Term 1 - Introduction

Question 1

Ductile vs Brittle

Answer 1

Brittle Deformation: When a rock breaks, it is called brittle deformation. Produces Faults, fractures, joints.

Ductile Deformation: When rocks bend or flow, like clay, it is called ductile deformation. Produces folds, shear fabrics.

Question 2

Rock change terminology

Answer 2

Strain: how the rock body has changed shape, volume, orientation and position during deformation.
What happened

Kinematics: how the geometry of the rock body, and the strain, evolved (changed) during deformation.
How it happened

Dynamics: the orientation, magnitude and origin of the forces (or stresses) that caused the deformation
Where, when and how intense?

Question 3

Deformation changes

Answer 3

Strain = distortion: = Change in length, size, shape

Rotation

Displacement

Elastic deformation: = Rocks return to original state when stress is removed.
Permanent deformation: = Preserved after stress relaxation

Question 4

What is strain?

Answer 4

σ = force/area

Confining pressure: = Force equal in all directions

Differential stress: = Force NOT equal in all directions
• Potential for deformation

Can get:
Compressive (reduction in size)
Tensile (extension)
Shear (slices in opposite directions)

Question 5

Principle stress orientations

Answer 5

3 orthogonal stress axis

σ1 = Maximum compressive stress
σ2 = Intermediate compressive stress
σ3 = Minimum compressive stress (i.e. tensile stress)

Question 6

Structure types

Answer 6

• Primary structures: = related to the original deposition & formation of the rock
• Secondary structures = formed by deformation  modify the primary structures
o e.g.
- Faults
- Folds
- Foliations

Question 7

Normal vs reverse:

Answer 7

Normal:
Footwall moves up, hanging wall remains
σ1 downwards, σ3 sidewards to hanging wall

Reverse:
Footwall moves up/over hanging wall
σ3 downwards, σ1 sidewards to hanging wall

Question 8

Fold tightness

Answer 8

• The more compression there is the tighter the fold will become
o so, we can use the steepness of the dips to infer how much compression there was
• Interlimb angle tightness:
o Gentle fold: 180-120 degrees
o Open fold: 120-70 degrees
o Closed fold: 70-30 degrees
o Tight fold: Less than 30 degrees
o Isoclinal: Negligible (very low) – folds are near vertical

Question 9

Orientation of fold axial plane

Answer 9

Upright fold: Axial plane 90 degrees to horizontal
Inclined fold: Axial plane angled
Recumbent fold : Over-turned - axial plane is horizontal

Question 10

Plunging folds

Answer 10

Non-plunging fold: Hinge line (axis) = horizontal

Plunging fold: Hinge line (axis) = angled

Question 11

Types of folds

Answer 11

Symmetric folds Asymmetric folds

Parasitic folds - contains Z and S structures which converge on the M structure at the axial plane

Question 12

Bedding-cleavage intersection

Answer 12

• Cleavage forms parallel to axial plane
• So we can infer folding occurred from presence of cleavage
• Cleavage verges up towards antiform hinge
• Cleavage points towards nearest antiform

Question 13

Finding σ1, σ2 and σ3 on a stereonet for faults and lineations

Answer 13

σ1 = between the two lineation pitches
σ2 = At the fault line intersection
σ3 = The σ1 and σ3 line intersection