Week 6

Question 1

Synform

Answer 1

U

Question 2

Antiform

Answer 2

n

Question 3

Antiformal anticline

Answer 3

Youngest top
Oldest bottom

Eroded = oldest in middle

Question 4

Synformal syncline

Answer 4

Youngest top
Oldest bottom

Eroded = oldest middle

Question 5

Synformal anticline

Answer 5

Oldest top
Youngest bottom

Eroded = youngest middle

Question 6

Antiformal syncline

Answer 6

Oldest top
Youngest bottom

Eroded = youngest middle

Question 7

Anticline =

Answer 7

Y points away from bed

Question 8

Syncline =

Answer 8

Y points towards bed

Question 9

Fold attitude

Answer 9

1. Dip of axial plane

2. Plunge of fold hinge line

Question 10

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

Answer 10

Upright (symmetric)

Inclined (asymmetric/overturned)

Horizontal (recumbent)

Question 11

Interlimb angles (with increasing strain)

Answer 11

GENTLE
180-120

OPEN
120-70

TIGHT
70-30

ISOCLINAL
30-0

N.B. 0’ = // limbs

Question 12

Folds and cleavage

Answer 12

Aligned minerals = cleavage planes

// to axial plane

Question 13

BCIL

Answer 13

Bedding Cleavage Intersection Line

// to fold hinge

= use to determine fold plunge

Question 14

Cleavage vergence =

Answer 14

Direction of travel to nearest antiform

Bedding line
Cleavage line

Direction from cleavage to bedding line

Question 15

Is extension in folds +ve or -ve?

Answer 15

-ve extension

= shortening structures

Question 16

Relating the FSE to folds

Answer 16

XY plane = axial plane

Y axis = hinge line

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

Question 17

Boundary conditions for folds and FSE

Answer 17

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

Question 18

Configurations of external forces for folding

Answer 18

1. LAYER // FOLDING
2. BENDING
3. SHEAR (FLOW) FOLDING

Question 19

1. Layer // folding

Answer 19

Force applied // to bedding

= buckling of single layers

a) homogeneous shortening
b) folding

Question 20

1. Factors affecting layer // folding

Answer 20

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

Question 21

2. Bending

Answer 21

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

Question 22

3. Shear (flow) folding

Answer 22

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

Question 23

When does passive folding occur?

Answer 23

If strength of layers doesn’t influence fold development

Question 24

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

Answer 24

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

Question 25

What is an ideal folding scenario?

Answer 25

Constant layer thickness "Parallel folds" "Class 1B"

Question 26

Buckle folds

Answer 26

Tangential longitudinal strain Outer arc extension Inner arc compression - with neutral surface (no strain) between Most deformation at fold hinge

Question 27

Flexural slip folding in multilayers

Answer 27

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

Question 28

Flexural flow folding

Answer 28

High T/"sandwiched" incompetent layer Layer // simple shear Decreases towards hinges

Question 29

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

Answer 29

Hinge becomes thicker than limbs Tends to produce "similar folds" or "class 2" folds

Question 30

Parasitic folds

Answer 30

``` 2' Systematic asymmetry (mergence) around 1' folds ``` Z M S W Less competent = deforms 1st

Question 31

Types of cleavage refraction (fanning)

Answer 31

1. CONVERGENT | 2. DIVERGENT

Question 32

Convergent cleavage refraction

Answer 32

Coarse grained Competent = buckles (more rotated at hinge)

Question 33

Divergent cleavage refraction

Answer 33

Fine grained Incompetent = flows (more rotated further from hinge)

Question 34

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

Answer 34

STEEPER

Question 35

Point maximum =

Answer 35

concentration of points

Question 36

Girdle distribution =

Answer 36

line of best fine = fold profile plane

Question 37

Beta axis =

Answer 37

Pole to girdle distribution = pole to fold profile plane = MEAN FOLD HINGE

Question 38

Axial planar cleavage =

Answer 38

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

Question 39

Can shear folds be used to estimate shortening?

Answer 39

NO

Question 40

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

Answer 40

1. Folding (buckling and cleavage refraction) | 2. Homogeneous shortening (cleavages become // again)

Question 41

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

Answer 41

Viscosity contrast