Rock Deformation Geological Structures Joints Faults Shear Zones

Rock Deformation

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 e. g. (faults/joints) • plastic/ductile deformation e. g. (folds/shear zones) • elastic deformation

Geological Structures What factors affect deformation? Deforming everyday materials exercise! Materials Room temp Brittle Chilled Ductile Warmed Elastic Quick Rubber Band Biscuit Chocolate Cling Film

Geological Structures What factors affect deformation? Deforming everyday materials exercise! Materials Rubber Band Room temp elastic brittle ductile Chilled elastic brittle ductile Ductile Warmed elastic brittle ductile Elastic Quick brittle Brittle Biscuit Chocolate Cling Film brittle

Geological Structures What factors affect deformation? Deforming everyday materials exercise! • temperature • rate/speed of deformation (strain rate) • properties of rock (competence)

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

Geological Structures 2. FAULTS • Normal fault • Reverse fault Vertical faults • Thrust fault • Tear fault Horizontal faults

Normal Faults NORMAL = lengthening of crust due to tensional stress ii i Upthrow side iii Dip of fault Downthrow side i – throw ii – net slip iii – dip slip Fault plane

Vertical Faults NORMAL = lengthening of crust due to tensional stress

Reverse Faults REVERSE = shortening of crust due to compressional stress ii i i – throw ii – net slip iii – dip slip iii Dip of fault Upthrow side Downthrow side Fault plane

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 • 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

Bedding Folds • Bedding • Anticline/syncline • Joints • Upright/overturned/recumbent • Dip direction/angle • Symmetrical/asymmetrical • Strike • Axial plane • Trend GEOLOGICAL STRUCTURES Faults • Normal • Reverse • Lava flows • Thrust • Dykes • Tear Igneous Bodies 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?

Question 1 • Oldest beds are A & B • Beds C to F dip NW • The unconformity predates B • The fault postdates A

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

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 A geologist measured the strike of some strata as due East & their dip as 10 degrees to the South. Which is the correct notation? • 180/10 E • 090/10 S • 010/090 S • 90/10 S

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