Structure

Rheology

Study of flow of any material under the influence of applied stress

Elastic deformation

Strain disappears when force is taken away ( not permanent)

Brittle deformation

Discontinuous deformation

Rocks change by breaking in response to compression/extension

Deformation is localised along a plane with rocks to the side unaffected

Features: Brittle deformation

• joints

• Veins

• Faults

• Fissures

Ductile deformation

Permanent continuous deformation

Rocks change shape smoothly without breaking

Compression/extension/shear

Affects all of the rock

Deformation

Change in shape, position and volume of a rock in response to applied forces

Features: ductile deformation

• shear zones

• boudinage

• Cleavage

• Folds

Stress

Force per unit area

Causes strain

Strain

Change in shape of a volume of material

Caused by stress

Mantle: Brittle deformation

• Near surface

• Lower pressure

• Lower temperature

Mantle: ductile deformation

• under brittle-ductile transition zone

• Deeper

• Higher pressure

• Higher temperature

Joints

• extension fracture

• Little/no offset and tectonic activity

• Control flow of groundwater/hydrocarbons

• Brittle deformation feature

• Due to cooling/drying out?

• Open as rocks pressure and temperature are uplifted

Veins

• fractures filled with minerals

• Brittle deformation feature

Fissure

• extension fractures

• Filled with gas/fluid

• Brittle deformation feature

Slickensides

• direct of relative fault motion

• Created by two fault blocks rubbing past each other

Slickenfibres

Type of lineation

• small voids open

• New minerals precipitated

• Fibrous

• Direction of fibre growth gives relative slip direction

• Sense fibre growth off the fault plane gives the sense of faulting

Principle stresses

Sigma 1 - Largest

Sigma 2 - medium

Sigma 3 - smallest

Normal fault

Thrust fault

Strike-slip fault

Normal fault

• sigma 1 vertical

• Hang wall moves down

• Dips 50-60

Thrust fault

• sigma 3 vertical

• Sigma 1 horizontal

• Places older rock on younger rock

• Dips 0-30 (steeper reverse)

Strike slip fault

• sigma 2 vertical

• Close to vertical dip

• Moves parallel left/right