Theory of Plate Tectonics: Mechanism 1

Theory of Plate Tectonics:
• Combined Seafloor Spreading and Subduction
– Lithosphere created at spreading ridges and consumed at
subduction zones
– Continents along for the ride
• Mechanisms?
Mechanism 1: Slab Sinking
• Subducting slab pulls lithosphere along
• Problems:
– Translate stress across slab
– No Atlantic subduction (next slide)
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Problem: no Atlantic Subduction
Except for
small
Scotia and
Puerto
Rico Arcs
Plate Tectonic Mechanism 2:
• Ridge Push: Sloped density contrast (Lith vs. Asth) is unstable
– Horizontal force works to flatten this slope to stable configuration
• Problem: Earthquakes are tensional (pulling) not compressional
(pushing).
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Plate Tectonics mechanism 3:
• Asthenosphere convection:
– Regional flow of asthenosphere shears plate
– Forces plate along
Plate boundaries are defined by locations of Earthquakes.
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Plate Boundaries
• ~15 plates
• Only 3 types of boundaries
• Divergent, convergent, transform
Divergent Boundaries:
• Plates spread apart
• Mantle rises, melts
and forms new
lithosphere
• OCEANS are
formed
• Continents rift.
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Continental Rifting:
animation
Cont. rifting culminates in mature ocean basins
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Transform Boundaries: plates slide past each other
• Very common in ocean b/c they link ridge segments
– They also link trenches to ridges, trenches to trenches, etc.
• Required because
– spreading ridges are linear & Earth is spherical, and
– Spreading rates vary along ridges
San Andreas:
Show animation
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3rd Type - Convergent Margins
• Where plates collide - continents
eventually grow
• Two types:
– collision zones (e.g. Himalaya) and
– Subduction zones (e.g. Japan, Andes,
Aleutians)
Subduction Zones:
Animation
• Build island arcs (e.g. Japan, Aleutians) and
continental arcs (e.g. Andes)
• Consume lithosphere
• Grow continents by addition of island arcs.
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Collision Zones:
• Evolve from
subduction zones
• Build large
mountains
• ‘stuff-up’ subduction
with continental
mass
Plate Motions
How can we understand plate motions: their velocities
and paths over time?
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Spreading Rate = Velocity (distance/time) of plate divergence
• Calculate velocity from
– age of crust
– Distance from axis
• Recall paleomagnetic
time scale?
– Brunhes-Matuyama 0.7
ma
– Matuyama-Gauss 2.4
ma
• Notice that Spreading rate
is RELATIVE to the axis!
If axis is moving…
• Spreading rate is
averaged over time
2.4 Ma
0.7 Ma
Hot-spots and rates of plates:
Absolute Velocities
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What is a hot-spot (or plume)?
• Anomalously hot mantle (from coremantle boundary)
• Rises Buoyantly (plume)
• Melts on assent (adiabatic decompressive)
• Feeds volcanoes on Lithosphere
Create hot-spot track
See the hot-spot tracks? Samoa, Hawaii-Emperor,
Ninety-East Ridge, Tristan/Walvus Ridge, etc.
(they are ‘everywhere’!)
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How to make a hot-spot track
• Plume ‘burns’ through
lithosphere
• Plate motion continues
• New volcanoes form
• Example: Candle
burning a piece of
paper.
Animation - good more cartoons!
What info do hot-spot tracks provide?
• Assuming fixed HotSpots:
– Direction of plate
motion (absolute)
– Absolute Velocity
• Do they agree with
spreading rates?
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Absolute vs. Relative plate vectors
Do they always agree? Blue vs. red vectors
Not always, spreading rate is plate motion relative to divergent boundary. Hotspot track/absolute velocity is velocity of plate relative to fixed reference
Plate Tectonics & Wilson Cycle
• ~15 plates interact
at margins
• Created at MOR
and consumed at
SZ
• Wilson Cycle…
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Wilson Cycle:
• Ocean basins open & close over geologic time
• Continents grow by
– Accumulation of sediment in intercontinental basins
– Accumulation of island arcs as SZ collide with continents
Wilson cycle: watch oceans
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Pangaea! Panthalassic & Tethys
Open Tethys rift Pangaea, open Atlantic, Pacific:
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Close Tethys, open Atlantic, begin closing Pacific:
Today:
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