Geological Structures
• Joints
• Faults
• Shear Zones
• Folds
• Igneous Bodies
• Unconformities
Geological Structures
Why do rocks deform?
STRESS (force per unit area)
• compressive stress
• tensional stress
• shear stress
Geological Structures
What is the result of stress?
STRAIN (deformation)
• brittle deformation
• plastic/ductile deformation
• elastic deformation
e.g. (faults/joints)
e.g. (folds/shear zones)
Geological Structures
What factors affect deformation?
Deforming everyday materials exercise!
Materials Rubber Band
Biscuit Chocolate Cling Film
Room temp
Chilled
Warmed
Quick
Brittle
Ductile
Elastic
Geological Structures
What factors affect deformation?
Deforming everyday materials exercise!
Materials Rubber Band
Biscuit Chocolate Cling Film
Room temp
Chilled
Warmed
Quick
Brittle
Ductile
Elastic
elasticelasticelasticbrittle
brittle
brittle
brittle
brittle
brittle
brittle
ductile
brittle
ductile
ductile
ductile
brittle
Geological Structures
What factors affect deformation?
Deforming everyday materials exercise!
• temperature
• rate/speed of deformation (strain rate)
• properties of rock (competence)
Geological Structures
• Shrinkage joints
• Tectonic joints
• Pressure release joints
1. JOINTS
Geological Structures
• Normal fault
• Reverse fault
• Thrust fault
2. FAULTS
• Tear fault
Vertical faults
Horizontal faults
Normal Faults
Downthrow sideUpthrow side
Fault plane
Dip of faulti
ii
iii
i – throw
ii – net slip
iii – dip slip
NORMAL = lengthening of crust due to tensional stress
Vertical Faults
NORMAL = lengthening of crust due to tensional stress
Reverse Faults
Upthrow side
Downthrow side
Fault plane
Dip of fault
i
ii
iii
i – throw
ii – net slip
iii – dip slip
REVERSE = shortening of crust due to compressional stress
Thrust Faults
Upthrow side
Downthrow side
Fault plane
Dip of fault
Tear Faults
Slickensides along Fault plane
Slickensides are polished striated rock surfaces caused by one rock mass moving across another on a fault.
Form minute steps facing direction of movement
Fault plane
Components of Principle Stress
Components of Principle Stress
Fault Structures
Thrust Fault Outcrop Pattern
Brampton BGS Map June 2000
Folds
Fold Geometry
Fold Geometry
Style of Folding
Fold Features
Interlimb Angles
Tight ~ 30°
Open ~ 120°
Close ~ 60°
Isoclinal ~ 0° (limbs parallel)
Fold Outcrop Patterns
Plunging Folds
Plunging Folds
Plunging Folds
Domes & Basins
Fold Mechanisms
Geological Structures
Flexural (parallel) Folds
•Thickness of individual layers are constant, whether on crest or trough•Impersistent – fold dies out as not enough room to fit in
•Movement occurs along the boundaries between layers by flexural movement•Formed in strong/competent layers•Thickness of the most competent layer determines the fold wavelength•Low temperature & pressure
•Slickenside lineations may occur between layers
•Strain is low enough not to alter thickness of the folded layer
•Outer zone of orogenic belt
•Helvetic Alps
Fold Mechanisms
Geological Structures
Flow (similar) Folds
•Maintains same curvature on the inner and outer surfaces of the folded area•Each layer is thinner on the flanks and thicker on the axis of the fold•Fold extends indefinitely – no space problem
•Movement within layer occurs within rock when it is ductile
•Develop in highly incompetent beds which behave more as a viscous fluid than a solid rock
•In slightly more competent rocks, shear folding occurs, this is produced by differential movement along cleavage planes e.g.. slate•Axial plane cleavage forms parallel to fold axis
• High temperatures & pressure
•Inner zone of orogenic belt
•Pennine Alps
Unconformities
Unconformity
unconformity
GEOLOGICAL STRUCTURES
Bedding
Bedding•
•
•
•
Joints
Dip direction/angle
Strike
Folds
Anticline/syncline
Faults
Normal
•
•
•
•
Reverse
Thrust
Tear
Unconformities
Formation
•
•
Recognition
Igneous Bodies
Lava flows
•
•
•
•
Dykes
Sills
Batholiths
•
•
•
•
•
Upright/overturned/recumbent
Symmetrical/asymmetrical
Axial plane
Trend
Geological Structures
1. What sort of fault is this?
Geological Structures
2. What sort of fault is this?
Geological Structures
3. What sort of fault is this?
Geological Structures
4. What sort of structure is this?
Geological Structures
5. Describe this fold structure?
Geological Structures
6. What sort of fold is this?
Geological Structures
7. What structure is shown here?
Question 1
• Oldest beds are A & B
• Beds C to F dip NW
• The unconformity predates B
• The fault postdates A
• The faults are reverse
• Unconformity predates the dyke
• Intrusions associated with the faults
• Anticline postdates the dyke
Question 2
Question 3
• Axial plane
• Fold axis
• Fold limb
• Fold dip
Question 4
• Fault below the outlier downthrows West
• There are 2 dykes
• The intrusions are associated with the faults
• The anticline postdates the intrusions
Question 5
• Thrust faults
• Horst
• Strike-slip fault
• Rift valley
Question 6
• Gently dipping to the south
• Horizontal
• Gently dipping to the north
• Steeply dipping to the north
Question 7
• 180/10E
• 090/10S
• 010/090S
• 90/10S
A geologist measured the strike of some strata as due East & their dip as 10 degrees to the South. Which is the correct notation?
Question 8
• Thrust fault
• Normal fault
• Strike-slip fault
• Reverse fault
Question 9
• Oldest rocks are A & B
• Beds C to F dip NW
• A & B are folded
• The fault is a strike-slip fault
Question 10
• Fold limb
• Fold axis
• Fold nose
• Axial plane
Question 11
• Syncline plunging West
• Anticline plunging East
• Syncline plunging East
• Anticline plunging West
Question 12
• Reverse fault
• Normal fault
• Strike-slip fault
• Thrust fault
Question 13
• Steeply dipping to the South
• Steeply dipping to the North
• Vertical
• Gently dipping to the South
Question 14
• Sill is older than bed D
• Outlier was formed by erosion
• Unconformity postdates dyke
• Faults are reverse faults
Question 15
• Syncline plunging West
• Anticline plunging West
• Syncline plunging East
• Anticline plunging East
Question 16
• Syncline plunging East
• Anticline plunging West
• Anticline plunging East
• Syncline plunging West
Question 17
• Thrust fault
• Reverse fault
• Normal fault
• Strike-slip fault
Question 18
• Anticline
• Plunging anticline
• Plunging syncline
• Syncline
Question 19
• Thrust fault
• Rift valley
• Horst
• Strike-slip fault
Question 20
• Gently dipping to the North
• Beds are folded
• The fault is a reverse fault
• Green bed is the youngest