Strain-dependent strength profiles
Implication of planetary tectonics
Laurent G.J. Montési1
Frederic Gueydan2, Jacques Précigout3 1University of Maryland
2Université de Montpellier 2, 3Université d’Orléans 11/21/2014 College de France 2014
Plate Tectonics: Only on Earth!
• Grand challenge problem in Solid Earth Sciences (NAS report, 2008)
• Affects planetary evolution, atmosphere, life?
Earth
Venus
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Strength envelopes (classical)
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Brittle failure
Depends on pressure
Ductile creep
Depends on Temperature
Depends on Strain rate
Arbitrary strength limit
Melosh, 2011
A plate tectonic recipe
• Vigorous
convection
• Weak
deformation
zones
Weakened
arbitrarily
• What mechanism weakens
plate boundaries?
• Which mechanisms are active
only on Earth
• Is the “strength” all that is
needed?
11/21/2014 College de France 2014 O’Neill et al., 2007
Weakness of
brittle faults
• Fault rocks contain
weak minerals
(smectite, talc and
minor chlorite).
• Low friction results
from slip on a network
of weak phyllosilicate-
rich surfaces that
define the rock fabric
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Complex I 2 m
Complex V
aF
ero
c tluL2
L3
0.6
0.8
0.4
0.2
0 20 40 60 80 100 120 140
L2 powder
L3 powder
L2 fault rock
L3 fault rock
L2 fault rock, wet
Normal stress (MPa)
Co
ef
cie
nt
of
fric
tio
n
Collettini et al., 2009
Which level is weak?
• Low yield strength of the lithosphere
• Solomatov (2004): <~3MPa
• Low brittle strength
• Friction coefficient of 0.15 (O’Neill et al.
2007)
• Serpentine? (Moore et al., 2007)
• High pore fluid pressure
• Low ductile strength
• Necessary to reconcile low coefficient of
friction and depth to brittle-ductile
transition
• Ductile shear zones
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The San Andreas Fault at depth?
Fabric transition
High strength
Brittle: earthquakes
Grain size reduction: silent?
11/21/2014 College de France 2014 Gueydan et al., 2014
Brittle failure Need fluids and/or serpentine
Ductile failure In the mantle when dis-GBS is possible (T<700°C) In the crust in presence of phillosilicates (T<500°C)
Protolith
Ductile shear zone structure
• Requires change in state or environment • Temperature
• Grain size
• Interconnection of weak phase
• Abundance of weak phase
• Composition (metamorphism, melt)
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F. Gueydan, personal communication, 2006
Fabric and rheology
Protolith (uniform strain) Shear zone (uniform stress)
Strength controlled by strong phase Strength controlled by weak phase 11/21/2014 College de France 2014
Grain Size Reduction
• Mylonite in oceanic peridotite, Shaka Fracture zone Shear direction
11/21/2014 College de France 2014 Warren and Hirth, 2006
Shear zone development
Strain1 10 100
2.5 1021 2.5 1022 2.5 1023
T=500°CT=400°CT=300°C
Viscosity
Strain
CRUSTAL SHEAR ZONES (g=2.3)
100 km
11/21/2014 College de France 2014 Gueydan et al., 2014
Strain1 10 100
2.5 1021 2.5 1022 2.5 1023
100 km
Viscosity (Pa.s)
MANTLE SHEAR ZONES (g=2.3)
T=800°CT=700°CT=600°C
Viscosity
Strain
In the crust: Weak phase interconnection if phyllosilicates
In the mantle: grain size reduction if dis-GBS
Oceanic lithosphere • Crust: mostly brittle
• Hydrated minerals and high pore fluid pressure
• In the mantle • Dis-GBS starts essentially at zero
age • Older lithosphere has a thicker
localizing mantle
• Melt embrittlement?
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Strain Weakening
1
Moho
800
800
Layering
development
Grain size reduction &
superplasticity
crust
Oceanic
mantle
1000
600
400
0.50
Te
mp
era
ture
(°C
)
No weakening
Strain Weakening
1
Moho
800
400
Layering
development
Grain size reduction &
superplasticity
crust
Oceanic
mantle
1000
600
200
0.50
Te
mp
era
ture
(°C
)
No weakening
Strain Weakening
1
Moho
800
400
Layering
development
Grain size reduction &
superplasticity
deep crust
mid crust
upper crust
subcontinental
mantle
No weakening
1000
600
200
0.50
Te
mp
era
ture
(°C
)
No weakening
Thermal structure from McKenzie et al., EPSL 2005
Venus
• Mantle • Dis-GBS possible if low
enough geotherm / thin crust • 15 K/km: 22km
• 10 K/km: 34 km thick
• Crust • High surface temperature, no
water: no phyllosilicates
• Shallow brittle failure
• More important in extension
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Crustal thickness
from James et al.,
2013
Localization easier in extension
• Deeper brittle layer =>
larger fault offsets
• Possibility of melting =>
melt weakening and melt
embrittlement
• Geometrical effects
(stress focusing)
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Mars
• Mantle:
• Dis-GBS if low enough
heat flux/ thin crust
• 30km crust (lowlands):
100 mW/m2
• 60km crust (highlands):
50 mW/m2
• Crust:
• Water likely: brittle
weakening
• Mafic composition does
not favor phyllosilicates at
depth: no layering effect
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Montési and Zuber, 2003
Localization in planetary lithospheres
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Earth
continents
Earth
Oceans
Mars Venus
~30 k
m
Convection and tectonics
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Bottom-driven tectonics
Edge-driven tectonics
Phillips and Hansen, 1998
Schmerr, 2012
Bottom-driven tectonics
• Need high basal stress: convection
reaches the brittle-ductile transition
• Need ductile layer for
accommodation of shear zones
11/21/2014 College de France 2014 Montési, LPSC 2013
Buoyancy-driven tectonics
• Continent spreading (Earth’s Archean)
• Upwellings (Beta Regio, East African Rift?)
11/21/2014 College de France 2014 Montési, LPSC 2013
Rey et al., 2014
Less efficient on Mars
Summary • The strength of the
lithosphere changes with
strain
• Layering in brittle regime
or in presence of micas
• Grain size reduction in
presence of dis-GBS
creep
• Strength profile
compared to the Earth
• Mars is similar to Earth’s
continents
• Venus is dominated by
brittle localization unless
low heat flux
• Issue of driving forces
• Bottom stresses are
inefficient to drive
tectonics
• Continents dominated by
buoyancy-driven flows?
• Buoyancy less efficient
on Mars
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