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FOLDS, FAULTS AND

GEOLOGIC MAPS

Objectives

• Define three types of stress.

• Describe the differences between elastic, brittle, and

ductile deformation.

• Define cratons and orogens.

• Define strike and dip.

• Define and describe synclines, anticlines, and other

types of folds.

• Distinguish between a topographic map and a

geologic map.

Rock Deformation

• Stress

– The force acting on a

surface, per unit area

– May be greater in

certain directions than

in others

• Pressure

– A kind of stress in

which the forces acting

on a body are the same

in all directions

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Rock Deformation

• Tension

– A stress that acts in a

direction perpendicular

to and away from a

surface

• Compression

– Stress that acts in a

direction perpendicular

to and toward surface

Rock Deformation

• Shear

– A stress that acts in a direction parallel to a surface

• Strain

– A change in shape or volume of a rock in response to

stress

• Uniform stress causes change in volume only

• Differential stress may causes change in shape

Rock deformation

• Elastic deformation

– Temporary change in shape or volume from which a

material rebounds after the stress is removed

Paper clip returns to

original shape

Paper clip permanently

deformed: stress was

greater than elastic limit

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• Ductile deformation

– Permanent but gradual change in shape or volume of a

material, caused by flowing or bending

Brittle at room temperature Ductile at high temperature

•Brittle deformationPermanent change in shape or volume, in which a

material breaks or cracks

Rock deformationDuctile Brittle

Rock Deformation

• Main factors that

affect rock

deformation

– Temperature

– Confining pressure:

uniform pressure due

to depth

– Rate of deformation

– Composition

High confining

pressure

Low confining

pressure

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Rate of deformation

Rock Deformation

• Where rock deformation occurs

– Craton

• A region of continental crust that has remained tectonically stable

for a very long time

– Orogen

• An elongated region of crust that has been deformed and

metamorphosed through a continental collision

• Isostasy

– Similar to an ice berg floating in water; it floats at a level determined

by the density contrast

– The flotational balance of the lithosphere on the asthenosphere

Rock Deformation

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Rock Deformation

Structural Geology

• Structural geology -

the study of:

– Stress and strain

• Processes causing

stress and strain

• Deformation and rock

structures that result

from stress and strain

Structural Geology• Strike

– The compass orientation of the line of intersection between a

horizontal plane and a planar feature (e.g. rock layer or fault)

• Dip

– The angle between the tilted surface and a horizontal plane

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Structural Geology

Structural Geology

• Faults and fractures

– Fault; fracture with movement

along the fracture surface

– Normal fault

• The block of rock above the

fault surface moves

downward relative to the

block below

Structural Geology

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Structural Geology• Faults and fractures

– Reverse fault

• The block on top of the fault surface moves up and over the block on the

bottom

– Thrust fault

• A reverse fault with a shallow angle of dip

Structural Geology

• Strike-slip fault

– A fault in which the direction of the movement is mostly

horizontal and parallel to the strike of the fault

• San Andreas Fault

Strike-slip fault

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Structural Geology

Offset drainages

San Andreas San Jacinto fault

The fault on the left is a right-lateral fault because the offset

stream is to your right as you face the fault.

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Structural Geology

• Folds

– A bend or warp in a layered

rock

• Monocline

– A local steepening in

otherwise uniformly

dipping strata

• Anticline

– A fold in the form of an

arch, with the rock strata

convex upward and the

older rocks in the core

Monocline

Parts of a fold

• Syncline

• A fold in the form of a

trough, with the rock

strata concave upward

and the younger rocks in

the core

• The geometry and the

orientation of a fold is

described by the:

– Axial plane

– Axial trace

– Limbs

– Axis

– Plunge

Limb

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Domes and basins

Dome

Michigan Basin

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Geologic Maps

• Geologic map…

– Shows the locations, kinds, and orientation of rock units

– Shows structural features such as faults and folds

• Topographic maps…• Show the shape of a ground surface, as well as the location and

elevation of surface features, usually by means of contour lines

Topographic Map

Geologic Maps

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Structural Geology

Structural Geology

Geologic Maps

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Geologic Maps

The Canadian Rockies