Faults I
Fault
• A fault is a mesoscopic to macroscopic plane (listric
faults are curved at large scale!) along which the two
blocks on either side have displaced (slipped) relative to
one another
– The slip is primarily due to brittle deformation
• This distinguishes faults from fault/shear zone
– Deformation in a fault zone is distributed along a set
of closely-spaced faults within a zone
– Deformation in a shear zone is ductile (i.e., high
strain without macroscopic loss of cohesion),
involving either crystal plastic or catraclastic flow
mechanisms (or a combination = semibrittle)
Scale of Faults
• The range of size for faults is from:
– microscopic, mm scale (10-3 m), to
– thousands of kilometer (106 m)
• (regional, lithospheric)
• A fault is called a shear fracture if its
dimensions are smaller than one meter
Net Slip
• The net slip of a fault is the magnitude and
direction of relative displacement on the fault
plane between two previously contiguous
points (piercing points).
• The net slip is a vector; it requires magnitude
(e.g., in meters) and a direction (trend/plunge)
• It can be resolved into its components
• We also need to define the sense of slip (or
shear) to completely define the net slip
Net slip
• The net slip vector can be resolved into any
arbitrary pair of components, for example
– along the strike (strike-slip)
– along the dip (dip-slip)
– oblique to the strike (oblique-slip)
• This is the most common case!
• The components for the dip-slip are:
– Heave: horizontal component of dip-slip
– Throw: vertical component of dip-slip
Measuring Net Slip
• Need two previously contiguous points
(piercing points) on the fault plane
• These two points (one on the hanging wall and
the other on the footwall) are the intersection
of a so-called piercing line with the fault
• The piercing line, defined by intersection of
two planes (e.g., two beddings, fault and
bedding), becomes broken after faulting
Slip Lineation
• Lineation on the fault plane that form parallel to the net slip,
for at least the last increment of slip
• Slip lineation forms parallel to the intersection of the fault plane
and the movement plane (M-plane, which is the 13 plane)
– The 13 plane, of course, is perpendicular to the 2, which
lies on the plane of the fault(s)
– It also contains the pole to the fault
– The M-plane is constructed by putting the pole to the fault
and the slip lineation on a same great circle
• The attitude of the slip lineation provides the attitude of latest
slip (trend/plunge)
• The sense of slip may be provided with shear indicators on the
fault surface
Slickensided
surface &
Slip fibers
Anderson Faulting Theory
• The surface of Earth is a principal plane of
stress (i.e., there is no shear stress along the
surface of Earth)
• The normal to the surface is therefore parallel
to one of the principal stresses (1, 2, 3)
Principal stresses and faults
• According to the Anderson theory of faulting,
one principal axis of stress is always
perpendicular to the earth surface
(i.e., is vertical), while the other two are
horizontal
• Normal fault: 1 is vertical
• Reverse fault: 3 is vertical
• Strike-slip fault: 2 is vertical
http://www4.uwsp.edu/geo/faculty/hefferan/geol320/faults.html
Terminology - Non-vertical faults
• Block above the fault plane is the hanging-wall
• Block below the fault plane is the footwall
• By convention, geologist keep track of the
movement of the hanging wall (not the
footwall)
– The hanging wall can move up or down
• This is the basis of the classification of
faults
Plots of slip lineation for kinematic analysis
• These plots (of slip linear) are used for
kinematic analysis, i.e., determining the
direction of motion along the fault
– For this we need the attitude of the fault, the
orientation of the slip lines, and the sense of slip
• In this plot, the slip line is decorated with an
arrow which indicates the direction (and
sense) of slip (along the M-Plane)
Procedure for plotting fault data
• Plot the trace of the fault and its pole
• If there are two conjugate faults, then plot both
Note:
– The conjugate faults develop if the difference in the value
between the maximum and the other two principal
stresses is significant
– If the intermediate and minimum principal stresses are
equal, and say 1 is vertical, then normal faults with
variable strikes would develop.
• For each fault, plot the slip lineation (using its
trend/plunge or pitch) on the fault plane
• The intersection of the two conjugate faults defines the
direction of the 2
• The bisector of the acute angle defines the 1, and the
bisector of the obtuse angle defines the 3
– Both 1 and 3 lie on the M-Plane
• Plot the M-plane (it contains the fault pole and slip line)
• Decorate the slip line, on the M-plane/fault, with a short line
(called ‘slip linear’), drawn along the M-plane
– If we know the sense of slip (e.g., normal, reverse), say
from slip fibers, decorate the line with an arrow to
indicate the relative movement of the HW block (e.g.,
arrow points updip along the M-plane)
– Distinguish the upward slip lines from downward ones
Fault-slip data collected at selected sites along the main
transverse faults in the Northern Apennines. Bonini, 2009
• Notice that the direction of the 1 should be steep for the case
of normal faults, and the slip linear(s) must be along the true
dip, downdip toward the premitive
• For the case of a reverse fault, 1 should be gently plunging,
near the primitive, and the slip linear(s) should point updip
• For the case of strike slip fault, 1 is near the primitive, and
the sense of the slip is indicated by a couple (remember that
the acute wedge facing 1 goes in!).
• For each case, conjugate faults intersect along the 2
Direction of shortening vs. extension
• From the slip linear and fault’s orientation, we
can find the shortening and extension axes for
the fault
These axes lie on the M-plane
• They are perpendicular to each other
• The slip linear arrow points toward the
extension axis, and away from the shortening
direction
A set of conjugate normal faults,
with slip lines and slip linears.
The corresponding M-planes and
M-axes (normal to the M-planes).
http://docvsoft.com/orient/Documentation/html/ch03_spherical.html
Tension axes or T-axes (yellow) and
pressure axes or P-axes (green) for the
same data set on last slide, with a
contoured gradient on the P-axes.
A beach ball diagram, can be plotted to
display the fault nodal planes. The nodal
planes are estimated based on the
eigenvectors of the M-axes and P-axes.
http://docvsoft.com/orient/Documentation/html/ch03_spherical.html
Differential stress and faulting
• In sandbox experiments, it has been shown
that:
– Normal faults form when 1 remains constant
while 3 weakens (i.e., differential stress increases
as 3 goes to the left on the Mohr diagram)
• i.e., vertical push is constant, while 3 (horizontal) is
reduced, i.e., circle grows to the left
– Reverse faults form when 3 remains constant
while increases 1 increases
• i.e., circle grows to the right on the Mohr diagram
Stress and Normal Faulting
Stress and Reverse Faulting
Terminology
• Emergent fault
– Active fault that cuts the surface of Earth
• Exhumed fault
– Exposure of an inactive fault at the surface
due to uplift or erosion
• Blind fault
– A fault that dies out in the subsurface
without intersecting the surface of Earth
General Types of Fault
• Faults are divided into the following three categories
based on the relative displacement of the fault blocks
with respect to the attitude of the fault plane:
• Dip slip fault - The hanging wall block moves (up or
down) parallel to the dip of the fault plane
– The net slip is pure dip-slip
Classification of Faults …
• Strike slip fault - Both blocks move parallel to the strike
of the fault plane
– There is no hanging wall in this case!
– The net slip is pure strike-slip
• Oblique slip fault - The displacement vector is oblique
to both strike and dip
• The senses of both the dip slip (normal or reverse) and
strike slip (left- or right-lateral) are needed for a
oblique-slip fault
– Left-lateral, normal, oblique-slip fault
– Right-lateral, reverse, oblique-slip fault
Extensional or Contractional
• Contractional fault
– Forms due to shortening of the layers
– Rock units become duplicated
– Includes reverse and thrust fault
• Extensional fault
– Forms due to lengthening of a layer
– Involves loss of stratigraphic section
– Includes normal fault
Extensional & contractional Faults
Dip-slip Faults
• Dip-slip - Motion is along the dip
– High-angle ( >60o)
– Intermediate angle (30o-60o)
– Low-angle <30o)
• Two types of dip-slip: Normal and Reverse
• Normal fault - If the relative motion of the hanging wall
block is down-dip on the fault
• Is caused by extension
• Forms horst and graben
– Example: Basin and Range, Mid-ocean ridge
Dip-slip Faults
• Reverse fault, if the motion of the hanging
wall block is up-dip on the fault.
– Caused by contraction
• e.g., faults in subduction zones
– Thrust is a low-angle reverse fault
• e.g. Grand Tetons; the Appalachians
Strike-slip Faults
• Strike-slip - one block moves horizontally past
another block:
– Are usually very long (100’s - 1000’s of km)
• NOTE:
– At a small scale, fault attitude may be constant
– At a larger scale, however, both the dip and/or
strike of a fault may change
Strike-Slip Faults - Types
• Left-lateral (sinistral) strike slip fault
– To an observer standing on one block and looking
across the fault, the other block seems to have
moved to the left
• Right-lateral (dextral) strike slip fault
– The block across the fault moved to the right of the
observer. e.g., San Andreas fault
• Oblique-slip
– motion is oblique to dip and strike
– e.g., normal, left-lateral, right-lateral, reverse
Fault Type
• Listric fault:
– The dip of the fault varies with depth.
• Fault bend:
– Is where both the dip and strike of a fault
changes.
• Flat:
– A fault which is locally parallel to the bedding (in
the hanging wall or the footwall).
– A fault parallel to bedding in the hanging wall
may be across the bedding in the footwall, and
vice versa!
• Ramp: A fault which is locally across bedding
Ramps/Flats before & after Thrusting
Bends
• The change in the attitude of the fault steps
the fault either to the left (left-step) or to the
right (right-step)
• Depending on the sense of displacement of
the fault, the right or left step may produces
either contraction (restraining bends) or
extension (releasing bends) across the step
Basement thrust over younger sediments in
transpressional segment of San Andreas fault
Fault Separation
• Distance between the displaced parts of a marker as
measured along a specific line, on a specific plane.
– Is usually not the same as the net slip, unless the
specified line is parallel to the net slip.
– It depends on the attitude of the displaced marker.
• NOTE:
– Two non-parallel markers will produce different
separation
– Separation along the fault for one marker may show
right-lateral, and for another, a left-lateral sense of
slip!
Fault Separation - Facts
• A strike-slip fault cutting a horizontal sequence of
layers produces no horizontal (strike) separation!
• A dip-slip fault cutting vertical layers produces no
dip separation
• Use linear features (e.g., fence, roads, etc.), or trace
of a planar feature, on a horizontal plane, to
determine the horizontal separation
• Heave and throw are components of the dip
separation
Faulting
• Faulting, as a mode of failure, is the most significant
way in which lithospheric masses are tectonically
transported relative to each other, especially in the
seismogenic upper crust
• Deformation in this brittle part of the crust takes place
by pressure-sensitive, strain rate-insensitive frictional
sliding on discrete fault planes with little inelastic
strain and dislocation activity
• Faults commonly involve frictional sliding along preexisting joints, veins, and other discontinuities, but
can also initiate and propagate in intact rocks
Fault Geomorphology & Scale
• Active faults such as the San Andreas:
– Show considerable variation in the irregularity of
their trace
– Commonly occur in variably-oriented strands or
segments
• The segments grow and link as the total
displacement increases
– Range in fault length is over eight orders of
magnitude (10-3 m to 105 m)
– Display a power law size distribution; i.e., fractal
Fault Surface Structures
• Fault displacement produces friction-related
striations (polishing and grooving) indicating the
latest, local direction of movement and
sometimes absolute direction of movement
• The slip lineation are called slickensides or
slickenlines
• Fiber growth in the direction of fault
displacement, on the slickensided surface,
provide clear indications of relative offset
• Extensional fractures occur at a high angle to
slip direction and dip steeply into fault plane
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Structural features to Recognize Faults
polishing and grooving
slickensides
breccia
gouge
mylonite
shear zone
associated fractures
drag of layers adjacent to fault
juxtaposition of dissimilar rock types
displacement of planar structures
Fault
Breccia
Clay
Gouge
Mylonite vs. Cataclasite
Geomorphic features
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fault scarp
fault-line scarps
triangular facets
alignment of facets
increase of stream gradients at the fault line
hanging valleys
aligned springs and vegetation
landslides
displaced stream courses
Fault Scarp
Fault-line scarp caused by faulting of
a resistant layer
Fault and Stress
• Conjugate shear fractures develop at about  = 30
degrees from 1
• 1 bisects the acute angle of about 60o between the
two fractures
• 3 bisects the obtuse angle between the two fractures
Faults and the Principal Stresses
• Reverse faults are more likely to form if 3 is vertical
and constant (at a standard state), while horizontal,
compressive 1 and 2 increase in value compared to
the standard state
• Normal faults form if 1 is vertical and constant, while
horizontal 3 and 2 decrease in value, or if horizontal
3 is tensile
• Strike-slip faults form if 2 is vertical and constant,
while horizontal 1 and 2 increase and decreases in
value, respectively
Semibrittle shear zone SC Folitaion
Riedel R,
R', and P
shears