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Folds, faults and deformation. Deformation of Rocks. Folds and faults are geologic structures caused by deformation. Structural geology is the study of the deformation of rocks and its effects. Fig. 11.1. Orientation of Deformed Rocks.

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deformation of rocks
Deformation of Rocks
  • Folds and faults are geologic structures caused by deformation.
  • Structural geology is the study of the deformation of rocks and its effects.
orientation of deformed rocks
Orientation of Deformed Rocks

We need some way to describe the distribution of geologic structures. So we use the terms strike and dip.

Strike:compass direction of a rock layer as it intersects with a horizontal surface.

Dip:acute angle between the rock layer and the horizontal surface, measured perpendicular to strike.

stress force per unit area
Stress:force per unit area

Types of directed stresses include:

• compression

• extension

• shear

strength
Strength
  • ability of an object to resist deformation
  • compressive or tensile

Fig. 11.7

strain
Strain

Any change in original shape or size of an object in response to stress acting on the object

Fig. 11.1

types of deformation
Types of Deformation
  • Elastic: Temporary change in shape or size that is recovered when the deforming force is removed
  • ductile (plastic):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 past each other in the deforming material
  • Brittle (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
factors that affect deformation
Factors that Affect Deformation
  • temperature
  • pressure
  • strain rate
  • rock type

The variation of these factors determines whether a rock will fault or fold.

effects of rock type on deformation
Effects of Rock Type on Deformation

Some rocks are stronger than others.

  • competent: rocks that deform only under great stresses
  • incompetent: rocks that deform under moderate to low stresses
joints
Joints

Cracks in rocks along which there has been no appreciable displacement.

faults
Faults
  • Fractures in rocks created by earthquakes.
  • Faults are characterized by the direction of relative movement: slip
  • Dip-slip faults
    • normal
    • reverse
  • Strike-slip faults
  • Oblique-slip faults
  • Hanging Wall: Term used by miners. They could “hang” their light on this side of the fault because it was above them.
  • Footwall: Also from the miners, this side of the wall upon which they could stand below the hanging wall.
dip slip faults
Dip-slip Faults

Motion of the fault blocks is parallel to the dip direction.

slide23

Rift Valley

Fig. 11.13

slide24

Rift Valley

Fig. 11.13

reverse fault
Reverse Fault

hanging wall

footwall

cross section

thrust fault
Thrust Fault

Thrust faults are low-angle reverse faults.

hanging wall

footwall

cross section

slide29

Scarp

Armenia 1988 Earthquake

strike slip faults
Strike-slip Faults

Motion of the fault blocks is parallel to the strike direction.

folding of rocks
Folding of Rocks
  • Produced by horizontal or vertical forces
  • Scale can be from cm to 100’s of km
fold terminology
Fold Terminology

axial plane:the plane of mirror symmetry dividing the fold into two limbs

axis:the line formed by the intersection of the axial plane and a bedding plane

horizontal fold:fold where the axis is horizontal

plunging fold:fold where the axis is not horizontal

slide36

Beds dip away symmetrically from the axial plane

Beds on one side of the axial plane dip steeper than those on the other side

more fold terminology
More Fold Terminology

syncline:a sequence of folded rocks with the youngest rocks on the inside of the fold

anticline:a sequence of folded rocks with the oldest rocks on the inside of the fold

fold terms
Fold terms

synform:a sequence of folded rocks in which the limbs form an arrow pointing down

antiform:a sequence of folded rocks in which the limbs form an arrow pointing up

and more fold terminology
And More Fold Terminology

dome:a sequence of folded rocks in which all the beds dip away from a central point

basin:a sequence of folded rocks in which all the beds dip towards a central point