Download Introduction to Faults

Chapter 3 Summary
that allow geologists to define a sense of
 Branch Line—Where two
Faulting is a very critical
shear along a fault.Certain structures
faults either separate or
means
of
brittle
crustal
found along a fault zone include:
emerge.
movement. The forces of

Slickenfiber
lineations
–
compression,
and
Note: Important
Definitionsextension
In Bold
 Splay
Faults—Subsidiary
Dilations within the layers along
shear
cause
three
major
fault
faults
that
from the
Faults are locations on crust that show brittle block movement via shear. They are important because if you have forces ofbranch
compression,
expansion,
a fault zone allow water to
types:
Thrust,
Normal,
and
fault.
or shear, the
solid blocks
on Earth
must cater
to these forces. These forces result in breaks called faults and themain
motion
of blocks around these faults
minerals that form
Transform.
kindsdoesofnot necessarilyprecipitate
accommodates
the forces; These
but the motion
go directly in the direction of the earth’s forces. To describe the motion lets define
along the scrapes in the fault
faulting
have
accessory
Hanging Wall and Footwall. The wall above the fault is the hanging wall and below the fault is the
footwall.and
In Ramping.
the Normal Fault, the hanging wall
Duplexes
plane.
indicate
goes down,geologies
extending thethat
crust. In
the Thrustthat
Fault, the footwall gets pushed up, compressing the crust. In Strike Slip Faults, the fault blocks slide past
world
faults
do notifoccur
are occurring.
 Stylotites—Compressional
each other they
to accommodate
shear. strike slip faults are right-lateral
if the block across the observerReal
move
right.
Theygenerally
are left-lateral
the opposite
forces
canFault.
sometimes cause a along flat smooth surfaces but contain
happens. Oblique Slip faults combine Strike-Slip with a Thrust or
Normal
serrated structure that results Ramps that connect various portions of
Note: Important Definitions In Bold
from material within the unit the fault. Various ramps are as follows:
See Figure
3.3
for
movement
references
leaving as a result of pressure
Faults are surfaces on the crust where
 Frontal ram
dissolution. This is common in
rocks are moved parallel to the plane of
 Lateral ramp
results &
in erosion,
an
the surface.
are are
a result
of brittle
Accessories
of this They
faulting
as follows
and show up in limestones
petrography,and
topography
and small scale structures. Rocks are important
When faults do not connect, there is a
overallTemperature
loweringgoesofup, rocks
total become
deformation
and leave
evidence
indicators
of this movement.
Because
as you gothat
beneath the surface,
less brittle and the Cataclastic Rocks which
characteristic step-over.
volume.,
indicate
the
driving
forces
experienced
have been ground up from the brittle movement give way to Mylonitic Rocks. Mylonitic zones are those where rocks have been stretched out. Scarps
which
can movement
either bemakes
compressional,
are where
the block
distinct changes in topography.
Also, these
Scarps can erode and then cause deposition. Small scale structures
 Secondary
Fractures—These
extensional,
or
shear.
To
describe can
the be Congruous
faultsare
migrate
along
surface
and
also indicate a fault location. These structures
steps ordevelop
Incongruous
features
that aface
downstream
and
structures
at an steps.
angle Congruous
to When steps
motion
along
faults,
we These
use the
terms are to follow.
incongruous
steps
are the
opposite.
structures
the main fault surface and can form successively younger ramps/faults, a
Hanging Wall and Foot Wall. The wall
faultForm
duplex
is formed.
Duplexes
canopposite
oftenreplace
house openings
secondaryinminerals.
•
Slickenfiber lineations – When fibrous looking minerals
the fault block.
in direction
of motion
of the
above some point on the fault is the
occur
as
a
result
of
normal,
reverse,
or
block.
hanging wall and below the point is the
strike slip movement.
•
Stylotites—When
contraction
occurs,
solution of minerals
occurs along
These structures
alongstylotites.
with striations
footwall.
In a Normal
Fault, the
hanging
andsurface
gouging
allow
to I fractures to relieve some of the force moving the
•
Secondary
at antoangle
and are
kindgeologists
of like mode
wall
goes Fracture—Occur
down relative
theto the fault
deduce much about the history of a See Figures 3.33 & 3.34 for Fault Ramp
blocks. footwall,this is a result of extensional
and Duplex geometry
fault.wall, a gouge may be dug on the wall.
forces..
In a Thrust
Fault, the
footwall
•
Gouging—When
an asperity
is fixed
to the opposite
Gouges show which direction the shear was.
pushed
up,acompressing
the gouging
crust. Inends.
Asperitygets
usually
leaves
mark where the
Strike Slip Faults, the fault blocks slide
Damagetales.
Zones
aretheir
a grains
result become
of brittle
Ductile
Shear zones
have that
theirform
ownasymmetric
•
Porphyroclasts and Porphyroblasts- Large crystal rocks
in ductile
shear zones
Also,
foliated.
past each other to accommodate shear
suite of indicative structures which deformation along a fault zone where
forces. Strike slip faults are rightrocks are ground and crack in various
include:
as dextral)
thehave a plane, like when Professor Burgmann showed
Howlateral(also
to determine known
fault movement.
If youif just
us that fault
cutout, you
cannot These
see true sense
orientations
in response
to stress..
 dip
Porphyroclasts
across from
the observer
moves
of blockblock
offset because
that cutout
could have
been cut out at any
from horizontal, thus making damage
your analysis
If you
a preexisting
zones unfruitful.
are strongest
nearhave
the fault..
right. (like
Theytheare
left-lateral(sinistral)
 displacement
Porphyroblasts
linear feature
intersection
of two planes)ifyou can tell the
of the fault. These points
are
called
piercing
points.
The
movement
of
Some structures in damage zones are
the will
opposite
happens.
Oblique
Slip
these points
give a vector
of block
movement.
 Sheath folds
 Wing
Cracks—Extension
combines
Strike-Slip faulting
Faultfaulting
Terminations
are as follows:
fractures
associated
with small
with Thrust or Normal faulting.
•
Termination Line—Line that lineates where the fault ends in every direction including inside the
amounts
ground
of (brittle
displacement
movement eventually
becomes ductile at some point).
 Horsetail Splay—Occur along
Because faults do not continue through
See
Figure
3.3 for
movement
•
Blind
fault—A
fault
that doesreferences
not break through
the
topographic
surface
larger faults and create a series of
the entire earth, there are certain
secondary
pinnate
shear
•
Branch Line—Where the fault ends and branches
off Branch
lines create
Splay Faults
definitions
to define
the boundaries
of a
fractures.
Some
common
features
of
faults
include:
zone:
•
Splay Faults—Subsidiary faults of the main fault.
Cataclastic Rocks, which have been
Synthetic
Branch
Faults—
 the cataclastic rocks become fractured indamage
•
Damage Zones—On the side of a fault or along the tip,
zones. These
damage
zones
ground into powder or fractured into
When deformation at a fault tip

Termination
Line—Where
clasts from the brittle movement, these
causes shear of the same sense of
the faults ends.
give way to Mylonitic Rocks. Mylonitic
the motion of the fault
zones
are
those
where
rocks
have
been

Fault
trace—Where
a
fault
Math:
 Antithetic
Faults—When
deformed by being stretched out.Scarps
terminates along a tographic
deformation
at a fault tip causes
The following
equation
has
been
derived
of
faults
and
shows
that
in
systems,
longer
faults
are
rarer
than
shorter
ones.
are where the block movement makes a
surface.
shear of the opposite sense as the
distinct change in topography There are
 Blind fault—A fault that
main fault. This creates rotation
also certain
structures
thatgreater
can imply
N = Number
of faults
with length
than L
does not break through the
of the block in the damage zone.
topographic surface
m, Kfaulting.
= DerivedCongruous
Constants or Incongruous
stepsare asymmetric kinematic indicators
N=K÷Lm→ log N=log K – m*log L
1
Ara Alexanian, 2011
Edited by Andrew Tholt &
Jessica Anderson, 2013
Math:
The following equation has been
derived from faulting and shows that in
fault systems, longer faults are rarer than
shorter ones.
N = Number of faults with length
greater than L
m, K = Derived Constants
N=K/Lm→ log N=log K – m*log L
References & Resources
Robert J. Twiss, Eldridge M. Moores,
Structural Geology 2nd edition, (W. H.
Freeman), p. 61-89, 2006
2
Ara Alexanian, 2011
Edited by Andrew Tholt &
Jessica Anderson, 2013