CHAPTER Va : CONTINUOUS HETEROGENEOUS .CHAPTER Va : CONTINUOUS HETEROGENEOUS DEFORMATION ... The

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Transcript of CHAPTER Va : CONTINUOUS HETEROGENEOUS .CHAPTER Va : CONTINUOUS HETEROGENEOUS DEFORMATION ... The

  • 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

    Va-2-1 FOLDS-Morphology

    CHAPTER Va : CONTINUOUS HETEROGENEOUS DEFORMATION

    Va-2 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

    Va-2-1 FOLDS-Morphology

    CHAPTER Va : CONTINUOUS HETEROGENEOUS DEFORMATION

    Va-2 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

    Va-2-1 FOLDS-Classification

    CHAPTER Va : CONTINUOUS HETEROGENEOUS DEFORMATION

    Va-2 STRUCTURES

  • So

    S1

    Cleavage fan

    Crenulation cleavage: The development of fine scale microfolding can produce systematic realignment of pre-existing 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

    Va-2-1 FOLDS-Associated linear and planar microstructures

    CHAPTER Va : CONTINUOUS HETEROGENEOUS DEFORMATION

    Va-2 STRUCTURES

  • Outer arc lengthens

    Inner arc shortens

    Neutral surface

    Extrado fracturesIntrado stylolites

    Flexural shear foldingOrthogonal flexure

    hinge

    shear planes

    Volume loss flexure

    Va-2-1 FOLDS-Kinematic models of folding

    CHAPTER Va : CONTINUOUS HETEROGENEOUS DEFORMATION

    Va-2 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 Volume-loss 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

  • Va-2-1 FOLDS-Kinematic models of folding

    CHAPTER Va : CONTINUOUS HETEROGENEOUS DEFORMATION

    Va-2 STRUCTURES

    Fault-bend fold Fault-propagation 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 faut-bend fold in association with a fault ramp. The second example (sketches on the right) illustrates the development of a fault-propagation 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

  • Va-2-1 FOLDS-Kinematic models of folding

    CHAPTER Va : CONTINUOUS HETEROGENEOUS DEFORMATION

    Va-2 STRUCTURES

    Progressive flatenning

  • Va-2-1 FOLDS-Kinematic models of folding

    CHAPTER Va : CONTINUOUS HETEROGENEOUS DEFORMATION

    Va-2 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 tube-shaped 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, bedding-cleavage 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

    Va-2-1 FOLDS-Fold systems

    CHAPTER Va : CONTINUOUS HETEROGENEOUS DEFORMATION

    Va-2 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

    bedding-cleavage

    4

    Vergence of displacement4

    The determination of two of these criteria leadto the determination of the two others.

    Scale independent microtectonic laws

    Va-2-1 FOLDS-Fold systems

    CHAPTER Va : CONTINUOUS HETEROGENEOUS DEFORMATION

    Va-2 STRUCTURES

  • So

    S1

    Vergence of the fold ?

    Scale independent microtectonic laws

    Va-2-1 FOLDS-Fold systems

    CHAPTER Va : CONTINUOUS HETEROGENEOUS DEFORMATION

    Va-2 STRUCTURES

  • Vergence of the fold ?

    Scale independent microtectonic laws

    Va-2-1 FOLDS-Fold systems

    CHAPTER Va : CONTINUOUS HETEROGENEOUS DEFORMATION

    Va-2 STRUCTURES

  • Boudin lines

    Boudin

    Neck lines

    Neck

    Pinch-and-swell structures

    Crystallization in pressure shadowNeck foldSymmetric boudinage

    Asymmetric boudinage, asymmetric pressure shadows

    Va-2-2 BOUDIN AND BOUDINAGE

    CHAPTER Va : CONTINUOUS HETEROGENEOUS DEFORMATION

    Va-2 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

    Va-2-3 DUCTILE SHEAR ZONES

    CHAPTER Va : CONTINUOUS HETEROGENEOUS DEFORMATION

    Va-2 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

    Va-2-3 DUCTILE SHEAR ZONES

    CHAPTER Va : CONTINUOUS HETEROGENEOUS DEFORMATION

    Va-2 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

    Va-2-3 DUCTILE SHEAR ZONES: The kinematic reference frame

    CHAPTER Va : CONTINUOUS HETEROGENEOUS DEFORMATION

    Va-2 STRUCTURES

  • 3

    1

    A B

    C

    D

    A'B'

    C'

    D'

    A' B' C' D' 1 1 1 1

    33

    Surfaces of non-deformation

    1

    3 2D 3D

    CHAPTER Va : CONTINUOUS HETEROGENEOUS DEFORMATION

    Va-3 ORIENTATION OF THE AXES OF THE FINITE STRAIN ELLIPSOIDVa-3-1 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

    Va-3 ORIENTATION OF THE AXES OF THE FINITE STRAIN ELLIPSOIDVa-3-1 FOLDS AND BOUDINS

    3

    3

    1

    2

    1

    2

    1

    33

    1

    2

    1

    2

    1 3

    3

    3

    2

    1

    3

    1

    2

    3

  • CHAPTER Va : CONTINUOUS HETEROGENEOUS DEFORMATION

    Va-3 ORIENTATION OF THE AXES OF THE FINITE STRAIN ELLIPSOIDVa-3-1 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 finit