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Faults
BASIC DEFINITIONS
►
FAULT: A surface or narrow zone along which one side
has moved relative to the other. A fault may form
through either brittle (fractures) or ductile (shear)
deformation. Faults are classified based upon the
orientation of the displacement vector along the fault
surface.
 DIP-SLIP FAULTS: Faults along which the movement is
approximately parallel to the dip
 STRIKE-SLIP FAULTS: Faults along which the movement is
approximately parallel to the strike
 OBLIQUE-SLIP FAULTS: Faults along which the movement is
inclined to both the strike and dip of the fault surface. It may be
described as the sum of a strike-slip and a dip-slip vector
HANGING WALL vs FOOTWALL
►
►
A fault divides rock into two fault blocks
For any inclined fault, the block above the fault is the
hanging wall block, and the block below the fault is the
footwall block
www.data.scec.org/Module/footnt02.html
CLASSIFICATION of DIP-SLIP FAULTS
►
NORMAL FAULTS: Dip-slip faults on which the hanging
wall moves down relative to the footwall
 Place younger rocks on older rocks
 May result in omission of strata
 Form in regions of lateral extension
www.data.scec.org/
Module/footnt02.html
EXAMPLES OF
NORMAL FAULTS
Outcrop scale normal faults
Conjugate Normal Faults,
Canyonlands National Park
darkwing.uoregon.edu/~millerm/conjN1.html
Normal Fault Scarps, Turkey
www.msnucleus.org/.../pt/hazards/4/pth4_1a.html
EXAMPLES OF
NORMAL FAULTS
FW
FW
HW
FW
HW
Outcrop scale normal faults
HW
FW
HW
HW
FW
Conjugate Normal Faults,
Canyonlands National Park
darkwing.uoregon.edu/~millerm/conjN1.html
Normal Fault Scarps, Turkey
www.msnucleus.org/.../pt/hazards/4/pth4_1a.html
CLASSIFICATION of DIP-SLIP FAULTS
►
THRUST or REVERSE FAULTS: Dip-slip faults on which the
hanging wall moves up relative to the footwall
 Place older rocks on younger rocks
 May result in repetition of strata
 Form in regions of lateral
compression
www.data.scec.org/
Module/footnt02.html
EXAMPLES OF
THRUST FAULTS
Outcrop scale thrust faults
www.pitt.edu/.../7Structures/ReverseFaults.html
Thrust Fault in Concrete from 1964 Quake,
Anchorage, Alaska
www.ucmp.berkeley.edu/.../alaska/0709log.html
Thrust Fault in Sediments
www001.upp.so-net.ne.jp/fl-fg/05-01.htm
EXAMPLES OF
THRUST FAULTS
Outcrop scale thrust faults
www.pitt.edu/.../7Structures/ReverseFaults.html
HW
FW
HW
FW
HW
Thrust Fault in Concrete from 1964 Quake,
Anchorage, Alaska
www.ucmp.berkeley.edu/.../alaska/0709log.html
FW
Thrust Fault in Sediments
www001.upp.so-net.ne.jp/fl-fg/05-01.htm
CLASSIFICATION of STRIKE-SLIP FAULTS
►
RIGHT-LATERAL (DEXTRAL) FAULTS: Strike-slip faults
across which the block moves to the right
►
LEFT-LATERAL (SINISTRAL) FAULTS: Strike-slip faults
across which the block moves to the right
EXAMPLES OF
STRIKE-SLIP FAULTS
Right Lateral Fault in Asphalt
www.uwsp.edu/.../fault_transform_photo.html
San Andreas Fault, CA
http://education.usgs.gov/california/
pp1515/chapter2/fig2-21.jpg
Right Lateral Slip, Izmit, Turkey, 1999 Quake
http://www.geo.uib.no/jordskjelv/
index.php?topic=earthquakes&lang=en
EXAMPLES OF
STRIKE-SLIP FAULTS
Right Lateral Fault in Asphalt
www.uwsp.edu/.../fault_transform_photo.html
San Andreas Fault, CA
http://education.usgs.gov/california/
pp1515/chapter2/fig2-21.jpg
Right Lateral Slip, Izmit, Turkey, 1999 Quake
http://www.geo.uib.no/jordskjelv/
index.php?topic=earthquakes&lang=en
STRATIGRAPHIC INDICATORS OF FAULTS
► Truncation
of Units
 Unconformity? Intrusion?
► Omission
of units
 Unconformity?
► Repetition
 Folding?
of Units
PHYSIOGRAPHIC INDICATORS OF FAULTS
► Scarps
 Terraces? River Valley?
► Linear
Valleys
 Erodable Strata?
► Pull-Apart
Basins or
Push-Up Ridges
http://www.bedford.k12.ny.us/flhs/
science/images/sanandreas1.jpg
MESOSCOPIC
STRUCTURAL
INDICATORS OF FAULTS
► Slickensides
► Drag
Folds
► Vein Sets
Horizontal Slickensides
http://wso.williams.edu/~cgarvin/photos/
slickensides.jpg
Drag Fold Associated with Thrust Fault
web.umr.edu/.../205-mile_landslides.htm
MESOSCOPIC
STRUCTURAL
INDICATORS OF FAULTS
► Slickensides
► Drag
Folds
► Vein Sets
Horizontal Slickensides
http://wso.williams.edu/~cgarvin/photos/
slickensides.jpg
Drag Fold Associated with Thrust Fault
web.umr.edu/.../205-mile_landslides.htm
Why do certain types of
faults have
predictable dips?
FRACTURE EXPERIMENTS
► To
understand the mechanics of fracturing,
geologists perform controlled experiments on
rock cores
► Rock
is wrapped in a metal jacket
 Provides confining pressure
 σ2 and σ3 are perpendicular to core length
► Piston
presses along the length of the core
 σ1 is parallel to core length
FRACTURE EXPERIMENTS
►
Core fractures with a
consistent pattern
FRACTURE EXPERIMENTS
►
Core fractures with a
consistent pattern
►
Two sets of fractures
with an acute angle of
approximately 60º
between them
 Conjugate fractures
FRACTURE EXPERIMENTS
σ1
►
Core fractures with a
consistent pattern
►
Two sets of fractures
with an acute angle of
approximately 60º
between them
 Conjugate fractures
► σ1 bisects the acute
σ1
angle between the
conjugate fractures
FRACTURE EXPERIMENTS
σ1
► σ1 bisects
the acute
angle between the
conjugate set
► σ3 bisects
the obtuse
angle between the
conjugate set
σ3
► σ2 runs
σ1
parallel to the
intersection of the
conjugate set
ANDERSON’S THEORY OF FAULTING
► The
surface of Earth is a “free surface”, that is, it
is a surface that cannot support shear stress
 Shear stress = 0
► Shear
stress = 0 along principal stress planes
 Principal stress directions must be parallel or
perpendicular to a principal stress plane
► The
surface of Earth is a principal stress plane
 At the surface of Earth, the principal stress directions
must be parallel or perpendicular to a principal stress
plane
COMBINING FRACTURE
EXPERIMENTS WITH
ANDERSON’S THEORY
► σ1 must
be either parallel or perpendicular to
Earth’s surface
► Fractures
form in conjugate sets around σ1
► The
orientation of fractures (faults) will vary with
the orientation of σ1 and the other principal
stresses
► There
are only 3 possible orientations of the
principal stresses relative to the Earth’s surface
σ1 Vertical…
► Produces
fractures/faults that
dip at 60º
► Motion
is along dip
with hanging-wall
down
► NORMAL
FAULTS
σ1 Horizontal, σ3 Vertical…
► Produces
fractures/faults that
dip at 30º
► Motion
is along dip
with hanging-wall
up
► THRUST
FAULTS
σ1 Horizontal, σ2 Vertical…
► Produces
fractures/faults that
dip at 90º
► Motion is along
strike
► STRIKE-SLIP
FAULTS