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Competence
1
0
2
3
4
0=1

Cylindrical folds have straight hinge lines (straight B axis).
BA
C
Hinge
Limb
Fold axes
B // HingeA hingeand // axial surfaceC axial surface
T
T diagram
In cylindrical fold poles areoriented at 90 of the B axis
circle
B axis
Va21 FOLDSMorphology
CHAPTER Va : CONTINUOUS HETEROGENEOUS DEFORMATION
Va2 STRUCTURES The hinge line joins the points of maximum curvature on a folded surface.The axial surface contains the hinge lines of many folded surfaces. This surface is not necessarily planar
On a stereonet, the distribution of poles gives information about the geometry of folds.

diagram
B axis
facet
It is not possible to determine the attitude of the axial surface from or diagram alone. For this, we need to plot the axial trace (trace of the axial surface on the ground surface).The B axis and the axial trace are two lines that belong the axial surface.
B axis
Axial trace
Ground surface
B axis
Axial trace
Axial surface
This construction assumes that the B axis and the axial trace are not parallel to each other.
Construction of the axial surface
Va21 FOLDSMorphology
CHAPTER Va : CONTINUOUS HETEROGENEOUS DEFORMATION
Va2 STRUCTURES

Isopach
hinge
Inclined fold Recumbent fold
Vertical foldPlunging fold
SimilarConcentric
younging
Open Tight
Isoclinal
Reclined fold
Axial surface
Overturned fold
Crest
hinge
Trough
horizontal
t
tt
t
t t
t
e
ee
e
Kink foldRootlessPtygmatic fold
Va21 FOLDSClassification
CHAPTER Va : CONTINUOUS HETEROGENEOUS DEFORMATION
Va2 STRUCTURES

So
S1
Cleavage fan
Crenulation cleavage: The development of fine scale microfolding can produce systematic realignment of preexisting layering.
S1So 1 cm
1 m
S1: A xial plane cleavage (12 plane)
S1So
Fracture cleavage
10 cm
Intersection lineation
Fold rodding lineationor crenulation lineation
Parasitic fold
Cleavagerefraction
Quartzite
Phyllite
Va21 FOLDSAssociated linear and planar microstructures
CHAPTER Va : CONTINUOUS HETEROGENEOUS DEFORMATION
Va2 STRUCTURES

Outer arc lengthens
Inner arc shortens
Neutral surface
Extrado fracturesIntrado stylolites
Flexural shear foldingOrthogonal flexure
hinge
shear planes
Volume loss flexure
Va21 FOLDSKinematic models of folding
CHAPTER Va : CONTINUOUS HETEROGENEOUS DEFORMATION
Va2 STRUCTURES
Neutral surface
The geometry of folds largely depends on the way they are formed. There are a limited number of kinematic models...
Flexural folding produces isopach folds. This mode of folding can be achieved through three mechanisms: Orthogonal Flexure, Shear Flexure or Volumeloss Flexure.
Dissolution
Shear // to limbs
Neutral surface
Shear planes
Axial surface
Passive shear folding produces similar folds. This mode of folding is achieved through heterogeneous simple shear. Folds develop with their axial surfaces parallel to the shearing planes.
Shearzone
Shearzone
Symmetric fold
Asymmetric fold

Va21 FOLDSKinematic models of folding
CHAPTER Va : CONTINUOUS HETEROGENEOUS DEFORMATION
Va2 STRUCTURES
Faultbend fold Faultpropagation fold
Fault tip
Fault ramp
Anticlinal stacks
Rollover anticline
Kink band
= kink angle
Axial surface
Kink band
Formation of kink and chevron folds.Folds with straight limbs and sharp hinge are chevron folds if they area symmetric and kink folds if they are asymmetric. They develop in strongly layered or laminated sequences that have a strong planar mechanical anisotropy.
Development of chevron folds by kinking.
Development of kink folds.
Geometry of a kink band and terminology.
Folds may develop in close association with and as the result of faulting. The first example (sketches on the left) illustrates the development of a fautbend fold in association with a fault ramp. The second example (sketches on the right) illustrates the development of a faultpropagation fold above the tip of a propagating thrust.
Finally, folds also develop has a consequences of extensional tectonics. The sketch on the rigth illustrate a rollover anticline in association with an extensional detachment fault.
Detachment

Va21 FOLDSKinematic models of folding
CHAPTER Va : CONTINUOUS HETEROGENEOUS DEFORMATION
Va2 STRUCTURES
Progressive flatenning

Va21 FOLDSKinematic models of folding
CHAPTER Va : CONTINUOUS HETEROGENEOUS DEFORMATION
Va2 STRUCTURES
Drag Folds. When rocks are subjected to shear, layers in the rock commonly form asymmetric folds whose sense of asymmetry reflects the sense of shear. Such folds are called drag folds and are the result of velocity gradient in the shear zone. Drag folds are noncylindrical and asymmetric. Their axial planar surface tends to be parallel to the shearing plane.
Sheath folds are a particular class of drag fold. They are tubeshaped fold with an elliptic or even a circular section. They develop with their a axis parallel to the direction of shearing.

Z fold S fold
Fold asymmetry, beddingcleavage relationships, stratigraphy up direction,and vergence.
A xial plane cleavage (12 plane)
Bedding
Stratigraphy up
Z fold S fold
100 m
M fold
Scale independent microtectonic laws
Va21 FOLDSFold systems
CHAPTER Va : CONTINUOUS HETEROGENEOUS DEFORMATION
Va2 STRUCTURES
Vergence is a term used to indicate the direction of movement and rotation that occured during deformation.

200 m1
1
3
2
23
Stratigraphy up direction
fold asymmetry
beddingcleavage
4
Vergence of displacement4
The determination of two of these criteria leadto the determination of the two others.
Scale independent microtectonic laws
Va21 FOLDSFold systems
CHAPTER Va : CONTINUOUS HETEROGENEOUS DEFORMATION
Va2 STRUCTURES

So
S1
Vergence of the fold ?
Scale independent microtectonic laws
Va21 FOLDSFold systems
CHAPTER Va : CONTINUOUS HETEROGENEOUS DEFORMATION
Va2 STRUCTURES

Vergence of the fold ?
Scale independent microtectonic laws
Va21 FOLDSFold systems
CHAPTER Va : CONTINUOUS HETEROGENEOUS DEFORMATION
Va2 STRUCTURES

Boudin lines
Boudin
Neck lines
Neck
Pinchandswell structures
Crystallization in pressure shadowNeck foldSymmetric boudinage
Asymmetric boudinage, asymmetric pressure shadows
Va22 BOUDIN AND BOUDINAGE
CHAPTER Va : CONTINUOUS HETEROGENEOUS DEFORMATION
Va2 STRUCTURES

Shearzone
Orientation and magnitude of finites strain ellipses and trajectories of S1 across a ductile shear zone resulting from inhomogeneous progressive simple shear.
Inhomogeneous progressive simple shear
= tg
SIMPLE SHEAR
0
S1 tra
jecto
ries
Va23 DUCTILE SHEAR ZONES
CHAPTER Va : CONTINUOUS HETEROGENEOUS DEFORMATION
Va2 STRUCTURES

Shearzone
Inhomogeneous progressive pure shear
Orientation and magnitude of finites strain ellipses and trajectories of S1 across a ductile shear zone resulting from inhomogeneous progressive pure shear.
= 1/
PURE SHEAR
Va23 DUCTILE SHEAR ZONES
CHAPTER Va : CONTINUOUS HETEROGENEOUS DEFORMATION
Va2 STRUCTURES

Mylonitic zonea, L
c, N
b, M
Shearplane
Movementplane
Sheardirection
(a, L), (b, M), (c, N) : kinematic axes
a, L
b, M
X, 1
Z, 3
Y, 2
Y
c, N Z
YX
protolith
Va23 DUCTILE SHEAR ZONES: The kinematic reference frame
CHAPTER Va : CONTINUOUS HETEROGENEOUS DEFORMATION
Va2 STRUCTURES

3
1
A B
C
D
A'B'
C'
D'
A' B' C' D' 1 1 1 1
33
Surfaces of nondeformation
1
3 2D 3D
CHAPTER Va : CONTINUOUS HETEROGENEOUS DEFORMATION
Va3 ORIENTATION OF THE AXES OF THE FINITE STRAIN ELLIPSOIDVa31 FOLDS AND BOUNDINS

1
2
2 2
1
3
FlatteningConstriction
FlatteningConstriction
1
3
2
3
3
1
2Plane strain
Plane strain
CHAPTER Va : CONTINUOUS HETEROGENEOUS DEFORMATION
Va3 ORIENTATION OF THE AXES OF THE FINITE STRAIN ELLIPSOIDVa31 FOLDS AND BOUDINS
3
3
1
2
1
2
1
33
1
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1
2
1 3
3
3
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1
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1
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3

CHAPTER Va : CONTINUOUS HETEROGENEOUS DEFORMATION
Va3 ORIENTATION OF THE AXES OF THE FINITE STRAIN ELLIPSOIDVa31 FOLDS AND BOUDINS
Development of cleavage during progressive flatenning
1
1
2
2
3
3
S1//12
Axial plane cleavage is parallel to the flattening plane (12) of the F.S.E.

Usually, shear zones wrap around less deformed domains. The geometry of the shear zones net changes with the characteristics of the regional finite strain ellipsoid.
Lineation 1 = Gliding line
Characteristic structure of reactivated basement
Contriction: L // 1 N close to 3 M close to 2
Flattening : N close to 3 ML close to 12
Plane strain: M // 2 N close to 3 L close to 1
1
32
1
32
1
23
CHAPTER Va : CONTINUOUS HETEROGENEOUS DEFORMATION
Va3 ORIENTATION OF THE AXES OF THE FINITE STRAIN ELLIPSOIDVa32 DUCTILE SHEAR ZONES

Two directions of stretching => pancacke shape ellipsoid
Two directions of shortening => cigar shape ellipsoidOne invariant direction (direction of nondeformation)=> plane strain ellipsoid
2
0
1
K=1
Ln (2/3)
Ln (
1/
2)
0 1 2
K = 8
K = 0
2
1
3
2
1
3
2
1
3
3
2
1
1
3
3
3
1
2
1
32
CHAPTER Va : CONTINUOUS HETEROGENEOUS DEFORMATION
Va4 CHARACTERISATION OF THE FINITE STRAIN ELLIPSOID (K)Va41 FOLDS AND BOUDINS

Constriction Plane strain Flattening
CHAPTER Va : CONTINUOUS HETEROGENEOUS DEFORMATION
Va4 CHARACTERISATION OF THE FINITE STRAIN ELLIPSOID (K)
Va42 PRESSURE SHADOWS

Two directions of stretching => pancacke like ellipsoid
Two directions of shortening => cigar like ellipsoidOne invariant direction (direction of nondeformation)=> plane strain ellipsoid
2
0
1Ln (X/Y)
Ln (Y/Z)
K=1
Ln (2/3)
Ln (1/2)
0 1 2
K = 8
K = 0
Uniaxial oblate
Uniaxial prolate
1
2
3
1
2
3
1
2
3
1
CHAPTER Va : CONTINUOUS HETEROGENEOUS DEFORMATION
Va4 CHARACTERISATION OF THE FINITE STRAIN ELLIPSOID (K)
Va43 DUCTILE SHEAR ZONES

3
1
3
1
3
1
3
1
Incrementalstrain ellipse
Finite strain ellipse
Nondeformation line
Line of nondeformation during progressive pure shear
The shortened domain increasesduring progressive pure shear.
Material lines rotate more rapidlythan the nondeformation lines.
Initial state
Incrementalextended domain
Finite extended domain
CHAPTER Va : CONTINUOUS HETEROGENEOUS DEFORMATION
Va5 STRAIN REGIME
Va51 DEFORMED VEINS

Line of nondeformation during progressive simple shear
3 1
Incrementalstrain ellipse
Nondeformation line
Finite strain ellipse
During simple shear the shearing planeis a plane of nondeformation, thereforethere is only one area in which lineswill be shortened then stretched.
On the field, one looks for directions alongwhich veins have been shortened thenstretched. If those veins are withinone quadrant then we concludefor the noncoxiality of the deformation.
Initial state
Finite extendeddomain
Incrementalextended domain
3 1
3 1
CHAPTER Va : CONTINUOUS HETEROGENEOUS DEFORMATION
Va5 STRAIN REGIME
Va51 DEFORMED VEINS
Incrementalshortened domain

CHAPTER Va : CONTINUOUS HETEROGENEOUS DEFORMATION
Va5 STRAIN REGIME
1
32
1
32
M N L
M N L
3 3 3
3 3 3
1 1 1
1 1 1
Coaxial deformation
Noncoaxial deformation
Va52 Anastomosed ductile shear zones

S planes: Schistosity
C planes: shear planes
C planes
S planes C/S fabricsThe number of C planes increase towardthe mylonite.
C/S planes
C' planes
C/S/C' fabrics
C' shear planes are extensional shearbands which tend to reduce the thicknessof the ductile shear zone.
Asymetric boudinage of a mylonitic zone C' planes
M N L
3 3 3
1 1 1
CHAPTER Va : CONTINUOUS HETEROGENEOUS DEFORMATION
Va5 STRAIN REGIME
Va53 C/S, C/S/C' fabrics
1 average trajectory

1mm
phg
grt
czo
omp
ENEWSW
b/
C plane
S plane 1mm
phg
grt
czo
kyomp
ENEWSW
a/
Pressure shadows and crystallization tails during simple shear
Tiling structure
pressure shadows
pressure shadows
CHAPTER Va : CONTINUOUS HETEROGENEOUS DEFORMATION
Va5 STRAIN REGIME
Va54 PRESSURE SHADOWS

CHAPTER Va : CONTINUOUS HETEROGENEOUS DEFORMATION
Va5 STRAIN REGIME
Va54 PRESSURE SHADOWS
Pressure shadows and crystallization tails during pure shear
Facecontrolled, deformable fibres formed and deformed in progresssive simple shear
Pyrite grain
Quartz fibres
pressure shadows

Crystal slip
CHAPTER Va : CONTINUOUS HETEROGENEOUS DEFORMATION
Va5 STRAIN REGIME
Va55 MICROSHEARS
Mica fish

1c axis
Slip plane
Macroscopic pure shear
Preferred crystallographic orientation by dislocation glide
Ductile deformation by dislocation creep produces characteristic preferred orientations of mineral crystallographic axes. The pattern of CPO depends on:
>the slip systems that are actived (depends on temperature and stress)
>the geometry and the magnitude of the deformation
Coaxial deformation > fabrics symmetric to the principal axes of finite strain
Noncoaxial deformation > asymmetric fabric
c axis
Slip plane
Macroscopic simple shear
c axis
1
c axis1
3
C axis fabrics
1
c axis
1
c axis
3
C axis fabrics
Symmetric
Asymmetric
CHAPTER Va : CONTINUOUS HETEROGENEOUS DEFORMATION
Va5 STRAIN REGIME
Va56 CRYSTALLOGRAPHIC FABRICS