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Geology > Geological Maps and Structures L4 > Flashcards

Geological Maps and Structures L4 Flashcards

(73 cards)

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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
How is a fold with a low axial surface dip described?
Upright
25
What is a symmetric fold?
A fold in which the limbs are bisected (equally divided) by the axial plane/surface and as a result the limbs are equal length
26
What is an asymmetric fold?
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
27
What is vergence?
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
28
What are the 3 ways in which vergence is described?
In terms of cardinal direction of shear (i.e. N, E, S, W) Clockwise or anticlockwise Z or S folds
29
How would an asymmetric fold in which the upper limb appears to have moved East relative to the originally upright fold be described?
East-verging
30
When using "s" and "z" to describe fold symmetry, what does the short limb of the fold represent?
The middle of the letter (z or s) i.e. if the asymmetric fold looks like the letter "z" or "s"
31
When using "clockwise/anticlockwise" or "z/s" to describe fold asymmetry, what must you bear in mind?
The direction you are facing when the observation is made
32
What are 4 characteristics of a fold that can be described?
Tightness Roundness Attitude Symmetry
33
What do folding mechanisms describe?
The way in which forces act on layered rocks to cause folding
34
What are the 4 mechanisms of folding?
Buckling Bending Simple shear Homogenous strain
35
What is buckling?
Buckling occurs when a layer is shortened parallel to the plane of layering caused by layer-parallel compressional forces
36
What does buckling require to occur and how does it work?
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
When do large wavelengths form during buckling?
Large wavelengths are formed when the competent layer (the layer that is folded) is highly viscous compared with the surrounding matrix
38
What is bending?
Folding caused by heterogenous forces acting transverse to the layering (pressure from above/ below the layering/at a high angle to layered rocks)
39
What is simple shear?
Folding that occurs where the forces act at intermediate angles to layering
40
What is homogenous strain?
Forces that can amplify folds, but not create them
41
What are the 3 types of flexural folding?
Flexural slip Flexural flow Tangential-longitudinal strain
42
What do the 3 types of flexural folding describe?
How folded layers respond to both active and passive folding
43
What is flexural slip?
Folded layers slip along layer interfaces to accommodate folding, like bending a stack of cards.
44
How does shear act on a fold in flexural slip?
The sense of shear is opposite on the 2 fold limbs but there is zero shear right at the fold hinge.
45
Where and in what conditions is flexural slip more common?
Cooler temperatures in sedimentary layers of the upper crust
46
What is flexural flow?
Fold limbs deform purely by shearing without any slip between layers
47
In what conditions is flexural flow more common?
Higher temperatures and under more ductile conditions
48
What is tangential-longitudinal strain?
Fold formed by tension in the outer arc and compression in the inner arc
49
Explain the mechanism of tangential-longitudinal strain in terms of tension and compression and their effects on the fold.
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
What type of folds does tangential-longitudinal strain produce and why?
Open folds - as the fold gets tighter, flexural slip and flexural flow take over
51
What is a parasitic fold?
Folds that are characterised by a small wavelength and are situated within folds with a larger wavelength.
52
How are parasitic folds formed?
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
In which direction does the parasitic fold verge?
Parasitic folds verge towards the hinge of the larger fold
54
Are parasitic folds symmetric or asymmetric?
Asymmetric
55
Why are the parasitic folds at the hinge of the large scale fold symmetric?
Shear is zero at the fold hinge
56
Where do parasitic symmetrical M-folds occur in relation to the larger fold?
At the hinge where shear is zero
57
Where do asymmetric Z-folds and S-folds occur?
Either side of the hinge.
58
What is cleavage in terms of folding?
A type of penetrative planar deformation fabric, or foliation, along with rocks split or "cleave"
59
What does cleavage form as a result of?
Stress/strain
60
In what conditions does cleavage form?
Low-grade metamorphic conditions
61
What does a rock need to contain in order to develop cleavage?
Sheet silicates
62
What is compaction cleavage?
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
What is pencil cleavage?
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
What is slaty cleavage?
Continued compression parallel to bedding will form form a closely-spaced slaty cleavage oblique to the bedding
65
What is crenulation cleavage?
Compression oblique to an existing slaty cleavage causing the folding of cleavage.
66
Why is cleavage useful to look at in the field?
There is a geometric relationship between folding and cleavage formed during the same deformation event
67
In a fold, what does cleavage commonly form parallel to and hence what is the name given to this cleavage?
Cleavage commonly forms parallel to the axial plane - this is known as axial planar cleavage
68
What are refolded/superposed folds ?
Folds modified by a later folding event
69
What is the name given to type 1 interference (refolded folds)?
Egg box interference
70
What is the name given to type 2 interference (refolded folds)?
Winged interference
71
What is the name given to type 3 interference (refolded folds)?
Hook interference
72
Why might refolded folds be useful in the field?
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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