Fold classification

Question 1

Four common folds classification

Answer 1

1. Sense of curvature
2. Symmetry and vergence
3. Dip of axial surface and plunge of hinge line
4. Interlimb angle

Question 2

limbs dip down and away from the hinge zone

convex Upward

Answer 2

Antiform

Question 3

hínge Zone is at the botom (outlining a valley):

convex downward.

Answer 3

Synform

Question 4

large (megascopic, regional-scale) of antifom and synforms

Answer 4

Anticlinorium and synclinorium

Question 5

The rocks in the core of the fold are the ______

Answer 5

youngest

Question 6

rock layers get
younger away from
the core BUT has the
shape of a synform.

Answer 6

Synformal anticline

Question 7

rock layers get
younger towards the
core BUT has the
shape of an antiform.

Answer 7

Antiformal anticline

Question 8

the enveloping surface and axial Surface are perpendicular to each other.

Answer 8

Symmetric folds

Question 9

the enveloping
surface and axial
surface are NOT
perpendicular to
each other.

Answer 9

Asymmetric folds

Question 10

Large folds commonly have
smaller-scale folds in their
limbs and crest. They are
USUally referred to as higher
order (2nd, 3rd order etc.)

Answer 10

parasitic folds.

Question 11

They form during flexure of
layered rock where slip
OCCUrs between rock layers

Answer 11

flexural slip

Question 12

The difference
between S- and Z-folds
lies in their sense of
rotation or _______

Answer 12

vergence

Question 13

Vergence of parasitic
folds is _____

Answer 13

towards the
hinge line

Question 14

Clockwise

Answer 14

Z-folds

Question 15

Anticlockwise

Answer 15

S-folds

Question 16

fold structures with only one tilted limb; the beds on either side of the tilted limb are horizontal.

Answer 16

Monoclinal fold/ monoclines

Question 17

fold structures wherein the layers dip uniformly in a single direction having the same general inclination.

Answer 17

Homoclinal fold/ homoclines

Question 18

folds can be describes by:

Answer 18

1. Dip of axial surface
2. Plunge of hinge line

Question 19

Dip of axial surface: Upright

Answer 19

70°-90°

Question 20

Dip of axial surface: Inclined

Answer 20

10°-70°

Question 21

Dip of axial surface: Recumbent

Answer 21

0°-10°

Question 22

angle enclosed by its
two limbs.

Answer 22

Interlimb angle

Question 23

Fold geometry can be classified by means of

Answer 23

dip isogons.

Question 24

5 Folding Mechanisms

Answer 24

Active folding or buckling
passive folding
bending
flexural folding
kinking and chevron folding

Question 25

Based on dip isogons, folds coan be classified into the three main types.

Answer 25

Class 1: Dip isogons converge toward the inner arc, which is tighter than the outer arc. Class 2: Similar folds, also called shear folds. Class 3: Dio isogons diverge toward the inner arc, which is more open than the outer arc.

Question 26

Class 1

Answer 26

1A 1B 1C

Question 27

Class 2

Answer 27

thinned limbs and thickened hinges.

Question 28

Class 3

Answer 28

even thinner limbs and thickened hinges.

Question 29

How do folds form?

Answer 29

Considerations: 1. The way stress act ona layered rock. 2. How layer(s) react to stress. 3. we will know if rocks fold Actively or passively.

Question 30

Fold mechanisms that can initiate when a layer is shortened parallel to the layering.

Answer 30

Active folding or Buckling

Question 31

_______ is typical for rocks where passive flow occurs, i.e. where the layering exerts no mechanical influence on the folding.

Answer 31

Passive folding

Question 32

Passive folds are frequently found in

Answer 32

shear zone

Question 33

this occurs when forces act **across layers** at a high angle (unlike buckling where the main force acts parallel to a layer.)

Answer 33

Bending

Question 34

________ maintains bed thickness.

Answer 34

Flexural folding

Question 35

In cases where strain is more evenly distributed in thelimbs in the form of shear strain, flexural slip turns into the closely related mechanism called

Answer 35

flexural shear or flexural flow.

Question 36

Kink folds & Chevron folds

Answer 36

straight limbs and sharp hinges. formed by displacements between individual laminae. sharp & angular folds.

Question 37

well-rounded hinges.

Answer 37

Concentric folds

Question 38

box-shaped; wide flat top and steep flanks. formed when a layer is detached from the underlying and overlying layers.

Answer 38

Box folds