# faults folds

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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 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?