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Plate Tectonics
Eric Calais
Purdue University
Department of Earth and Atmospheric Sciences
West Lafayette, IN 47907-1397
[email protected]
http://www.eas.purdue.edu/~calais/
Plate tectonics
• Developed in the 1960’s and 70’s from two basic ideas:
– Paleomagnetism ⇒ Apparent Polar Wander paths for continents
⇒ Continental drift [plus paleontological and climatological
evidence]
– Ocean floor magnetic anomalies (reversals) ⇒ Sea floor
spreading
• Other pieces of the puzzle:
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⇒
Earthquake distribution
Earthquake focal mechanisms
Structure of the Earth
Definition of plates and of their motions
• Has become the unifying theory of geology and geophysics
for explaining earthquakes, volcanoes, mountains, ocean
basins and other major earth phenomena
History…
• Fit between coasts of South
America and Africa: Abraham
Ortelius (1596) and Francis
Bacon (1620)
• Alfred Wegener, 1912,
observed mismatch of climate
features:
• Coal in Antarctica.
• Glacial deposits in now arid
regions of Southern Continents.
History…
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Additional observations:
– The same fossils found on several
continents: Glossopteris (Fern),
Lystrosaurus and Cynognathus
(Triassic reptiles), Triassic reptile
(freshwater reptile)
– Paleozoic mountain belts
•
Wegener proposed that:
– Continents move about the Earth’s
surface = theory called Continental
Drift.
– All continents were part of single
supercontinent in Paleozoic = Pangea
•
Pangaea starts to break apart 200
million years ago:
– Two continents: Laurasia (Northern)
and Gondwanaland (Southern)
– Oceans form in between.
History…
Wegener’s ideas were rejected because:
– His theory proposed no physical mechanism to
move the continents
– There was no direct measurement or
quantitative proof that continents had moved
– It was unclear how continents could move
through ocean basins.
Mapping the ocean floor
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Prior to 1900s, sea floor was thought to be flat.
WWI and WWII: sonar used to map the ocean floor.
Linear underwater mountain ranges in every ocean = Mid-Ocean Ridges
1947: the sediment layer on the ocean floor is very thin.
If oceans were the same age as the continents (billions of years), where was all
the sediment?
Oceanic crust not as old as continental crust.
Magnetic maps of the seafloor made in the 1950s showed evidence of polarity
reversals mirrored on both sides of a mid-ocean ridge.
Harry Hess, Princeton, 1962:
– The sea floor separates along the mid-ocean ridges = sea-floor spreading.
– New crust is formed at the ridge from upwelling magma, pushed (or pulled?) laterally
away from the ridge.
– Older crust destroyed at trenches.
Mapping the ocean floor
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Mid-ocean ridges
Fracture zones
Continental margins
Trenches
Seamounts:
– Isolated
– In chains
Heezen and Tharp, 1977
Cont.
margin
Abyssal plain
Mid-oceanic ridge
Continental shelf
Continental
slope
Seamount
Bathymetric profile across the Central Atlantic
Mapping the ocean floor
Earthquake distribution
• Vast aseismic areas
• Earthquakes concentrated along narrow bands
Earthquake focal mechanisms
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Extension at mid-ocean ridges
Strike-slip perpendicular to MOR
Compression at trenches
Earth structure
• Cold and rigid crust and
upper mantle =
lithosphere, brittle
• Hot and weak layer
underneath =
asthenosphere, plastic
flow
• Lithospheric thickness
depends on temperature:
~1300oC at the base of the
lithosphere (beginning of
partial melting of mantle
peridotites)
Putting it all together: 1. plates
Present-day plates and their boundaries
Putting it all together:
2. Plate boundaries
Spreading centers
•
FAMOUS project, 1975 (FrenchAmerican Mid-Ocean Undersea Study)
– Used Alvin to dive on a segment of the
Mid-Atlantic Ridge
– Observed pillow basalts, normal faults
•
1979: discovery of hot springs (“black
smokers”) at MOR (East Pacific Ridge
near Galapagos)
Spreading
centers
• Anomalies mapped by
magnetometers towed
behind research vessels
• Vine and Matthew,
1963:
– Linear magnetic
anomalies
– Parallel to ridges
– Symmetrical pattern
w.r.t. the ridge
Spreading centers
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Harry Hess proposed the idea of
sea floor spreading (“An essay in
geo-poetry”): seafloor is created at
ridges by volcanism and spreads
outward from them
Magnetic anomalies result from
new oceanic crust formed at the
ridge axis
Continuous process, as the Earth’s
magnetic field changes polarity
every ~0.5 My or so
Magnetic reversals give the age of
the crust
Distance from the ridge axis shows
how fast spreading occurs.
Fast versus slow ridges
Spreading
centers
• Magma chambers below
ridges
• Magma rises (buoyant)
forming dikes that feed
pillow basalts
• Part of magma that slowly
cools at depth forms gabbros
• Residual = dense minerals
(= olivine) “sediment” in the
magma chamber =>
cumulates
• In addition:
– Sedimentation
– Hydrothermal system
Ophiolites
• “Green rocks” found
in mountain belts
• Show the
superposition of
rocks found at midoceanic ridges:
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Pillow basalts
Sheeted dikes
Gabbros
Peridotite cumulates
A famous ophiolite
complex: Oman
Hydrothermal Systems
Depth and heat flow
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Oceanic lithosphere cools (and thickens) as it moves away from ridge axis =>
contracts and becomes denser
Heat flow decreases as the square-root of age
Isostasy => ocean basins deepen as the square root of age of the oceanic crust
Gravitational potential difference between ridges and basins => ridge push
Age of ocean
basins
• Ocean Drilling
Program
• Age of seafloor
inferred from
magnetic anomalies
confirmed
Transform faults
Transform
faults
Transform faults
• Arrows show velocities on plates A and B. Spreading
centers are linked by transform faults.
• Transform faults accommodate strike-slip motion
between plates A and B.
• There is no slip occurring outside of the transform fault
(dashed line = fracture zone)
A famous transform fault:
the San Andreas fault
Subduction zones
• 1930: earthquake
depth increases with
distance from the
trench
• Defines an inclined
plane, sometimes
with a complex shape
• Wadati-Benioff plane
(1935)
The Wadati-Benioff zone under northern
Honshu, Japan, showing two parallel planes of
earthquake loci. VF indicates the volcanic
front, at the center of the land area (from
Hasegawa, 1989).
Earthquakes at subduction zones
• Usually compressive mechanisms dominate for the deepest
earthquakes (depths > 300-350 km). This is due in part to
the increased resistance to slab penetration in response to
higher mantle viscosity; and in part to the presence of the
olivine-spinel phase change.
Seismic tomography at
subduction zones
Seismic tomography shows the subduction of
cold oceanic lithosphere under continents
Subduction zones
Distribution of down-dip stresses in inclined seismic zones. Open circles show mechanisms with the
compressional axis parallel to the dip of the zone. Solid circles show mechanisms with the tensional
axis parallel to the dip of the zone. Crosses indicate mechanisms with other orientations. Solid lines
show the approximate configuration of the seismic zone (from Isacks and Molnar, 1971).
Temperature structure
Mantle
wedge
Model of the thermal structure of a subduction zone
(convergence at 6 cm/year)
⇒ oceanic crust of subducted slab will only melt at
depths greater than 600 km
⇒ BUT: volcanic arc above temperature of 200-300C
in the the slab (=depths of 120-140 km), which is too
cool for the ocean crust too melt, but warm enough to
drive off fluid into the overlying mantle wedge.
⇒ Fluids decrease the melting point of mantle material
in mantle wedge => magmatism
Back-arc (=marginal) basins
http://epsc.wustl.edu/seismology/old_website/Labatts/labatts.html
Accretionary prisms
(Nankai trench)
A “typical”
subduction
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Trench
Accretionary prism
Magmatism:
– Volcanoes lie about 125 to
175 km from the oceanic
trench
– Produce andesitic lavas (more
silicic than basaltic)
– Some of the subducted
material (mostly sediments
and recycled oceanic crust) is
incorporated in these lavas.
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Back-arc basin
A “typical” subduction
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The ultimate fate of subducted
slab is not certain:
– The absence of earthquakes
below 700 km does not mean
that the slab has been totally
integrated into the mantle
– There is evidence that beneath
some long-lived subduction
zones, slabs have penetrated
into the lower mantle through
the 670 km discontinuity
– There is evidence of large
"anomalously cold blobs" in
the lower mantle that could be
the diffused remnants of
ancient subducted slabs.