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GG303 Lecture 25
9/4/01
1
FAULTS (I)
I
Main Topics
A Why are faults important?
B Observations of faults
C Geologic classification of faults
I I Why are faults important?
A Faults generate earthquakes
B Faults reveal how the earth has deformed through time
C Faults (including deformation bands) play critical roles in fluid
transport in the earth's crust (e.g., water, magma, petroleum &
natural gas, and hydrothermal fluids [ore minerals])
D Faults are zones of weakness to account for in engineering projects
III Observations of faults
A Gross geometry
1 Thin relative to their in-plane dimensions
2 Bounded in extent
3 Grossly planar (usually)
B Relative displacement (slip) of opposing fault walls is
parallel to the fault
C Structural details of faults
1 Overall structure: ~planar segments
Affects internal structure & fault behavior
2 Internal structure: complicated fracture pattern
3 Common for faults to be paralleled by other fractures. Typical
a s s u m p t i o n is that fault slip causes fault-parallel fractures to
form (note that cross-cutting relationships don't help out).
4 Common for hydrothermal/geothermal activity to occur near ends
of fault segments (San Andreas, Wasatch faults)
D Composition
1 Breccia
2 Fault gouge
3 Mineralization
Stephen Martel
25-1
University of Hawaii
GG303 Lecture 25
9/4/01
2
E Kinematics
1 Cut adjacent material; faulting post-dates the host rock
2 Relative (not absolute) d i s p l a c e m e n t (slip) of originally
neighboring points is parallel to the fault; relative
displacement may or may not be small relative to fault length
F Surface textures of faults
1 Slickensides (polished surfaces)
2 S l i c k e n l i n e s (striations); parallel to most recent(?) slip vector
IV Geologic classification of faults (see Fig. 25.1)
A Geologic classification
1 Based on orientation of slip vector relative to the strike and dip
of a fault
2 Slip determined by the relative displacement of piercing points
that were originally neighbors on opposite faces of a fault.
3 Piercing points mark intersection of a line with a fault
4 The slip vector connects offset piercing points
5 Slip is not the same as "movement" or "displacement"
B Strike-slip fault: slip vector is predominantly horizontal (i.e.,
parallel or anti-parallel to the line of strike)
1 Right lateral: in map view across a fault, a marker is offset to
the right
2 Left lateral: in map view across a fault, a marker is offset to the
right
C Dip-slip fault: slip vector is parallel (or anti-parallel) to dip
1 Normal fault: hanging wall moves down relative to footwall
2 Thrust fault: hanging wall moves up relative to footwall
*Deeper (older) rocks thrust over shallower (younger) rocks*
D Oblique-slip: combination of strike slip and dip slip
E Slip vs. Separation (see Fig. 25.2)
1 Slip: True relative displacement of originally neighboring points
2 Separation: Apparent relative displacement of an offset feature
as seen in a map or a cross-section
F The amount and direction of slip can change with time
and/or position along a fault!
Stephen Martel
25-2
University of Hawaii
GG303 Lecture 25
9/4/01
3
Fig. 25.1
Geologic Classification
of Faults
Strike-slip Faults
Left lateral
Right lateral
Piercing
points
Slip
Vector
y
wa
ive
Dr
y
wa
ive
Dr
Slip
Vector
Folded unit,
with sharp hinge
Dip-slip Faults
Pure normal slip
Pure reverse (thrust) slip
w
ive
Dr
ll
wa
ot
ll
Fo
wa
ot
Fo
Slip vector
ay
wa
ive
Dr
y
l
al
gw
in
l
al
gw
in
ng
ng
Slip
Vector
Ha
Ha
Folded unit,
with sharp hinge
Stephen Martel
25-3
University of Hawaii
GG303 Lecture 25
9/4/01
4
Contrast between slip and separation
Fig. 25.2
Pure dip slip.
Slicklenlines plunge
directly down-dip.
Piercing
point
Fault Scarp
Slip vector
y
wa
ive
Dr
After slip,
Before erosion
Folded unit, with sharp hinge
Apparent right-lateral
offset (right-lateral
separation)
Separation
Separation
Apparent left-lateral
offset (left-lateral
separation)
After slip,
After erosion,
After driveway removal,
After new home construction
Stephen Martel
25-4
University of Hawaii