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June 29, 2009 EURISPET 1
Strength of the lithosphereIntroduction to mantle rheology from
laboratory approachShun-ichiro Karato
Yale UniversityNew Haven, USA
June 29, 2009 EURISPET 3
Outline
• rheology and geological problemsplate tectonics, survival of continents
• fundamentals of non-elastic deformation• oceanic lithosphere• importance of opx
• continental lithosphere• water, pressure effects
June 29, 2009 EURISPET 5
Plate Tectonics (bending)
Survival of Continents
(strength of the oceanic lithosphere)
(oceanic lithosphere)
(strength of the continental lithosphere)
Geodynamic issues in subduction zones related to rheological properties
June 29, 2009 EURISPET 6
A conventional model of lithosphere strength(Kohlstedt et al., 1995)
This model does not explain major geological features:too strong oceanic lithosphere for plate tectonicstoo weak continents to preserve deep continental roots
oceanic lithosphere continental lithosphere
June 29, 2009 EURISPET 7
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
June 29, 2009 EURISPET 10
Ductile deformation by thermally activated processes
τ~1/n exp (G*/nRT)
G*: material dependent, P dependent (G*=E*+PV*-TS*)The rate depends on defect concentration.
G*
.
June 29, 2009 EURISPET 11
brittle versus plastic deformation
Sensitivity to brittle fracture (regime2 B2) plastic flow (regime2 A1) pressure temperature materials rate of deformation water content stress state1
~linear weak weak weak strong yes
~exponential strong (exponential) strong strong strong weak3
Material dependence (opx versus olivine)P-dependenceWater dependence
June 29, 2009 EURISPET 13
homogeneous deformation
Rheology (of oceanic lithosphere) and mantle convection
(Tackley, 2000)
plate tectonics
stagnant lid
(Solomatov-Moresi, 1996, 1997)
June 29, 2009 EURISPET 14
Plate tectonics would not occur on Earth for this model.
Δρ⋅g⋅υ η⋅h3=&ε⋅τ⋅h3 τ1(~20MPa)<τ<τ2(~200MPa) : plate tectonics
τ1(~20MPa)>τ : homog. deformation
τ>τ2(~200MPa) : stagnant lid
energy release by subduction
energy dissipation by plate bending ⇔
(Kohlstedt et al., 1995)
plat
e t e
c to n
ics
June 29, 2009 EURISPET 15
How has a continental root survived?
~200 km thick continental lithosphere has survived for ~3Gyrs
June 29, 2009 EURISPET 16
In order to preserve the deep continental root, it must have a high viscosity (>10 -10 higher than the surrounding mantle).
Lenardic and Moresi (1999)
2 3
June 29, 2009 EURISPET 17
A conventional model(Kohlstedt et al., 1995)
Continental roots would be weaker than deep oceanic mantle --> continental roots would not have survived for this model.
June 29, 2009 EURISPET 18
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
June 29, 2009 EURISPET 19
needs for deformation experiments at higher P1. Deformation of minerals that are stable only at high P (opx, wadsleyite,
ringwoodite etc.)
2. Characterization of water and pressure effects
June 29, 2009 EURISPET 20
Deformation apparatus
Paterson apparatusP<0.5 GPa, T<1550 K
Rotational Drickamer apparatusP<17 GPa, T<2300 K
Griggs apparatusP<3 GPa, T<1600 K
Oceanic lithosphere: P to 3 GPa, T to 1500 K
Continental lithosphere: P to 10 GPa, T to 1700 K
June 29, 2009 EURISPET 21
Oceanic lithosphere(why plate tectonics on Earth?)
Oceanic lithosphere is (nearly) dry and cold.
brittle fracture + dry olivine (power-law creep) --> 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.
June 29, 2009 EURISPET 22
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
June 29, 2009 EURISPET 27
How has a continental root survived?
(Kohlstedt model) Continental roots would be weaker than deep oceanic mantle --> continental roots would not have survived.Rheology of the deep continental roots.
June 29, 2009 EURISPET 29
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).
• --> hardening of continental roots by partial melting?
June 29, 2009 EURISPET 30
Water weakening
low-P dataMei and Kohlstedt (2000)
water content -->
Str
ain
rate
June 29, 2009 EURISPET 31
partial melting removes water
“batch melting” CsCs0=kφ+1−φ()k≈1−1−kkφ (CsCm=k,φ: dgρ of m lτing)
“fρacτional m lτing” CsCs0=1−φ()1−kk≈exp−1−kkφ()≈1−1−kkφ
June 29, 2009 EURISPET 32
dept
h
strength
Quantify the water weakening effectQuantify the P-effect on dry rheology
June 29, 2009 EURISPET 33
Water weakeningneed to find a formula for extrapolation to high-pressures
low-P data (<0.45 GPa)Mei and Kohlstedt (2000)
June 29, 2009 EURISPET 34
&wet=A⋅σn⋅fH2OrP,T()⋅exp−E*+PV*RT( )∝η−1()
Data from a broad pressure range are needed to characterize the water effect.
fH2OrP,T() exp−E*+PV*RT( ) Mei-Kohlstedt
Karato-Jung
(Karato, 1989)
June 29, 2009 EURISPET 35
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.
fugacity effect
V* effect
Karato and Jung (2003)
June 29, 2009 EURISPET 36
A two-parameter (r, V*) equationfits nicely to the data.
&wet=A⋅fH2OrP,T()⋅exp−E*+PV*RT( ) Karato and Jung (2003)
June 29, 2009 EURISPET 37
Need to know “dry” rheology to evaluate the effect of de-watering
&≈&wetCW()+&εdry ξ=ηfinal()ηinitial()=&εCWcont()&εCW0()
The degree of hardening due to de-watering depends on &εwetCW,T,P( ) and &εdryT,P().
June 29, 2009 EURISPET 38
High-P deformation
gas-medium apparatusP<0.5 GPa, T<1550 K
Rotational Drickamer apparatusP to 17 GPa, T<2300 K
Griggs apparatusP<3 GPa, T<1600 K
June 29, 2009 EURISPET 39
RDA (rotational Drickamer apparatus)1. High P-T (good support, nearly homogeneous T (P))2. Large strain (torsion tests)3. Relatively large sample size (broad range of grain-size)
June 29, 2009 EURISPET 42
Incident X-ray
Geometry of X-ray diffraction for the rotational Drickamer apparatus
Diffracted X-ray
2
Observed part
June 29, 2009 EURISPET 44
(dry) olivine, deformation
Important to conduct high-P experiments (low-P experiments are not useful even though they are high-resolution).
Kawazoe et al. (2009)
June 29, 2009 EURISPET 45
Hardening due to de-watering (ΔT=0)
ocea
nic,
wed
ge m
antle
cont
inen
tal l
ithos
pher
e
June 29, 2009 EURISPET 47
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.
2 3
June 29, 2009 EURISPET 48
Summary-II(issues to be studied further)
Role of opx in an opx+ol 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?
June 29, 2009 EURISPET 50
volume fraction of fine-grained region (%)
mod
al fr
actio
n
Peridotite from the shear zone (Italy)
opx ribbon
June 29, 2009 EURISPET 52
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.
June 29, 2009 EURISPET 53
Topography
Vp tomography at 600 km depth
Huang & Zhao (2006) JGR
Bouguer gravity
North-South gravity lineament
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?
June 29, 2009 EURISPET 56
Ductile rheology
• Plastic deformation in minerals occurs due to the thermally activated motion of crystalline defects. σ=σ&,T,P:X( )
X= composition (water, Fe/Mg), grain-size σ∝&ε1nexpE*+PV*nRT() ‡ strength in the ductile regime depends on T, P, strain-rate (+ composition (water content), grain-size)
June 29, 2009 EURISPET 57
• Rate of deformation (strain-rate)
(density of defect)*(velocity of defect motion)
(velocity of defect motion) (driving force [stress])*(mobility)
(mobility) exp[-H(σ,C)/RT]: depends on mechanisms
(density of defect): function of T, σ, C: depends on mechanisms
Many mechanisms exist for plastic deformation.
-> “strength” depends on mechanisms.
∝
∝∝
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