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Geology > Geological Maps and Structures L4 > Flashcards

Geological Maps and Structures L4 Flashcards

1
Q

What are folds

A

When initially planar structures transform into bent or curved structures during ductile deformation

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2
Q

What is primary deformation?

A

Folding that occurs during the formation of the rock such as slump folds or dewatering structures

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3
Q

What is secondary deformation?

A

Folding that occurs due to tectonic activity

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4
Q

What is an antiform?

A

A fold that closes upwards

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5
Q

What is a synform?

A

A fold that closes downwards

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6
Q

In an antiform, is the hinge point near to a crest point or a trough point?

A

Crest point

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7
Q

In a synform, is the hinge point near to a crest point or a trough point?

A

Trough point

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8
Q

What is a neutral fold?

A

A fold that closes sideways

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9
Q

What is an anticline?

A

Anticlines have oldest rocks in the core (they young outwards)

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10
Q

What is a syncline?

A

Synclines have youngest rocks in the core (they young inwards)

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11
Q

What is an antiformal syncline?

A

A fold that closes upwards which has the youngest rocks in the core (they young inwards)

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12
Q

What is a synformal anticline?

A

A fold that closes downwards which has the oldest rocks in the core (they young outwards)

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13
Q

What is fold tightness defined by?

A

The angle between two fold limbs (the interlimb angle)

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14
Q

What is the interlimb angle?

A

The angle between two fold limbs.

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15
Q

How is the interlimb angle calculated?

A

Tangents are drawn through the inflection point (points of minimal curvature) of each limb. The angle at which these tangents intersect is the interlimb angle.

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16
Q

What are the 5 ways in which the interlimb angle is categorised?

A

Gentle (120-180)
Open (70-120)
Close (30-70)
Tight (5-30)
Isoclinal (0-5)

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17
Q

What is an isoclinal fold?

A

A fold in which the limbs are roughly parallel (0-5 degrees)

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18
Q

What is fold roundness?

A

A measure of hinge zone curvature

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19
Q

What are the 3 ways fold roundness is categorised?

A

Rounded
Sub-angular
Angular

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20
Q

What chevron folds and kink folds?

A

Special types of angular folds with straight limbs and sharp hinges

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21
Q

What is fold attitude?

A

The dip (inclination) of the axial surface and plunge of the hinge line

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21
Q

What is fold attitude?

A

The dip (inclination) of the axial surface and plunge of the hinge line

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22
Q

What is the attitude of a fold classified according to?

A

The dip of the axial surface
The plunge of the hinge line

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23
Q

What is a recumbent fold?

A

A fold in which the axial surface is greatly dipping, >80 degrees (horizontal/on its side)

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24
Q

How is a fold with a low axial surface dip described?

A

Upright

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25
Q

What is a symmetric fold?

A

A fold in which the limbs are bisected (equally divided) by the axial plane/surface and as a result the limbs are equal length

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26
Q

What is an asymmetric fold?

A

A fold in which the axial plane is closer in orientation to one of the limber than the other and as a result the limbs are different lengths

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27
Q

What is vergence?

A

A measure of the sense of symmetry of asymmetric folds, described as the direction of apparent shear of the upper, longer limb of asymmetric folds

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28
Q

What are the 3 ways in which vergence is described?

A

In terms of cardinal direction of shear (i.e. N, E, S, W)
Clockwise or anticlockwise
Z or S folds

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29
Q

How would an asymmetric fold in which the upper limb appears to have moved East relative to the originally upright fold be described?

A

East-verging

30
Q

When using “s” and “z” to describe fold symmetry, what does the short limb of the fold represent?

A

The middle of the letter (z or s)
i.e. if the asymmetric fold looks like the letter “z” or “s”

31
Q

When using “clockwise/anticlockwise” or “z/s” to describe fold asymmetry, what must you bear in mind?

A

The direction you are facing when the observation is made

32
Q

What are 4 characteristics of a fold that can be described?

A

Tightness
Roundness
Attitude
Symmetry

33
Q

What do folding mechanisms describe?

A

The way in which forces act on layered rocks to cause folding

34
Q

What are the 4 mechanisms of folding?

A

Buckling
Bending
Simple shear
Homogenous strain

35
Q

What is buckling?

A

Buckling occurs when a layer is shortened parallel to the plane of layering caused by layer-parallel compressional forces

36
Q

What does buckling require to occur and how does it work?

A

Buckling requires a contrast in viscosity between adjacent layers - the more viscous layer will buckle and the less viscous material will flow round it

37
Q

When do large wavelengths form during buckling?

A

Large wavelengths are formed when the competent layer (the layer that is folded) is highly viscous compared with the surrounding matrix

38
Q

What is bending?

A

Folding caused by heterogenous forces acting transverse to the layering (pressure from above/ below the layering/at a high angle to layered rocks)

39
Q

What is simple shear?

A

Folding that occurs where the forces act at intermediate angles to layering

40
Q

What is homogenous strain?

A

Forces that can amplify folds, but not create them

41
Q

What are the 3 types of flexural folding?

A

Flexural slip
Flexural flow
Tangential-longitudinal strain

42
Q

What do the 3 types of flexural folding describe?

A

How folded layers respond to both active and passive folding

43
Q

What is flexural slip?

A

Folded layers slip along layer interfaces to accommodate folding, like bending a stack of cards.

44
Q

How does shear act on a fold in flexural slip?

A

The sense of shear is opposite on the 2 fold limbs but there is zero shear right at the fold hinge.

45
Q

Where and in what conditions is flexural slip more common?

A

Cooler temperatures in sedimentary layers of the upper crust

46
Q

What is flexural flow?

A

Fold limbs deform purely by shearing without any slip between layers

47
Q

In what conditions is flexural flow more common?

A

Higher temperatures and under more ductile conditions

48
Q

What is tangential-longitudinal strain?

A

Fold formed by tension in the outer arc and compression in the inner arc

49
Q

Explain the mechanism of tangential-longitudinal strain in terms of tension and compression and their effects on the fold.

A

Tension on the outside of the arc - develops small tension cracks that may be filled with minerals as well as cleavage parallel to the layer surface.

Compression on the inside of the arc - can produce small compressional faults or cleavage perpendicular to the base of the layer or pressure solution veins called stylolites

50
Q

What type of folds does tangential-longitudinal strain produce and why?

A

Open folds - as the fold gets tighter, flexural slip and flexural flow take over

51
Q

What is a parasitic fold?

A

Folds that are characterised by a small wavelength and are situated within folds with a larger wavelength.

52
Q

How are parasitic folds formed?

A

1) A thin viscous layer sits on top of a thicker viscous layer
2) Pressure is applied causing the thicker layer to thicken, and the thin layer to buckle
3) With more deformation, large scale folds develop (controlled by the thicker layers)
4) We are left with a large wavelength fold (the thick layer), containing a small wavelength parasitic fold (thin layer).

53
Q

In which direction does the parasitic fold verge?

A

Parasitic folds verge towards the hinge of the larger fold

54
Q

Are parasitic folds symmetric or asymmetric?

A

Asymmetric

55
Q

Why are the parasitic folds at the hinge of the large scale fold symmetric?

A

Shear is zero at the fold hinge

56
Q

Where do parasitic symmetrical M-folds occur in relation to the larger fold?

A

At the hinge where shear is zero

57
Q

Where do asymmetric Z-folds and S-folds occur?

A

Either side of the hinge.

58
Q

What is cleavage in terms of folding?

A

A type of penetrative planar deformation fabric, or foliation, along with rocks split or “cleave”

59
Q

What does cleavage form as a result of?

A

Stress/strain

60
Q

In what conditions does cleavage form?

A

Low-grade metamorphic conditions

61
Q

What does a rock need to contain in order to develop cleavage?

A

Sheet silicates

62
Q

What is compaction cleavage?

A

Cleavage that forms as a rock compacts such that the direction of compression is perpendicular to the bedding. The compaction causes the collapse of pore space and the alignment of minerals.

63
Q

What is pencil cleavage?

A

Cleavage that forms when tectonic stress that is parallel to the bedding is applied causing shortening and a realignment of minerals, yielding a second cleavage oblique to the original compaction cleavage

64
Q

What is slaty cleavage?

A

Continued compression parallel to bedding will form form a closely-spaced slaty cleavage oblique to the bedding

65
Q

What is crenulation cleavage?

A

Compression oblique to an existing slaty cleavage causing the folding of cleavage.

66
Q

Why is cleavage useful to look at in the field?

A

There is a geometric relationship between folding and cleavage formed during the same deformation event

67
Q

In a fold, what does cleavage commonly form parallel to and hence what is the name given to this cleavage?

A

Cleavage commonly forms parallel to the axial plane - this is known as axial planar cleavage

68
Q

What are refolded/superposed folds ?

A

Folds modified by a later folding event

69
Q

What is the name given to type 1 interference (refolded folds)?

A

Egg box interference

70
Q

What is the name given to type 2 interference (refolded folds)?

A

Winged interference

71
Q

What is the name given to type 3 interference (refolded folds)?

A

Hook interference

72
Q

Why might refolded folds be useful in the field?

A

Interference patterns of smaller scale folds can reveal a large-scale structure that may not be immediately obvious

Geology (17 decks)

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