folds, faults & mountains

8/12/2019 Folds, Faults & Mountains

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Folds Faults and

Mountains


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Fold and Thrust Mountains

Enormous mountain ranges form when

plates converge.

Contorted rocks show the power of

plate tectonics.


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Rock

Distortion


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Convergent Plate Boundaries

and Folding

Ocean-Ocean

collision forms

Island Arc: Japan,

Aleutians, Cent. Am.

Continent-Continent

collision forms

Folded Mountain Belt:Alps,

Himalayans, Appalachians


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Evidence of Lateral Compression

Formerly horizontal layers are twisted,

bent, or broken.

Some folded rocks are pushed over on

their sides, or even upside down.


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Folded Sandstone

Source: Martin Bond/Science Photo Library/Photo Researchers, Inc.


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Studying Faults and Folds

The branch of geology that studies crustal

deformation is called Structural Geology.

Geologic structures determine ground

stability, and where to build cities.


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Stress

Units are Pressure: Force/Area

Three types of stressa) Compression causes bending

b) Tension causes thinning

c) Shearing causes one type of faults


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Compression, Tension,

and Shearing Stress

Convergent Divergent Transform


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Types of deformation

Elastic deformation up to elastic limit Springs back to original shape Demo: Pencil

Brittle failure (it breaks) Demo Pencil Causes: 1. subjected to great stress that exceeds the

yield point AKA elastic limit, OR Subjected to sudden stress AKA impact

Plastic deformation Does not spring back keeps deformed shape

Demo Chewing gum Cause can be high temperaturenear melting

or high pressure squeezed like a ball of clay


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Relation

shipBetween

Stress

andStrain

Strain can be a change in shape (a deformation) due to an applied stress


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Relationship

BetweenStress and

Strain at low

Temps andPressure or

Sudden Stress


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Relationship

Between

Stress andStrain under

high Temps

or Pressure


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Factors affecting rock deformation

Intensity of applied stress

HeatTemperature of the Rock Amount of Time the Stress is applied

Rock Composition


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Interpreting Deformed Rocks

Most apparent in sedimentary rocks

Importance of deformation Indicates past plate motions

Indicates other past geological events

Locates specific natural resources

Mapping 101: Rock orientation: strike and dip


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Strike and Dip

Strike intersection w horizontal, dip perpendicular, angle from horizontal down toward surface

Strike is long line, dip is short line

Note the angle of dip given 45o


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Folds

Folds def:Bends in rock layers

Types: synclines and anticlines

Syncline (downfold) innermost rocks youngest

Anticline (upfold) innermost rocks oldest

Parts of a fold (limbs, axial plane, axis)

Note: Anticlines and synclines are structures inrocks, not surface landforms


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Folded Rocks, Hwy 23

Newfoundland, New Jersey

Source: Breck P. Kent

Adjacent Anticline and Syncline

Note highest point


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Folded Rocks (Dorset, England)

Center has overturned area

Source: Tom Bean

Lucky we have ways of

recognizing right side up

What are they?

OlderYounger

Overturned

Area

Older

Younger


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Folded Rock Before Erosion


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Folded Rock After Erosion

Eroded Anticline, older rocks in center. Syncline is opposite.


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Topography may be opposite of Structure

AnticlineBefore/After Erosion

Notice center rock oldest


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Topography may be opposite of Structure

Syncline Before/After Erosion

Notice center rock youngest


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Fold symmetry

a) Symmetrical or open folds

b) Asymmetrical foldsc) Overturned folds

d) Recumbent folds

e) Plunging folds


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Various Folds


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Various Folds (cont'd)


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Not a good drawing, axial plane should be closer to horizontal


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Plunging Folds

Nose of anticline points direction of plunge, syncline nose in opposite direction

Up

End Down

End

Demo: Plastic box, water, paper folds


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Plunging Folds

Source: GEOPIC, Earth Satellite Corporation


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Again: Strike and Dip


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Domes and basins

1. Domes

2. Basins3. Occur within plates

4. Result from vertical forces

5. Geographic examples (later)


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3-D: Dome and Basin


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Fractures

Fractures

- Joints: fractures with no relativemovement

- Faults: fractures with relative

movement


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Joints: Fractureswith no movement

Source: Martin G. Miller/Visuals Unlimited


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Dip-Slip

Faults


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Source: John S. Shelton

Normal Fault: Hanging Wall Down

Key

Bed

Hanging wall overhangs

the fault plane

Es eciall common in diver ent mar ins


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Normal Fault

(Hanging Walldown)

Reverse Fault Typical of convergent


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Reverse Fault(called Thrust Fault if shallow angle)

Younger

(Hanging wall Up)

Miners pay geologists to

find their lost orebody

One friend earned

enough to buy a house

This poor guy is out of luck

What phase of magma fractionation would result in the

placement of this ore body?

Which formed first, the ore body or the fault?

What common mineral is mostly likely in the ore body?

Structural Geology is taught by Dr. Krall

yp g

margins


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Evidence of faults

a) Visible displacement of rocks

b) Pulverized rock and Slickensidesc) Key beds cut out by faulting reappear

elsewhere.

htt // t f d d / l t/ li h/ h/Sli k id if


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Fracture Zones and Slickensideshttp://pangea.stanford.edu/~laurent/english/research/Slickensides.gif

T f F lt 2


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Types of Faults - 2

Strike-slip faults 11) Example: San Andreas Transform fault

2) Distinctive landforms (linear valleys,chains of lakes, sag ponds, topographicsaddles)

3) Fresh pulverized rock. Transform fault

through granite: Arkose sandstone4) Evidence of Shear stress


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San

AndreasFault

Source: Georg Gerster/Wingstock/Comstock

H i t l M t Al


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Horizontal Movement Along

Strike-Slip Fault


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Oblique Slip

Also seen in Transform Faults such as San Andreas

Both strike slip and dip-slip

T f f lt


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Types of faults

Strike-slip faults 2

1) Example: Mid-Ocean Ridge Transform

faults

2) Small offsets in ridge

3) San Andreas is also ridge offset,but on a huge scale with a historical twist


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Faults & Plate Tectonics

Divergence

Convergence

Transform


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Plate tectonics and faulting

Normal faults: mid-ocean ridges and

continental rifts are the same thing.

Divergent Margins

Surface rock is pulled apart Hanging wall drops down


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Horst and Graben Formation

H t d G b F ti


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Horst and Graben Formation


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Graben in

Iceland

Source: Simon Fraser/Science Photo Library/Photo Researchers, Inc.


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Reverse and thrust faults: convergent

plate boundaries

Hanging Wall is pushed up.


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Lewis Thrust Fault


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Lewis Thrust Fault (cont'd)


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c) Strike-slip faults: Transform Boundaries


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San

AndreasFault


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Types and processes of mountain-

building (Orogenesis)

1. Volcanic mountains

2. Fold-and-thrust mountains3. Fault-block mountains

4. Upwarped mountains


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Types of Mountains

2. Fold-and-thrust mountains

Formed by Continent-ContinentCollisions


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Appalachian

MountainSystem

Model for the Evolution of the


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Southern AppalachiansSupercontinent breaks up, rifts apart.

Another rift starts moving Africa west. The ocean floor breaks

and one side subducts, starting a new island arc.

AnotherRift

Over here

somewhere

rift



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Southern Appalachians (contd)The ocean floor breaks again, new subduction adds volcanics to an existing microcontinent

Net westward movement pushes the ridge, subduction zone and fragment into N.America

Rifting restarts to the East

Weak rifts


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Southern Appalachians (contd)

Rifting Restarts

Collisional Mountains ???


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Collisional Mountains ???

(The Grand Tetons in Wyoming)

Source: Peter French/DRK Photo

Paradigm shifts: What is wrong with our model? More on this later


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Fault-block mountains

Rift Valleys, Mid Ocean Ridges

Basin and Range province ???

Normal Fault Blocks as in East Africa

Divergent Margins? Paradigm Shifts

Origin of the Basin and Range


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Southwestern North America

Looks different

Paradigm Shifts


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Upwarped mountains

a) Gently bent without much deformation

b) Ascent of buoyant mantle material

c) Far from plate boundaries

d) Adirondack Mountains: Uplift of deep

PreCambrian Igneous and Metamorphic

rocks

The Adirondack Mountains


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The Adirondack Mountains

of Northern New York

Source: Clyde H. Smith/Allstock/Tony Stone Images


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Anticlines and Oil

Early USA

petroleum

exploration, e.g.

Pennsylvaniaanticlines


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Faults and Oil


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End of Chapter 9

folds, faults & mountains

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