1. Strain Localization and Ductile Failure in Feldspar Rocks Georg Dresen and Erik Rybacki GFZ German Research Center of Geosciences

2. 25 km 250 m SE Madagaskar Quartzofeldspathic Granulites 700°C-800°C ~ 600-800 MPa Cap de Creus Metasediments 400°C-500°C ~ 250 MPa Shear Zones Cutting Through Lower Crust

3. 40 mm µm-scale mm-scale Grain Size  >> 10 4 mm  ~ 1-10 UM Plag/Amph/CPX Clinopyroxene Plagioclase Anorthositic Granulites, Norway Shear Zone in Metabasites, Ivrea Zone Mylonite

4. Rybacki et al., JGR 2006; Dimanov and Dresen, JGR 2005; Mei and Kohlstedt, JGR 2000 Synthetic Rocks at Hydrous Conditions Mylonite grain size Viscosity from postseismic relaxation models Stress estimates from shear zones Lab Data vs. Field Observations

5. Ductile Failure in Feldspar Rocks Mag. x 50,  = 4 Mag. x 50,  < 2.0 Mag. x 100000,  = 4  2 – 80 MPa, T: 900°C-1200°C, P c : 100 - 400 MPa, 26 samples, 40% deformed in linear viscous creep to failure at  < 5 Failure Cavity Crack

6. Localization and Failure  ≈ 2·10 -4 s -1.  ≈ 5·10 -5 s -1.  ≈ 2·10 -5 s -1. AnDi-mixture ~2·10 -5 s -1 pure An 1100°C Cavitation, Failure 1100°C 1050°C 1000°C

7. 1 mm200 µm 20 µm10 µm SEM BSE Images of Cavity Bands  ~ 4 Rybacki, Wirth and Dresen, GRL, 2008, JGR, 2010  ~ 2

8. 20 µm Cavity bands in optical thin sections

9. Pores, Cavities in TEM BF 1150 °C,  ~ 3.5

10. SiO 2 Glass in Shear Bands (FIB STEM) 1µm 200 nm SiO 2 Glass

11. Conclusions Where strength at lower crustal depth is limited by fine-grained mylonite shear zones it is expected to be low Accelerated postseismic creep in fine-grained mylonitic shear zones in the near field is probably linear viscous Cavitation in fine-grained feldspar aggregates occurs at flow stresses 5-20 times lower than confining pressure Cavitation in ultramylonite shear zones may lead to episodic slip acceleration, porosity/permeability increase and ductile failure

13. Cavity nucleation mechanisms Vacancy condensation Wedging at grain triple points Tensile grain boundary ledges Twinning Dislocation pile-up Zener-Stroh mechanism Cooperative GBS (i.e. Riedel, 1986; Kassner & Hayes, 2003)

14. Monkman – Grant (1956) Relation

15. Melt-enhanced grain boundary sliding and cavitation in qtz-fsp mylonites 800-900°C 900 - 1000 MPa Melt 2-4 vol% 500 µm Kfs Qtz-Pl Melt 200 µm Zavada et al., JGR, 112, 2007

16. Field evidence for cavitation±failure  voids in natural quartz-feldspar mylonites (e.g., White & White, 1981; Behrmann, 1985; Behrmann & Mainprice, 1987; Mancktelow et al., 1998; Hiraga et al., 1999; Zavada et al., 2007, 2012; Kilian et al., 2011)  enhanced fluid flow in HT shear zones (e.g., Geraud et al. 1995; Regenauer-Lieb, 1999; Fusseis et al., 2009)  pseudotachylytes (e.g., White, 1996, 2012)  seismicity / slow earthquakes? (e.g., Shigematsu et al., 2004; 2009) (Geraud et al., 1995)

17. Stress (MPa) 20 40 60 100 200 400 Log Strain Rate (s -1 ) -3.0 -4.0 -5.0 1200°C 1120°C 3 1 1.3 2.1 2.9 Log Stress (MPa) n=1 Temperatures: ca. 900°C – 1200°C Stresses: 30 MPa - 600 MPa Strain Rates: 2x10 -6 – 1x10 -3 s -1 20 mm Experimental Techniques Axial Load Pressure Vessel Sample

18. Effect of Feldspar Water Content on Flow Regimes and Viscosity „wet“ „dry“

19. 19 Shear Zone In-plane slip-induced shear stress vs depth Rupture depth Z =15 km Uniform slip u = 5 m Shear Modulus G = 30 GPa Okada, 1992 Montesi, 2004

20. 20 Byerlee‘s Law PLB