Class 1: Geometry of Folds

Question 1

What is the distinction between the terms ‘axial plane’ and ‘axial trace’?

Answer 1

Axial plane: contains the hinge lines of a nested stack of folded surfaces

Axial trace: the line of intersection between the axial surface and another surface of interest

Question 2

How to describe the scale of folds

Answer 2

Amplitude, wavelength, arc length, layer thickness

Question 3

Define antiform, anticline, synform and syncline

Answer 3

Synform: bowl or U

Antiform: Hill or n

Syncline: Youngest rocks in the core

Anticline: Oldest rocks in the core

Question 4

What is a recumbent fold?

Answer 4

Axial pane is horizontal

Question 5

What is an isoclinal fold?

Answer 5

the axial surface and limbs slope in the same direction and at approximately the same angle

interlimb angle ≤30

Question 6

What is a chevron folds?

Answer 6

fold with very angular hinges

Question 7

What is a parallel fold?

Answer 7

have constant orthogonal layer thickness around the fold

Question 8

What is a similar fold?

Answer 8

have a constant layer thickness parallel to the axial trace

Question 9

What is a congruent minor structure?

Answer 9

Structures that form at the same time as a large scale structure, and have a geometry that is systematically related to it

Form around fold and faults

Question 10

What is axial planar cleavage?

Answer 10

Cleavage/schistosity that forms parallel or sub-parallel to the axial surfaces of the folds

Can determine orientation of the axial surface

bedding/cleavage intersection determines fold axis orientation

Question 11

How does the asymmetry of minor folds relate to the direction of vergence (direction to the major antiform)?

Answer 11

Question 12

What is crenulation cleavage

Answer 12

the cleavage or schistosity is refolded on a sub-millimetre to centimetre scale

Question 13

S,L,D,F notation

Answer 13

•Note the notation: S₀ is the original surface (bedding), S₁ is the first cleavage, S₂ is the second cleavage, and so on. We can also use D₁, D₂, . . . to refer to deformation phase 1, 2, etc., F₁, F₂, . . . to refer to folds formed in the 1^st phase of deformation, 2^nd phase of deformation, etc., and L₁, L₂, . . . to refer to lineations formed in the 1^st phase of deformation, 2^nd phase of deformation, etc.

Question 14

What is a stretching lineation?

Answer 14

mineral elongation lineations formed parallel to the maximum extension direction

Question 15

What is pinch-and-swell? and what is the distinction between it and boudinage?

Answer 15

gentle oscillations of the thickness of the bed

boudinage is when the layer periodically separates