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    Leung Kar FaiVale Exploration Geologist

    Geological Society of Hong Kong

    Faults and Folds

    Geological Structures

    Geological Structures

    Geological structures = rockdeformation resulted from the changein stress through geological time.

    Why change in stress?

    Tectonic processes are responsible for thechange in stress

    Geological Structures

    Common structures

    1. Faults

    2. Folds

    3. Joints

    4. Unconformities

    Common Structures - Faults

    Common Structures - Folds

    Common Structures - Joints

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    Common Structures -Unconformities

    Geological Structures

    Implications

    1. Tectonic history

    2. Mineral exploration

    3. Gas and oil exploration4. Geotechnical engineering

    Stress and Strain

    The concepts of stress, strain andmaterial behavior are fundamental to theunderstanding of geological structuresincluding faults and folds

    Stress

    z Stress is the force applied to each unitarea in a particular direction

    z Vector quantity

    z Measured in pascals, N/m2

    z Normal stress

    z Shear stress

    Type of Stresses

    Normal stress

    Perpendicular to plane

    Extensional stress

    Compressional stress

    Shear stress

    Parallel to plane

    Reality: Stress Decomposition

    ShearstressNormal

    stress

    Appliedstress

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    Strain

    z When rocks deform they are said to strain

    z A strain is a change in size, shape, or volumeof a material in response to applied stress

    z Strain is given in

    fraction, no unit

    z Linear strain = L / L

    Linear strain

    Shear Strain

    Simple shear

    z Causes rotation

    z Changes in angular relation

    z Shear strain differs in different directions

    1

    3

    1

    3

    Relationship between principalstresses and faults

    Faulting occursalong directionof maximum

    shear stress

    Principal Stress Directions

    The stress state at any given point can bedescribed by a system of three principal stresses,normal to each other and along which there are noshear stress components.

    1

    3

    2

    1 (P) maximumprincipal stress

    3 (T) minimumprincipal stress

    Pictures courtesy of IRIS

    Strike, Dip and Slip

    Dip slip fault - Reverse

    Strike slip fault - Left-lateral

    Dip slip fault - Normal

    Oblique-slip fault

    strik

    e

    dip

    SecondaryStructures

    1st order fault

    2nd order fault

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    Classification of Faults

    z Dip-slip fault

    Normal fault

    Reverse fault

    Thrusting fault

    Listric normal fault

    z Strike-slip fault

    Right-lateral strike slip

    Left-lateral strike slip

    z Oblique-slip fault

    Fault and Stress P: maximumB: intermediate

    T: minimum

    Normal Reversed Strike-slip

    Normal Fault

    Maximum principal

    stress 1 vertical

    Minimum principal

    stress 3 horizontal

    Reverse Fault

    Maximum principal

    stress 1 horizontal

    Minimum principal

    stress 3 vertical

    Strike-slip Fault

    Both maximum andminimum principal

    stresses horizontal

    Intermediate

    principal stress 2vertical

    Field Evidence for Faults

    z Offsets & sheared pebbles

    z Fault breccia

    z Fault gouge

    z Quartz veins

    z Mineralization and bleached zones

    z Cleavages and Riedels

    z Fault-related landforms

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    Displaced Bedding

    PortIsland

    Displaced Bedding

    Fractured pebbles

    Tension cracks

    Fault Breccia

    Fault Breccia Fault Gouge

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    Nam Chung

    Offset of quartz vein

    Offset of quartz vein

    Stepping structure: left-lateral faults

    Bleached rocks inFractured zone

    Mineralization along faults

    Fault zone on Sha Chau

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    Crushed granite

    Intact granite

    Quartz mesh

    Tolo Channel FaultExposed at Fung Wang Wut

    Fault-related landforms

    z Linear depressions

    z Displaced stream channels

    z Fault scarps

    z Springs & ponds

    z Raised terraces and waterfalls

    Displacedchannels alongSan Andreas

    Fault

    Photo courtesy of USGS

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    Earthquake & Landform

    waterfall

    Waterfalls can form inmatters of seconds

    Drag folds

    Australian Plate

    Pacific Plate

    Sag pond along fault

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    Northern Tibet: Triangular facets are fault scarps

    Horst-and-Graben

    Illustration courtesy of USGS

    Horst-and-Graben

    Polished surface

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    Cooling joints

    Tension joints

    Joints

    Cooling joints: Ninepin Islands

    Columnar Joint

    Columnar Joint

    As solidifying material contracts, because thewhole volume of rock is contracting, evenly-spaced centers of contraction develop.

    Cracks open up to accommodate that

    contraction. This makes a honeycomb-stylepattern, because 3 crack orientations is theminimum number necessary to allowcontraction in every direction.

    Tension joints

    Conjugate joints

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    Ping Chau

    Material Behavior

    Source:http://wapi.isu.edu/envgeo/EG2_earth/brittle_deformation.htm

    Brittle materials fracture without suffering anysignificant plastic deformation. Plastic deformation is

    not recoverable, i.e. the change is permanent.

    Brittle vs Ductile Deformation

    1

    1

    3 3

    Brittle vs Ductile Deformation

    Brittle deformation: material deform byrupturing, e.g. joints, faults

    Ductile deformation: material deform bybending, flowing or with deformationspread over a zone of numerous closedmicroscopic scale fractures, e.g. fold,shear zone

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    Factors

    1. Material composition and properties

    2. Temperature

    3. Pressure

    4. Strain rate & time

    5. Presence of fluid

    Brittle vs Ductile Deformation

    Folds: Description andClassification

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    Folding at Bluff Head

    Folding at Bluff Head

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    Folding at Bluff Head

    Folding at Bluff Head

    Folding at Ma Shi Chau

    Folding in Lai Chi Chong

    Folding in Lai Chi Chong

    Folding in Lai Chi Chong

    Micro-fault

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    Kink fold (sharp hinge)

    Ptygmatic folds in migmatite

    Folds - Regional Scale

    Jingxi Region,Guangxi

    50 km

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    Landsat image of Mirs Bay

    Geological Map of Hong Kong (GEO)

    Flower Structure

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    How many?

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