Transcript Chapter 9 of Earth

Folds Faults and
Mountains
Fold and Thrust Mountains
• Enormous mountain ranges form when
plates converge.
• Contorted rocks show the power of
plate tectonics.
Rock
Distortion
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
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.
Folded Sandstone
Source: Martin Bond/Science Photo Library/Photo Researchers, Inc.
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.
Stress
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,
and Shearing Stress
Convergent
Divergent
Transform
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 temperature – near melting
or high pressure … squeezed like a ball of clay
Relation
ship
Between
Stress
and
Strain
Strain can be a change in shape (a deformation) due to an applied stress
Relationship
Between
Stress and
Strain at low
Temps and
Pressure or
Sudden Stress
Relationship
Between
Stress and
Strain under
high Temps
or Pressure
Factors affecting rock deformation
•
•
•
•
Intensity of applied stress
Heat –Temperature of the Rock
Amount of Time the Stress is applied
Rock Composition
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
Strike and Dip
Strike is long line, dip is short line
Note the angle of dip given 45o
Strike intersection w horizontal, dip perpendicular, angle from horizontal down toward surface
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 in
rocks, not surface landforms
Folded Rocks, Hwy 23
Newfoundland, New Jersey
Note highest point
Source: Breck P. Kent
Adjacent Anticline and Syncline
Folded Rocks (Dorset, England)
Center has overturned area
Older
Overturned
Area
Younger
Lucky we have ways of
recognizing right side up
What are they?
Source: Tom Bean
Younger
Older
Folded Rock Before Erosion
Folded Rock After Erosion
Eroded Anticline, older rocks in center. Syncline is opposite.
Topography may be opposite of Structure
Anticline Before/After Erosion
Notice center rock oldest
Topography may be opposite of Structure
Syncline Before/After Erosion
Notice center rock youngest
Fold symmetry
a)
b)
c)
d)
e)
Symmetrical or open folds
Asymmetrical folds
Overturned folds
Recumbent folds
Plunging folds
Various Folds
Various Folds (cont'd)
Various Folds (cont'd)
Various Folds (cont'd)
Not a good drawing, axial plane should be closer to horizontal
Plunging Folds
Demo: Plastic box, water, paper folds
Up
End
Down
End
Nose of anticline points direction of plunge, syncline nose in opposite direction
Plunging Folds
Source: GEOPIC©, Earth Satellite Corporation
Interpreting 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 Dip
Domes and basins
1.Domes
2.Basins
3.Occur within plates
4.Result from vertical forces
5.Geographic examples (later)
3-D: Dome and Basin
Fractures
Fractures
- Joints: fractures with no relative
movement
- Faults: fractures with relative
movement
Joints: Fractures – with no movement
Source: Martin G. Miller/Visuals Unlimited
Fault Type 1 - Dip-slip faults
1)
2)
Terms: Hanging wall and footwall
Normal faults
(a) Grabens
(b) Horsts
3)
Reverse faults
a) low angle called Thrust faults
4)
Oblique-slip faults
Dip-Slip
Faults
Normal Fault: Hanging Wall Down
Hanging wall overhangs
the fault plane
Key
Bed
Source: John S. Shelton
Especially common in divergent margins
Normal Fault
(Hanging Wall
down)
Reverse Fault
Typical of convergent
margins
(called “Thrust Fault” if shallow angle)
Structural Geology is taught by Dr. Krall
(Hanging wall Up)
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?
Younger
Miners pay geologists to
find their lost orebody
One friend earned
enough to buy a house
This poor guy is out of luck
Evidence of faults
a) Visible displacement of rocks
b) Pulverized rock and “Slickensides”
c) Key beds cut out by faulting reappear
elsewhere.
http://pangea.stanford.edu/~laurent/english/research/Slickensides.gif
Fracture Zones and Slickensides
Types of Faults - 2
•
Strike-slip faults 1
1) Example: San Andreas Transform fault
2) 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
Andreas
Fault
Source: Georg Gerster/Wingstock/Comstock
Horizontal Movement Along
Strike-Slip Fault
Oblique Slip
Both strike slip and dip-slip
Also seen in Transform Faults such as San Andreas
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
Faults & Plate Tectonics
Divergence
Convergence
Transform
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
Horst and Graben Formation
Horst and Graben Formation
Graben in
Iceland
Source: Simon Fraser/Science Photo Library/Photo Researchers, Inc.
Plate tectonics and faulting
• Reverse and thrust faults: convergent
plate boundaries
• Hanging Wall is pushed up.
Lewis Thrust Fault
Lewis Thrust Fault (cont'd)
Lewis Thrust Fault (cont'd)
Source: Breck P. Kent
PreCambrian Limestone over
Cretaceous Shales
Plate tectonics and faulting
• c) Strike-slip faults: Transform Boundaries
San
Andreas
Fault
Types and processes of mountainbuilding (Orogenesis)
1.
2.
3.
4.
Volcanic mountains
Fold-and-thrust mountains
Fault-block mountains
Upwarped mountains
Types of Mountains
• 2. Fold-and-thrust mountains
– Formed by Continent-Continent Collisions
Appalachian
Mountain
System
Model for the Evolution of the
Southern Appalachians
Supercontinent breaks up, rifts apart.
rift
Another rift starts moving Africa west. The ocean floor breaks
and one side subducts, starting a new island arc.
Another
Rift
Over here
somewhere
Model for the Evolution of the
Southern Appalachians (cont’d)
The ocean floor breaks again, new subduction adds volcanics to an existing microcontinent
Weak rifts
Net westward movement pushes the ridge, subduction zone and fragment into N.America
Rifting restarts to the East
Model for the Evolution of the
Southern Appalachians (cont’d)
Arc and subduction zone collide w/ N.Am., westward subduction starts
The continents collide
Model for the Evolution of the
Southern Appalachians (cont’d)
Rifting Restarts
Collisional Mountains ???
(The Grand Tetons in Wyoming)
Source: Peter French/DRK Photo
Paradigm shifts: What is wrong with our model? More on this later
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
Southwestern North America
Looks different
Paradigm Shifts
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
of Northern New York
Source: Clyde H. Smith/Allstock/Tony Stone Images
Anticlines and Oil
Early USA
petroleum
exploration, e.g.
Pennsylvania
anticlines
Faults and Oil
End of Chapter 9
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