Strain-dependent strength profiles Implication of planetary tectonics

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1 Strain-dependent strength profiles Implication of planetary tectonics Laurent G.J. Montési 1 Frederic Gueydan 2, Jacques Précigout 3 1 University of Maryland 2 Université de Montpellier 2, 3 Université d Orléans

2 Plate Tectonics: Only on Earth! Grand challenge problem in Solid Earth Sciences (NAS report, 2008) Affects planetary evolution, atmosphere, life? Earth Venus

3 Extension on Earth and Venus Main Ethiopian Rift Devana Chasma

4 Shortening on Earth and Venus Sierra Pampaneas Yalyane Dorsa

5 Strength envelopes (classical) Brittle failure Depends on pressure Arbitrary strength limit Ductile creep Depends on Temperature Depends on Strain rate Melosh, 2011

6 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 O Neill College de et France al.,

7 Coef cient of friction F a ult cor e Weakness of Complex V brittle faults L2 Fault rocks contain weak minerals (smectite, talc and minor chlorite). Complex I L3 2 m Low friction results from slip on a network of weak phyllosilicaterich surfaces that define the rock fabric L2 powder L3 powder L2 fault rock L3 fault rock L2 fault rock, wet Normal stress (MPa) 11/21/2014 Collettini et al., 2009 College de France 2014

8 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

9 The San Andreas Fault at depth? 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) Brittle: earthquakes Fabric transition High strength Grain size reduction: silent? 11/21/2014 Gueydan et al., 2014 College de France 2014

10 Ductile shear zone structure Requires change in state or environment Temperature Grain size Interconnection of weak phase Abundance of weak phase Composition (metamorphism, melt) Protolith L-S tectonites, South Armorican Shear Zone F. Gueydan, personal communication, 2006

11 Fabric and rheology Protolith (uniform strain) Shear zone (uniform stress) Strength controlled by strong phase Strength controlled by weak phase

12 Grain Size Reduction Mylonite in oceanic peridotite, Shaka Fracture zone Shear direction 11/21/2014 Warren College and de Hirth, France

13 Shear zone development In the crust: Weak phase interconnection if phyllosilicates CRUSTAL SHEAR ZONES (g=2.3) Strain T=300 C T=400 C T=500 C Viscosity 100 km Strain In the mantle: grain size reduction if dis-gbs MANTLE SHEAR ZONES (g=2.3) Strain T=600 C T=700 C Viscosity (Pa.s) T=800 C Viscosity 100 km Strain /21/2014 Gueydan et al., 2014 College de France 2014

14 Final weakening 11/21/2014 Gueydan College et de al., France 2014

15 Strain-dependent strength envelopes 11/21/2014 Gueydan et al., 2014 College de France 2014

16 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? Thermal structure from McKenzie et al., EPSL 2005

17 Venus Mantle Crust Dis-GBS possible if low enough geotherm / thin crust 15 K/km: 22km 10 K/km: 34 km thick High surface temperature, no water: no phyllosilicates Shallow brittle failure More important in extension Crustal thickness from James et al., 2013

18 Localization easier in extension Deeper brittle layer => larger fault offsets Possibility of melting => melt weakening and melt embrittlement Geometrical effects (stress focusing)

19 Mars Mantle: Dis-GBS if low enough heat flux/ thin crust 30km crust (lowlands): 100 mw/m 2 60km crust (highlands): 50 mw/m 2 Crust: Water likely: brittle weakening Mafic composition does not favor phyllosilicates at depth: no layering effect 11/21/2014 Montési College and Zuber, de France

20 Localization in planetary lithospheres Earth continents Earth Oceans Mars Venus ~30 km

21 Convection and tectonics Bottom-driven tectonics Edge-driven tectonics Phillips and Hansen, 1998 Schmerr, 2012

22 Bottom-driven tectonics Need high basal stress: convection reaches the brittle-ductile transition Need ductile layer for accommodation of shear zones 11/21/2014 Montési, LPSC 2013 College de France 2014

23 Buoyancy-driven tectonics Continent spreading (Earth s Archean) Less efficient on Mars Rey et al., 2014 Upwellings (Beta Regio, East African Rift?) Montési, LPSC 11/21/ College de France 2014

24 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 Understanding Earth textbook