folds faults and mountains. fold and thrust mountains enormous mountain ranges form when plates...
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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)
Various Folds (cont'd)Various Folds (cont'd)
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.
Plate tectonics and faultingPlate tectonics and faulting
• 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
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
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
Model for the Evolution of the Model for the Evolution of the Southern Appalachians (cont’d)Southern Appalachians (cont’d)
Arc and subduction zone collide w/ N.Am., westward subduction starts
The continents collide
Model for the Evolution of the Model for the Evolution of the Southern Appalachians (cont’d)Southern Appalachians (cont’d)
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