Folds Faults and Mountains. Fold and Thrust Mountains Enormous mountain ranges form when plates converge. Contorted rocks show the power of plate tectonics

Folds Faults and Folds Faults and MountainsMountains

Fold and Thrust MountainsFold and Thrust Mountains

• Enormous mountain ranges form when plates converge.

• Contorted rocks show the power of plate tectonics.

Rock Rock DistortionDistortion

Convergent Plate Boundaries Convergent Plate Boundaries and Foldingand Folding

Ocean-Oceancollision forms Island Arc: Japan,Aleutians, Cent. Am.

Continent-Continentcollision formsFolded Mountain Belt:Alps, Himalayans, Appalachians

Evidence of Lateral CompressionEvidence of Lateral Compression

• Formerly horizontal layers are twisted, bent, or broken.

• Some folded rocks are pushed over on their sides, or even upside down.

Folded SandstoneFolded Sandstone

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

Studying Faults and FoldsStudying 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.

StressStress

Units are Pressure: Force/Area

Three types of stress

a) Compression causes bending

b) Tension causes thinning

c) Shearing causes one type of faults

Compression, Tension, Compression, Tension, and Shearing Stressand Shearing Stress

Convergent Divergent Transform

Types of deformationTypes 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 temperature – near melting or high pressure … squeezed like a ball of clay

RelationRelationship ship Between Between Stress Stress and and StrainStrain

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

Relationship Relationship Between Between Stress and Stress and Strain at low Strain at low Temps and Temps and Pressure or Pressure or Sudden StressSudden Stress

Relationship Relationship Between Between Stress and Stress and Strain under Strain under high Temps or high Temps or Pressure Pressure

Factors affecting rock deformationFactors affecting rock deformation

• Intensity of applied stress

• Heat –Temperature of the Rock

• Amount of Time the Stress is applied

• Rock Composition

Interpreting Deformed RocksInterpreting 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

Strike and DipStrike and Dip

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

Strike is long line, dip is short lineNote the angle of dip given 45o

FoldsFolds

• 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 in rocks, not surface landforms

Folded Rocks, Hwy 23 Folded Rocks, Hwy 23 Newfoundland, New JerseyNewfoundland, New Jersey

Source: Breck P. Kent

Adjacent Anticline and Syncline

Note highest point

Folded Rocks (Dorset, England)Folded Rocks (Dorset, England)Center has overturned areaCenter has overturned area

Source: Tom Bean

Lucky we have ways of recognizing right side upWhat are they?

OlderYounger

OverturnedArea

Older

Younger

Folded Rock Before ErosionFolded Rock Before Erosion

Folded Rock After ErosionFolded Rock After Erosion

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

Topography may be opposite of Structure Topography may be opposite of Structure

AnticlineAnticline Before/After Erosion Before/After Erosion

Notice center rock oldest

Topography may be opposite of Structure Topography may be opposite of Structure

Syncline Before/After ErosionSyncline Before/After Erosion

Notice center rock youngest

Fold symmetryFold symmetry

a) Symmetrical or open folds

b) Asymmetrical folds

c) Overturned folds

d) Recumbent folds

e) Plunging folds

Various FoldsVarious Folds

Various Folds (cont'd)Various Folds (cont'd)



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

Plunging FoldsPlunging Folds

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

UpEnd Down

End

Demo: Plastic box, water, paper folds

Plunging Folds Plunging Folds

Source: GEOPIC©, Earth Satellite Corporation

Interpreting FoldsInterpreting Folds

• Determine if center rocks are older or younger than flanks: fossils, right side up clues (graded bedding and mudcracks)

• Are limbs parallel or “Nosed”?

• Determine limb dips from measurements, stream V’s. Strike and Dip

• Use nose rules for anticlines and synclines

Again: Strike and DipAgain: Strike and Dip

Domes and basinsDomes and basins

1.Domes

2.Basins

3.Occur within plates

4.Result from vertical forces

5.Geographic examples (later)

3-D: Dome and Basin3-D: Dome and Basin

FracturesFractures

Fractures

- Joints: fractures with no relative movement

- Faults: fractures with relative movement

Joints: Fractures – with no movementJoints: Fractures – with no movement

Source: Martin G. Miller/Visuals Unlimited

Fault Type 1 - Dip-slip faultsFault Type 1 - Dip-slip faults

1) Terms: Hanging wall and footwall

2) Normal faults(a) Grabens

(b) Horsts

3) Reverse faultsa) low angle called Thrust faults

4) Oblique-slip faults

Dip-Slip Dip-Slip FaultsFaults

Source: John S. Shelton

Normal Fault: Hanging Wall Down

KeyBed

Hanging wall overhangs the fault plane

Especially common in divergent margins

Normal Fault (Hanging Wall down)

Reverse Fault Reverse Fault (called “Thrust Fault” if shallow angle)(called “Thrust Fault” if shallow angle)

Younger

(Hanging wall Up)

Miners pay geologists to find their lost orebodyOne 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

Typical of convergent margins

Evidence of faultsEvidence of faults

a) Visible displacement of rocks

b) Pulverized rock and “Slickensides”

c) Key beds cut out by faulting reappear elsewhere.

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

Types of Faults - 2Types of Faults - 2

• Strike-slip faults 11) Example: San Andreas Transform fault2) Distinctive landforms (linear valleys,

chains of lakes, sag ponds, topographic saddles)

3) Fresh pulverized rock. Transform fault through granite: Arkose sandstone

4) Evidence of Shear stress

San San Andreas Andreas FaultFault

Source: Georg Gerster/Wingstock/Comstock

Horizontal Movement Along Horizontal Movement Along Strike-Slip FaultStrike-Slip Fault

Oblique SlipOblique Slip

Also seen in Transform Faults such as San Andreas

Both strike slip and dip-slip

Types of faultsTypes of faults

• Strike-slip faults 21) 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

Faults & Plate TectonicsFaults & Plate Tectonics

Divergence

Convergence

Transform

Plate tectonics and faultingPlate 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

Horst and Horst and Graben FormationGraben Formation

Horst and Horst and Graben FormationGraben Formation

Graben in Graben in IcelandIceland

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


• Reverse and thrust faults: convergent plate boundaries

• Hanging Wall is pushed up.

Lewis Thrust FaultLewis Thrust Fault

Lewis Thrust Fault (cont'd)Lewis Thrust Fault (cont'd)

Lewis Thrust Fault (cont'd)Lewis Thrust Fault (cont'd)

Source: Breck P. Kent

PreCambrian Limestone over Cretaceous Shales


• c) Strike-slip faults: Transform Boundaries

San San Andreas Andreas FaultFault

Types and processes of mountain-Types and processes of mountain-building (Orogenesis)building (Orogenesis)

1. Volcanic mountains

2. Fold-and-thrust mountains

3. Fault-block mountains

4. Upwarped mountains

Types of MountainsTypes of Mountains

• 2. Fold-and-thrust mountains

– Formed by Continent-Continent Collisions

Appalachian Appalachian Mountain Mountain SystemSystem

Model for the Evolution of the Model for the Evolution of the Southern AppalachiansSouthern Appalachians

Supercontinent 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

Model for the Evolution of the Model for the Evolution of the Southern Appalachians (cont’d)Southern Appalachians (cont’d)

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.AmericaRifting restarts to the East

Weak rifts


Arc and subduction zone collide w/ N.Am., westward subduction starts

The continents collide


Rifting Restarts

Collisional Mountains ??? Collisional Mountains ??? (The Grand Tetons in Wyoming)(The Grand Tetons in Wyoming)

Source: Peter French/DRK Photo

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

Fault-block mountainsFault-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 Origin of the Basin and Range Southwestern North AmericaSouthwestern North America

Looks differentLooks different

Paradigm Shifts

Upwarped mountainsUpwarped 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 The Adirondack Mountains of Northern New Yorkof Northern New York

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

Anticlines and OilAnticlines and Oil

Early USA petroleum exploration, e.g. Pennsylvania anticlines

Faults and OilFaults and Oil

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End of Chapter 9End of Chapter 9

Documents

Folds Faults and Mountains. Fold and Thrust Mountains Enormous mountain ranges form when plates converge. Contorted rocks show the power of plate tectonics