Week 6 Flashcards
Synform
U
Antiform
n
Antiformal anticline
Youngest top
Oldest bottom
Eroded = oldest in middle
Synformal syncline
Youngest top
Oldest bottom
Eroded = oldest middle
Synformal anticline
Oldest top
Youngest bottom
Eroded = youngest middle
Antiformal syncline
Oldest top
Youngest bottom
Eroded = youngest middle
Anticline =
Y points away from bed
Syncline =
Y points towards bed
Fold attitude
- Dip of axial plane
2. Plunge of fold hinge line
Different dips of axial planes (and resulting form of fold)
Upright (symmetric)
Inclined (asymmetric/overturned)
Horizontal (recumbent)
Interlimb angles (with increasing strain)
GENTLE
180-120
OPEN
120-70
TIGHT
70-30
ISOCLINAL
30-0
N.B. 0’ = // limbs
Folds and cleavage
Aligned minerals = cleavage planes
// to axial plane
BCIL
Bedding Cleavage Intersection Line
// to fold hinge
= use to determine fold plunge
Cleavage vergence =
Direction of travel to nearest antiform
Bedding line
Cleavage line
Direction from cleavage to bedding line
Is extension in folds +ve or -ve?
-ve extension
= shortening structures
Relating the FSE to folds
XY plane = axial plane
Y axis = hinge line
- valid in certain circumstances
- need boundary conditions/mechanism
- strain history
Boundary conditions for folds and FSE
- 3 configurations external forces can act on layers for folding
- Not mutually exclusive
- Active vs passive folding
Configurations of external forces for folding
- LAYER // FOLDING
- BENDING
- SHEAR (FLOW) FOLDING
- Layer // folding
Force applied // to bedding
= buckling of single layers
a) homogeneous shortening
b) folding
- Factors affecting layer // folding
COMPETENCE
~=viscosity miu relative to matrix
a) miulayer ~=miu matrix
b) miu layer»_space; miu matrix
THICKNESS
Thicker = greater wavelength and amplitude
Same competence but far apart = fold independently of each other
- Bending
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
- Shear (flow) folding
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
When does passive folding occur?
If strength of layers doesn’t influence fold development
How to determine the viscosities of different folded beds (practical exercise)
- Layer // shortening i.e. when strain accommodated by thickening before folding
- stretch out
- difference in length - Homogeneous shortening
- longest length(x) - length after folding = e
- minimum estimates
What is an ideal folding scenario?
Constant layer thickness
“Parallel folds”
“Class 1B”
Buckle folds
Tangential longitudinal strain
Outer arc extension
Inner arc compression
- with neutral surface (no strain) between
Most deformation at fold hinge
Flexural slip folding in multilayers
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
Flexural flow folding
High T/”sandwiched” incompetent layer
Layer // simple shear
Decreases towards hinges
What does it mean when homogeneous shortening is superimposed on a pre-existing fold?
Hinge becomes thicker than limbs
Tends to produce “similar folds” or “class 2” folds
Parasitic folds
2' Systematic asymmetry (mergence) around 1' folds
Z M S W
Less competent = deforms 1st
Types of cleavage refraction (fanning)
- CONVERGENT
2. DIVERGENT
Convergent cleavage refraction
Coarse grained
Competent
= buckles
(more rotated at hinge)
Divergent cleavage refraction
Fine grained
Incompetent
= flows
(more rotated further from hinge)
In a pole to plane, if it plots closer to the edge…
STEEPER
Point maximum =
concentration of points
Girdle distribution =
line of best fine
= fold profile plane
Beta axis =
Pole to girdle distribution
= pole to fold profile plane
= MEAN FOLD HINGE
Axial planar cleavage =
aligned minerals produce cleavage planes oriented // to fold axial planes
Can shear folds be used to estimate shortening?
NO
To come full circle, under what conditions is an axial planar cleavage likely to develop in a multilayer sequence?
- Folding (buckling and cleavage refraction)
2. Homogeneous shortening (cleavages become // again)
What controls the extent to which homogeneous occurs prior/after folding?
Viscosity contrast
