Transcript Lecture 1b: Plate Tectonics: the Earth as a System

Lecture 1b: Plate Tectonics: the Earth as a System
• Up to ~40 years ago, geology was a large collection of somewhat
disconnected observations and local knowledge. The advent of
plate tectonics organizes most of geology into a coherent,
physically-based framework, and is therefore of paramount
importance in surficial geology as well as in geophysics and the
study of the deep interior.
• The postulates of plate tectonics are as follows:
– The silicate earth is divided into a lithosphere, which is cold and therefore
brittle and rigid, and an asthenosphere, which is hot and therefore ductile.
• Simple scaling arguments show that the asthenosphere is likely to convect.
The lithosphere is the cold, upper boundary layer of this convecting system,
but the extreme temperature dependence of rheology makes the system very
different from simple convection models
– The lithosphere organizes itself into a series of internally (almost) rigid
plates. These plates are mobile with respect to the asthenosphere and with
respect to each other.
• By various mechanisms, the strain needed to allow convective heat to escape
and cold material to return to the asthenosphere is concentrated into narrow
zones at the boundaries of plates
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Plate Tectonics: the Earth as a System
• Plate tectonics is a kinematic theory
– It specifies that the plates move and describes where deformation,
seismicity, volcanism, etc. occur
– The issue of dynamics, i.e., the driving forces for plate motion, is a
separate question
– Details aside, however, plate motions are the surface expression of the
Earth’s heat engine: the interior is hot, space is cold, the second law of
thermodynamics states that this gradient will drive spontaneous processes
in pursuit of equilibrium
• Only the Earth has plate tectonics at the present era. This implies
that it requires a combination of:
– the right heat budget (mostly a matter of size; compare Mars)
– the right rheology
• The surface must be rigid enough to make plates (compare Jupiter)
• but weak enough to localize strain (compare Venus)
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Plate Tectonics: the Earth as a System
• The boundaries between lithospheric plates are of three kinds
– Divergent, Convergent, Transform
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Divergent Plate Boundaries
• Where lithospheric plates are moving away from one another at
their boundary, new lithosphere must be created. This is
accomplished by mid-ocean ridges and continental rifts.
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Convergent Plate Boundaries
• Where lithospheric plates are moving towards one another at
their boundary, lithospheric area must be consumed. This is
accomplished by subduction or thickening and delamination.
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Transform Plate Boundaries
• Where the motion of two plates is parallel
to their boundary, lithosphere is neither
created nor deformed, but strain is
concentrated and seismicity is common.
In all three cases, the response of the
lithosphere is dominated by faulting, the
primary mechanism for strain concentration to
narrow boundaries
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Hotspots
• To this simple scheme we must add at least one other important
element of mantle convection that is not directly associated with
the rheology of the lithosphere:
– Plumes: some 10% of heat flow across the mantle is thought to be carried
by rising plumes rather than subduction of cold lithosphere. This mode of
convection is driven by basal heating rather than surface cooling. Plumes
are presumed to lead to long-lived centers of intraplate volcanism as well
as large episodes of volcanism often associated with the formation of new
divergent boundaries.
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Evidence for Plate Tectonics
The continents move? Humbug…can you prove it?
• The fit of the continents. This was
noted almost as soon as good maps
were available, namely after the
solution of the problem of longitude
in 1735 (maps before this are
distorted since only latitude could
be measured precisely).
• Lithologic and Paleontological
correlations across the Atlantic. In
1915 Alfred Wegener added
observations of Paleozoic
similarities between S. America and
Africa as evidence for the former
proximity of the continents. He
proposed that all landmasses were
formerly joined in the
supercontinent Pangaea.
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Evidence for Plate Tectonics
• Mapping of the seafloor: discovery of mid-ocean ridges and deepsea trenches, 1950-1960, by sonar bathymetry measurements.
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Evidence for Plate Tectonics
• Mapping of the seafloor: this is now done globally and precisely
with satellite gravity data (calibrated by ship tracks)
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Evidence for Plate Tectonics
• Distribution of volcanism
– Volcanoes are found along narrow belts; although volcanism has many
causes, it is a sign of activity (vertical motions, large heat or mass transfers)
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Evidence for Plate Tectonics
• Seismicity: earthquake locations are
mostly restricted to narrow zones
along mid-ocean ridges, oceanic
trenches, and continental margins.
In cross-section, the Wadati-Benioff
zone clearly suggests the
mechanism of subduction.
Seismicity is a sideeffect of the
deformation of rocks;
hence the seismicity
distribution is almost
prima facie evidence of
the internal rigidity of
plates and the strain
concentration at plate
boundaries.
Red: shallow
Green: intermediate depth
Blue: Deep
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Evidence for Plate Tectonics
• Age of the seafloor from sediment thickness and biostratigraphy.
The age of the oldest sediment in a section (deposited on igneous
basement) clearly gets systematically older with distance from the
mid-ocean ridges. This data is gathered by deep-sea drilling.
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Evidence for Plate Tectonics
• Magnetic lineation of the seafloor. When new ocean crust forms it
acquires a readily measurable remanent magnetism. The pattern of
polarity reversals known from terrestrial magnetostratigraphy can
be read as symmetric lineations around the mid-ocean ridges that
demonstrate seafloor spreading and establish its rate.
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Evidence for Plate Tectonics
• Geodetic observation. To all the traditional lines of evidence, we
must now add direct measurement of plate velocities by geodesy
(VLBI and GPS).
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Mantle Convection
• Convection is the transport of heat across a gravitational potential by
bulk motion driven by buoyancy forces due to thermal expansion
• Whether a layer will convect rather than remain stationary and
transport heat by conduction depends on the following variables:
–
–
–

–
–


Q: the magnitude of heat flow across the bottom of the layer
A: the magnitude of heat generated within the layer
d: the thickness of the layer
a: the thermal expansivity
g: the acceleration due to gravity
k: the thermal conductivity
k: the thermal diffusivity
n: the kinematic viscosity
• In particular, a layer will be unstable to overturning if the
dimensionless Rayleigh number Ra exceeds a critical value ~103
agd (Q + Ad )
Ra =
kkn
4
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Mantle Convection
• What is Ra for the Earth’s mantle?
– We know g and d, obviously. a and k are measured in the laboratory on
mantle minerals. n is measured by postglacial rebound or other geophysical
methods as well as laboratory measurements. (Q+Ad)/k follows from heat
flow measured at the surface.
Layer
Upper mantle
Lower mantle
Whole mantle
thickness (km)
700
2000
2700
Ra
106
3 x 107
108
• Conclusion: even though the mantle is rigid enough to
transmit shear waves, it is grossly unstable to convective
overturn, and expected to convect vigorously.
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Geodynamic setting of major rock suites
• Most geology can be understood by situating the
environment of rock formation in a plate tectonic
context.
– That is, the active regions where rocks are being formed today
are situated either at particular kinds of plate margins or in
continental or oceanic plate interiors, but most of the action is
at plate boundaries
– When we recognize characteristic sequences of ancient rock
types and deformation patterns, we can connect them to
processes that we see today.
– The modern, active examples help us to interpret ancient
formations.
– Here are the major environments where rocks are made, with
modern and ancient examples of each…
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Geodynamic setting of major rock suites
Oceanic crust
– Plate spreading creates a very characteristic sequence of rocks.
It is found anywhere one drills into oceanic basement. It was
recognized as a characteristic assemblage on land and named
ophiolite before the nature of the oceanic crust was known.
Ophiolites are pieces of ocean crust obducted onto continents
by the tectonics of convergent margins.
The characteristic
assemblage, from the top
down is:
• Deep-sea marine sediments
• Massive sulfide deposits
• Pillow basalts
• Sheeted basaltic dikes
• Layered gabbro
• Serpentinized peridotites
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Geodynamic setting of major rock suites
• Oceanic crust
– Modern example: Hess Deep, Equatorial Pacific
This sequence results
from both the tectonic and
petrogenetic processes at
mid-ocean ridges. More
on mid-ocean ridges in the
next lecture
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Geodynamic setting of major rock suites
• Oceanic crust
– Ancient example: Oman ophiolite
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Geodynamic Setting of Major Rock Suites
Passive continental margin
• When a continent is rifted and a new continental margin
forms, it soon becomes a passive margin in the middle of a
plate as the coast moves further from the ridge axis (example:
east coast of North America).
• A passive margin is the edge of a continent, but not a plate boundary
• A characteristic and reproducible geological sequence results:
• massive basaltic volcanism and normal faulting
associated with initiation of rifting
• shallow water sediments: evaporites, delta deposits,
carbonate shelf
• a thick wedge of terrigenous sediment, notably turbidites,
as thermal subsidence and loading of the margin provide a
sink for material eroded from the continent
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Geodynamic
Setting of Major
Rock Suites
Passive continental
margin
Modern example:
US Atlantic Coast
Ancient examples:
Paleozoic Cordilleran
miogeocline
Proterozoic greenstone
belts of Africa, Canada
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Geodynamic Setting of Major Rock Suites
Ocean-ocean subduction
Subduction of one oceanic plate under another oceanic plate
occurs frequently, without the involvement of continental
material. Examples are the Aleutian arc, Marianas arc,
Antilles arc.
• Volcanic sequences mostly basaltic
• Accretionary prisms small because subducted plate typically carries only thin
coating of pelagic sediment
• Overriding plate behind the arc under extension and a back-arc basin may
develop, with a spreading center much like a mid-ocean ridge
Island arcs may be accreted onto continental margins, e.g. along
west coast of North America. Much of central Asia is constructed
from a series of Paleozoic island arcs and accretionary prisms.
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Geodynamic Setting of Major Rock Suites
Ocean-ocean subduction
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Geodynamic Setting of Major Rock Suites
Ocean-continent subduction
Where an oceanic plate subducts readily under a continental
margin, we generate an Andean type margin. High mountains,
stratovolcanoes, and large plateaux are characteristic.
• Thick igneous crust forms by intrusion and eruption of calc-alkaline magma
(basalt-andesite-dacite-rhyolite). At depth there are major batholiths of
intermediate to silicic intrusive rocks
• Forearc basin accumulates volcaniclastic sediment
• Accretionary prism is scraped off downgoing plate and piled up inboard of the
trench: a melange of oceanic rocks metamorphosed at high pressure and low
temperature
• The association of a high-pressure, low-temperature metamorphic belt with a
high-temperature belt of batholithic rocks parallel and ~200 km away is called a
paired metamorphic belt. Many can be recognized in ancient terrains.
Modern example: Cascades
Ancient example: California (the paired metamorphic belt here is the
Franciscan Complex and the Sierra Nevada)
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Geodynamic Setting of Major Rock Suites
Ocean-continent subduction
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Geodynamic Setting of Major Rock Suites
Continental collision zone
At the closing of an ocean basin, two continents collide. Continental crust is too
buoyant to subduct. Instead the crust is doubled-up, shortened, folded, and
overthrust in a mountain building event or orogeny.
Many important ancient rock sequences (e.g. Appalachians) are recognized, by
comparison to active (Himalaya) or recent (Alpine) mountain belts, to be
derived from collisional orogenies.
• Central part of range is built from uplifted and heavily deformed oceanic and
continental shelf sediments and slices of ophiolite from the vanished ocean.
• Collisional orogens begin as ocean-continent subduction zones; associated rocks are
incorporated in the collision. Volcanism may continue during the collision.
• Large mountain belts shed lots of sediment into surrounding basins. During mountainbuilding, deep-sea basins receive turbidite deposits called flysch which is folded and
deformed as the orogeny continues. Later sediments called molasse are deposited in
shallow-water or subaerial basins.
• Extensive regional metamorphism exposed at the surface as rocks from depth are
brought to the surface by thrust faulting and erosion of cover.
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Geodynamic Setting
of Major Rock Suites
Contintental Collision
Modern example:
Himalayan orogeny
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Geodynamic
Setting of Major
Rock Suites
Contintental
Collision
Ancient
example:
Appalachian
orogeny
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