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Structural > Week 6 > Flashcards

Week 6 Flashcards

1
Q

Synform

A

U

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2
Q

Antiform

A

n

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3
Q

Antiformal anticline

A

Youngest top
Oldest bottom

Eroded = oldest in middle

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4
Q

Synformal syncline

A

Youngest top
Oldest bottom

Eroded = oldest middle

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5
Q

Synformal anticline

A

Oldest top
Youngest bottom

Eroded = youngest middle

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6
Q

Antiformal syncline

A

Oldest top
Youngest bottom

Eroded = youngest middle

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7
Q

Anticline =

A

Y points away from bed

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8
Q

Syncline =

A

Y points towards bed

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9
Q

Fold attitude

A
  1. Dip of axial plane

2. Plunge of fold hinge line

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10
Q

Different dips of axial planes (and resulting form of fold)

A

Upright (symmetric)

Inclined (asymmetric/overturned)

Horizontal (recumbent)

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11
Q

Interlimb angles (with increasing strain)

A

GENTLE
180-120

OPEN
120-70

TIGHT
70-30

ISOCLINAL
30-0

N.B. 0’ = // limbs

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12
Q

Folds and cleavage

A

Aligned minerals = cleavage planes

// to axial plane

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13
Q

BCIL

A

Bedding Cleavage Intersection Line

// to fold hinge

= use to determine fold plunge

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14
Q

Cleavage vergence =

A

Direction of travel to nearest antiform

Bedding line
Cleavage line

Direction from cleavage to bedding line

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15
Q

Is extension in folds +ve or -ve?

A

-ve extension

= shortening structures

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16
Q

Relating the FSE to folds

A

XY plane = axial plane

Y axis = hinge line

  • valid in certain circumstances
  • need boundary conditions/mechanism
  • strain history
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17
Q

Boundary conditions for folds and FSE

A
  1. 3 configurations external forces can act on layers for folding
  2. Not mutually exclusive
  3. Active vs passive folding
18
Q

Configurations of external forces for folding

A
  1. LAYER // FOLDING
  2. BENDING
  3. SHEAR (FLOW) FOLDING
19
Q
  1. Layer // folding
A

Force applied // to bedding

= buckling of single layers

a) homogeneous shortening
b) folding

20
Q
  1. Factors affecting layer // folding
A

COMPETENCE
~=viscosity miu relative to matrix

a) miulayer ~=miu matrix
b) miu layer&raquo_space; miu matrix

THICKNESS
Thicker = greater wavelength and amplitude
Same competence but far apart = fold independently of each other

21
Q
  1. Bending
A

Force applied across bedding

= boudins

= fault bend folds
- can be used to estimate shortening assuming bed length constant during deformation

= forced folds

= forceful intrusion folds

22
Q
  1. Shear (flow) folding
A

Force applied oblique to bedding

= shear zones

Progressive shear on initial irregularity

N.B. if homogeneous = just foliation (need heterogeneity)

CAN’T USE TO ESTIMATE SHORTENING

23
Q

When does passive folding occur?

A

If strength of layers doesn’t influence fold development

24
Q

How to determine the viscosities of different folded beds (practical exercise)

A
  1. Layer // shortening i.e. when strain accommodated by thickening before folding
    - stretch out
    - difference in length
  2. Homogeneous shortening
    - longest length(x) - length after folding = e
    - minimum estimates
25
Q

What is an ideal folding scenario?

A

Constant layer thickness

“Parallel folds”

“Class 1B”

26
Q

Buckle folds

A

Tangential longitudinal strain

Outer arc extension
Inner arc compression
- with neutral surface (no strain) between

Most deformation at fold hinge

27
Q

Flexural slip folding in multilayers

A

Think: phone book

Max slip on fold limbs, 0 at hinge
No difference between inner/outer portion of fold

Common in thin beds separated by weak layers e.g. shale

Slickenlines preserved on bedding planes in fold limbs

28
Q

Flexural flow folding

A

High T/”sandwiched” incompetent layer

Layer // simple shear

Decreases towards hinges

29
Q

What does it mean when homogeneous shortening is superimposed on a pre-existing fold?

A

Hinge becomes thicker than limbs

Tends to produce “similar folds” or “class 2” folds

30
Q

Parasitic folds

A 
2'
Systematic asymmetry (mergence) around 1' folds

Z M S W

Less competent = deforms 1st

31
Q

Types of cleavage refraction (fanning)

A
  1. CONVERGENT

2. DIVERGENT

32
Q

Convergent cleavage refraction

A

Coarse grained
Competent

= buckles
(more rotated at hinge)

33
Q

Divergent cleavage refraction

A

Fine grained
Incompetent

= flows
(more rotated further from hinge)

34
Q

In a pole to plane, if it plots closer to the edge…

A

STEEPER

35
Q

Point maximum =

A

concentration of points

36
Q

Girdle distribution =

A

line of best fine

= fold profile plane

37
Q

Beta axis =

A

Pole to girdle distribution

= pole to fold profile plane
= MEAN FOLD HINGE

38
Q

Axial planar cleavage =

A

aligned minerals produce cleavage planes oriented // to fold axial planes

39
Q

Can shear folds be used to estimate shortening?

A

NO

40
Q

To come full circle, under what conditions is an axial planar cleavage likely to develop in a multilayer sequence?

A
  1. Folding (buckling and cleavage refraction)

2. Homogeneous shortening (cleavages become // again)

41
Q

What controls the extent to which homogeneous occurs prior/after folding?

A

Viscosity contrast

Structural (17 decks)

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