Strain Localization and Ductile Failure in Feldspar Rocks Georg Dresen and Erik Rybacki

1. Strain Localization and Ductile Failurein Feldspar RocksGeorg Dresen and Erik Rybacki GFZGerman Research Center of Geosciences

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

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

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

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

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

7. SEM BSE Images of Cavity Bands g ~ 2 1 mm 200 µm g ~ 4 g ~ 4 20 µm 10 µm Rybacki, Wirth and Dresen, GRL, 2008, JGR, 2010

8. Cavity bands in optical thin sections 20 µm 20 µm

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

10. SiO2 Glassin Shear Bands (FIB STEM) 1µm 1µm Glass SiO2 200 nm 1 µm

11. Conclusions • Wherestrengthatlowercrustaldepthis limited by • fine-grainedmyloniteshearzonesitisexpectedtobelow • Acceleratedpostseismiccreep in fine-grainedmylonitic • shearzones in thenearfieldisprobably linear viscous • Cavitation in fine-grainedfeldsparaggregatesoccursatflow • stresses 5-20 timeslowerthanconfiningpressure • Cavitation in ultramyloniteshearzonesmay • leadtoepisodicslipacceleration, porosity/permeability • increaseandductilefailure

13. Cavity nucleation mechanisms Vacancy condensation Wedging at grain triple points Tensile grain boundary ledges Twinning Cooperative GBS Dislocation pile-up Zener-Stroh mechanism (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% Qtz-Pl Melt Kfs Zavada et al., JGR, 112, 2007 500 µm 200 µm

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

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

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. PLB Byerlee‘s Law