Chapter 8 PM

8/9/2019 Chapter 8 PM

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D efinitions and Characteristics

A fault consists of a zonealong which slip (shear displacement) hasoccurred. The zone isseparated from in-tact rock by two surfaces. Thezone can vary in widthfrom less than a mm to

more than a km. A fissure is essentially a joint with a large aperturemeasured in cm to M.


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F ault Rock is the material in a fault zone it consists of:gouge

BrecciaCataclasitePseudotachylite


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G ouge is mostly clay sized, poorly consolidated material pulverized by fault slip.

Breccia consists of angular, poorly sorted clasts up toboulder size that have beenbroken up by fault slip.


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Cataclasite is a fine grained poorly sorted breccia that is usually well lithified.


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Tachylite is volcanic glass.

Pseudotachylite is

also glass, but formed by melting rock due thefrictional heat along a fault .

Normally forms at depth where rock isalready heated.


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D efinitions and CharacteristicsF ault terminology


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Classification of FaultsS lip and S eparation:

S lip is a displacement vector that connects two points oneither side of the fault zone that were connected beforefaulting. A bedding surface alone can never be used todetermine slip.

S eparation is an apparent displacement parallel to thestrike and/or parallel to the dip. It is not the slip, but may bea component of the slip.S trike separation is not the same as strike slip (Next S lide)


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Classification of FaultsTerms use to describe fault displacement


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Classification of Faults

E xamples of S lip and S eparation of bedding surfaces:


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F aults are classified based on their orientation relative to thesurface (strike and dip) and sense of slip i.e. relativedisplacement of the fault blocks.

There are four general categories of faults:

S trike-slipDip-slipOblique-slipRotational-slip


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S trike-slip F aultsS ense of slip:

L eft- L ateral Right- L ateral


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Classification of FaultsH igh angle Dip-slip

F aults (>50 o dip)H anging Wall F oot Wall S ense of slip:

Normal (extension)Reverse (shortening)


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Classification of FaultsL ow angle Dip-slip

F aults (


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Classification of FaultsOblique slipS ense of slip:

Normal/left slipNormal/right slip(Not shown)

Reverse/left slipReverse/right slip(not shown)


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Classification of FaultsRotational slipL ooking across fault:

ClockwiseCounter clockwise


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Classification of FaultsClassification also described in terms of horizontal strain:

S hortening (contractional) faultsE xtensional faults


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Slip D irectionDetermination of slip

S lip direction can frequently be determined from slickenlinesand fiber lineations.


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Slip direction

S lickenlines give only thedirection of slip.S ometime the sense of slip can be determined aswell. F or example, theclast causing the groove

is present indicating theoverlying fault block moved upward.


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Slip D irection

F ibrous mineral growths may also give thesense of slip. The

fibers tend to bestepped becausethey grow fromirregularities onthe fault surface.

The stepsindicate sense of motion of theblock abovethem.


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What is the sense of slip of the fault blocks in the picture


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Fault BendsWhen ever a fault has a bend it will commonly produce folds

in layered sediment.Dip slip fault bends produce fault bend foldsS trike slip bends produce pop-up folds or sags


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Thr ust Fault

Bends

Thrust faults usually follow bedding plane surfaces and then bend up(ramp) to another bedding plane

producing a stair-step geometry.

A rampgenerates a fault- bend fold whichmay develop aflat top if there isenoughdisplacement.


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Thr ust Fault Bends

Because of the ramp-flat geometry,thrust belts can be very complicated.

A ramp cuts off layering

producing ahanging wall cutoff and afootwall cutoff.

L ikewise there

are bothhanging wall flats and footwall flats.


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Norm al Fault

BendsL istric normal faults

also produce fault bend folds. Thesehave been called avariety of names:

Rollover foldsReverse Drag F olds


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S t rik e sl ip

Fault BendsS trike slip fault bends have

two types:Releasing bends

produce extensional structures: sags (basins)and normal faultsRestraining bends

produce shortening structures: uplifts, foldsand thrust faults


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S t rik e sl ip

Fault BendsThe strike slip fault at left

has a bend. What type of bend is it? That is the

sense of displacement on the fault?


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Fault Terminations

Terminology Blind fault E mergent fault E xhumed F ault


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Fault Terminations

When faults terminate,folding commonly occurs toaccommodate thechange indisplacement.

Both fault bends and fault

terminations cangenerate folds


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Fault Terminations

F olds at fault terminations are called fault propagation folds if they form at a ramp (left) or detachment folds if they formalong a flat


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Fault TerminationsIdentify the type of folds depicted below


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Sense of Shear on Faults

The sense of displacement on faultsis often important, so be familiar with the following ways this can bedetermined.Offset piercing points

Asymmetry of folds related to the

fault S teps on fiber lineationsE n echelon extension veinsPinnate fractures


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Determine slip direction and fold asymmetry


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Sense of Shear on FaultsDetermine slip direction using Z and S asymmetric folds in

fault zones


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Determine slip direction using Tension G ashes in fault zones

The extension direction pointsin the shear direction along the zone


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Sense of Shear on FaultsS lip direction from F iber

lineations


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Sense of Shear on FaultsPinnate fractures are small

shear fractures (Reidel shears) that areassociated with a larger fault. The shortening direction bisects theacute angle between thesmall fractures and thelarger one.

The extension direction

(Perpendicular to max shortening) again pointsin the direction of shear,

just like the en echelonveins.


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R ecognition of Faults

A fault scarp is an offset of the topographic surface that is

produced by recent movement on a fault.


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A fault line scarp is also a step in the topography, but represents differential erosion along an old fault that hasrock 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.


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I dentification and Expression of FaultsF aults truncate and offset layering and other types of

boundaries in rock.F

aults may be difficult to identify on amap if they parallel the strike of the layering.


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I dentification and Expression of Faults

Depending onthe type of

fault and itsdip relativeto layering,faults either omit strata,duplicatethem, or truncatethem.


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I dentification and Expression of Faults

F aults in thesubsurface canbe detected by

contouring stratigraphic boundaries fromdrill hole data(structural

contour maps


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I dentification and Expression of FaultsS ubsurface faults can be detected by geophysical methods

such as seismic (below) as well as gravity and magnetic methods.


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Strain, Stress & FaultsNormal faults produce horizontal extension. And vertical

shortening (sag, rift, basin)


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Strain & FaultsReverse faults produce horizontal shortening. And vertical

extension (uplift)


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Strain & FaultsS trike slip faults produce both horizontal shortening and

extension. No vertical strain (ideally). L ook at the figurebelow as a map, not a x-section.


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Stress & Faults A nde r sons Th eory of Fault i ng

Andersons Theory of F aulting relies on theMohr-Coulomb failurecriterion:

( X c X o )/ W = Tan( J );

X o and Tan( J vary with therock type and are called the cohesion and internal friction.

Its the ratio of shear stressto normal stress on asurface that determineswhether a fault will occur.


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Mohr-Coulombfailure criterion:

( X c X o )/ W = Tan( J )Is usually written as

X c = X o + W Tan( J )This plots as a

straight line onthe Mohr diagram

J is the angle of thefailure line and Tan( J ) is theslope.


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The Mohr circleshows thenormal and

shearing stresses on all the surfaces in arock.

When the circlebecomestangent to thefailure line, thenone surface hasthe right amount of normal and shearing stressto fracture.


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The orientation of

that fracturesurface relativeto W 1 can bedetermined fromthe Mohr circle.

(Remember that Uis the angle tothe surfacenormal!)


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There is a

relationshipbetween Uand J

U= 45 + J /2


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The Mohr fracturecriterion predicts that fault normalswill always

occur about 60 o

to the maximumcompressivestress W 1

Or, the fault surfacewill be about 30 o

to W 1


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The Mohr fracturecriterion also predicts that there will be twofault surfaces

about 30 o

oneither side of W 1

These are called aconjugate set


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Thrust faults aregenerated when W 1is horizontal and W 3 is vertical. W 2 is horizontal and

parallel to theintersection of thefault surfaces.


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Stress & Faults A nde r sons Th eory of

Fault i ng

Normal faults occur when W 1 is vertical and W 3 is horizontal.W 2 is horizontal and

parallel to theintersection of thefault surfaces.


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Stress & Faults A nde r sons Th eory of

Fault i ng

S trike-slip faults occur when both W 1 and W 3are horizontal. W 2 isvertical and parallel tothe intersection of thefault surfaces.


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Stress & Faults

CaveatsThe Mohr-Coulomb fracture

criteria applies to uniform,intact rock. If there areweak zones in the rock like bedding surfaces and

preexisting fractures, then Andersons theory of faulting cannot be applied.


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O ri g i n of Li st ric Faults

All stress (like politics) is local, and changes bothhorizontally and vertically. F aults are curved because thestress that generates them changes from place to place.


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P r ob lems wi t h Fault i ng

There are two problemsassociated with theorigin of faults:S trength of rock increases with

pressure (depth) sothat the rocks at depth are too strong for the availablestresses in the E arthto break them.


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The other problem isL arge, nearly horizontal thrust faults cannot movebecause thehorizontal stress onthe hanging wall block would crushthe end before it moved the block.


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One solution is low effective pressure produced by high pore pressure ( W * = W p p )

If there is no fluid in the fault zone, the shear stress(related to the radius of theMohr diagram) is not great enough to overcome thestrength of the rock at high

pressures (the center of theMohr Diagram)


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If pore pressure in the fault zoneincreases to reduce theeffective stress, then theeffective stress may exceed

the rock strength as theeffective pressure is reduced (the center of the Mohr Diagram).


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Fault A rr a ys

G roups of faults that formed at the same time are called fault systems, or fault arrays. S ome general terminology:Thin S kinned F ault arrays that are confined to thesedimentary sequence and do not penetrate basement Thick skinned F ault arrays that do penetratebasement Master faults large regionally significant faultsS ynthetic faults smaller faults that parallel the master fault

Antithetic faults smaller faults that are conjugate tothe master fault.


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Fault A rr a ys

Terms describing fault arrays inmap view.Parallel array

Anastomosing E n echelonRelay Conjugate

Nonsystematic usually fromreactivation of older faults.

T d ibi i l f l i fil


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Fault A rr a ys

Terms describing extensional fault arrays in profile.L istric faults are usually thin skinned (but not always)H orsts & grabenH alf grabenRift large regional feature that contains horsts & grabenand/or half graben.


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Fault A rr a ysTerms describing shortening fault arrays in

profile.L

istric faults are usually thin skinned (but not always)S tructures occur in fold-thrust beltsF lats & rampsImbricate fans


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Fault A rr a ysMore terms describing shortening fault

arrays in profile.Duplex & horses (multiple, closely spaced

ramps)Roof Thrust & floor thrust


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Fault A rr a ysS trike slip systems in profile

F lower structures master fault

branches upward forming a stem with petalsNegative & positive flowers.


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Faults, Res ourc es &

Ea r t hq ua k es

F aults 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 isnot only accumulated over time, but also over space only a small area of a fault will displace at one timewith another area displacing at another time.


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Faults, Res ourc es & Ea r t hq ua k es

An E arthquake and fault displacement at one particular zone along a large fault may occur at regular intervalsof time the recurrence interval

The recurrence interval has a large standard deviation its not Old F aithful.

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.


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Faults, Res ourc es & Ea r t hq ua k es

F aults are also economically important for resourcerecovery:

They can form barriers or channels for fluid flow, whether groundwater or petroleumThey are sources and zones for mineralization and oresThey may offset economically valuable strata (coal,

petroleum reservoirs) leading to recovery complications.

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