Rock Deformation

Joints Faults Shear Zones Folds Igneous Bodies Unconformities Geological Structures Joints Faults Shear Zones Folds Igneous Bodies Unconformities

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

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

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

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

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)

Pressure release joints Geological Structures 1. JOINTS Shrinkage joints Tectonic joints Pressure release joints

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

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

Vertical Faults NORMAL = lengthening of crust due to tensional stress

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

Thrust Faults Dip of fault Upthrow side Downthrow side Fault plane

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 Open ~ 120° Close ~ 60° Tight ~ 30° 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 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

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 Inner zone of orogenic belt Pennine Alps High temperatures & pressure

Unconformities

Unconformity unconformity

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

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?

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

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

Question 3 Axial plane Fold axis Fold limb Fold dip

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

Question 5 Thrust faults Horst Strike-slip fault Rift valley

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

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

Question 8 Thrust fault Normal fault Strike-slip fault Reverse fault

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

Question 10 Fold limb Fold axis Fold nose Axial plane

Syncline plunging West Anticline plunging East Syncline plunging East 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

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

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

Syncline plunging West Anticline plunging West Syncline plunging East Question 15 Syncline plunging West Anticline plunging West Syncline plunging East Anticline plunging East

Syncline plunging East Anticline plunging West 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

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