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EART28202 Structural Geology > Geometrical features of folds and their associated minor structures > Flashcards

Geometrical features of folds and their associated minor structures Flashcards

1
Q

What is reflected in the geometry of a fold?

A

The way it was formed

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2
Q

What is a cylindrical fold?

A

A fold that can be generated by translating a straight line with no rotation

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3
Q

What is the property of a cylindrical fold that is used to generate the fold?

A

Fold axis

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4
Q

What is the profile plane of a fold?

A

The plane that is perpendicular to the fold axis

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5
Q

What is the hinge line of a fold?

A

The line connecting points of maximum curvature

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6
Q

What is the inflection line of a fold?

A

The lines joining points of zero curvature

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7
Q

What is the region around the hinge line called?

A

Hinge zone

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8
Q

What is the region around the hinge zone called?

A

Limbs

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9
Q

What is the relationship between the hinge line and fold axis?

A

They are parallel but the hinge line is at a specfic location of the fold, the fold axis is not

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10
Q

What defines the boundaries of a single fold?

A

The portion of the folded surface that lies between two adjacent inflection points

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11
Q

What is the enveloping surface?

A

The surface that contains the crest lines or trough lines

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12
Q

What is the sheet dip?

A

The dip of the enveloping surface, it is effectively the average dip of the folded surface

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13
Q

What is the median surface?

A

The surface containing all the inflection lines

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14
Q

What is the axial surface?

A

The surface containing all the hinge lines of a nested stack (at least 2) of folded surfaces (becomes the axial plane is the surface is planar)

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15
Q

What is the difference between the hinge plane and axial plane?

A

They are parallel but the hinge plane is a specific location wheras the axial plane is not

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16
Q

What is the axial trace?

A

The line of intersection of the axial surface with any other surface of interest (usually the ground surface)

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17
Q

What is the difference between plunge and dip?

A

Plunge is measured along the axis of a fold and dip is measured along the limb

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18
Q

What is a synform?

A

Folds that close down

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19
Q

What is an antiform?

A

Folds that close up

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20
Q

What is a syncline?

A

Folds with the youngest rocks in the core

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21
Q

What is an anticline?

A

Rocks with the oldest rocks in their core

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22
Q

In which direction is it said that folds face?

A

The direction that they young within their axial plane

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23
Q

What is a homocline?

A

Uniformly tilted layering of a regional scale

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24
Q

What is a monocline?

A

A fold pair with two long horizontal limbs connected by a short inclined limb

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25
Q

What is a box fold?

A

Double-hinged fold

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26
Q

What is a kink fold?

A

Half a box fold

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27
Q

When is a fold symmetric?

A

The median surface and axial surface are perpendicular and they divide an antiform-synform pair into mirror-symmetric quarter waves

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28
Q

When is a fold asymmetric?

A

When it violate one or both (usually both) of the conditions for symmetry

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29
Q

Describe an asymmetric fold

A

The median surface and axial plane are not perpendicular and the fold limbs are of unequal length

30
Q

What are the two types of asymmetric fold?

A

Z-fold and S-fold

31
Q

What is vergence?

A

The direction of the antiform if you are on the long limb

32
Q

When do asymmetric folds only violate one condition of symmetry?

A

When the limbs have different thicknesses

33
Q

What happens when the limbs of an asymmetric fold are different thicknesses?

A

The axial plane doesn’t bisect the angle between the limbs

34
Q

How is the scale of folds measured?

A

Amplitude and wavelength in reference to the enveloping and median surfaces

35
Q

When are two amplitudes required?

A

When the median surface is not midway between the enveloping surfaces

36
Q

What are two other measurements that are important in the formation of folds?

A

Arc length and layer thickness

37
Q

What is a vertical fold?

A

Where the fold axis (and therefore the axial plane) is vertical

38
Q

What is a reclined fold?

A

Where the plunge/plunge direction of the fold axis is the same as the dip/dip direction of the axial plane

39
Q

What is used to describe the attitude of folds?

A

The plunge of the fold axis and the dip of the axial plane

40
Q

What three quantitative measurements can be used to define fold shape?

A

Fold aspect ratio, fold bluntness, fold tightness

41
Q

How do you work out fold aspect ratio?

A

Amplitude / half wavelength

42
Q

What is fold bluntness?

A

A measure of how rounded or angular the hinge is (e.g. chevron, rounded, concentric)

43
Q

What is fold tightness?

A

Defined by the interlimb angle (minimum angle between limbs as measured in the profile plane)

44
Q

What is a parallel fold?

A

One that has constant orthogonal layer thickness around the fold

45
Q

What is a similar fold?

A

One that has constnat layer thickness parallel to the axial trace

46
Q

What is the dip isogon?

A

The line connecting points of equal dip on either side of a folded layer, classes of fold range from strongly convergent (1A) to divergent (3)

47
Q

How can fold classes be shown graphically?

A

Orthogonal thickness / orthogonal thickness in the hinge vs. angle

48
Q

What are primary and secondary structures in folds?

A

Primary structures provide an indication of orientation at the time of formation whilst secondary structures are produced by deformation

49
Q

Are minor folds primary or secondary structures?

A

Secondary

50
Q

How can minor folds help locate the fold hinge?

A

Their vergence changes across the fold hinge

51
Q

Are planar fabrics/lineations primary or secondary structures?

A

Secondary

52
Q

What are three examples of how lineations are formed?

A

Passive rotation of elongate crystals, mineral shape change by plasticity, oriented nucleation and growth of new grains

53
Q

What is the cleavage of a rock?

A

The fabric formed in fine-grained rocks that can be seen with the naked eye, applies at low metamorphic grade

54
Q

What is schistosity?

A

Coarser-grained foliations formed at higher metamorphic grades than cleavage

55
Q

What is the relationship btween cleavage/schistosity and the axial surface?

A

Cleavage/schistosity formes parallel/sub-paralle ot the axial surface of a fold, this is described as axial planar cleavage

56
Q

What happens when the cleavage is not parallel with the axial plane?

A

It fans about the fold in a convergent or divergent way

57
Q

What is cleavage refraction?

A

The changing of the dip of the cleavage when it passes between lithologies, caused by the competence (resistance to flow) of the layers

58
Q

What does cleavage in more competent lithologies look like?

A

Typically at a higher angle to the bedding

59
Q

What are crenulation lineations?

A

The hinges of the small folds formed by the deformation of cleavage, their axial planes are crenulation cleavage

60
Q

What are S0, S1, and S2?

A

S1 is the original bedding surface, S1 is the first cleavage, and S2 is the second cleavage (crenulation)

61
Q

What are four examples of lineations formed during deformation and often lie within the cleavage plane?

A

Preferred orientation, small elongate concentrations, elongate mineral overgrowths, well-alighned elongate shapes

62
Q

What are stretching lineations?

A

Mineral elongations that form parallel with the maximum extension direction of the structure on which they occur

63
Q

What is pitch-and-swell?

A

Gentle oscillations of the thickness of the bed

64
Q

What is boudinage?

A

When the pitch-and-swell is periodically separated

65
Q

What does the shape of the boudins depend on?

A

The contrast in the competence with the adjacent layers

66
Q

What are the three types of boudin shapes?

A

Block, barrel, fish-mouth

67
Q

What is between separate boudins?

A

Secondary mineral deposits

68
Q

When are boudins asymmetric?

A

When formed in the presence of a layer parallel shear, their rotation may be with the sense of shear

69
Q

What are mullions?

A

Convex surfaces with intervening cusps that form at the interface of layers of very different competence

70
Q

What do the cusps point to?

A

The more competent unit

EART28202 Structural Geology (10 decks)

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