FractFault.pptx

7/27/2019 FractFault.pptx

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Structural Geology


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FRACTURES AND FAULTS


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ObjectivesThis unit of the course discusses Fractures and Faults

By the end of th is u ni t you wi l l be able to:

Differentiate between the differenttype of fractures

Differentiatebetween the dif ferent type of faults

Understand the relationship between the different type of

stresses and faults

Where faults form and how?

Faults mechanics

Role of fl uid in faulting

Faul ts movement mechanisms

Shear, Shear zones and dif ferent type of shears


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FRACTUREFRACTURE: is defined by Twiss and

Moores (1992)as ..surfaces alongwhich rocks or minerals have broken;

they are therefore surfaces across

which the material has lost cohesion

Characteristics of fractures according to

Pollard and Aydin (1988) fractures have two parallel surfaces that

meet at the fracture front

these surfaces are approximately planar

the relative displacement of originally

adjacent points across the fractures issmall compared to the fracture length..


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Fracture, Joint and Fault

The term fracture encompasses both joints

and faults.

JOINTS:are fractures along which there has

been no appreciable displacement parallel tothe fracture and only slight movement normal

to the fracture plane.Joints are most common of all structures present in all settings in all kind of rocks as well

as consolidated and unconsolidated sediment


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Types of Fractures

Extensional FractureIn extensional fractures the Fracture plane is oriented parallel to 1and 2 and perpendicular to 3.

Three types of fractures have been identified:

Mode Ifractures (joints)it is the extensional fractures and formed byopening with no displacement parallel to the fracture surface (see

above figure). Mode IIand Mode IIIare shear fractures. These are faults like

fractures one of themis strike -slip and the other is dip-slipSame fracture can exhibit both mode II and mode IIIin different parts of the region.


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Importance of studying joints and

shear fractures To understand the nature and sequence of

deformation in an area.

To find out relationship between joints and

faul ts and or folds. Help to find out the bri ttle deformation in an

area of construction (dams, bridges, and power

plants.

I n mineral explorationto find out the trend andtype of fractures and joints that host

mineralization which wi ll help in exploration.


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Importance of studying joints and

shear fractures

Joints and fractures serve as the plumping system for

ground water f low in many area and they are the only

routes by which ground water can move through igneous

and metamorphic rocks.

Joints and fractures porosity and permeabil i ty is very

important for water supplies and hydrocarbon reservoirs.

Joints orientations in road cuts greatly affect both

construction and maintenance. Those oriented parallel toor dip into a highway cut become hazardous dur ing

construction and later because they provide potential

movement surfaces.


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TYPES OF JOINT Systemat ic jo ints :have a

subparallel orientation andregular spacing.

Jo int set:joints that share asimilar orientation in same area.

Joint system :two or more joints

sets in the same area

Nonsystemat ic jo ints:jointsthat do not share a commonorientation and those highlycurved and irregular fracturesurfaces. They occur in mostarea but are not easily related toa recognizable stress.

Some times both systematic and nonsystematic jointsformed in the same area at the same time butnonsystematic joints usually terminate atsystematic joints which indicates thatnonsystematic joints formed later.


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Type of Fractures

Plumose jo ints:joints that

have feathered texture on

their surfaces, and from this

texture the direction of

propagation of joints can be

determined.

Veins:are filled joints and

shear fractures and the

filling range from quartz and

feldspar (pegmatite and

aplite) to quartz, calcite and

dolomite.


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Type of Fractures Conjugate fractures:paired

fracture systems, formed in thesame time, and produced bytension or shear. Many of themintersect at an acute angle which

will be bisected by the Curved fractures:occur

frequently and may be caused bythe textural and compositionaldifferences within a thick bed or

large rock mass or they may aresult of changes in stressdirection or analysis.

Cross cutting relationship and materialfilling the fractures can help in resolving

the chronological order of deformation.


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FRACTURE ANALYSISStudy of joints in an area will give information about the

sequence and timing of formation. It will also provideinformation on the timing and geometry of the brittledeformation of the crust and the way fractures propagatethrough the rocks.


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Importance of Fracture OrientationStudy of orientation of systematic fractures

provides information about theorientation of one or more principlestress directions involved in the brittle.

Parameters measured for fractures are str ike

and dip.

Or str ike of l inear features from aer ial photos

and landsat images.Data obtained from fractures is plotted in

rose diagram or equal area net. Equalarea net for strike and dip and rosediagram for strike only.

Studies of joint and fracture orientationfrom LANDSAT and other satelliteimagery and photographs have a varietyof structural, geomorphic, andengineering applications.


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Strain -ellipsoid analysis

of joints in area mayhelp to determine

dominant crystal

extension directions


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Fold and Joints

Join ts may form dur ingbri t t le folding in a

posi t ion related to the

fold axis and axial

su rface as fo l low s

parallel

normal

obliquedepend ing on stress

condi t ion.


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Fault Related Joints Join ts are also fo rmed

adjacent to bri t t le faults, and

movement along faul ts

usu ally produces a series of

systematic fractures.

Most joints form by extensional fracturing of rock

in the upper few kilometers of the Earth's crust.

The limiting depth formation of extension fractures

should be the ductile-brittle transition.


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Factors Affecting the Formation of Joints

Rock type

Fluid pressure

Strain rate

Stress difference at a particular

time


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Characteristics of Fractures Plumose structure:is the

structures formed on thejoint surface during itspropagation and providesinformation about the jointpropagation direction.

Hackle marks:indicatezones where the jointpropagate rapidly.

Arrest line:forms

perpendicular to thedirection of propagationand is parallel to theadvancing edge offractures.


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Characteristics of Fractures Bedding and foliation planes in coarse-

grained rocks constitute barriers to joinpropagation. Bedding in uniformly fine-grained rocks, such as shales andvolcanicalstic rocks, appears to be lessof barriers.

In sandstone bed propagation ofjoints through the bed is slightlyoffset from the layers above orbelow.

Variation in bed thickness also affectspropagation direction.

In horizontal layering joints will notpropagate from sandstone into shale

if the least principle horizontal stressin shale is greater than that insandstone.

Fractures will be terminated at thecontact between the two rocks.


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Joints Classified According to their Environment

and Mechanism of Formations (Engelder, 1985)

Tectonic fracture

Hydraulic fracture

Unloading fracture

Loading fracture

A ll of these types are based on the

assumpt ion that fai lure mechanism is

tensile.


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Tecton ic fractures:

Form at depth in response to abnormal f lu id pressureand

invo lve hyd rofractur ing. They form m ainly by tectonic stress

and the horizonta l com pact ion of sedimentat depth less

than 3 km, where the escape of f luid is hin dered by low

permeabi l i ty and abno rmally high pore pressu reis c reated.

Hydraul ic fractures:Form as tectonic fractures by th e pore pressu recreated due to

the con f ined pressed f lu id d uring buria l and vertica l

com pact ion o f sediment at depth greaterthan 5 km. Filled

veins in low metamorphic roc ks are one of the best of

examples of hyd raul ic fractures.


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Unloading fractures:Form near sur face as erosion removes overburden

and thermalelastic contraction occurs.They form

when more than half of the original overburdenhas been removed. The present stress and tectonicactivity may serve to ori ent these joints. Verticalunl oading f ractures occur duri ng cool ing andelastic contr action of rock mass and may occur atdepths of 200 to 500 m.

Release fractures:

Simi lar to unloading fr actures but they form byrelease of stress. Orientation of release joints iscontrolled by the rock fabric. Released joints formlate in the history of an area and are orientedperpendicular to the original tectonic compressionthat formed the dominant fabr ic in the rock.

Release joints may also develop paral lel to the fold

axes when erosion begins and rock mass that wasunder buri al depth and li thi f ication begins to cooland contract, these jointsstar t to propagateparallel to an existing tectonic fabri c.

Sheared fractures may be straight or curved butusually can't be traced for long distance.


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Joints within PlutonsFractures form in pluton in response to

cooling and later tectonic stress. Many ofthese joints are filled with hydrothermalminerals as late stage products. Differenttypes of joints are present with pluton(i.e. longitudinal, and cross joints)


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NONTECONIC FRACTURES

Sheeting join ts:

Those joints form subparallel tothe surface topography.These joints may be moreobserved in igneous rocks.Pacing within these fracturesincreases downward. These

fractures thought that theyform by unloading overlongtime when erosion removeslarge quantities of theoverburden rocks.

Columnar joints and MudCracks:

Columnar joints form in flows,dikes, sills and volcanic necksin response to cooling andshrinking of the magma.


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FAULT CLASSIFICATION AND

TERMINALOGYFaults:Are fractures that have

appreciable movement parallel totheir plane. They produced usual ly

be seismic activity.

Understanding faults is useful in

design for long-term stabi l i ty of

dams, bridges, bui ldings and powerplants. The study of fault helps

understand mountain buil ding.

Faults may be hundred of meters or afew centimeters in length. Theiroutcrop may have as knife-sharpedges or fault shear zone. Faultshear zones may consist of aserious of interleavinganastomosing brittle faults andcrushed rock or of ductile shear

zones composed of mylonitic rocks.


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Parts of the Fault Fault plane: Sur face that the movement hastaken place within the fault.On this sur face

the dip and str ike of the faul t is measured.

Hanging wall:The rock mass resting on thefault plane.

Footwall:The rock mass beneath the faul t

plane. Slip:Describes the movement parallel to the

fault plane.

Dip slip: Describes the up and downmovement parallel to the dip direction of thefault.

Strike slip:Applies where movement isparallel to str ike of the faul t plane.

Oblique slip:I s a combination of str ike sl ipand dip sl ip.

Net slip (true displacement): I s the totalamount of motion measured parallel to the

direction of motion


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Separation:The amount opapparent offset of a faul ted

sur face, measured in specif ied

direction. There are str ike

separation, dip separation, and

net separation.

Heave:The hor izontalcomponent of dip separation

measured perpendicular to str ike

of the faul t.

Throw:The vertical componentmeasured in vertical plane

containing the dip.


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Features on the fault surface Grooves (parallel to the

movement direction) Growth of fibrous minerals

(parallel to the movementdirection)

Slickensidesare the polishedfault surfaces.

Small steps.Al l are considered a kind of

l ineation. They indicate themovement relative trend NW,

NE etc.

Small steps may also be used todetermine the movementdirection and direction ofmovement of the opposingwall. Slicklines usuallyrecord only the last momentevent on the fault.


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ANDERSON FAULTS CLASSIFICATION

Anderson (1942) definedthree types of faul ts:

Normal Faul ts

Thrust Faul tsWrench Faults

(str ike sl ip)


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Different Type of Faults


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Normal FaultNormal Fault :The hanging wall has moved down

relative to the footwall.

Graben:consists of a block that has dropped downbetween two subparllel normal faults that dip towards

each other.Horst:consists of two subparallel normal faults that dip

away from each other so that the block between thetwo faults remains high.

Listric:are normal faults that frequently exhibit (concave-up) geometry so that they exhibit steep dip near surfaceand flatten with depth.

Normal faul ts usu ally found in areas where extensio nal regime

is present.


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Normal Faults


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Thrust FaultThrust Faul ts:In the thrust

faults the hanging wallhas moved up relative tothe footwall (dip angle30 or less)

Reverse Faults :Are similarto the thrust faultsregarding the sense ofmotion but the dip angleof the fault plane is 45or more

Thrust faul ts usually

formed in areas of

com perssional regime.

Thrust Fault


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Thrust Faults


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Strike-Slip FaultStrike-sl ip Faults :Are faults

that have movement alongstrikes.

There are two types of strikeslip faults:

A] Righ t lateral str ik e-sl ip faul t

(dextral):Where the sideoppo si te the observermo ves to th e r ight .

B] Left lateral str ik e-sl ip fau l t(sinistral):Where the sideoppo si te the observermoves to th e lef t .

Note that the same sense ofmovement will also beobserved from the other sideof the fault.

Strike-Slip

Faults


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Transform FaultsTransform Faults:Are a

type of strike-slip fault(defined by Wilson 1965).They form due to thedifferences in motionbetween lithosphericplates.They arebasically occur wheretype of plate boundaryis transformed intoanother.

Main types of transform

faults are: Ridge-Ridge

Ridge-Arc

Arc-Arc


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Other types of fault en-echelon faults: Faults that

are approximately parallel oneanother but occur in shortunconnected segments, andsometimes overlapping.

Radial faults :faults that areconverge toward one point

Concentr ic faul ts:faults that areconcentric to a point.

Bedding faul ts (bedding plane

faults):follow bedding or occurparallel to the orientation ofbedding planes.


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CRITERIA FOR FAULTING Repet i tion or om issionof strat igraphic uni ts asymmetr ical

repet i t ion Displacement of recognizable markersuch as fossi ls ,

co lor, composit ion , texture ..etc.).

Truncat ion of structures, beds or rock uni ts.

Occurrence of faul t rock s(mylon i te or cataclast ic or b oth)

Presence of S or C stru ctu resor both , rotated po rphy ry

clasts and other evidenc e of shear zone.

Abundant ve ins, si l ic i f ic at ion or other m ineral izat ion alongfracture may indicate fault ing.

Drag Unitsappear to b e pul led in to a faul t du r ingmovement (usu al ly with in the drag fold and the resul t isthru st fault)

Reverse dragoccurs along l is t r ic norm al faul ts.

Slickensidesand s l ickenl in es along a fault su rface

Topog raphic character ist icssu ch as drainges that arecon tro l led by faul ts and faul t scarps.


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FAULTS MECHANICSAnderson 1942 defined three fundamental possibilities of stress regimes and stress

orientation that produce the three types of faults (Normal, thrust, and strike-slip)

no te that 1> 2> 3

Thrust fault:1 and 2 are horizon tal and 3 is vert ical. Thus a s tate ofhor izonta l com pressionis d ef ined for thrust faul ts . Shear plane is orientedto 1w ith ang le = o r < 45 and // 2.

Strike-Slip faults: 1 and 3 are horizon tal and 2 is v ert ical. Shear plane isorientedto 1 w ith ang le = or 45 and // 3. Form also du e to hor izontalcompress ion.

Normal faults: 1 is vert ical and 2 and 3 are horizon tal. Shear plane isoriented45 or les s to 1 and // 2. Form d ue to horizontal extension o r vert icalcompress ion.


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Role of fluids in faulting

Fluid s p lays an impo rtant ro le in faul t ing .

They have a lub r icating ef fect in the faul t

zone as buoyancy that reduces the shear

stress necessary to permit the faul t tosl ip. The effect of f lu id on movement is

represented as in landsl ide and snow

avalanches.


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Faults movement mechanismsMovement on faul ts oc curs in tw o d i f ferent ways:

Stick sl ip :(unstable frictional sliding) involvessudden movement on the fault after a long-termaccumulation of stress. This stress probably the causeof earthquakes.

Stable sl id ing:involves uninterrupted motion along afault, so stress is relieved continuouslyand does notaccumulate.

The two types of movement may be produced along thesegments of the same fault. Stable sliding whereground water is abundant, whereas, stick-slip occurwith less ground water


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Other factor that control the type of movement isthe curvature of the fault surface.

Withdrawal of ground watermay cause nearsurface segments of active faults to switch

mechanisms from stable sliding to stick slip, therebyincreasing the earthquake hazard.

Pumping fluid into a faultzone has been proposedas a way to relieve accumulated elastic strain

energy and reduce the likelihood of largeearthquake, but the rate at which fluid should bepumped into fault zone remains unknown.


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Fault Surfaces and Frictional sliding

Fault surfaces between twolarge blocks are alwaysnot planar especially onthemicroscopic scale. Thisirregularities and

imperfections are calledasperit ies increase theresistance to frictionalsliding. They also reducethe surface area actually incontact. The init ia l contact

area may be as lit t l e as10%, but as movement

started the asperit ies w il l

break and contact w i l l be

more.


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Shear (frictional) Heating in Fault zones

During movement of faults frictional heatis generated due to the mechanical

work. The heat generated can berelated to an increase in temperature.This friction heat is indicted by theformation of veins pseudotachylite(false glass) in many deep seated faultzones and the metamorphism alongsubduction zones (greenschist and

blueschist facies).In some areas there is indication of

temperature of 800c and 18 to 19 kb(60km depth). This indicate that they can

form in the lower crust or upper mantle.

Fault zones may also serve as conduit for

rapid fluxing of large amounts of waterand dissipation of heat duringdeformation.

Generally friction-related heating alongfaults is a process that clearly occurs inthe Earth, but difficult to demonstrate.


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BRITTLE AND DUCTILE FAULTSBrittle faults occur in the upper 5 to 10 km

of the Earths crust. In the upper crust

consist of :Single movement

Anastomosing complex of fracturesurfaces.

The individual fault may have knife-sharpcontacts or it may consist of zone of

cataclasite.At ductile-brittle zone 10-15km deep incontinental crust, faults arecharacterized by mylonite. At surfaceof the crust mylonite may also occurlocally where the combination ofavailable water and increased heat

permits the transition.The two types of faul t may occur within onefaul t where close and at the sur facebr ittl e the associated rocks are cataclastsand at deep where ducti le and brittl ezone myloni te is present


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SHEAR ZONEShear zones are produced by both

homogeneous and

inhomogenous simple shear, or

oblique motion and are thought

of as zones of ducti le shear.

Shear zones are classified by

Ramsay (1980) as:

1) brittle

2) brittle-ductile

3) ductile


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Characteristics of Shear ZonesShear zones on all scales are zones

of weakness. Associate with the formation of

mylonite.

Presence of sheath folds.

Shear zones may act both asclosed and open geochemical

systems with respect to f luids

and elements.

Shear zones generally have

parallel sides.

Displacement profi les along

any cross section through

shear zone should be identical.


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INDICATORS OF SHEAR SENSE OF MOVEMENT

1. Rotated porphyroblasts

and porphyroclasts.

2. Pressure shadows

3. Fractured grains.4. Boudins

5. Presence of C- and S-

sur faces (parallel

alignment of platymineral)

6. Riedel shears.

FractFault.pptx

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