PLATE TECTONICS

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PLATE TECTONICS

• Last chapter in Davis and Reynolds

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OUTLINE OF LECTURE

• Earth engine
• Plumes
• Basic ingredients in plate tectonics
• Plate kinematics
• In 2-D
• On a sphere

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Review of major questions

• Earth layering
• The composition of the crust
• Rheology of the Earth (lithosphere,
  asthenosphere)
• Types of plate boundaries

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Magnetic anomalies
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Spreading at mid-ocean ridges must be compensated
by subduction. In addition,there are transform
faults in the oceans. Note no volcanism on
diagram.
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What drives plate motion?
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Mantle drag forces and assembly of supercontinents
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Mantle convection

• Time scales
• Length scales
• Plume heads and continental breakup

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T - scale plate motions Length scales - appear
much more complicated than the ridge-trench
systems
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Model linking subduction to plume magmatism
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Continental break-up plume-caused?
Sometimes clearly not. Other times, major oceans
appear to form during times of major flood
basalts -short lived, vigorous plume heads that
may have broken the continents apart
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Plate T throughout Earth history

• How far back in the past?
• Different in the past?
• How much longer will it last?

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Evidence for PT goes back to the Archean. Faster
motions, more melt, smaller continents (the
continental nuclei known as cratons or croutons)
Granite-greenstone belts old zircons
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Zircons - as old as 40- 4.2 Ga evidence for
continental crust
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Continents-succession of orogenic events
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Future is fairly bright as far as PT goes. But
after a while, (4 more Ga?), the Earths engine
wont have enough power to drive plate.
Convection will stop, so will PT.
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Basic kinematic elements

• Plate boundaries, triple junctions
• Absolute plate motion, relative plate motion
• Euler poles
• Worked examples

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Ridges, trenches, transforms Triple junctions,
quadruple js 3riple junctions are stable more
plates at a point - not stable
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Absolute plate motions - velocity in an absolute
reference frame- say relative to a point outside
the Earth. Or an assumed stationary long lived
plume. E.g. Hawaii Otherwise, one uses a
relative velocity reference frame. One plate is
kept stationary the velocity of the others
relative to the stationary plate is monitored.
The understanding is that the entire system
(including the stationary plate) is actually
moving on the globe. In the case of ridges, we
use the half spreading rate for velocity
calculations.
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Absolute framework - consider Hawaii a stationary
plume (it delivers melts in exactly the same spot
over its entire history). We can calculate the
velocity vector of the Pacific plate.
75-43 - N20W x cm/yr 43-0 Ma N70 W, y
cm/yr.
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There are very few such long lived plume products
and it is questionable whether they remain fixed.
The common way of tracking plate motions is in a
relative framework.
Some useful rules 1. Plate motions are transform
parallel 2. Plate moves away from
ridge 3. The sum of relative plate
velocities is zero.
- that is because by definition plates are
rigid.
Velocity is a vector magnitude, direction and
sense.
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Examples
2.
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Worked exercise
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Its a right lateral transform boundary
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Finding the relative velocity of Farallon to
North America
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Complicating a bit- what if the transforms are
curved? We then have to admit theres some
rotation involved. Any rotation is achieved
around a pole. From geometry, this is called the
Euler pole. Transforms form arcs that are
segments of circles centered in the Euler pole of
a plate.
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Euler poles
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Example Australia and New Zeeland
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Plate tectonics on a sphere

• Angular velocity, linear velocity
• Rotations around Euler poles
• Projections on stereonets

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Tectonics on a sphere requires that we use
angular velocities ??v/r and r R sin g where
R is the radius of the Earth.
So what? Check out the fig - predicts motion away
from Euler pole. In this case - 2 plates with E
at N pole
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Find distances on a sphere use lat long and g
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The projections used in 3D plate tectonics are
stereonets - equal area - however unlike your
usual down view with geo structures, this is a
side view. All calculations (angles etc) are
similar.
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What you need to know

• The fundamentals of plate tectonics, driving
  forces link to mantle convection
• Differences between present day and past
  characteristics of PT
• Be able to handle simple 2-3-4- plate geometry
  problems in 2D involving only translations.
• Calculate velocity vectors for such examples
• Know what the Euler pole is and angular vs.
  linear velocity. Be able to find one if you have
  the other.

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• The lithosphere is divided too into layers upper
  mantle, lower crust, upper crust. There are
  essentially two types of lithosphere oceanic and
  continental. You should know the approximate
  composition and dimensions of each of these.

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• The lithosphere is broken into about 8 large
  plates and a number of smaller ones. You should
  know the geography of these plates where the
  boundaries are, what types of boundaries these
  are, and roughly how the plates are moving with
  respect to the hot spot

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• Earthquakes reveal the subsurface geometry of
  subducting plates (or slabs as they are often
  called), and show that the configuration of the
  slab can be quite variable. These seismic zones
  are called Wadati-Benioff zones.

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Extensional Tectonics

• Extension of the lithosphere may occur by several
  means by a whole-scale pure shear (in which
  extension of the whole column occurs, the lower
  crust and upper mantle homogeneously) or by
  various asymmetric means (in which extension of
  the upper crust is laterally offset from the
  lower crust or upper mantle extension may also
  be relatively discrete rather than homogeneous).

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• Normal faults may be planar or listric, the
  latter is more commonly shown. Faults in the
  upper crust are thought to sole into detachment
  faults that then transfer extension of the lower
  crust to another location. Faults are often
  considered passive or inert features where the
  hanging-wall slides down the fault and does all
  the deforming. But in fact, the footwalls of
  normal faults do a lot of deforming, too. Hence,
  the margins of rift or extensional zones are
  often lifted to form imposing flanks

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Convergent Tectonics
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Thermal Convectionand Viscosity of a Fluid
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Convection in the Earth

• Thermal convection is inferred to exist on a
  large scale in at least two regions in the Earth.
  The liquid outer core and the upper mantle that
  behaves as a solid for seismic wave propagation
  and as a very viscous fluid for long duration
  geologic processes including convection.

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Convection Reasons

• Original heat from accretion and heat released
  during radioactive decay of unstable isotopes.
• The natural, spontaneous, radioactive decay of
  unstable isotopes of elements that are
  distributed throughout the Earth, particularly in
  the crust and mantle.

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Viscosity Experiments

• Newtonian viscosity is a law of friction for
  fluids.
• Viscosity is defined as the shearing stress
  divided by the rate of shear for the fluids.
• Viscosity can be thought of as resistance of a
  fluid to flow

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Possible driving forces for plate tectonics

• bottom tractions by convection currents.
• trench pull.
• ridge push (sliding off a high)
• trench suck.
• global expanding or contracting forces
• membrane forces on spinning ellipsoid (e.g.
  variants of polar fleeing forces)

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• coupled currents in the mantle raft the
  overlying plates around. Traction stresses at the
  base of the plates would be critical.
• decoupled plates move due to internal body
  forces, and influence the shallow convection
  current pattern in the mantle.
• locally exclusive, but not globally.

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• Bouyancy driven by gravity acting on density
  contrasts caused by thermal differences and phase
  changes.

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Driving Forces
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