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

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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 Earths 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)
• and 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? Humbugcan 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 deep-sea 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 side-effect 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

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Mantle Convection

• What is Ra for the Earths 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. (QAd)/k follows from heat flow
  measured at the surface.
• Layer thickness (km) Ra
• Upper mantle 700 106
• Lower mantle 2000 3 x 107
• Whole mantle 2700 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 new oceanic crust forms
  outboard of the continental margin and the coast
  moves further from the ridge axis (example east
  coast of North America).
• A passive margin, once formed, is the edge of a
  continent, but it is not a plate boundary
• A characteristic and reproducible geological
  sequence is likely to result
• 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 tend to be mostly basaltic,
  with small volumes of rhyolite.
• Accretionary prisms are small because subducted
  plate typically carries only thin coating of
  pelagic sediment.
• The crust of the overriding plate behind the arc
  is frequently put under extension and a back-arc
  basin develops, in which a spreading center much
  like a mid-ocean ridge may form.
• Island arcs may be accreted onto continental
  margins during collisional orogenies. Many
  fragments of old island arcs have been
  incorporated into North America along the coast
  of British Columbia and elsewhere. Much of
  central Asia is constructed from a whole 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 and spectacular
  stratovolcanoes as well as elevated plateaux are
  characteristic of the topography.
• A thick igneous crust is generated by intrusion
  and eruption of calc-alkaline magma
  (basalt-andesite-dacite-rhyolite). At depth there
  are major batholiths of intermediate to silicic
  intrusive rocks.
• A forearc basin adjacent to the volcanic
  mountains accumulates volcaniclastic sediment.
• An accretionary prism is scraped off the
  downgoing plate and piled up inboard of the
  trench. It is likely to include a melange of
  oceanic rocks metamorphosed by rapid subduction
  to high pressure without time to reach high
  temperatures.
• 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 cannot readily be
  subducted because it is too buoyant. Instead the
  crust is doubled-up, shortened, folded,
  overthrust, etc. 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.
• The central part of the range is often built from
  uplifted and heavily deformed oceanic and
  continental shelf sediments and slices of
  ophiolite from the vanished ocean.
• Collisional orogens begin life as ocean-continent
  subduction zones and many parts of a
  suprasubduction zone assemblage become
  incorporated in the collision. Volcanism may
  continue during the collisional orogen.
• These large mountain belts shed huge amounts of
  sediment into surrounding basins. During
  mountain-building, adjacent deep-sea basins
  receive voluminous turbidite deposits called
  flysch. Early flysch may be folded and deformed
  as the orogeny continues and mountain building
  propagates into the foreland. Later, they shed
  sediments called molasse into shallow-water or
  subaerial basins.
• There is extensive regional metamorphism exposed
  at the surface as rocks from deep in the crust
  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