wooden blocks, and then moving
one block opposite the other, as shown in the
figure
• The clay will accommodate some of the strain,
but will then rupture.
Formation of
Reidel Shears
• The first fractures are short, shear fractures inclined
to the trace of the through-going fault
• They are called Reidel shears, and generally occur
as a conjugate pair
• The acute bisectrix of the Reidel shears gives the
local orientation of σ1.
Fault-Related Folding
We have several types of fault-related folding:
• Fault-propagation folds.
• Fault-bend folds
• Folding accompanies faulting (in fault zone)
• Detachment folds
• Drag fold, or drape folds or forced folds
Fault-propagation
folds.
Development of Folds
1. Stages in development of
folds, leading to a fault.
2. This might reflect simply an
increase in the regional strain
rate, or it might reelect a
“lock-up”
3. Lock-up means that the folds
reach a point where continued
folding is very difficult
Fault-Propagation Fold Photo
• Fault propagation
fold in Mesozoic
sedimentary rocks
in the Salt Range,
northern Pakistan.
Fault-Bend
Folds
• A bend in the fault surface may cause folding of
strata that move past the bend
• The moving layers must accommodate the bend,
without gaps or overlaps
• Folds that form in this manner are called fault-bend
folds
• They develop in association with all kinds of faults,
but have been most studied in dip-slip faults.
Diagram of Fault-Bend Fold
Development
• Steps in the formation of a fault-bend fold
Fault-Bend Folds
Detachment folds
Folds in a shear zone
Drag fold, or drape folds
Fault systems
1. Normal Fault systems
2. Reverse Fault systems
3. Strike-slip Fault systems
Types of fault Arrays
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a. Parallel array
b. Anastamosing array
c. en echelon array
d. Relay array
e. Conjugate array
f. Random array
1.Normal Fault systems
a. Half-Graben Blocks
• Rotation of the hanging-wall block tilts the surface of
the hanging-wall toward the fault, which creates a
half-graben
• Half-graben blocks are bounded by a fault on one
side only.
Basin and Range Province
• In the Basin and
Range Province, most
of the blocks are halfgrabens
• The ranges are the
tilted tips of the fault
blocks
Bear Island, Norway
• Half graben tilting: Beds
dipping about 30º to the
west.
• Cross section indicates the
dip is most likely due to
half-graben development
Normal Fault systems
b. Horst and Graben
• When two adjacent normal faults dip toward each
other, the central block slides down to form a graben
• The remaining high ground in between is called a
horst
• This type of faulting is common in rift systems.
2. Reverse Fault System
a. Imbricate Fan
• Thrust fault arrays are usually either parallel or
relay arrays
• If there is no upper confining layer, an imbricate
fan forms
• These faults die out up dip.
2. Reverse Fault System
b. Duplex Structures
• If an upper and lower confining layers are part of the
thrust, the intermediate lays form duplex structures,
where the thrust spans the gap between the lower
and upper thrusts sheets
• The lower confining layer is the floor thrust, and the
upper confining layer is the roof thrust.
3. Strike-slip system
Flower Structures
• Their cross-sectional view looks like the head of a flower,
so they are called flower-structures.
• They are of two types positive flower structure and
negative flower structure.
Relation of Faulting to Stress
• If faults initiate as Coulomb shear fractures,
they will form at about 30° to the σ1
direction and continued at the σ2 direction.
• The ratio of shear stress to normal stress
on planes orientated at about 30 ° to σ1 is at
a maximum.
FAULTS AND SOCIETY
1. Earthquakes
2. Mining
3. Oil Reservoirs
4. Groundwater