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PLATE TECTONICS
Last chapter in Davis and Reynolds
OUTLINE OF LECTURE
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2.
Earth engine
Plumes
Basic ingredients in plate tectonics
Plate kinematics
In 2-D
On a sphere
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
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?
Mantle drag forces and assembly of supercontinents
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Mantle convection
• Time scales
• Length scales
• Plume heads and continental breakup
T - scale ~ plate motions
Length scales - appear much more complicated than
the ridge-trench systems
Model linking subduction to plume magmatism
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Continental break-up: plumecaused?
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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
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
Zircons - as old as 40- 4.2 Ga; evidence for continental crust
Continents-succession of orogenic events
Future is fairly bright as far as PT goes. But after a
while, (4 more Ga?), the Earth’s engine won’t have
enough power to drive plate. Convection will stop, so
will PT.
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Basic kinematic elements
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Plate boundaries, triple junctions
Absolute plate motion, relative plate motion
Euler poles
Worked examples
Ridges, trenches, transforms
Triple junctions, quadruple j’s
3riple junctions are stable; more plates at a point - not stable
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.
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.
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.
Examples
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Worked exercise
It’s a right lateral transform boundary
Finding the relative
velocity of Farallon to
North America
Complicating a bit- what if the transforms are curved?
We then have to admit there’s 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.
Euler poles
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
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
Find distances on a sphere; use lat long and g
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.
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.
• 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.
• 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
• 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.
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).
• 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
Convergent Tectonics
Thermal Convection
and Viscosity of a Fluid
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.
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.
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
Possible driving forces for plate
tectonics:
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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)
• 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.
• Bouyancy driven by gravity acting on
density contrasts caused by thermal
differences and phase changes.
Driving Forces