Strength of the lithosphere Introduction to mantle rheology from laboratory approach

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Strength of the lithosphereIntroduction to
mantle rheology fromlaboratory approach

• Shun-ichiro KaratoYale UniversityNew Haven, USA

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Yale University
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Outline

• rheology and geological problems
• plate tectonics, survival of continents
• fundamentals of non-elastic deformation
• oceanic lithosphere
• importance of opx
• continental lithosphere
• water, pressure effects

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Geodynamic issues in subduction zones related to
rheological properties
(strength of the oceanic lithosphere)
Plate Tectonics (bending)

• (oceanic lithosphere)

Survival of Continents
(strength of the continental lithosphere)
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A conventional model of lithosphere
strength(Kohlstedt et al., 1995)
oceanic lithosphere continental lithosphere

• This model does not explain major geological
  features
• too strong oceanic lithosphere for plate
  tectonics
• too weak continents to preserve deep continental
  roots

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ABC of rock deformation

• How to construct a strength profile?
• Brittle deformation
• generation and propagation of a fault
• Plastic deformation
• permanent strain due to microscopic atomic
  motion

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Processes controlling the strength
brittle deformation

• ductile
• deformation

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Strength in the brittle regimeByerlees law
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Ductile deformation by thermally activated
processes
G
.
??1/n exp (G/nRT) G material dependent, P
dependent (GEPV-TS) The rate depends on
defect concentration.
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brittle versus plastic deformation
Material dependence (opx versus
olivine) P-dependence Water dependence
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strength

• depth

strength
depth
ductile branch
ductile branch
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Rheology (of oceanic lithosphere) and mantle
convection
homogeneous deformation
plate tectonics

• stagnant lid

(Solomatov-Moresi, 1996, 1997)
(Tackley, 2000)
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plate tectonics
(Kohlstedt et al., 1995)

• Plate tectonics would not occur on Earth for this
  model.

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How has a continental root survived?
200 km thick continental lithosphere has
survived for 3Gyrs
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Lenardic and Moresi (1999)

• In order to preserve the deep continental root,
  it must have a high viscosity (gt10 -10 higher
  than the surrounding mantle).

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A conventional model(Kohlstedt et al., 1995)

• Continental roots would be weaker than deep
  oceanic mantle --gt continental roots would not
  have survived for this model.

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A conventional model of lithosphere strength
(Kohlstedt et al.,1995) fails to explain the most
important features of geological processes plate
tectonics and long-term stability of
continents.What are wrong with that model ?

• Limited experimental conditions (low pressure)
• Uncertainties in water content in the
  continental mantle
• olivine-based model
• continental lithosphere was assumed to be wet
• water, P-effects are poorly constrained

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• needs for deformation experiments at higher P
• Deformation of minerals that are stable only at
  high P (opx, wadsleyite, ringwoodite etc.)
• Characterization of water and pressure effects

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Deformation apparatus
Rotational Drickamer apparatus Plt17 GPa, Tlt2300 K
Griggs apparatus Plt3 GPa, Tlt1600 K

• Paterson apparatus
• Plt0.5 GPa, Tlt1550 K

Oceanic lithosphere P to 3 GPa, T to 1500
K Continental lithosphere P to 10 GPa, T to 1700
K
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Oceanic lithosphere(why plate tectonics on
Earth?)

• Oceanic lithosphere is (nearly) dry and cold.
• brittle fracture dry olivine (power-law creep)
  --gt too strong
• How can one make the lithosphere weak at low T
  (and dry)?
• Plastic deformation is material sensitive.
• Lithosphere is made of olivine opx.
• How about opx (orthopyroxene)?
• Little previous studies on opx deformation.
• Opx is stable only above 1 GPa (at high T)
• A conventional gas-apparatus can be used only
  below 0.5 GPa.

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Plastic deformation of opx(Ohuchi and Karato,
2009)

• Griggs apparatus (1.3 GPa, 973-1273 K)
• CsCl pressure medium
• Simple shear
• With a small amount of water

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opx
ol
opx
ol

• (Ohuchi and Karato (2009))

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stress
opx
(Ohuchi and Karato (2009)
strain
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Role of a weak opx on the strength of an oli
opx mixture
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opx (IWL)-model
ol (LBF)-model

• (Ohuchi and Karato (2009))

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How has a continental root survived?

• (Kohlstedt model) Continental roots would be
  weaker than deep oceanic mantle --gt continental
  roots would not have survived.
• Rheology of the deep continental roots.

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Temperature difference? Continent versus ocean
temperature difference
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Causes for a strong continent

• Temperature difference is too small.
• Water content difference?
• Water enhances deformation.
• Continental upper mantle is depleted(large
  degree of partial melting).
• --gt hardening of continental roots by partial
  melting?

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Water weakening
Strain rate
low-P data Mei and Kohlstedt (2000)

• water content --gt

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partial melting removes water
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• depth

strength
Quantify the water weakening effect Quantify the
P-effect on dry rheology
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Water weakeningneed to find a formula for
extrapolation to high-pressures
low-P data (lt0.45 GPa) Mei and Kohlstedt (2000)
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(Karato, 1989)
Mei-Kohlstedt
Karato-Jung
Data from a broad pressure range are needed to
characterize the water effect.
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Pressure effects on creep strength of olivine
(wet)

• Variation in the strength of olivine under wet
  conditions is different from that under dry
  conditions.
• The strength changes with P in a non-monotonic
  way.
• High-P data show much higher strength than low-P
  data would predict.

Karato and Jung (2003)
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A two-parameter (r, V) equation fits nicely to
the data.
Karato and Jung (2003)
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Need to know dry rheology to evaluate the
effect of de-watering
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High-P deformation

• gas-medium apparatus
• Plt0.5 GPa, Tlt1550 K

Rotational Drickamer apparatus P to 17 GPa,
Tlt2300 K
Griggs apparatus Plt3 GPa, Tlt1600 K
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• RDA (rotational Drickamer apparatus)
• High P-T (good support, nearly homogeneous T (P))
• Large strain (torsion tests)
• 3. Relatively large sample size (broad range of
  grain-size)

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Synchrotron facility at Brookhaven National Lab
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Strain measurements by X-ray imaging
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Geometry of X-ray diffraction for the rotational
Drickamer apparatus
Diffracted X-ray
Y
2q
Incident X-ray
Observed part
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wadsleyite
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(dry) olivine, deformation

• Kawazoe et al. (2009)

Important to conduct high-P experiments (low-P
experiments are not useful even though they are
high-resolution).
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Hardening due to de-watering (?T0)

• oceanic, wedge mantle

continental lithosphere
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Summary-I

• In order to obtain critical data on the
  rheological properties from experimental studies,
  one needs to conduct deformation experiments
  beyond 1 GPa.
• With a pure olivine lithosphere, plate tectonics
  is difficult to operate opx may weaken the
  lithosphere to allow plate tectonics to operate.
• The de-watering in the deep upper mantle can
  increase the viscosity 10 - 10 times that
  would stabilize the continental roots.

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Summary-II(issues to be studied further)

• Role of opx in an opxol mixture experimental
  study on deformation of an opx-ol mixture, study
  of naturally deformed rocks
• Is the continental lithosphere really dry?
• Does subduction help growth of continents or
  destroy them?

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Conditions for the survival of continental roots
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Peridotite from the shear zone (Italy)

• modal fraction

volume fraction of fine-grained region ()
opx ribbon
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water
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Big Mantle Wedge (BMW) model (Zhao et al., 2004)
Big Mantle Wedge
(1) Shallow deep slab dehydration (Ohtani et
al., 2004) (2) Corner flow (convection) in the
Big Mantle Wedge (3) Thinning fracture of the
continental lithosphere (4) Upwelling of the
asthenospheric materials to form the intraplate
volcanoes.
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Bouguer gravity North-South gravity lineament
Topography
Vp tomography at 600 km depth
Ma (1989) Xu (2007)
The western edge of the stagnant slab roughly
coincides with the surface topographic boundary
NSGL. The stagnant slab has affected the surface
structure and tectonics?
Huang Zhao (2006) JGR
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Global mantle tomography
Zhao (2004) PEPI 146, 3-34.
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Deformation mechanism map
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Ductile rheology

• Plastic deformation in minerals occurs due to the
  thermally activated motion of crystalline defects.

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• Rate of deformation (strain-rate)
• (density of defect)(velocity of defect motion)
• (velocity of defect motion) (driving force
  stress)(mobility)
• (mobility) exp-H(??C)/RT depends on
  mechanisms
• (density of defect) function of T, ???C
  depends on mechanisms
• Many mechanisms exist for plastic deformation.
• -gt strength depends on mechanisms.

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(Ohuchi and Karato (2009)
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• (Ohuchi and Karato (2009)