Plate%20Tectonics:%20The%20Unifying%20Theory

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Plate TectonicsThe Unifying Theory
Peter W. Sloss, NOAA-NESDIS-NGDC
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Plate Tectonics

• Fundamental concept of geoscience
• Integrates from many branches
• First suggested based on geology and paleontology
• Fully embraced after evidence from geophysics

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What tectonics theory explains

• Distribution of earthquakes and volcanoes
• Relationship of age and height of mountain
  belts
• Age distribution of oceanic crust
• Magnetic information in rocks

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What is Plate Tectonics Theory?

• Earths upper mantle (lithosphere) is broken into
  rigid plates which move with respect to each
  other
• Plates rest on and move in the asthenosphere
• There are 3 kinds of plate margins that are
  marked by earthquakes and volcanoes
• Divergent
• Convergent
• Transform
• The movement is driven by uneven distribution of
  heat within Earth and the mechanism that drives
  plate movement is not well known

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Mosaic of Earths Plates
Peter W. Sloss, NOAA-NESDIS-NGDC
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Theory evolved from 2 concepts

• Continental Drift - by Alfred Wegener
• Fit of continents - Pangaea
• Ancient climatic evidence - glacial deposits
• Fossil evidence - Glossopteris Mesosaurus
• Matched mineral zones mountain chains
• (Problem - no acceptable method of motion)

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Fit of the Continents
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Anomalous Distribution of Fossils(Mesosaurus)
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ANDPALEOMAGNETIC EVIDENCE

• Polar Wandering - either
• the North Pole or the continents had moved
• both Europe and North America had apparently
  moved as a single continent for several hundred
  million years

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Apparent polar wandering curves
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Seafloor Spreading

• Geopoetry of Harry Hess Robert Dietz
• New seafloor forms by upwelling at the center of
  MOR and moves laterally
• Older crust is destroyed in the subduction zones
  at the trenches
• Seafloor is younger than 200 MY
• Solved Continental Drift problem

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Modern Proof of Seafloor Spreading

• Deep Sea Drilling Project - sampled sea floor
  sediment crust
• Age thickness increased with distance from MOR
• Ages were symmetrical about MOR
• Magnetic Anomalies - found weak strong signal
• Alternating normal reversed polarization
• Stripes to symmetrical about MOR

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Magnetic Anomalies
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Ocean crust records magnetic reversals
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Age of Seafloor Crust
Fig. 20.11
R. Dietmar Muller, 1997
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More Proofs

• Hot Spots - deep, long-lived, stationary mantle
  magma sources
• Expressed at the surface by
• linear chain of volcanoes
• aged with distance from hot spot
• Over 100 identified
• Used as fixed points against which plate motion
  is measured

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Hawaiian-Emperor chain
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Long-lived global hot spots
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PUTTING IT ALL TOGETHER
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I. Plates

• Group of rocks all moving in the same direction
• Can have both oceanic and continental crust or
  just one kind
• Plate interior are relatively quiet
• Most activity is located at plate margins

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II. Types of plate boundaries

• divergent mid-ocean ridges
• convergent collision zones volcanic
  arcs
• strike-slip San Andreas Fault Alpine Fault,
  N.Z.

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A. Divergent plate boundaries

• Usually begin within continents as hot spot burns
  thru crust and eventually grows to become ocean
  basin
• Two kinds of divergent margins
• Mid Ocean Ridges (MOR)
• Continental Rift Valleys

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Features of Mid Ocean Ridges

• Central rift valley (width is inversely
  proportional to the rate of spreading)
• Region of high heat flow
• Shallow-focus earthquakes
• Almost exclusively basalt volcanism

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Earthquakes Associated with Divergent Margins
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Rifting and Seafloor Spreading Along the
Mid-Atlantic Ridge
Peter W. Sloss, NOAA-NESDIS-NGDC
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Rifting and Seafloor Spreading
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Features of Continental Rifts

• East Africa, Rio Grande rift
• Beginning of ocean formation (may not get that
  far)
• Rifting begins at a triple junction (two
  spreading centers get together to form ocean
  basin, one left behind).
• Rock types basalt and sandstone

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Hot Spot induced rifting

• Hot spot burns thru crust -gt 3 branched rift
• Divergence begins
• 2 branches are active
• 1 branch is not active
• Linear sea develops

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Inception of Rifting Along theEast African Rift
System
Peter W. Sloss, NOAA-NESDIS-NGDC
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Inception of Rifting Within a Continent
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Nile Delta
Gulf of Aqaba
Gulf of Suez
Red Sea
Earth Satellite Corp.
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The Gulf of California Formed by Rifting of Baja
California from Mainland Mexico
Worldsat International/Photo Researchers
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B. Convergent boundaries

• New crust created at MORold crust destroyed
  (recycled) at subduction zones
• 2 kinds subduction collision
• Relative important densities
• continental crust 2.8 g/cm3
• oceanic crust 3.2 g/cm3
• asthenosphere 3.3 g/cm3

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Convergent boundaries

• Three types
• oceanocean Philippines
• oceancontinent Andes
• continentcontinent Himalaya

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OceanOcean

• Island arcs Chain of volcanic islands
• Highly seismic tectonic belt of
• shallow to deep earthquakes
• High heat flow arc of active andesitic
  volcanoes
• Bordered by a submarine trench

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OceanOcean Subduction Zone
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OceanContinent

• Continental arcs
• Magmatic belt of active volcanoes (andesite to
  rhyolite)
• Often accompanied by compression of upper crust
  which builds mountains
• Bordered by a submarine trench

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Ocean-ContinentSubduction Zone
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ContinentContinent

• Continentcontinent boundaries, convergence is
  accommodated by
• Folding (shortening and thickening)
• Strike-slip faulting
• Underthrusting (intracontinental subduction)

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ContinentContinent Convergent Boundary
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Continent-Continent Collision
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Himalayas and Tibetan Plateau

• Product of the collision between India and
  Asia.
• Collision began about 45 M yr. ago, continues
  today.
• Before collision, southern Asia looked
  something like the Andes do today.

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Stages in the collision of India with Asia
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C. Transform Boundary Offsetting Spreading Centers
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III. Rates of plate motion

• Mostly obtained from magnetic
• anomalies on seafloor
• Slow spreading 3 cm/year
• Fast spreading 10 cm/year
• Very fast spreading 17cm/year

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Relative Velocity and Direction of Plate Movement
Data from C. Demets, R.Ggt Gordon, D.F. Argus, and
S. Sten, Model Nuvel-1, 1990
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IV. Rock assemblages and plate tectonics

• Each plate tectonic environment produces a
  distinctive group of rocks.
• By studying the rock record of an area, we can
  understand the tectonic history of the region.

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Layer of ophiolite suite.
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Precambrian Ophiolite Suite
Pillow basalt
M. St. Onge/Geological Survey of Canada
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V. Exotic or Microplate Terranes

• Large blocks that contrast sharply with
  surrounding area
• Wrong faulting, folding, fossils, rock types,
  metamorphism, magnatism
• Thought to be fragments of continents, seafloor,
  seamounts, island arcs that rafted in docked in
  new place

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Approaching Arc or Microcontinent
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Collision
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Accreted Microplate Terrane
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Microplate terranes Added to Western North
America Over the Past 200 Million Years
After Hutchinson, 1992-1993
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VI. Driving mechanism of plate tectonics

• Thought to be convection of the mantle.
• Friction at base of the lithosphere transfers
  energy from the asthenosphere to the lithosphere.
• Convection may have overturned asthenosphere 46
  times.

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Other factors

• Trench pull
• Plate sliding
• Ridge push

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Three possible driving factors
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Three possible mechanisms for the movement of
lithosphere over the asthenosphere
Fig. 17.17
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VII. Tectonic reconstructions

• A variety of evidence traces the motion of
  continents over time

• Paleomagnetism
• Deformational structures
• Environments of deposition
• Fossils
• Distribution of volcanoes

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Assembly of Pangaea
I.W.D. Dalziel, 1995
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Breakup of Pangaea
200 million years ago
After Dietz Holden, 1970
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Breakup of Pangaea
140 million years ago
After Dietz Holden, 1970
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Breakup of Pangaea
65 million years ago
After Dietz Holden, 1970
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Breakup of Pangaea
Today
After Dietz Holden, 1970
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Examining Deep-sea Drill Cores
Texas AM University
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Questions about plate tectonics

• What do we really know about convection cells
  in the mantle?
• Why are some continents completely surrounded
  by spreading centers?
• Why are tectonics in continental crust and
  oceanic crust so different?

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Cross Section of Western Canada
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Fig. 20.25a
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Formation of Magnetic Anomalies
Fig. 20.10
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Himalayas and Tibetan Plateau

• Models
• Underthrusting
• Distributed shortening
• Strike-slip faulting

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Wilson cycle

• Plate tectonics repeats itself rifting, sea-
• floor spreading, subduction, collision,
• rifting,
• Plate tectonics (or something like it)
• seems to have been active since the
• beginning of Earths history.

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After Hutchinson, 1992-1993
Fig. 20.22
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Examples of Plate Boundaries
O-C convergent
O-O divergent
C-C divergent
O-O divergent
O-O convergent
O-O divergent
O-C convergent
Fig. 20.8a,b
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Volcanic and Nonmarine sediments are deposited in
rift valleys
Fig. 20.17a
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Cooling and subsidence of rifted margin allows
sediments to be deposited
Fig. 20.17b
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Carbonate platform develops
Fig. 20.17c
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Continental margin continues to grow supplied
from erosion of the continent
Fig. 20.17d
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OceanContinent Convergent Boundaries
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Opening of the Atlantic by Plate Motion
Fig. 20.13
After Phillips Forsyth, 1972
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Idealized Ophiolite Suite - Oc. Crust
Deep-sea sediments
Pillow basalt
Gabbro
Peridotite
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Model for Forming Oceanic Crust at Mid-ocean
Ridges
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The growth of oceanic basin 1
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The growth of oceanic basin 2
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The growth of oceanic basin 3
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Age of the Ocean Basins
After map by Sclater Meinke
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Parts of an OceanOcean Convergent Plate Boundary
Fig. 20.18
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Parts of an OceanContinentConvergent Plate
Boundary
Fig. 20.19
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Continued Subduction
Fig. 20.20a
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Continent Continent Collision
Fig. 20.20b