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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

11/21/2014 College de France 2014

Extension on Earth and Venus

Devana Chasma Main Ethiopian Rift


Shortening on Earth and Venus

Yalyane Dorsa Sierra Pampaneas


Strength envelopes (classical)


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


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


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)

11/21/2014 College de France 2014 L-S tectonites, South Armorican Shear Zone

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


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

Final weakening


Strain-dependent strength envelopes


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?


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


superplasticity

crust

Oceanic

mantle

1000

600

200

0.50

Te

mp

era

ture

(°C

)

No weakening

Strain Weakening

1

Moho

800

400

Layering

development


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


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)


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


Montési and Zuber, 2003

Localization in planetary lithospheres


Earth

continents

Earth

Oceans

Mars Venus

~30 k

m

Convection and tectonics


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

11/21/2014 College de France 2014 Understanding Earth textbook

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