FAULT IN TERMS OF GEOLOGY

RAJRRAJA,BIBEK & PALLAVI

EARTH SCIENCENIT ROURKELA

RAJA,BIBEK & PALLAVIEARTH SCIENCE

NIT ROURKELA

Main objectiveso What is fault ?

o Parts of the fault

o classification of faults based on relative movement between wall

o Some global examples

o Relation between slip and separation

o Transform fault & transcurrent fault

o Mechanism of faulting(Anderson’s Theory)

o Recognition of faults

o Relation with tectonics

o Focal mechanism

o Role of fluids in faulting

o Significance of fault

o others

• Brittle fault > A fault is a discrete fracture between blocks of rock that have been displaced relative to each other, in a direction parallel to the fracture plane.

> A fault zone is a region containing several parallel or anastomosing (i.e. branching and reconnecting) faults.

> Any fault-bounded sliver in a fault zone is a horse. Fault and fault zones are identified either when an earthquake occurs or by geological mapping showing that motion across a discontinuity has occurred in the past. On geologic maps, only faults that affect the outcrop pattern are usually shown.

FAULT- faults are defined when two adjacent blocks of rock have moved past

each other in response to induced stresses. The notion of localized movement leads to two genetically different classes of faults reflecting the two basic responses of rocks to stress: brittle and ductile.

Ductile fault >Shear zones are the analogues in a ductile material of faults in a brittle material.

>Shear zones are regions of localised but continuous ductile displacement, formed under conditions of elevated temperature and/or confining pressure, in contrast to fault zones that are regions of localised brittle deformation.

>Shear zones are thus ductile faults, by contrast to the brittle faults.

Geometrical classification Fault plane Faults dipping more than 45° are called high angle faults; Faults dipping less than 45° are called low angle faults. In general, fault surfaces are curved. Undulation of fault-surfaces is commonly seen in 3D seismic data. The fault corrugations thereby identified are attributed to the linkage of fault-segments through time. A listricfault is a curved, concave upward fault, that is, it gradually flattens with depth. Where low-angle faults affect a set of nearly horizontal bedded rocks, they generally follow a staircase path made up of alternating ramps and flats. The flats are where the overlying rocks slide along a relatively weak bedding plane also called a décollement plane, which refers to a surface across which there is a discontinuity in displacement, strain or fold style. The ramps are fault sections climbing through the stratigraphic sequence, typically at around 30° to the horizontal, across stiff, competent layers. Ramps do not necessarily strike perpendicular to the movement direction (frontal ramp) but are also found oblique or parallel to the transport direction (lateral ramp or tear fault).

Parts of the Fault• Fault plane: Surface that the movement has

taken place within the fault.On this surface the dip and strike of the fault is measured.

• Hanging wall: The rock mass resting on the fault plane.

• Footwall: The rock mass beneath the fault plane.

• Slip: Describes the movement parallel to the fault plane.

• Dip slip: Describes the up and down movement parallel to the dip direction of the fault.

• Strike slip: Applies where movement is parallel to strike of the fault plane.

• Oblique slip: Is a combination of strike slip and dip slip.

• Net slip (true displacement): Is the total amount of motion measured parallel to the direction of motion

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

of the Earth’s crust. In the upper crust consist of :

Single movement

Anastomosing complex of fracture surfaces.

The individual fault may have knife-sharp contacts or it may consist of zone of cataclasite.

At ductile-brittle zone 10-15km deep in continental crust, faults are characterized by mylonite. At surface of the crust mylonite may also occur locally where the combination of available water and increased heat permits the transition.

The two types of fault may occur within one fault where close and at the surface brittle the associated rocks are cataclasts and at deep where ductile and brittle zone mylonite is present

• Separation: The amount op apparent offset of a faulted surface, measured in specified direction. There are strike separation, dip separation, and net separation.

• Heave: The horizontal component of dip separation measured perpendicular to strike of the fault.

• Throw: The vertical component measured in vertical plane containing the dip.

DESCRIPTION OF FAULT DIPHorizontal faults Faults with a dip of about 0°; ifthe fault dip is between about10° and 0°, it is called subhorizontal.Listric faults Faults that have a steep dip close to the Earth’s surface and have a shallowdip at depth .Because of the progressive decrease in dip with depth, listricfaultshave a curved profile that is concave up.Moderately dipping Faults with dips between about faults 30° and 60°.Shallowly dipping Faults with dips between aboutfaults 10° and 30°; these faults arealso called low-angle faults.Steeply dipping faults Faults with dips between about60° and 80°; these faults are also called high-angle faults.Vertical faults Faults that have a dip of about90°; if the fault dip is close to90° (e.g., is between about 80°and 90°), the fault can be called sub vertical Rake angle Net slip Dip-slip.

FAULT CLASSIFICATION AND TERMINALOGY

Faults: Are fractures that have appreciable movement parallel to their plane. They produced usually be seismic activity.

Understanding faults is useful in design for long-term stability of dams, bridges, buildings and power plants. The study of fault helps understand mountain building.

Faults may be hundred of meters or a few centimeters in length. Their outcrop may have as knife-sharp edges or fault shear zone. Fault shear zones may consist of a serious of interleaving anastomosing brittle faults and crushed rock or of ductile shear zones composed of myloniticrocks.

Normal Fault

• Maximum principalstress σ3(compressive stress) vertical• Minimum principalstress σ1(tensile stress) horizontal

Normal fault have dominant dip slip component, where hanging wall move down relative to the foot wall. The hanging wall of normal fault either dipping in same direction or in the opposite direction these are described as synthetic fault and antithetic fault.

Displacement On Normal Fault

Normal FaultNormal Fault: The hanging wall has moved down relative to

the footwall.

Graben: consists of a block that has dropped down between 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 the two faults remains high.

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

Normal faults usually found in areas where extensional regime is present.

• Growth faults are faults that operate during sedimentation and therefore displace an active surface of sedimentation. They are dominantly listric. They form characteristically, but not exclusively, in unconsolidated sediments freshly deposited in basins that are actively growing in breadth and depth .As a clear result, sediments or specific layers are thicker on the hanging-wall than sediments and layers of the same age on the footwall.

Pure shear or simple shear??

Planar faults Planar, rotational normal faults occur above a basal detachment or a brittle-ductile transition. They separate juxtaposed and tilted blocks without internal deformation. Both the faults and fault-blocks rotate simultaneously about an axis roughly parallel to the strike of the faults (rigid body rotation resulting in domino or bookshelf faulting).

Each fault block has its own half graben. Each fault must have the same amount of displacement and tilting or there are space problems at the bottom of the system (opening of voids). Planar, rotational faults and blocks generally abut against transfer, scissors faults.

Structures at Divergent Boundaries

• Tensional Stresses cause brittle strain and formation of sets of normal faults

• i.e., Horsts and Grabens

Horsts and Grabens

• Older Rocks are exposed along the ridges formed by the horsts

• Younger rocks lie beneath the grabens

• Sediment fills in the linear valleys

Hr Horst

tGraben

Horst

GrabenHorst

Graben

Nevada

• “Washboard topography” is the result of Horsts and Grabens

• A.k.a, Basin and Range

• E.g., Humbolt Range

• E.g., Death Valley (Graben)

Horst and Graben, Nevada

Humboldt Range, Northern Nevada

Graben

Horst

Tectonic Settings for Extension

• Divergent plate motions

• Gravitational collapse

– Over-thickened crust

– Continental margins

• Salt Domes

Continental Extension

Rifts

Bounded by normal faults

Lithospheric Extension

•Thinning of the lithosphere

•Normal faulting at the surface

•High heat flow (rising of the isotherms)

•Thermal subsidence - > sedimentary basins

North Sea Extensional Basin

Normal Faults(global examples)

IRAN

STRIKE-SLIP FAULT & OBLIQUE SLIP FAULT

Strike-slip Fault

• Both maximum and

minimum principal

stresses horizontal

• Intermediate

principal stress σ2

Vertical.

Strike-slip fault which have dominant strike slip component. It is mainly two type Dextral and Sinistral. If looking across the fault plane the opposite wall appears to

moved towards the right, is called “Dextral or right handed fault”.If looking across the fault plane the opposite wall appear to move toward left, is called “Sinistral

or left handed fault”. Example of strike-slip fault are Alpine Fault of New Zealand, San Andreas Fault of California etc.

Transform Faults Transform fault are a type of

strike-slip fault (defined by Wilson 1965).

Transform faults end at the junction of another plate boundary or fault type.

In addition, transform faults have equal deformation across the entire fault line.

They are basically occur where type of plate boundary is transformed into another.

Main types of transform faults are:

• Ridge-Ridge

• Ridge-Arc

• Arc-Arc

Transcurrent or Wrench Fault:-

In strike-slip fault the relative displacement of block is horizontal.

The fault plane of a strike-slip fault is vertical and often extends for long distances.

Transcurrent faults have greater displacement in the middle of the fault zone and less on the margins.

Because these fault are strike across they are called “transcurrent”, or “wrench” fault.

Tear fault:- It is a strike slip fault that runs across the strike of a contractionalor extensional belt and accommodates differential displacement between

two segments of the belt.

Transfer fault:-

A transfer fault is a strike slip fault that transfers displacement between two similarly oriented fault segments (e.g. two normal faults).

Transfer faults are usually confined to hanging walls of detached systems (i.e. not affecting the basement) and terminate where they connect the linked faults.

transfer faults linking two thrusts or normal faults are therefore nearly parallel to the movement direction .

Transfer zones (faults) usually terminate where they connect to and terminate other faults or structures .

Large strike slip faults are often curve or made up of zone of “ En echelon faults” (short faults are overlap each other) movement on such fault

system can cause extension or shortening in the bent region of the fault.Whether an extension or shortening will occur will depend on whether

the fault is right lateral or left lateral.

Development of a Pull-Apart Basin(Transtension):-

A right lateral -right stepping or left lateral-left stepping fault will produce extension on bend zone causes large scale pull apart movement by produce normal fault on this zone. Pull- apart may also underlain by flower structures(negative flower structure) . The fault associated with these structure combination of Normal and Strike slip faults .

Normal (-) Flower Structure - Tulip

Development of a Restraining Bend(Transpression):-

A right lateral- left stepping or left lateral-right stepping fault will produce shortening on the bend zone causes uplift or push up with development of flower or palm tree structures by produce reverse fault on this zone. Similarly push apart may also contain flower structures(Positive flower structure). The fault associated with this structure combination of reverse and strike slip faults.

Reverse (+) Flower Structure - Palm

Summary of Flower Structures - Palms & Tulips

Oblique Slip Fault:-• Where the strike slip and dip slip displacements are similar in magnitude,

the fault can be called an oblique slip fault.

• Depending on whether the opposite wall moves towards right or left and whether the hanging wall moves relatively up or down oblique slip fault further sub divided into four type

Evidence of Faulting

• Recognition of fault in the field can divided into three group:- I)geological evidences, II)Fault plane evidences, III) Physiographic evidences.

• A)Geological evidences:- The most imp geological evidences of faulting are:-• 1)Offset of rock unit:- Displacement of rock bed, dyke, vein, etc. Occurs on opposite

site of a fault.

• Offset of quartz vein

• 2) Repetition and omission of strata:- In a traverse line, the outcrop of a bed may berepeated in cyclic order or it may disappear. Such repetition or omission of bed oftenestablishes a fault.

• 3) Stratigraphic sequence:- The normal stratigraphic sequence of a region maydisturbed by faulting. When older strata occur above younger strata, this typedisturbence is mainly done by thrust fault.

B) Fault Plane Evidences:-• Fault plane contain some structures which are found associated with faults. Most

imp evidences are :-

• 1)Feather joints:- Feather joint are the tension joint formed due to fault movement, and is found in the fault plane. These joint intersect the fault plane at an acute angle. This acute angle points towards the direction of movement along fault plane.

2)Slickensides:-The movement of one block against another block result polishing and grooving of fault surfaces. These grooved and striations are called slickensides.

Slickensides are useful to knowing the direction of the last movement on a fault surface. This also indicate the direction of the net slip on the fault plane.

3)Drag:- This structure found in the fault zone. The end of strata may bend up and down formed due to frictional resistance of the bed on the fault plane.

For normal dip slip fault the hanging wall dragging up and the foot wall dragging down.

4)Fault Breccia and Gouge

• Along some faults the rocks are highly fracture or crushed to angular fragment.

• Angular grains are embedded in a finely grounded rock are called “faulted breccia”.

• Secondary mineral such as quartz, calcite, or some pyrite fill the open space of faulted breccia.

• Faulted breccia are crused again by faulting it may be ground to fine clay like powder called “Gouge”. The gouge is frequently polished and striated by fault movement.

Fault Breccia

5)Silicification and Mineralization:-

Fault are basicaly fracture they often act as a channel way of moving solution. The solution may replace the country rock with quartz grain in called silicification. Fault plane also act as passages for mineralizing solution and many mineral deposit are formed in the fault plane is called mineralization.

Shiny slickensided surface in Paleozoicstrata of the Appalachians (Maryland, USA); coin for scale.(b) Slip fibers on a fault surface, showing steps that indicatesense of shear; compass for scale

(b)

Banded clay gouge from theLewis Thrust (Alberta, Canada).

(a) Fault breccia from the Buckskin detachment (Battleship Peak, Arizona, USA)

C) Physiographic Evidences

The physiographic evidences are seen clearly from a distance or an aerial photograph . The chief physiographic evidences are i) Fault scrap, ii)Fault line scrap, iii)Fault Control of Streams.

I) Fault Scrap:- A steep straight slope is called the scrap. It formed as a result of faulting. It found only in those area where fault has been geologically very recent. Fault scrape face in the direction of down throw side.

A reverse-motion, fault-line scarp from Mongolia.

II)Fault Line Scrap:- Fault frequently bring together resistant and non resistant rock beds. A ridge may formed along a fault due to process of unequal erosion.

Such ridge are called the “fault line scrap.

MECHANISM OF FAULT(ANDERSON’S THEORY)

Anderson’s theory of faulting

• Anderson has proposed a theory which explain a large number of fault in the shallow crust.

• Anderson argues that there can not be any shear stress on the free surface of the earth, one of the principle stress axes should be vertical, consequently the other two stress axes are horizontal.

• We get three type of fault depending on whether the tensile stress axis(o3), the compressive stress axis(o1),the intermidiate stress axis(o2).

• A) Normal fault:- In this case, the compressive stress(o1) will be

vertical and hence the fault will be at an angle with the vertical. Thus, the

fault will strike parallel to the o2 axis and will dip at angle 60-70°. The

upper block will move down along the dip direction and will give rise to

dip slip normal fault.

B) Reverse fault:- In this case, the tensile stress (o3) is vertical and the stresses o2 and o1 in

the horizontal direction will be unequal. The fracture will strike parallal to o2 and inclined to the

horizontal o2 axis. So the upper block move up along the dip direction and will give rise to dip slip

reverse fault. Moreover, the upper block will move up the slope in a direction perpendicular to o2 .

Thus, trust fault dipping at an angle of abut 20-30° will be produced.

C) Strike- slip fault:- In this case , the intermediate o2axis is vertical, the fault s will also be

vertical. The movement along fracture will be at right angle to the o2 axis, the fault will be strike-

slip fault.

The general conclusion of Anderson’s theory is that among three types of faults in the shallow crust,

the reverse fault will be low dipping(<45°), the normal fault will moderately dipping(>45°), and

strike-slip fault will be vertical.

REVERSE FAULTREVERSE FAULT

If the hanging wall has moved up with respect to the foot wall then the fault is called reverse fault.

Reverse fault are those which have dominant dip-slip component.

Here older rocks occure above younger rocks.

Reverse fault with a low angle (<45) are called thrust fault.

Low angle thrust fault is called overthrust (here net slip is large).

Reverse fault results by compression

Older rocks above younger rockshere in over thrust , net slip= stratigraphic throw/dip of

the fault plane

TERMINOLOGY• Klippe :- A closed

outcrop of a thrust sheet isolated from the main mass by erosion is called klippe (outlier)

• Window:- A closed outcrop of the substratum of a thrust sheet framed on all sides by the thrust surface is known as window(inlier)

Autochthonous:-The rocks of the foreland are essentially found where

they were deposited.

Allochthonous:-The rocks in the overthrust sheets have traveled many

miles from their original place of deposition.

0

LEADING IMBRICATE FAN- Here the thrust infronthas the maximum slip.

TRAILING IMBRICATE FAN- Here the thrust at the rear shows the maximum slip.

DUPLEX

. Duplex is a thrust mass which is bounded by a floor thrust and a roof thrust.

Duplex structure are imbricated.

A prominant low angle thrust occure beneath some thrust system is called floor thrust or sole thrust.

The upper thrust is called roof thrust

DUPLEX STRUCTURE

DEVELOPMENT OF THRUST DUPLEX

HINTERLAND-DIPPING DUPLEXThe imbricate faults in a

duplex dip towards the

hinterland is called a

hinterland dipping

duplex.

Blocks are surrounded

by thrust are called

horses.

Usually composed of

upward facing horses.

Here

displacement<length of

horse

ANTIFORMAL STACK

Here displacement in the individual horses is greater such that each horses lies more or less vertically above the other.

Horses are bunched up in the form of antiform.

Displacement=Length of horse.

FORELAND-DIPPING DUPLEX

Usually composed of downward facing horses.

Here displacement>length of horse

If the individual displacement is greater still then the horses have a foreland dip.

THRUST GEOMETRY

Tip line- The relative displacement must fade out outward,where it drops to zero is called tip line.

Tip line separates slipped rocks from non-slipped rocks.

Branch line-Two faults meet along a branch line.

A tip line and branch line meet along a tip point and branch point.

A subsidiary thrust may branch out as a splay from the main thrust.

4 types of splay- Isolated splay

Diverging splay

Rejoining splay

Connecting splay

• Isolated splay-Two tip points of the splay are exposed. Trace of the fault is isolated from the main fault.

• Diverging splay-Here single tip point and branch point are exposed on the erosion surface.

• Connecting splay-It connects two different faults. Two branch point of the splay are exposed.

• Rejoining splay-Tip line is not exposed. The ground surface meets the main fault at two branch points.

RAMP AND FLAT

• Ramps and flats are step like structure.

• Flats are fault surfaces that form parallel to the strata(usually weak rock units evaporites and shale).

• Ramps cut across a dip angle of 30 to 45(usually resistant rock units sand stone and lime stone).

• Ramps are classified in to 3 types(according to transport direction).

1. Frontal ramp

2. Lateral ramp

3. Oblique ramp

• Frontal ramp- Strikes roughly perpendicular to the transport direction(dominant reverse dip slip).

• Lateral ramp- Strikes roughly parallel to the transport direction(dominant strike slip).

• Oblique ramp- Strikes oblique to the transport direction(both strike slip and reverse dip slip).

THRUST RELATED FOLDS

• Backlimb thrust:-

A thrust cutting across gentler backlimb of an asymmetric fold is called backlimb thrust.

Forelimb thrust:-

A thrust cutting across steeper forelimb of an asymmetric fold is called forelimb thrust.

• Stretch thrust:-

It developes at an advance stage of folding by stretching and shearing out of overturned limbs.

Break thrust:-

Here the thrust cuts the forelimb at a large angle.

FAULT PROPAGATION FOLD A fold generated by propagation of

a thrust tip over a ramp in to undeformed strata.It forms at the tip of a thrust fault where propagation along the decollementhas ceased but displacement on the thrust behind the fault tip is countinuing.

• The countinuing displacement is accomodated by the formation of an asymmetric anticline syncline pair.

• As displacement continues the thrust tip starts to propagate along the axis of syncline. Such str is also known as tip line fold.

FAULT BEND FOLD

• Fault bend fold:- Fold generated by movement of a thrust sheet over a ramp.

POP UP STRUCTURE

• It is a lifted up triangular area.

• A section of hanging wall strata that has been uplifted by the combination of foreland thrust and hinterland thrust.

:

• There are two models of thrust propagation:-

Piggy-back model:-older thrust are structurally higher up and younger thrust are lower in sequence.

It is consider the normal mode of propagation in thrust system.

Over step model:- Opposite to piggy-back model.

Other types of fault• en-echelon faults: Faults that are

approximately parallel one another but occur in short unconnected segments, and sometimes overlapping.

• Radial faults: faults that are converge toward one point

• Concentric faults: faults that are concentric to a point.

• Bedding faults (bedding plane faults): follow bedding or occur parallel to the orientation of bedding planes.

(a) Microscopicfaults, showing fractured and displaced feldspar grains.

(b) Mesoscopic faults cutting thin layers in an outcrop.

SHEAR-SENSE INDICATORS FOR BRITTLE FAULTS AND FAULT ZONES

Steps on slickensides Microscopic steps develop along slickensided surfaces. Typically the face of the step is rougherthan the flat surface. However, slickenside steps may be confused with the intersection between pinnate fractures and the fault, giving an opposite shear sense.

Offset markers You can define shear sense if you are able to define the relative displacement of two piercing points on opposite walls of the fault, or can calculate the net-slip vector based on field study of the separation of marker horizons.

Fault-related folds The sense of asymmetry of fault-related folds defines the shear sense. Typically, fault-inceptionfolds verge in the direction of shear (see Chapter 11 for a definition of fold vergence). If the hingesof folds in a fault zone occur in a range of orientations, you may need to use the Hanson slip-linemethod to determine shear sense. Note that the asymmetry of rollover folds relative to shearsense is opposite to that of other fault-related folds. Fiber-sheet imbrication The imbrication of slip-fiber sheets on a fault provides a clear indication of shear sense. Fibersheets tilt away from the direction of shear.

Carrot-shaped grooves Grooves on slickensides tend to be deeper and wider at one end and taper to a point at the other,thus resembling half a carrot. The direction in which that “carrot” points defines the direction of shear.

Chatter marks As one fault block moves past another, small wedge-shaped blocks may be plucked out of theopposing surface. The resulting indentations on the fault surface are known as chatter marks.

Pinnate fractures The inclination of pinnate fractures with respect to the fault surface defines the shear sense

En echelon veins En echelon veins tilt toward the direction of shear. If the veins are sigmoidal, the sense of rotationdefines the shear sense.

Faulting & Stress

Focal Mechanism

Role of fluids in faulting

Fluids plays an important role in faulting. They have a lubricating effect in the fault zone as buoyancy that reduces the shear stress necessary to permit the fault to slip. The effect of fluid on movement is represented as in landslide and snow avalanches.

Other factor that control the type of movement is

the curvature of the fault surface.

• Withdrawal of ground water may cause near surface segments of active faults to switch mechanisms from stable sliding to stick slip, thereby increasing the earthquake hazard.

• Pumping fluid into a fault zone has been proposed as a way to relieve accumulated elastic strain energy and reduce the likelihood of large earthquake, but the rate at which fluid should be pumped into fault zone remains unknown.

Fault Surfaces and Frictional sliding

Fault surfaces between two large blocks are always not planar especially on the microscopic scale. This irregularities and imperfections are called asperities increase the resistance to frictional sliding. They also reduce the surface area actually in contact. The initial contact area may be as little as 10%, but as movement started the asperities will break and contact will be more.

Shear (frictional) Heating in Fault zones During movement of faults frictional heat

is generated due to the mechanical work. The heat generated can be related to an increase in temperature.This friction heat is indicted by the formation of veins pseudotachylite (false glass) in many deep seated fault zones and the metamorphism along subduction zones (greenschist and blueschist facies).

In some areas there is indication of temperature of 800ºc 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 water and dissipation of heat during deformation.

Generally friction-related heating along faults is a process that clearly occurs in the Earth, but difficult to demonstrate.

Structural Oil Traps

• REFERENCES-S.K.GHOSH-STRUCTURAL GEOLOGY ROBERT J. TWISS ELDRIDGE M.MOORES-STRUCTURAL GEOLOGY STEPHEN MARSHAK-EARTH STRUCTURE BILLINGS-STRUCTURAL GEOLOGY & INTERNET

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