Rock Deformation/Principles of Structural Geology

Question 1

Refers to the change in shape, position and/or volume of an object in response to applied forces.

Answer 1

Deformation

Question 2

The permanent change in shape (in 1D, 2D or 3D) of a rock body as a result of deformation.

Answer 2

Strain

Question 3

It is strain in a single direction (1D) or the change in the length of a line. It maybe measured in units (centimetres, kilometres etc) or expressed as the ratio elongation (e)

Answer 3

Longitudinal strain

Question 4

In a longitudinal strain _______ is negative (-) for _______ (the line has decreased in length) and positive (+) for ______ (the line has increased in length)

Answer 4

Elongation; Contraction; Extension

Question 5

Refers to the strain in a plane (2D) or the change in angles. It is also the deflection of an orthogonal marker. This is the change in angle between a pair of lines that were originally orthogonal.

Answer 5

Angular shear strain

Question 6

It is the tangent of the change in angle between the originally orthogonal pair of lines. It ______ for a clockwise deflection of the marker and _______ for an anticlockwise deflection.

Answer 6

Shear strain (γ); Positive; Negative

Question 7

A method of representing the amount of strain a rock has undergone. It uses an initially circular marker that is deformed to an ellipse.

Answer 7

Strain Ellipse

Question 8

Defined as a pair of equal and opposite forces acting on a unit area of a rock body.

Answer 8

Stress

Question 9

Refers to a surface or narrow zone with parallel displacement and offset of layers either side of the fracture.

Answer 9

Fault

Question 10

Movement parallel to dip of the fault plane. Both normal, thrust and reverse faults are dip-slip faults.

Answer 10

Dip-slip

Question 11

Other term for Double plunging folds.

Answer 11

Pericline

Question 12

The angle between the limbs and relates to the amount of strain during deformation.

Answer 12

Interlimb Angle

Question 13

What is the difference between brittle and ductile deformation?

Answer 13

• During brittle deformation rocks fracture with strain localised along a plane whilst the rocks to either side remaining unaffected (e.g. faults and joints). • During ductile deformation rocks change shape smoothly and strain is pervasive throughout the rock body (e.g. folds).

Question 14

Define what is a rigid body deformation.

Answer 14

• Rigid body deformation occurs where a rock mass moves or rotates with no change of shape • During translation every point in a rock body undergoes the same displacement. There is no distortion and no change of shape • Rotation is the same as translation only with a rotational component

Question 15

Define what is a non-rigid body deformation.

Answer 15

• Strain is the change in shape of a rock body during deformation • Volume change occurs where a rock body increases or decreases in volume. In two dimensions this is a change in area.

Question 16

Define what is homogeneous deformation.

Answer 16

• Homogeneous deformation occurs where deformation is constant across a rock body, that is different parts of an object deform by the same amount. In homogeneous deformation straight lines remain straight, parallel lines remain parallel and circles deform to ellipses.

Question 17

Define what is heterogeneous deformation.

Answer 17

• Heterogeneous deformation occurs where the deformation varies across a rock body, so different parts of an object deform by different amounts • A large area of heterogeneous deformation may be broken down into smaller areas of homogeneous deformation for analysis.

Question 18

Differentiate Pure Shear and Simple Shear.

Answer 18

• Pure shear is coaxial deformation. Contraction and elongation are parallel to the strain axes, so there will be no rotation of the axes from their original positions. • Simple shear is non-coaxial deformation. During deformation the strain axes rotate.
• The amount of rotation depends on amount of strain; the greater the strain, the greater the rotation.

Question 19

What is the difference between normal and shear stress?

Answer 19

Stress on a surface such as bedding can be broken down into normal stress σn (sigma n) which acts perpendicular to the surface and shear stress σs (sigma s) oriented parallel to the surface.

Question 20

Define hydrostatic and deviatoric stress.

Answer 20

• Hydrostatic stress occurs where the principal stresses have the same value: σ1 = σ2 = σ3 . • This is the state of stress found in fluids and does not cause strain, although it may cause a loss in volume • For strain to occur a deviatoric stress state where σ1 > σ2 > σ3 is needed

Question 21

What is a fault bend fold?

Answer 21

In thrust faults, where the thrust cuts upwards, the rocks in the hanging-wall are forced to fold to accommodate the shortening along the thrust. The resulting fold is known as a fault bend fold. The layers in the footwall remain underformed.

Question 22

Differentiate active folding form passive folding.

Answer 22

• Active folding is the response of layers of different competence to layer parallel compression. • Passive folding occurs where layering has no mechanical influence on the folds formed.

Question 23

Is antiform and anticline the same? Why or why not?

Answer 23

• Anticline: A fold with older rocks in its core. • Antiform: A fold where the limbs close upwards.

Question 24

What is the difference between an upright fold and inclined fold?

Answer 24

• An upright fold has a vertical axial plane. • An inclined fold has a dipping plane and a recumbent fold has a horizontal axial plane.

Question 25

What is the difference between a non-plunging fold and plunging fold?

Answer 25

• A non-plunging fold has a horizontal hinge line. • A plunging fold has a inclined hinge line.

Question 26

Define the following: symmetric folds, asymmetric folds, and vergence.

Answer 26

• Symmetric folds have equal limb lengths and the two sides of the fold are mirror images • Asymmetric folds have a shorter and a longer limb • A series of folds with the same asymmetry are said to have vergence.

Question 27

Discuss what is a normal fault and its respective subtypes.

Answer 27

• Normal faults usually dips at about 50 – 60° , although local conditions can cause this to vary. Sigma one is vertical and sigma three is horizontal. • Listric faults are normal faults that decrease in dip with depth. The hangingwall is folded into a roll-over anticline. Listric faults often occur at a kilometre scale. • Domino faults are a series of parallel normal faults and fault blocks that rotate during extension. This style of faulting leads to space problems at the base of the fault system which are usually accommodated by ductile deformation along a weak layer such as salt or a thick series of shales.

Question 28

Explain how thrust faults occur.

Answer 28

• It usually dips at about 0 – 30° , although local conditions can cause this to vary. • Sigma one is horizontal and sigma three is vertical. • As the hangingwall moves up the footwall older rocks are placed on top of younger rocks. • Thrust often run parallel to bedding (flat) then cut up through the layers (ramp) before finding another weak layer to follow.

Question 29

Enumerate and illustrate the different parts of a duplex in a fold and thrust belt occurence.

Answer 29

Fold and thrust belts • Thrust tip: Leading edge of a thrust fault where displacement ceases. • Blind thrust: The foremost thrust of a sequence, which dies out before it reaches the surface. • Imbricate fan: A series of thrusts extending from a floor thrust to the surface. • Imbricates: A series of thrusts with the same dip and branching from the same floor thrust, may be part of a duplex or imbricate fan. • Branch lines: Where a thrust branches away from or onto a floor or roof thrust. • Horse: A fault bound block within a duplex. • Roof thrust: Uppermost thrust of a duplex. • Floor thrust: Basal thrust of a duplex or imbricate fan, also known as a sole thrust.

Question 30

Discuss how a strike-slip fault occurs.

Answer 30

• A strike-slip fault is one where the fault blocks move horizontally past each other. • Sigma one and three are both horizontal and sigma two is vertical • Where relative displacement is to the right the fault it dextral, where it is to the left the fault is sinistral. Strikeslip faults are usually vertical or close to vertical.

Question 31

Explain how a bend, releasing bend, and restraining bend forms in a strike-slip fault.

Answer 31

• Bends in strike-slip faults result in areas of compression or extension depending on the direction of movement relative to the bend. • Where the bend is in the same direction as movement, an area of extension develops and a pull-apart basin bound by the strike-slip fault and normal faults forms. This is known as a releasing bend. • Where the bend is in the opposite direction to movement an area of compression develops with localised thrusting and folding. This is known as a restraining bend.

Question 32

Explain the mechanism of a ductile deformation.

Answer 32

• Ductile deformation occurs where rocks change shape smoothly, without breaking, in response to stress. • This shape change is pervasive – it affects all of the rock. • Large scale ductile deformation occurs at higher temperatures and pressures and so deeper in the crust than brittle deformation.

Question 33

Enumerate and define each part of a fold.

Answer 33

• Anticline: A fold with older rocks in its core. • Antiform: A fold where the limbs close upwards. • Axial plane: A plane that connects all the hinge lines through a fold, also known as the axial surface (3D term). • Axial trace: Where only an edge of the axial plane is seen, e.g. when a fold is seen in cross section or on a map. The hinge line runs along the top of the axial plane and so is also an axial trace (2D term). • Facing: The direction of younging along the fold axial plane. In an upward facing fold the beds are the right way up and get younger from the bottom to the top of the axial plane. • Hinge: Point of maximum curvature on a fold (2D term). • Hinge line: Line of maximum curvature along a fold (3D term). • Interlimb angle: The angle between the two limbs of a fold. • Limb: Bed segments between hinges. • Profile plane: The true cross section through a fold, perpendicular to the axial plane. • Syncline: A fold with younger rocks at in the core. • Synform: A fold where the limbs close downwards.

Question 34

Enumerate and define the different types of folds.

Answer 34

• Upright/symmetric fold - the angle of the limb is same on both sides • Isoclinal folds - are similar to symmetrical folds, but these folds both have the same angle and are parallel to each other. • Overturned fold - is an asymmetrical fold with inclined axial plane in which both the limbs are dipping essentially in the same general direction usually at different angles. • Overtold or Recumbent fold - when the axial plane becomes increasingly inclined approaches the horizontal. • Chevron fold - folded beds with straight limbs and sharp hinges,

Question 35

How does pressure solution occurs?

Answer 35

• Pressure solution occurs where soluble minerals go into solution in areas of high pressure and are reprecipitated in areas of lower pressure. • Clay minerals are concentrated into bands. • This results in a cleavage with fine layers of soluble and insoluble minerals giving the rock a banded appearance.

Question 36

Explain the difference between cylindical and non-cylindrical folds.

Answer 36

• Where a hinge line is straight the fold is said to be cylindrical. • Where a hinge line curves the fold is non-cylindrical • Curvature of the hinge line results in folds that close in the direction in which their limbs converge. • Double plunging folds (or pericline) form elongate domes or basins.

Question 37

How does dip isogons classify folds?

Answer 37

• Dip isogons are a means of classifying folds based on the geometric relationship between their outer and inner profiles • Lines are drawn on the profile plane and connect points of equal dip between the inner and outer surfaces of the fold