1. Two-phase deformation of peridotite: localization by recrystallization and phase-mixing Robert Farla 1,2*, Shun-ichiro Karato 1 and Zhengyu Cai 1 1 Department of Geology and Geophysics, Yale University, New Haven, USA 2 Now at Bayerisches Geoinstitut, Bayreuth University, Bayreuth, Germany * robert.farla@uni-bayreuth.de EGU presentation on 02/05/2014 Images from Wikipedia: Rob Lavinsky, iRocks.com – CC-BY-SA-3.0 https://creativecommons.org/licenses/by-sa/3.0/deed.en – http://www.darkroastedblend.com/2011/03/mega-earthquake-and-tsunami.html

2. Strength reduction in the dry oceanic lithosphere After Kohlstedt et al. 1995 The strength of the lithosphere controls the tectonic style of terrestrial planets and must be modest for plate tectonics (e.g., Solomatov and Moresi, 1997, Richards et al., 2001; van Heck and Tackley, 2011]). Strength reduction in the brittle regime limited to friction coefficient ≥ 0.6 (Byerlee’s rule). Therefore, additional mechanism needed to reduce strength in the ductile (and/or semi-brittle) regimes: localization of deformation.

3. Natural peridotite shear zones Photo by Dr. Lars HansenWarren and Hirth, 2006 Toy et al. 2009 Exhumed mylonite and ultra- mylonite shear zones of peridotite composition provide evidence of strain localization down to the lower lithosphere/upper mantle. However, deformation conditions not well known! 0.5 mm 3

4. Mechanisms for strain localization Dynamic recrystallization of olivine and orthopyroxene leads to nucleation of small grains During shear, at a given strain the small grains may locally form interconnected layers IF the recrystallized grains are prevented to grow, these layers may deform by grain-size sensitive creep, which leads to a reduction in strength! Load bearing frame-workInterconnected weak layers Handy, 1994 4

5. Experimental procedure 5

6. Stress evolution with strain for a mixture of 75% olivine and 25% orthopyroxene Bystricky et al. 2000, Zhang et al. 2000 Pure Fo 90 olivine 6

7. Resulting microstructures - part 1/2 At 1100°C to 1200°C dynamic recrystallization is concentrated in layers -> possible source of weakening. At ~1300°C, dynamic recrystallization is more homogeneous. 1000°C 1100°C 1200°C ~1300°C 7

8. Resulting microstructures - part 2/2 High stress at lower temperatures results in smaller rex. grain size -> possible switch in dominant deformation mechanism and stress reduction? Again, at highest temperature new grain size is more homogeneous and larger 1100°C1200°C ~1300°C 8 opx ol

9. Deformation mechanism maps At lower temperature (≤1200°C), olivine rheology predicts weakening at small grain size. At high temperature, weakening impeded by too large rex. grain size. But, weakening only persists if grain growth can be inhibited. 1200°Cversus ~1300°C Grain size (µm) Stress (MPa) 9 Limited weakening Weakening Constant strain rate contours Rex. grain size piezometer

10. Evidence for grain boundary pinning in mixed regions Mixed-phase regions (Area 2) show smaller grain size than in the olivine-only regions (Area 1) Grain size analysis demonstrates two slopes in plot; olivine grain size D ol determined by second-phase control (opx) and by dynamic rex. control (see Linckens et al. 2011). 10 opx ol Zener parameter 1200°C

11. Model for predicting strain localization (and weakening) across the dislocation-diffusion creep boundary We reconcile our observations at high-temperature, high-strain rate with deformation in nature at lower temperatures and strain rates. ! Fast grain growth kinetics in pure olivine prevents switch to diffusion creep at geological strain rates (so no localization) 11

12. Role of water – some considerations Water is known to weaken silicates (olivine, orthopyroxene, etc.). However, low stress leads to larger rex. grains and water enhances grain growth kinetics, thus preventing deformation from localizing! Careful control of T, fH 2 O, strain rate, grain size parameters is necessary. 12 Localization No localization Similar strain rates of ~10 -4 s -1

13. Conclusions Constant strain rate experiments show weakening associated with grain size reduction of both olivine and opx phases. New grains formed in layers, also tend to be smaller in mixed-phase regions than in olivine-only regions. The fine-grained layers may have deformed by grain- size-sensitive creep, explains weakening. We suggest some measure of grain boundary pinning slowed grain growth and enhanced strain localization. Our model suggests that a second-phase is necessary to explain formation of natural shear zones, offset to lower temperatures but also to lower strain rates. For more: Farla, Karato, & Cai, PNAS, 2013. 13

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15. Grain size reduction vs grain growth Large grains deforming by dislocation creep will dynamically recrystallize to a new grain size as function of stress (e.g. van der Wal et al. 1993) : d rex = aσ -p However, rapid grain growth in fine-grained regions deforming by grain size sensitive creep may prevent strain from localizing. The rate equation is: When q and H gg are large, growth kinetics is sluggish -> possibility of grain boundary pinning 15

16. DMM – Dis-GBS creep regime 16

17. EBSD - Pole figures 17 1200°C Weaker crystallographic preferred orientation – transition to diffusion creep MUD

18. FTIR – water content measurement 18

19. Strength profile based on olivine rheology 19 Diffusion creep 10 um grain size Dislocation Glide (Peierls mechanism) Dislocation creep (Power-law) Strength reduction Frictional sliding (Byerlee’s rule) Brittle-ductile transition Strain rate: 10 -15 s -1 Dry rheology (olivine)

20. Continental lithosphere 20 Précigout and Gueydan, 2009 20