Folds, Faults, and Folds, Faults, and MountainsMountainsPencil

Rubber bandGumFoam sedimentsCardboard fault modelsPlastic boxFood coloringPaper

Begin Chewing Gum

Fold and Thrust MountainsFold and Thrust Mountains

• Enormous mountain ranges form when Enormous mountain ranges form when plates converge.plates converge.

• Contorted rocks show the power of Contorted rocks show the power of plate tectonics.plate tectonics.

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

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

Evidence of Lateral CompressionEvidence of Lateral Compression

Convergent Plate Boundaries Convergent Plate Boundaries and Foldingand Folding

Continent-Oceancollision forms Continental Arc: Andes Cascades.

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

Compression, tension and shear forces stress Compression, tension and shear forces stress the rocks, causing them to strain i.e. “give”the rocks, causing them to strain i.e. “give”

Convergent Divergent TransformConvergent Divergent Transform

Units of Stress Force / Area

RelationRelationship ship Between Between Stress Stress and and StrainStrain

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

Rubber Band

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

Ruler, Pencil

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

Chewing Gum

Strike and DipStrike and Dip

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

Map Symbols: Strike shown as long line, dip as short line. Note the angle of dip shown: 45o

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

Source: Breck P. Kent

Adjacent Anticline and Syncline

Note highest point

Foam Strata

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

Foam strata

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

Various FoldsVarious Folds

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

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

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

Axial plane near axis should be close to horizontal

Axis

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

Nosed folds, therefore plunging

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

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

FracturesFractures

Source: Martin G. Miller/Visuals Unlimited

•

• Fractures

• - Joints: fractures with no relative movement

• - Faults: fractures with relative movement

Dip-Slip Dip-Slip FaultsFaults

Demo: Cardboard Models

Source: John S. Shelton

Normal Fault: Hanging Wall Down

Hanging wall overhangs the fault plane

Especially common in divergent margins

Foot wall under the fault plane

Hanging wall is down

KEY BED

KEY BED

Dip Slip Faults Dip Slip Faults

Younger

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?

This guy is rich

Normal

Reverse

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

a)a) Visible displacement of rocksVisible displacement of rocksb)b) Pulverized rock and “Slickensides”Pulverized rock and “Slickensides”c)c) Key beds cut out by faulting reappear elsewhere.Key beds cut out by faulting reappear elsewhere.

Types of Faults - 2Types of Faults - 2

• Strike-slip Strike-slip faultsfaults

1)1) Example: San Andreas Example: San Andreas Transform faultTransform fault

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

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

4)4) Evidence of Shear stressEvidence of Shear stress

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

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

Graben in Graben in IcelandIceland

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

Plate tectonics and faultingPlate tectonics and faulting

• Shallow dipping Reverse Fault called a “Thrust Fault”.

• 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)

Same layer

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

Source: Breck P. Kent

PreCambrian Limestone over Cretaceous Shales

Plate tectonics and faultingPlate tectonics and faulting

• 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

The Grand Tetons in WyomingThe Grand Tetons in Wyoming

Source: Peter French/DRK Photo

Mostly high angle normal faults ~9mya

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

We will discuss Buoyant subduction later

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