Download Plate Tectonics – How it Works

SIO 226: Introduction to
Marine Geophysics
Plate Tectonics
John Hildebrand
Scripps Institution of Oceanography
May, 2004
Plate Tectonics, Sea-Floor Spreading and
Continental Drift
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Main Parody
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Plate Geography
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Lineated Magnetic Anomalies in the Oceanic Crust
From:
R.G. Mason and
A.D. Raff
“Magnetic Anomalies
Off the West Coast
Of North America”
GSA Bulletin 72, 1259
1961.
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Three Interpretations of Magnetic Profiles
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Volcanic Intrusions
Basement Faulting
Variations in
Magnetization
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Horizontal Displacements in the Floor of the Pacific Ocean
V. D. Vacquier, “Magnetic Evidence for Horizontal Displacements in
The Floor of the Pacific Ocean” GSA Bulletin 72, 1250, 1961.
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Magnetic Anomalies Offset by Deep-Sea Escarpments
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Magnetic Anomalies Over Oceanic Ridges
F. J. Vine and D. H. Matthews -- Nature 199, 947, 1963
1.
Magnetic Field Reversals
2.
Ocean Crust Records Field Direction
3.
Sea Floor Spreading at Ridges
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Magnetic Reversal Time Scale
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Plate Tectonics: How it Works –Cox and Hart
Definitions and Symbols
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Plate Tectonics – How it Works
Motion of B is oblique to all
boundaries
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Motion of B is parallel to one
boundary – a transform.
Plate Tectonics – How it Works
Linear velocity
of point b
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The velocity fields of (b) and (c) are both
consistent with the assumed relative motion
of plates A and B about Euler pole E.
Plate Tectonics – How it Works
The Velocity Field of a Geologically Realistic Plate
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Plate Tectonics – How it Works
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Two sets of plates with the same spreading rates, but
different relative velocities
Plate Tectonics – How it Works
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Find a velocity field consistent with all boundaries …
Which are impossible?
Juan de Fuca Plate Motions
The Juan de Fuca Ridge is
adjacent to a series of
transforms that are not all
parallel.
 See (1), (2), (3), (4) and (5).
 Is the JdF Ridge at the
boundary between the Pacific
Plate and the North America
Plate?
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Juan de Fuca Plate Motions
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Introducing the JDF Plate
eliminates the inconsistency
in the directions of the
transforms.

Taking the NA Plate as
fixed, the velocity of the
JDF Plate relative to NA is
toward the northeast, a
convergent margin
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Mercator Projection
•Projection of a globe upon a cylinder
•Progressive distortion at high latitudes
•Great circle paths are generally NOT straight lines
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Oblique Mercator Projection
•Defines a new “pole” for projection and “equator” of
new pole is a straight line on the resulting map
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Mercator Projection of Fracture Zones
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After LePichon et al.
Oblique Mercator Projection as a
tool for finding poles relative motion
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Pole of projection for South Pacific 69-N 157-W
Absolute Motion: Hot Spot Tracks
Model of formation of Hawaiian Island chain by Hot Spot Volcanism
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Absolute Motion: Hot Spot Tracks
Age in Ma for volcanoes in the Hawaiian-Emperor Chain
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Comparison of Hawaiian and Louisville
Hot Spot Chains

Matching of hotspot
chains in the North
and South Pacific
about inferred poles
of Pacific plate
motion
After: Lonsdale JGR
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Comparison of Age Progression
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Plate Driving Forces: Thermal Convection
Do Ridges and Trenches delimit the limbs of a convection cell?
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Driving Forces Acting on Plates: After Forsyth and Uyeda
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Mantle and Continental Drag Forces – proportional to the area
and velocity of plate
Ridge Push – gravitational potential energy
Slab Pull – subducting slab density
Transform Fault Resistance – faulting occurs at critical yield so
force is velocity independent
Colliding Resistance – faulting occurs, velocity independent
Trench Suction – flow of astenosphere related to subduction
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Review of Plate Tectonic Model
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Oceanic crust is created at ridge crests
Ridge crest is segmented by transform faults
Oceanic crust is re-cycled at subduction zones
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