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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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