Structural GeologyStructural Geology(Geol(Geol 305) 305)

Semester (071)Semester (071)Dr. Mustafa M. HaririDr. Mustafa M. Hariri

FRACTURES AND FRACTURES AND FAULTSFAULTS

ObjectivesObjectivesThis unit of the course discusses Fractures and FaultsThis unit of the course discusses Fractures and Faults

By the end of this unit you will be able to:By the end of this unit you will be able to: Differentiate between the differentdifferent type of fractures type of fractures Differentiate between the different type of faultsdifferent type of faults Understand the relationship between the different type of different type of

stresses and faultsstresses and faults Where faults form and how? Faults mechanicsFaults mechanics Role of Role of fluid in faulting fluid in faulting Faults movement mechanismsFaults movement mechanisms Shear, Shear zones and different type of shearsShear, Shear zones and different type of shears

FRACTUREFRACTURE FRACTURE: FRACTURE: is defined by Twiss and is defined by Twiss and

Moores (1992)Moores (1992) as as “..surfaces along “..surfaces along which rocks or minerals have broken; which rocks or minerals have broken; they are therefore surfaces across they are therefore surfaces across which the material has lost cohesion”which the material has lost cohesion”

Characteristics of fractures according to Characteristics of fractures according to Pollard and Aydin (1988)Pollard and Aydin (1988)

fractures have two parallel surfaces that fractures have two parallel surfaces that meet at the fracture frontmeet at the fracture front

these surfaces are approximately planarthese surfaces are approximately planar the relative displacement of originally the relative displacement of originally

adjacent points across the fractures is adjacent points across the fractures is small compared to the fracture length..small compared to the fracture length..

Fracture, Joint and Fracture, Joint and FaultFaultThe term fracture encompasses both joints The term fracture encompasses both joints

and faults.and faults.

JOINTS:JOINTS: are fractures along which there has are fractures along which there has been no appreciable displacement parallel to been no appreciable displacement parallel to the fracture and only slight movement normal the fracture and only slight movement normal to the fracture plane. to the fracture plane.

Joints are most common of all structures present in all settings in all kind of rocks as well Joints are most common of all structures present in all settings in all kind of rocks as well as consolidated and unconsolidated sediment as consolidated and unconsolidated sediment

Types of FracturesTypes of Fractures Extensional FractureExtensional Fracture

In extensional fractures the Fracture plane is oriented parallel to σ1 and σ 2 and perpendicular to σ 3.

Three types of fractures have been identified: Three types of fractures have been identified: Mode IMode I fractures (joints) fractures (joints) it is the extensional fractures and formed

by opening with no displacement parallel to the fracture surface (see above figure).

Mode IIMode II and and Mode IIIMode III are shear fractures are shear fractures. These are faults like fractures one of them is strike -slip and the other is dip-slip is strike -slip and the other is dip-slip

Same fracture can exhibit Same fracture can exhibit both mode II and mode IIIboth mode II and mode III in different parts of the in different parts of the regionregion..

Importance of studying joints Importance of studying joints and shear fracturesand shear fractures

To understand the nature and sequence of To understand the nature and sequence of deformation in an area.deformation in an area.

To find out relationship between joints and To find out relationship between joints and faults and or folds.faults and or folds.

Help to find out the brittle deformation in an Help to find out the brittle deformation in an area of construction (dams, bridges, and power area of construction (dams, bridges, and power plantsplants..

In mineral explorationIn mineral exploration to find out the trend and to find out the trend and type of fractures and joints that host type of fractures and joints that host mineralization which will help in exploration.mineralization which will help in exploration.

Importance of studying joints Importance of studying joints and shear fracturesand shear fractures Joints and fractures serve as the plumping system for Joints and fractures serve as the plumping system for

ground water flow in many area and they are the only ground water flow in many area and they are the only routes by which ground water can move through igneous routes by which ground water can move through igneous and metamorphic rocks.and metamorphic rocks.

Joints and fractures porosity and permeability is very Joints and fractures porosity and permeability is very important for water supplies and hydrocarbon reservoirs.important for water supplies and hydrocarbon reservoirs.

Joints orientations in road cuts greatly affect both Joints orientations in road cuts greatly affect both construction and maintenance. Those oriented parallel to construction and maintenance. Those oriented parallel to or dip into a highway cut become hazardous during or dip into a highway cut become hazardous during construction and later because they provide potential construction and later because they provide potential movement surfaces.movement surfaces.

TYPES OF JOINTTYPES OF JOINT Systematic joints:Systematic joints: have a

subparallel orientation and regular spacing.

Joint set:Joint set: joints that share a similar orientation in same area.

Joint system:Joint system: two or more joints sets in the same area

Nonsystematic joints:Nonsystematic joints: joints that do not share a common orientation and those highly curved and irregular fracture surfaces. They occur in most area but are not easily related to a recognizable stress.

Some times both systematic and nonsystematic joints formed in the same area at the same time but nonsystematic joints usually terminate at systematic joints which indicates that nonsystematic joints formed later.

Type of Type of FracturesFractures

Plumose joints:Plumose joints: joints that joints that have feathered texture on have feathered texture on their surfaces, and from this their surfaces, and from this texture the direction of texture the direction of propagation of joints can be propagation of joints can be determined.determined.

Veins:Veins: are filled joints and are filled joints and shear fractures and the shear fractures and the filling range from quartz and filling range from quartz and feldspar (pegmatite and feldspar (pegmatite and aplite) to quartz, calcite and aplite) to quartz, calcite and dolomite.dolomite.

Type of Type of FracturesFractures Conjugate fractures:Conjugate fractures: paired paired

fracture systems, formed in the fracture systems, formed in the same time, and produced by same time, and produced by tension or shear. Many of them tension or shear. Many of them intersect at an acute angle which intersect at an acute angle which will be bisected by the will be bisected by the

Curved fractures:Curved fractures: occur occur frequently and may be caused by frequently and may be caused by the textural and compositional the textural and compositional differences within a thick bed or differences within a thick bed or large rock mass or they may a large rock mass or they may a result of changes in stress result of changes in stress direction or analysis.direction or analysis.

Cross cutting relationship and material Cross cutting relationship and material filling the fractures can help in resolving filling the fractures can help in resolving the chronological order of deformation.the chronological order of deformation.

FRACTURE ANALYSISFRACTURE ANALYSIS Study of joints in an area will give information about the Study of joints in an area will give information about the

sequence and timing of formation. It will also provide sequence and timing of formation. It will also provide information on the timing and geometry of the brittle information on the timing and geometry of the brittle deformation of the crust and the way fractures propagate deformation of the crust and the way fractures propagate through the rocks.through the rocks.

Importance of Fracture Importance of Fracture OrientationOrientation Study of orientation of systematic fractures Study of orientation of systematic fractures provides information about the provides information about the orientation of one or more principle orientation of one or more principle stress directions involved in the brittle.stress directions involved in the brittle.

Parameters measured for fractures are strike Parameters measured for fractures are strike and dip.and dip.

Or strike of linear features from aerial photos Or strike of linear features from aerial photos and landsat images.and landsat images.

Data obtained from fractures is plotted in Data obtained from fractures is plotted in rose diagram or equal area net. Equal rose diagram or equal area net. Equal area net for strike and dip and rose area net for strike and dip and rose diagram for strike only.diagram for strike only.

Studies of joint and fracture orientation Studies of joint and fracture orientation from LANDSAT and other satellite from LANDSAT and other satellite imagery and photographs have a variety imagery and photographs have a variety of structural, geomorphic, and of structural, geomorphic, and engineering applicationsengineering applications..

Strain -ellipsoid analysis of joints in area may help to determine dominant crystal extension directions

Fold and JointsFold and Joints

Joints may form during Joints may form during brittle folding in a brittle folding in a position related to the position related to the fold axis and axial fold axis and axial surface as follows surface as follows

parallelparallel normalnormal obliqueobliquedepending on stress depending on stress

condition.condition.

Fault Related JointsFault Related Joints Joints are also formed Joints are also formed

adjacent to brittle faults, and adjacent to brittle faults, and movement along faults movement along faults usually produces a series of usually produces a series of systematic fractures.systematic fractures.

Most joints form by extensional fracturing of rockMost joints form by extensional fracturing of rock in the upper few kilometers of the Earth's crustin the upper few kilometers of the Earth's crust . .

The limiting depth formation of extension fracturesThe limiting depth formation of extension fractures should be the ductile-brittle transitionshould be the ductile-brittle transition..

Factors Affecting the Formation Factors Affecting the Formation of Jointsof Joints

Rock typeRock type Fluid pressureFluid pressure Strain rateStrain rate Stress difference at a particular Stress difference at a particular

time time

Characteristics of Characteristics of FracturesFractures Plumose structure: is the

structures formed on the joint surface during its propagation and provides information about the joint propagation direction.

Hackle marks:Hackle marks: indicate zones where the joint propagate rapidly.

Arrest line:Arrest line: forms perpendicular to the direction of propagation and is parallel to the advancing edge of fractures.

Characteristics of Characteristics of FracturesFractures

Bedding and foliation planes in coarse-grained rocks constitute barriers to join propagation. Bedding in uniformly fine-grained rocks, such as shales and volcanicalstic rocks, appears to be less of barriers.

In sandstone bed propagation of joints through the bed is slightly offset from the layers above or below.

Variation in bed thickness also affects propagation direction.

In horizontal layering joints will not propagate from sandstone into shale from sandstone into shale if the least principle horizontal stress if the least principle horizontal stress in shale is greater than that in in shale is greater than that in sandstone.sandstone.

Fractures will be terminated at the Fractures will be terminated at the contact between the two rocks. contact between the two rocks.

Joints Classified According to their Joints Classified According to their Environment and Mechanism of Environment and Mechanism of Formations Formations (Engelder, 1985) Tectonic fractureTectonic fracture Hydraulic fractureHydraulic fracture Unloading fractureUnloading fracture Loading fractureLoading fracture

All of these types are based on the All of these types are based on the assumption that failure mechanism is assumption that failure mechanism is tensile.tensile.

Tectonic fractures:

Form at depth in response to Form at depth in response to abnormal fluid pressureabnormal fluid pressure and and involve hydrofracturing. They form mainly by involve hydrofracturing. They form mainly by tectonic stresstectonic stress and the and the horizontal compaction of sedimenthorizontal compaction of sediment at depth at depth less less than 3 kmthan 3 km, where the escape of fluid is hindered by , where the escape of fluid is hindered by low low permeability and abnormally high pore pressurepermeability and abnormally high pore pressure is created. is created.

Hydraulic fractures:Form as tectonic fractures by the Form as tectonic fractures by the pore pressurepore pressure created due to created due to

the the confined pressed fluid during burial and vertical confined pressed fluid during burial and vertical compaction of sediment at depth greatercompaction of sediment at depth greater than 5 kmthan 5 km. Filled . Filled veins in low metamorphic rocks are one of the best of veins in low metamorphic rocks are one of the best of examples of hydraulic fractures. examples of hydraulic fractures.

Unloading fractures: Unloading fractures: Form near Form near surface as erosion removes overburden surface as erosion removes overburden

and thermalelastic contraction occurs.and thermalelastic contraction occurs. They form They form when more when more than half of the original overburden than half of the original overburden has been removedhas been removed. The present stress and tectonic . The present stress and tectonic activity may serve to activity may serve to orient these jointsorient these joints. Vertical . Vertical unloading fractures occur unloading fractures occur during cooling and during cooling and elastic contraction of rock mass and may occur at elastic contraction of rock mass and may occur at depths of 200 to 500 m.depths of 200 to 500 m.

Release fractures: Release fractures: Similar to unloading fractures but they form by Similar to unloading fractures but they form by

release of stressrelease of stress. Orientation of release joints is . Orientation of release joints is controlled by the rock fabric. Released joints form controlled by the rock fabric. Released joints form late in the history of an area and are oriented late in the history of an area and are oriented perpendicular to the original tectonic compression perpendicular to the original tectonic compression that formed the dominant fabric in the rock.that formed the dominant fabric in the rock.

Release joints may also develop parallel to the fold Release joints may also develop parallel to the fold axes when axes when erosion begins and rock mass that was erosion begins and rock mass that was under burial depth and lithification begins to cool under burial depth and lithification begins to cool and contract, these jointsand contract, these joints start to propagate start to propagate parallel to an existing tectonic fabricparallel to an existing tectonic fabric..

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

Joints within Plutons Fractures form in pluton in response to Fractures form in pluton in response to cooling and later tectonic stress. Many of cooling and later tectonic stress. Many of these joints are filled with hydrothermal these joints are filled with hydrothermal minerals as late stage products. Different minerals as late stage products. Different types of joints are present with pluton types of joints are present with pluton (i.e. longitudinal, and cross joints)(i.e. longitudinal, and cross joints)

NONTECONIC NONTECONIC FRACTURESFRACTURES Sheeting joints:Sheeting joints:

Those joints form subparallel to Those joints form subparallel to the surface topography. the surface topography. These joints may These joints may be more be more observed in igneous rocksobserved in igneous rocks. . Pacing within these fractures Pacing within these fractures increases downward. These increases downward. These fractures thought that they fractures thought that they form by unloading overlong form by unloading overlong time when erosion removes time when erosion removes large quantities of the large quantities of the overburden rocks.overburden rocks.

Columnar joints and Mud Columnar joints and Mud Cracks:Cracks:

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

FAULT CLASSIFICATION AND FAULT CLASSIFICATION AND TERMINALOGYTERMINALOGY

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

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

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

Parts of the Parts of the FaultFault

Fault planeFault plane: Surface that the movement has Surface that the movement has taken place within the fault.On this surface taken place within the fault.On this surface the dip and strike of the fault is measured.the dip and strike of the fault is measured.

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

Footwall:Footwall: The rock mass beneath the fault The rock mass beneath the fault plane.plane.

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

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

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

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

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

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

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

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

Features on the fault Features on the fault surface surface

Grooves Grooves (parallel to the (parallel to the movement direction)movement direction)

Growth of fibrous mineralsGrowth of fibrous minerals (parallel to the movement (parallel to the movement direction)direction)

SlickensidesSlickensides are the polished are the polished fault surfaces.fault surfaces.

Small steps.Small steps.All are considered a kind of All are considered a kind of

lineation. They indicate the lineation. They indicate the movement relative trend NW, movement relative trend NW, NE … etc. NE … etc.

Small steps may also be used to Small steps may also be used to determine the movement determine the movement direction and direction of direction and direction of movement of the opposing movement of the opposing wall. Slicklines usually wall. Slicklines usually record only the last moment record only the last moment event on the fault.event on the fault.

ANDERSON FAULTS ANDERSON FAULTS CLASSIFICATION CLASSIFICATION

Anderson (1942) defined Anderson (1942) defined three types of faults:three types of faults:

Normal FaultsNormal Faults Thrust FaultsThrust Faults Wrench Faults Wrench Faults

(strike slip)(strike slip)

Different Type of Faults

Normal FaultNormal FaultNormal Fault: The hanging wall has moved down

relative to the footwall.Graben:Graben: consists of a block that has dropped down consists of a block that has dropped down

between two subparllel normal faults that dip towards between two subparllel normal faults that dip towards each other.each other.

Horst : : consists of two subparallel normal faults that dip consists of two subparallel normal faults that dip away from each other so that the block between the two away from each other so that the block between the two faults remains high. faults remains high.

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

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

Normal FaultsNormal Faults

Thrust FaultThrust FaultThrust Faults: In the thrust

faults the hanging wall has moved up relative to the footwall (dip angle 30º or less)

Reverse Faults: Are similar to the thrust faults regarding the sense of motion but the dip angle of the fault plane is 45º or more

Thrust faults usually Thrust faults usually formed in areas of formed in areas of comperssional regime. comperssional regime.

Thrust Thrust FaultFault

Thrust Thrust FaultsFaults

Strike-Slip FaultStrike-Slip FaultStrike-slip Faults: Are faults

that have movement along strikes.

There are two types of strike slip faults:

A] Right lateral strike-slip fault (dextral): Where the side opposite the observer moves to the right.

B] Left lateral strike-slip fault (sinistral): Where the side opposite the observer moves to the left.

Note that the same sense of Note that the same sense of movement will also be movement will also be observed from the other side observed from the other side of the fault.of the fault.

Strike-Slip Strike-Slip FaultsFaults

Transform FaultsTransform FaultsTransform Faults:Transform Faults: Are a Are a

type of strike-slip faulttype of strike-slip fault (defined by Wilson 1965). (defined by Wilson 1965). They form due to They form due to the the differences in motion differences in motion between lithospheric between lithospheric plates.plates. They are They are basically occur where basically occur where type of plate boundary type of plate boundary is transformed into is transformed into anotheranother..

Main types of transform Main types of transform faults arefaults are::

Ridge-RidgeRidge-Ridge Ridge-ArcRidge-Arc Arc-ArcArc-Arc

Other types of Other types of faultfault en-echelon faults:en-echelon faults: Faults that Faults that

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

Radial faults:Radial faults: faults that are faults that are converge toward one pointconverge toward one point

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

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

CRITERIA FOR CRITERIA FOR FAULTINGFAULTING Repetition or omissionRepetition or omission of stratigraphic units asymmetrical of stratigraphic units asymmetrical

repetitionrepetition Displacement of recognizable markerDisplacement of recognizable marker such as fossils, such as fossils,

color, composition, texture ..etc.)color, composition, texture ..etc.).. Truncation of structuresTruncation of structures, beds or rock units., beds or rock units. Occurrence of fault rocksOccurrence of fault rocks (mylonite or cataclastic or both) (mylonite or cataclastic or both) Presence of S or C structuresPresence of S or C structures or both, rotated porphyry or both, rotated porphyry

clasts and other evidence of shear zone.clasts and other evidence of shear zone. Abundant veinsAbundant veins, silicification or other mineralization along , silicification or other mineralization along

fracture may indicate faulting.fracture may indicate faulting. Drag UnitsDrag Units appear to be pulled into a fault during appear to be pulled into a fault during

movement (usually within the drag fold and the result is movement (usually within the drag fold and the result is thrust fault)thrust fault)

Reverse dragReverse drag occurs along listric normal faults. occurs along listric normal faults. SlickensidesSlickensides and slickenlines along a fault surface and slickenlines along a fault surface Topographic characteristicsTopographic characteristics such as drainges that are such as drainges that are

controlled by faults and fault scarps.controlled by faults and fault scarps.

FAULTS FAULTS MECHANICSMECHANICS

Anderson 1942 defined three fundamental possibilities of stress regimes and stress Anderson 1942 defined three fundamental possibilities of stress regimes and stress orientation that produce the three types of faults (Normal, thrust, and strike-sliporientation that produce the three types of faults (Normal, thrust, and strike-slip)

note that note that σσ1> 1> σσ 2> 2> σσ 3 3 Thrust fault:Thrust fault: σ 1 and σ 2 are horizontal and σ 3 is vertical. Thus a state of

horizontal compression is defined for thrust faults. Shear plane is oriented to σ 1 with angle = or < 45º and // σ 2.

Strike-Slip faultsStrike-Slip faults: σ 1 and σ 3 are horizontal and σ 2 is vertical. Shear plane is oriented to σ 1 with angle = or 45º and // σ 3. Form also due to horizontal compression.

Normal faultsNormal faults: σ 1 is vertical and σ 2 and σ 3 are horizontal. Shear plane is oriented 45º or less to σ 1 and // σ 2. Form due to horizontal extension or vertical compression.

Role of fluids in Role of fluids in faultingfaultingFluids plays an important role in faulting. Fluids plays an important role in faulting.

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

Faults movement Faults movement mechanismsmechanismsMovement on faults occurs in two different ways:Movement on faults occurs in two different ways: Stick slip: (unstable frictional sliding) (unstable frictional sliding) involves involves

sudden movement on the fault after a long-termsudden movement on the fault after a long-term accumulation of stress. This stress probably the cause accumulation of stress. This stress probably the cause of earthquakes.of earthquakes.

Stable sliding: involves uninterrupted motion along a involves uninterrupted motion along a fault, so stress is relieved continuouslyfault, so stress is relieved continuously and does not and does not accumulate.accumulate.

The two types of movement may be produced along the The two types of movement may be produced along the segments of the same fault. Stable sliding where segments of the same fault. Stable sliding where ground water is abundant, whereas, stick-slip occur ground water is abundant, whereas, stick-slip occur with less ground water with less ground water

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

Withdrawal of ground waterWithdrawal 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 faultPumping 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 Fault Surfaces and Frictional slidingsliding

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

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

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

In some areas there is indication of In some areas there is indication of temperature of 800ºc and 18 to 19 kb temperature of 800ºc and 18 to 19 kb (60km depth). This indicate that they can (60km depth). This indicate that they can form in the lower crust or upper mantleform in the lower crust or upper mantle.

Fault zones may also serve as conduit for Fault zones may also serve as conduit for rapid fluxing of large amounts of water rapid fluxing of large amounts of water and dissipation of heat during and dissipation of heat during deformationdeformation.

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

BRITTLE AND DUCTILE BRITTLE AND DUCTILE FAULTSFAULTS Brittle faults occur in the upper 5 to 10 km Brittle faults occur in the upper 5 to 10 km of the Earthof the Earth’’s crust. In the upper crust s crust. In the upper crust consist ofconsist of :

Single movement Single movement Anastomosing complex of fracture Anastomosing complex of fracture

surfacessurfaces.The individual fault may have knife-sharp The individual fault may have knife-sharp

contacts or it may consist of zone of contacts or it may consist of zone of cataclasitecataclasite.

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

SHEAR ZONESHEAR ZONEShear zones are produced by both Shear zones are produced by both

homogeneous and homogeneous and inhomogenous simple shear, or inhomogenous simple shear, or oblique motion and are thought oblique motion and are thought of as zones of ductile shear.of as zones of ductile shear.

Shear zones are classified by Shear zones are classified by Ramsay (1980) as:Ramsay (1980) as:

1) brittle 1) brittle

2) brittle-ductile 2) brittle-ductile

3) ductile 3) ductile

Characteristics of Shear Characteristics of Shear ZonesZones Shear zones on all scales are zones of weakness.

Associate with the formation of mylonite.

Presence of sheath folds. Shear zones may act both as

closed and open geochemical systems with respect to fluids and elements.

Shear zones generally have parallel sides.

Displacement profiles along any cross section through shear zone should be identical.

INDICATORS OF SHEAR SENSE OF INDICATORS OF SHEAR SENSE OF MOVEMENTMOVEMENT

1.1. Rotated porphyroblasts Rotated porphyroblasts and porphyroclasts.and porphyroclasts.

2.2. Pressure shadowsPressure shadows

3.3. Fractured grains.Fractured grains.

4.4. Boudins Boudins

5.5. Presence of C- and S- Presence of C- and S- surfaces (parallel surfaces (parallel alignment of platy alignment of platy mineral)mineral)

6.6. Riedel shears.Riedel shears.