june 29, 2009eurispet1 strength of the lithosphere introduction to mantle rheology from laboratory...

June 29, 2009 EURISPET 1

Strength of the lithosphereIntroduction to mantle rheology from

laboratory approachShun-ichiro Karato

Yale UniversityNew Haven, USA


Yale University


Outline

• rheology and geological problemsplate tectonics, survival of continents

• fundamentals of non-elastic deformation• oceanic lithosphere• importance of opx

• continental lithosphere• water, pressure effects


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


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


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


Processes controlling the “strength”

brittledeformation

ductiledeformation


Strength in the brittle regimeByerlee’s law

τ=τc+μfσn=τc+μfP


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*

.


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


dept

h

dept

h

strength strength

ductile branch

ductile branch


homogeneous deformation

Rheology (of oceanic lithosphere) and mantle convection

(Tackley, 2000)

plate tectonics

stagnant lid

(Solomatov-Moresi, 1996, 1997)


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


How has a continental root survived?

~200 km thick continental lithosphere has survived for ~3Gyrs


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


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.


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


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


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


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.


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


(Ohuchi and Karato (2009))

opx

ol

opx

ol


(Ohuchi and Karato (2009)

strain

stre

ss

opx


Role of a weak opx on the strength of an oli + opx mixture


(Ohuchi and Karato (2009))

opx (IWL)-modelol (LBF)-model


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.


Temperature difference?

Continent versus ocean: temperature difference


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?


Water weakening

low-P dataMei and Kohlstedt (2000)

water content -->

Str

ain

rate


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


dept

h

strength

Quantify the water weakening effectQuantify the P-effect on dry rheology


Water weakeningneed to find a formula for extrapolation to high-pressures

low-P data (<0.45 GPa)Mei and Kohlstedt (2000)


&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)


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)


A two-parameter (r, V*) equationfits nicely to the data.

&wet=A⋅fH2OrP,T()⋅exp−E*+PV*RT( ) Karato and Jung (2003)


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().


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


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)


Synchrotron facility at Brookhaven National Lab


Strain measurements by X-ray imaging


Incident X-ray

Geometry of X-ray diffraction for the rotational Drickamer apparatus

Diffracted X-ray

2

Observed part


wadsleyite


(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)


Hardening due to de-watering (ΔT=0)

ocea

nic,

wed

ge m

antle

cont

inen

tal l

ithos

pher

e


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


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?


Conditions for the survival of continental roots


volume fraction of fine-grained region (%)

mod

al fr

actio

n

Peridotite from the shear zone (Italy)

opx ribbon


water


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.


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?


Zhao (2004) 　 PEPI 146, 3-34.

Global mantle tomography


Deformation mechanism map


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)


• 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.

∝

∝∝

june 29, 2009eurispet1 strength of the lithosphere introduction to mantle rheology from laboratory...

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