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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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