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    Folds and Faults

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    Folds and Faults

    Deformation of Rocks

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    Deformation of rocks

    Folds and faults are geologic

    structures.

    Structural geology is the study of thedeformation of rocks and the effects

    of this movement.

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    Small-scale Folds

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    Small-scale Fault

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    Orientation of deformed rocks

    Strike: compass direction of a line

    formed by the intersection of the inclined

    plane and a horizontal plane. Dip: angle between the inclined plane

    and a horizontal plane, measured

    perpendicular to strike.

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    Dipping Sedimentary Beds

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    Cockscomb Ridge, S. Utah

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    Cockscomb Ridge, S. Utah

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    Stress

    (force per unit area)

    Types of directed stresses include:

    Compression

    Extension

    Shear

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    Compression

    Action of coincident oppositely directed forces

    acting towards each other

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    Tension

    Action of coincident oppositely directed forces

    acting away from

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    Shear

    Action of coincident oppositely directed forces

    acting parallel to each other across a surface

    in a couple

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    Strength

    Ability of an object to resist deformation

    Compressive or tensile

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    Strain

    Any change in original shape or size of an

    object in response to stress acting on the

    object

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    Types of deformation

    Elastic

    Ductile (plastic)

    Brittle (rupture)

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    Elastic deformation

    Temporary change in shape or size that is

    recovered when the deforming force is

    removed

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    Ductile (plastic) deformation

    Permanent change in

    shape or size that is not

    recovered when the

    stress is removed

    Occurs by the slippage

    of atoms or small

    groups of atoms pasteach other in the

    deforming material,

    without loss of cohesion

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    Brittle deformation (rupture)

    Loss of cohesion of a body

    under the influence of

    deforming stress

    Usually occurs along sub-

    planar surfaces that

    separate zones of coherent

    material

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    Factors that affect deformation

    Temperature

    Pressure

    Strain rate

    Rock type

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    Tectonic Forces and Resulting

    Deformation

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    Types of folds

    (bent planar structures)

    anticline: older rocks on the inside

    syncline: older rocks on the outside

    (scale - from mm to tens of km)

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    Fold terms

    axial Plane: the plane of mirror

    symmetry dividing the fold into two limbs

    axis: line formed by the intersection of

    the axial plane and a bedding plane

    horizontal fold: where the fold axis is

    horizontal

    plunging fold: where the fold axis is not

    horizontal

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    Fold Terminology

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    Symmetrical, Asymmetrical

    and Overturned Folds

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    Anticline

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    Asymmetric Folds

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    Overturned Folds

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    Overturned

    Syncline,Israel

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    Saudi Arabian Fold

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    Map View of

    Plunging

    Folds

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