Structural Geology 3443 Ch' 8 Faults

1
Structural Geology (3443)Ch. 8 Faults
faulting
Department of Geology University of Texas at
Arlington
2
Definitions and Characteristics
A fault consists of a zone along which slip
(shear displacement) has occurred. The zone is
separated from in-tact rock by two surfaces. The
zone can vary in width from less than a mm to
more than a km. A fissure is essentially a joint
with a large aperture measured in cm to M.
3
Definitions and Characteristics

• Fault Rock is the material in a fault zone it
  consists of
• gouge
• Breccia
• Cataclasite
• Pseudotachylite

4
Definitions and Characteristics
Gouge is mostly clay sized, poorly consolidated
material pulverized by fault slip.
Breccia consists of angular, poorly sorted clasts
up to boulder size that have been broken up by
fault slip.
5
Definitions and Characteristics
Cataclasite is a fine grained poorly sorted
breccia that is usually well lithified.
6
Definitions and Characteristics
Tachylite is volcanic glass. Pseudotachylite is
also glass, but formed by melting rock due the
frictional heat along a fault . Normally forms at
depth where rock is already heated.
7
Definitions and Characteristics
Fault terminology
8
Classification of Faults

• Slip and Separation
• Slip is a displacement vector that connects two
  points on either side of the fault zone that were
  connected before faulting. A bedding surface
  alone can never be used to determine slip.
• Separation is an apparent displacement parallel
  to the strike and/or parallel to the dip. It is
  not the slip, but may be a component of the slip.
• Strike separation is not the same as strike slip
  (Next Slide)

9
Classification of Faults
Terms use to describe fault displacement
10
Classification of Faults
Examples of Slip and Separation of bedding
surfaces
11
Classification of Faults
Examples of Slip and Separation of bedding
surfaces
12
Classification of Faults
Examples of Slip and Separation of bedding
surfaces
13
Classification of Faults

• Faults are classified based on their orientation
  relative to the surface (strike and dip) and
  sense of slip i.e. relative displacement of the
  fault blocks.
• There are four general categories of faults
• Strike-slip
• Dip-slip
• Oblique-slip
• Rotational-slip

14
Classification of Faults

• Strike-slip Faults
• Sense of slip
• Left-Lateral
• Right-Lateral

15
Classification of Faults

• High angle Dip-slip Faults (gt50o dip)
• Hanging Wall
• Foot Wall
• Sense of slip
• Normal (extension)
• Reverse (shortening)

16
Classification of Faults

• Low angle Dip-slip Faults (lt50o dip)
• Hanging Wall
• Foot Wall
• Sense of slip
• Detachment (Extension)
• Thrust (shortening)

17
Classification of Faults

• Oblique slip
• Sense of slip
• Normal/left slip
• Normal/right slip (Not shown)
• Reverse/left slip
• Reverse/right slip (not shown)

18
Classification of Faults

• Rotational slip
• Looking across fault
• Clockwise
• Counter clockwise

19
Classification of Faults

• Classification also described in terms of
  horizontal strain
• Shortening (contractional) faults
• Extensional faults

20
Slip Direction
Determination of slip Slip direction can
frequently be determined from slickenlines and
fiber lineations.
21
Slip direction
Slickenlines give only the direction of slip.
Sometime the sense of slip can be determined as
well. For example, the clast causing the groove
is present indicating the overlying fault block
moved upward.
22
Slip Direction
Fibrous mineral growths may also give the sense
of slip. The fibers tend to be stepped because
they grow from irregularities on the fault
surface. The steps indicate sense of motion of
the block above them.
23
Definitions and Characteristics
What is the sense of slip of the fault blocks in
the picture
24
Fault Bends
When ever a fault has a bend it will commonly
produce folds in layered sediment. Dip slip fault
bends produce fault bend folds Strike slip bends
produce pop-up folds or sags
25
Thrust Fault Bends
Thrust faults usually follow bedding plane
surfaces and then bend up (ramp) to another
bedding plane producing a stair-step geometry.
A ramp generates a fault-bend fold which may
develop a flat top if there is enough
displacement.
26
Thrust Fault Bends
Because of the ramp-flat geometry, thrust belts
can be very complicated.
A ramp cuts off layering producing a hanging wall
cutoff and a footwall cutoff. Likewise there are
both hanging wall flats and footwall flats.
27
Normal Fault Bends
Listric normal faults also produce fault bend
folds. These have been called a variety of
names Rollover folds Reverse Drag Folds
28
Strike slip Fault Bends

• Strike slip fault bends have two types
• Releasing bends produce extensional structures
  sags (basins) and normal faults
• Restraining bends produce shortening structures
  uplifts, folds and thrust faults

29
Strike slip Fault Bends
The strike slip fault at left has a bend. What
type of bend is it? That is the sense of
displacement on the fault?
30
Fault Terminations
Faults are approximately elliptical in shape with
maximum displacement near the center and
displacement diminishing outward to the edge. The
edge of the fault, with zero displacement, is
called the tip line.
31
Fault Terminations

• Terminology
• Blind fault
• Emergent fault
• Exhumed Fault

32
Fault Terminations
When faults terminate, folding commonly occurs to
accommodate the change in displacement. Both
fault bends and fault terminations can generate
folds
33
Fault Terminations
Folds at fault terminations are called fault
propagation folds if they form at a ramp (left)
or detachment folds if they form along a flat
34
Fault Terminations

Identify the type of folds depicted below
35
Sense of Shear on Faults

• The sense of displacement on faults is often
  important, so be familiar with the following ways
  this can be determined.
• Offset piercing points
• Asymmetry of folds related to the fault
• Steps on fiber lineations
• En echelon extension veins
• Pinnate fractures

36
Sense of Shear on Faults
Determine slip direction and fold asymmetry
37
Sense of Shear on Faults
Determine slip direction using Z and S asymmetric
folds in fault zones
38
Sense of Shear on Faults
Determine slip direction using Tension Gashes in
fault zones The extension direction points in the
shear direction along the zone
39
Sense of Shear on Faults
Slip direction from Fiber lineations
40
Sense of Shear on Faults
Pinnate fractures are small shear fractures
(Reidel shears) that are associated with a larger
fault. The shortening direction bisects the acute
angle between the small fractures and the larger
one. The extension direction (Perpendicular to
max shortening) again points in the direction of
shear, just like the en echelon veins.
41
Recognition of Faults
A fault scarp is an offset of the topographic
surface that is produced by recent movement on a
fault.
42
Definitions and Characteristics
A fault line scarp is also a step in the
topography, but represents differential erosion
along an old fault that has rock on one side that
erodes faster than that on the other side. In the
photo the fault block on which the person is
standing has actually moved up. It appears the
opposite because of differential erosion.
43
Identification and Expression of Faults
Faults truncate and offset layering and other
types of boundaries in rock. Faults may be
difficult to identify on a map if they parallel
the strike of the layering.
44
Identification and Expression of Faults
Depending on the type of fault and its dip
relative to layering, faults either omit strata,
duplicate them, or truncate them.
45
Identification and Expression of Faults
Faults in the subsurface can be detected by
contouring stratigraphic boundaries from drill
hole data (structural contour maps
46
Identification and Expression of Faults
Subsurface faults can be detected by geophysical
methods such as seismic (below) as well as
gravity and magnetic methods.
47
Strain, Stress Faults
Normal faults produce horizontal extension. And
vertical shortening (sag, rift, basin)
48
Strain Faults
Reverse faults produce horizontal shortening. And
vertical extension (uplift)
49
Strain Faults
Strike slip faults produce both horizontal
shortening and extension. No vertical strain
(ideally). Look at the figure below as a map, not
a x-section.
50
Stress FaultsAndersons Theory of Faulting
Andersons Theory of Faulting relies on the
Mohr-Coulomb failure criterion (tc to)/s
Tan(f) to and Tan(f vary with the rock type
and are called the cohesion and internal
friction. Its the ratio of shear stress to
normal stress on a surface that determines
whether a fault will occur.
51
Stress FaultsAndersons Theory of Faulting
Mohr-Coulomb failure criterion (tc to)/s
Tan(f) Is usually written as tc to s
Tan(f) This plots as a straight line on the Mohr
diagram f is the angle of the failure line and
Tan(f) is the slope.
52
Stress FaultsAndersons Theory of Faulting
The Mohr circle shows the normal and shearing
stresses on all the surfaces in a rock. When the
circle becomes tangent to the failure line, then
one surface has the right amount of normal and
shearing stress to fracture.
53
Stress FaultsAndersons Theory of Faulting
The orientation of that fracture surface relative
to s1 can be determined from the Mohr
circle. (Remember that q is the angle to the
surface normal!)
54
Stress FaultsAndersons Theory of Faulting
There is a relationship between q and f q 45
f/2
55
Stress FaultsAndersons Theory of Faulting
The Mohr fracture criterion predicts that fault
normals will always occur about 60o to the
maximum compressive stress s1 Or, the fault
surface will be about 30o to s1
56
Stress FaultsAndersons Theory of Faulting
The Mohr fracture criterion also predicts that
there will be two fault surfaces about 30o on
either side of s1 These are called a conjugate set
57
Stress FaultsAndersons Theory of Faulting
Thrust faults are generated when s1 is horizontal
and s3 is vertical. s2 is horizontal and
parallel to the intersection of the fault
surfaces.
58
Stress FaultsAndersons Theory of Faulting
Normal faults occur when s1 is vertical and s3
is horizontal. s2 is horizontal and parallel to
the intersection of the fault surfaces.
59
Stress FaultsAndersons Theory of Faulting
Strike-slip faults occur when both s1 and s3 are
horizontal. s2 is vertical and parallel to the
intersection of the fault surfaces.
60
Stress FaultsCaveats
The Mohr-Coulomb fracture criteria applies to
uniform, intact rock. If there are weak zones in
the rock like bedding surfaces and preexisting
fractures, then Andersons theory of faulting
cannot be applied.
61
Origin of Listric Faults
All stress (like politics) is local, and changes
both horizontally and vertically. Faults are
curved because the stress that generates them
changes from place to place.
62
Problems with Faulting

• There are two problems associated with the origin
  of faults
• Strength of rock increases with pressure (depth)
  so that the rocks at depth are too strong for the
  available stresses in the Earth to break them.

63
Problems with Faulting

• The other problem is
• Large, nearly horizontal thrust faults cannot
  move because the horizontal stress on the hanging
  wall block would crush the end before it moved
  the block.

64
Problems with Faulting
One solution is low effective pressure produced
by high pore pressure (s s pp) If there is
no fluid in the fault zone, the shear stress
(related to the radius of the Mohr diagram) is
not great enough to overcome the strength of the
rock at high pressures (the center of the Mohr
Diagram)
65
Problems with Faulting
If pore pressure in the fault zone increases to
reduce the effective stress, then the effective
stress may exceed the rock strength as the
effective pressure is reduced (the center of the
Mohr Diagram).
66
Fault Arrays

• Groups of faults that formed at the same time are
  called fault systems, or fault arrays. Some
  general terminology
• Thin Skinned Fault arrays that are confined to
  the sedimentary sequence and do not penetrate
  basement
• Thick skinned Fault arrays that do penetrate
  basement
• Master faults large regionally significant
  faults
• Synthetic faults smaller faults that parallel
  the master fault
• Antithetic faults smaller faults that are
  conjugate to the master fault.

67
Fault Arrays

• Terms describing fault arrays in map view.
• Parallel array
• Anastomosing
• En echelon
• Relay
• Conjugate
• Nonsystematic usually from reactivation of
  older faults.

68
Fault Arrays

• Terms describing extensional fault arrays in
  profile.
• Listric faults are usually thin skinned (but not
  always)
• Horsts graben
• Half graben
• Rift large regional feature that contains
  horsts graben and/or half graben.

69
Fault Arrays

• Terms describing shortening fault arrays in
  profile.
• Listric faults are usually thin skinned (but not
  always)
• Structures occur in fold-thrust belts
• Flats ramps
• Imbricate fans

70
Fault Arrays
More terms describing shortening fault arrays in
profile. Duplex horses (multiple, closely
spaced ramps) Roof Thrust floor thrust
71
Fault Arrays

• Strike slip systems in profile
• Flower structures master fault branches upward
  forming a stem with petals
• Negative positive flowers.

72
Faults, Resources Earthquakes
Faults and earthquakes are usually associated,
but aseismic fault creep does occur. Displacement
on large faults is accumulated from smaller,
sudden displacements producing earthquakes An
earthquake displacement event does not occur over
the whole length of a large fault so
displacement is not only accumulated over time,
but also over space only a small area of a
fault will displace at one time with another area
displacing at another time.
73
Faults, Resources Earthquakes
An Earthquake and fault displacement at one
particular zone along a large fault may occur at
regular intervals of time the recurrence
interval The recurrence interval has a large
standard deviation its not Old
Faithful. Predicting regions with a high
probability of a major earthquake is critical for
land use planning, especially for critical
installations like nuclear power plants and dams.
74
Faults, Resources Earthquakes
Faults are also economically important for
resource recovery They can form barriers or
channels for fluid flow, whether groundwater or
petroleum They are sources and zones for
mineralization and ores They may offset
economically valuable strata (coal, petroleum
reservoirs) leading to recovery complications.