Folding mechanisms

BYK.Manikandabharath

Anna university,chennai.

Fold•The bending of rock-strata due to compressional forces acting tangentially or horizontally towards a common point or plane from opposite directions is known as folding.•It result in the crumbling of strata forming wavy undulation on the earth which are know as fold

CAUSES AND MECHANISM OF FOLD:

•Most rocks are plastic and bend Under great pressure•Pressure acting on a rock It exceeds Elastic limit they become permanently bent or folded •Rock barrid on considerable depth high temperature plastic deformed under stress •Forces connected in the earth crest.

Fold mechanism is influenced by factors affecting deformation:

• temperature• pressure• fluid• properties of the rock as determined by

composition, texture, and anisotropy.

Fold mechanismsFold mechanisms include:• Buckling• Bending• Flexure folding• Flexural slip• Flexural flow• Passive (ductile) flow• Kink folding

BUCKLING • Folds form by buckling where force is applied parallel

to layering in rocks. The product of buckling is buckled fold.

• Produce layers shortening. • Thin sheet under lateral pressure• Strain produced with in the layer is determined by

extension around the outer Arc and compression in the Inner Arc .

BENDING • Bending involves application of force

across layers.

• Generally produce folds that are very gentle with large inter limb angles.

• Layers in bending are bent like an elastic beam the has been supported at the ends and loaded in the middle.

• Simple bending due to lateral compression• Stress axis horizontal • Strata with or without slide one another

Flexure folding

FLEXURAL SLIP • Act usually in low temperature and pressure found at

shallow depth within the Earth Crust.

• Parallel concentric folds form by buckling or bending. Slip in these folds is parallel to the layering and characterized by slickenside's, fibers. They have constant layer thickness.

Flexural-slip folds

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FLEXURAL FLOW • Flexural flow requires moderate-to high ductility contrast between

layers.

• In flexural fold amplitude and wavelength may be controlled by the original thickness, spacing and strength of the strong layers.

• Form in rocks from low and moderate metamorphic grade. They are similar like folds.

Example: shale (change thickness) and quartzite (fixed thickness)

Flexural-flow folds

PASSIVE FLOW Passive-flow folds: •Are similar folds that involve plastic deformation.• The layering acts only as a displacement marker. •Passive flow folds form in metamorphic rocks with low mean ductility and ductility contrast.

Example: salt, glacial ice and water saturated unconsolidated sediments

Passive-flow folds

Passive-slip folds• Passive-slip folds: type of similar folds, form by

shearing along planes inclined by layering, form by simple shear and not pure shear.

KINK FOLDING • Kink and chevron folds have straight limbs and narrow angular hinges. • Kink folds requires local slippage (flexural slip) between layers.• The fold is produced by the Rotation of set of layers on either side of kink-

plane (Axial surface) the layer deform partly by flexural slip

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