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Rock Deformation
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Joints Faults Shear Zones Folds Igneous Bodies Unconformities
Geological Structures Joints Faults Shear Zones Folds Igneous Bodies Unconformities
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Why do rocks deform? STRESS (force per unit area) compressive stress
Geological Structures Why do rocks deform? STRESS (force per unit area) compressive stress tensional stress shear stress
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What is the result of stress?
Geological Structures What is the result of stress? STRAIN (deformation) brittle deformation e.g. (faults/joints) plastic/ductile deformation e.g. (folds/shear zones) elastic deformation
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What factors affect deformation?
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
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What factors affect deformation?
Geological Structures What factors affect deformation? Deforming everyday materials exercise! Materials Rubber Band Biscuit Chocolate Cling Film Room temp Chilled Warmed Quick elastic brittle brittle ductile elastic brittle brittle ductile Brittle Ductile Elastic elastic brittle ductile ductile brittle brittle brittle brittle
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What factors affect deformation?
Geological Structures What factors affect deformation? Deforming everyday materials exercise! temperature rate/speed of deformation (strain rate) properties of rock (competence)
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Pressure release joints
Geological Structures 1. JOINTS Shrinkage joints Tectonic joints Pressure release joints
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2. FAULTS Normal fault Vertical faults Reverse fault Thrust fault
Geological Structures 2. FAULTS Normal fault Reverse fault Vertical faults Thrust fault Horizontal faults Tear fault
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Normal Faults NORMAL = lengthening of crust due to tensional stress ii iii Dip of fault i Downthrow side Upthrow side i – throw ii – net slip Fault plane iii – dip slip
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Vertical Faults NORMAL = lengthening of crust due to tensional stress
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Reverse Faults REVERSE = shortening of crust due to compressional stress Dip of fault ii i iii Upthrow side Downthrow side i – throw ii – net slip Fault plane iii – dip slip
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Thrust Faults Dip of fault Upthrow side Downthrow side Fault plane
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Tear Faults
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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
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Components of Principle Stress
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Components of Principle Stress
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Fault Structures
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Thrust Fault Outcrop Pattern
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Brampton BGS Map June 2000
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Folds
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Fold Geometry
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Fold Geometry
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Style of Folding
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Fold Features
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Interlimb Angles Open ~ 120° Close ~ 60° Tight ~ 30° Isoclinal ~ 0° (limbs parallel)
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Fold Outcrop Patterns
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Plunging Folds
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Plunging Folds
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Plunging Folds
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Domes & Basins
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Fold Mechanisms
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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 Outer zone of orogenic belt Slickenside lineations may occur between layers Helvetic Alps Strain is low enough not to alter thickness of the folded layer
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Fold Mechanisms
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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 Inner zone of orogenic belt Pennine Alps High temperatures & pressure
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Unconformities
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Unconformity unconformity
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GEOLOGICAL STRUCTURES
Bedding Folds Bedding Anticline/syncline Joints Upright/overturned/recumbent Dip direction/angle Symmetrical/asymmetrical Axial plane Strike Trend Faults GEOLOGICAL STRUCTURES Normal Igneous Bodies Reverse Thrust Lava flows Tear Dykes Unconformities Sills Batholiths Formation Recognition
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Geological Structures
1. What sort of fault is this?
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Geological Structures
2. What sort of fault is this?
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Geological Structures
3. What sort of fault is this?
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Geological Structures
4. What sort of structure is this?
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Geological Structures
5. Describe this fold structure?
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Geological Structures
6. What sort of fold is this?
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Geological Structures
7. What structure is shown here?
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The unconformity predates B The fault postdates A
Question 1 Oldest beds are A & B Beds C to F dip NW The unconformity predates B The fault postdates A
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Unconformity predates the dyke Intrusions associated with the faults
Question 2 The faults are reverse Unconformity predates the dyke Intrusions associated with the faults Anticline postdates the dyke
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Question 3 Axial plane Fold axis Fold limb Fold dip
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Fault below the outlier downthrows West There are 2 dykes
Question 4 Fault below the outlier downthrows West There are 2 dykes The intrusions are associated with the faults The anticline postdates the intrusions
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Question 5 Thrust faults Horst Strike-slip fault Rift valley
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Gently dipping to the south Horizontal Gently dipping to the north
Question 6 Gently dipping to the south Horizontal Gently dipping to the north Steeply dipping to the north
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Question 7 A geologist measured the strike of some strata as due East & their dip as 10 degrees to the South. Which is the correct notation? 180/10E 090/10S 010/090S 90/10S
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Question 8 Thrust fault Normal fault Strike-slip fault Reverse fault
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The fault is a strike-slip 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
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Question 10 Fold limb Fold axis Fold nose Axial plane
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Syncline plunging West Anticline plunging East Syncline plunging East
Question 11 Syncline plunging West Anticline plunging East Syncline plunging East Anticline plunging West
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Question 12 Reverse fault Normal fault Strike-slip fault Thrust fault
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Steeply dipping to the South Steeply dipping to the North Vertical
Question 13 Steeply dipping to the South Steeply dipping to the North Vertical Gently dipping to the South
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Outlier was formed by erosion Unconformity postdates dyke
Question 14 Sill is older than bed D Outlier was formed by erosion Unconformity postdates dyke Faults are reverse faults
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Syncline plunging West Anticline plunging West Syncline plunging East
Question 15 Syncline plunging West Anticline plunging West Syncline plunging East Anticline plunging East
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Syncline plunging East Anticline plunging West Anticline plunging East
Question 16 Syncline plunging East Anticline plunging West Anticline plunging East Syncline plunging West
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Question 17 Thrust fault Reverse fault Normal fault Strike-slip fault
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Question 18 Anticline Plunging anticline Plunging syncline Syncline
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Question 19 Thrust fault Rift valley Horst Strike-slip fault
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Gently dipping to the North Beds are folded
Question 20 Gently dipping to the North Beds are folded The fault is a reverse fault Green bed is the youngest
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