Download presentation
Presentation is loading. Please wait.
Published byTyree Balle Modified over 10 years ago
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
Similar presentations
© 2025 SlidePlayer.com. Inc.
All rights reserved.