Faults
Introduction
• Faults—“fractures along which there is visible offset by shear
displacement parallel to the fracture surface” (Davis & Reynolds, p.
269).
• Because we have a fairly good understanding of how faults form,
interpreting them is relatively straightforward and provides a great
deal of useful information
Recognizing Faults
• Fault scarps
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The actual fault surface exposed
Presently active faulting
Visible offset of natural and man-made objects
Approximate the total recent displacement
• Weathering will reduce apparent displacement
• Deposition of sediment on the down-thrown block will also reduce apparent
displacement
• Fault-line scarps
– Located at or near the fault
– Displacement reflects relative resistance to erosion
Borah Peak Scarp
Borah Peak Earthquake Scarp
Borah Peak Earthquake Scarp
Faulted Moraine
Recognizing Faults
• Triangular facets
– Form when a fault surface is cut by streams
– Suggests recent faulting
Pleasant Valley, Nevada
Fault Surfaces
• Fault surfaces are rarely exposed
– Fault surfaces are easily eroded because the rock is fractured
– They are sometimes preserved by mineralization
– Most often in the case of recent faulting
• Characteristics
– Planar
– Made of planar segments of differing orientations
– Systematically curved
– Highly irregular
Fault Surfaces
• Must end
– They don’t go on forever
– Most are probably elliptical
• Aspect ration of 2 or 3 to 1
• Greatest displacement near center
• Decreases toward tip line loop (line marking the end of the fault or zero displacement
Guatemala, Feb. 1976
San Fernando, 1971
Features of Fault Surfaces
• Slickensides are polished fault surfaces
– Shine is due to neomineral coatings
• Slickenlines are straight lines on the fault that record the direction of slip
– Most are striations produced by scraping of the fault surface by frictional abrasion
– Grooves and mullions are larger scale versions of the same phenomenon
• Slickenfibers are minerals that grew in the direction of movement during
faulting
– Enhanced growth on the “protected” sides of steps in on the fault plane
– Davis uses the terms slip-fiber or crystal fiber
Features of Fault Surfaces
• Slickolites
– Pressure solution feature
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Based on stylolites
Occur along fault surfaces
Aligned with direction of motion
Penetrate into bumps on fault
• Chatter marks
– Step like features
– Perpendicular to striations
– Not the most reliable motion indicator
Slickenside
Fault Surface Features
Fault Rocks
• Gouge
– Light-colored very fine-grained clayey fault rock
– Commonly found on fault surfaces and in fault zones
– Particles less than 0.1 mm
– When dry feels like loosely compacted talcum powder
– When wet feels sticky and clayey
– Forms under low temperature and pressure conditions
Fault Rocks
• Breccias
– Angular fragments of wall rock set in a finer-grained matrix of crushed wall
rock
• Megabreccia has clasts larger than 0.5 m
• Breccia has clasts 1 mm to 0.5 m
• Microbreccia has clasts 0.1mm to 1 mm
– Dilation and volume increase are characteristic
– Low confining pressure and/or high fluid pressure
– High fluid pressures can produce breccia dikes
Fault Rocks
• Cataclasite
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Very fine grained, hard fault rock
Many are ultra fine grained and nearly glassy in appearance
Resistant to erosion
Indicates elevated temperature and pressure conditions
• Pseudotachylite
– Fault-melted rock the cooled instantly to glass
– Implies high pressure conditions and instantaneously imposed strain—
shock
Map Expression of Faults
• Most faults are recognized in geologic mapping
– Truncation or offset of map units
– Classic offset occurs when faults are at a high angle to the strike of bedding
– Repetition and omission of layers occur when faults are nearly parallel to
the strike of bedding
– Repetition can be particularly pronounced when faults are nearly parallel to
bedding
– Faults can be found from drill data
• Drilling can encounter actual fault zones
• Sudden changes in depth at which units are encountered
Map Clues
Geophysical Expression
• Gravity and Magnetic data
– Abrupt contrasts in gravity or magnetic signatures
• High gravity values imply dense rock such as mafic igneous rocks
• Magnetic highs imply rocks with a high magnetite content
• Low gravity values and magnetic lows imply a thick cover of low density, non-magnetic
sediments
– Mid-Continent Gravity High
• Drill hole data
• Seismic Expression
– Reflection seismology
Drill Hole Data
Mid-Continent Gravity High
Mid-Continent Gravity High
Seismic Reflection
Fault Classification
• Slip versus Separation
– Slip is the actual relative displacement
• Direction of displacement
• Sense of displacement
• Amount of displacement
– Separation
• The apparent relative displacement
• Hanging wall and footwall
– Miners’ terms
– Think of mine tunnel
Slip Classification
• Dip-slip
– Motion down the dip
– Normal-slip (normal)
• Hanging wall moves down relative to the footwall
– Reverse-slip (reverse)
• Hanging wall moves up relative to the footwall
– Thrust-slip (thrust)
• Reverse fault with a dip less than 45°
Slip Classification
• Strike-slip
– Left-handed slip (left-lateral strike-slip)
• Opposite side shifts horizontally to the left
– Right-handed slip (right-lateral strike-slip)
• Opposite side shifts horizontally to the right
• Oblique-slip
– Combination of strike-slip and dip-slip
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Left-handed normal slip
Left-handed reverse slip
Right-handed normal slip
Right-handed reverse slip
Fault Classification
Separation Classification
• Refers to the apparent sense of motion
• Apparent offset in outcrops, map patterns, and cross sections
– Normal, reverse, or thrust separation
– Left-lateral or right-lateral separation
• If map and cross section views are available you can combine the
above
– The dominant separation is given last
• Left-lateral reverse separation is a fault that is mostly a reverse separation with a leftlateral component
Other terms
• Stratigraphic separation—measured along the fault perpendicular
to offset layering
• Dip separation—distance between offset beds measured parallel to
the dip of the fault
• Throw—vertical component of dip separation
• Heave—horizontal component of dip separation
• Horizontal separation—measured along a line perpendicular to the
offset surface
• Strike separation—measure parallel to the strike of the fault
Separation
Motion sense
• Slickensides and grooves
– Generally indicate the last movement
– Give movement direction
– May give sense
• Drag folds
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Layers next to the fault are dragged by faulting
Sigmoidal drag on closely spaced faults
S and Z folds
Reverse drag on listric faults
Drag Folds
Z- and S-Folds
Listric Faults