Faults Folds and Mountains

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Transcript Faults Folds and Mountains

Folds, Faults, and
Mountains
Pencil
Rubber band
Gum
Foam sediments
Cardboard fault models
Plastic box
Food coloring
Paper
Begin Chewing Gum
Fold and Thrust Mountains
• Enormous mountain ranges form when
plates converge.
• Contorted rocks show the power of
plate tectonics.
• Formerly horizontal layers are twisted,
bent, or broken.
• Some folded rocks are pushed over on
their sides, or even upside down.
Evidence of Lateral Compression
Convergent Plate Boundaries
and Folding
Continent-Ocean
collision forms
Continental Arc:
Andes Cascades.
Continent-Continent
collision forms
Folded Mountain Belt:Alps,
Himalayans, Appalachians
Compression, tension and shear forces stress
the rocks, causing them to strain i.e. “give”
Units of Stress Force / Area
Convergent
Divergent
Transform
Relation
ship
Between
Stress
and
Strain
Rubber Band
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
Ruler, Pencil
Relationship
Between
Stress and
Strain under
high Temps
or Pressure
Chewing Gum
Strike and Dip
Map Symbols: Strike shown as long line, dip
as short line. Note the angle of dip shown: 45o
Strike intersection w horizontal, dip perpendicular, angle from horizontal down toward surface
Folded Rocks, Hwy 23
Newfoundland, New Jersey
Note highest point
Foam Strata
Source: Breck P. Kent
Adjacent Anticline and Syncline
Folded Rocks (Dorset, England)
Center has overturned area
Foam strata
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
Various Folds
Various Folds (cont'd)
Various Folds (cont'd)
Various Folds (cont'd)
Axis
Axial plane near axis should be close 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
Nosed folds, therefore plunging
Source: GEOPIC©, Earth Satellite Corporation
3-D: Dome and Basin
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
Fractures
•
• Fractures
•
- Joints: fractures with no relative
movement
•
- Faults: fractures with relative
movement
Source: Martin G. Miller/Visuals Unlimited
Demo: Cardboard Models
Dip-Slip
Faults
Normal Fault: Hanging Wall Down
Hanging wall overhangs
the fault plane
Foot wall under the
fault plane
Hanging wall is down
Source: John S. Shelton
Especially common in divergent margins
Dip Slip Faults
This
guy is
rich
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
Reverse
Normal
This poor guy is out of luck
Miners pay geologists to
find their lost orebody
One friend earned
enough to buy a house
http://pangea.stanford.edu/~laurent/english/research/Slickensides.gif
Fracture Zones and Slickensides
a)
b)
c)
Visible displacement of rocks
Pulverized rock and “Slickensides”
Key beds cut out by faulting reappear elsewhere.
Types of Faults - 2
•
1)
2)
3)
4)
Strike-slip faults
Example: San Andreas
Transform fault
Distinctive landforms (linear
valleys, chains of lakes, sag
ponds, topographic saddles)
Fresh pulverized rock.
Transform fault through granite:
Arkose sandstone
Evidence of Shear stress
Horizontal Movement Along
Strike-Slip Fault
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
Graben in
Iceland
Source: Simon Fraser/Science Photo Library/Photo Researchers, Inc.
Plate 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 Fault
Lewis Thrust Fault (cont'd)
Same layer
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
Mostly high angle normal faults ~9mya
The Grand Tetons in Wyoming
Source: Peter French/DRK Photo
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
We will discuss Buoyant subduction later
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