Strain Localization and Ductile Failurein Feldspar Rocks
Georg Dresen and Erik RybackiGFZ German Research Center of Geosciences
25 km25 km 250 m250 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 CrustShear Zones Cutting Through Lower Crust
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
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
po
stseismic re
laxatio
n mo
de
ls S
tres
s e
stim
ate
s fr
om s
he
ar z
one
sLab Data vs. Field Observations
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, Pc: 100 - 400 MPa, 26 samples, 40% deformed in
linear viscous creep to failure at < 5
Failure
Cavity
Crack
Localization and Failure
≈ 2·10-4s-1.
≈ 5·10-5s-1.
≈ 2·10-5s-1.
AnDi-mixture ~2·10-5s-1pure An 1100°CC
avitation, Failure
1100°C
1050°C
1000°C
1 mm 200 µm
20 µm 10 µm
SEM BSE Images of Cavity Bands
~ 4 ~ 4
Rybacki, Wirth and Dresen, GRL, 2008, JGR, 2010
Rybacki, Wirth and Dresen, GRL, 2008, JGR, 2010
~ 2
20 µm20 µm
Cavity bands in optical thin sections
Pores, Cavities in TEM BF
1150 °C, ~ 3.5 1150 °C, ~ 3.5
SiO2 Glass in Shear Bands (FIB STEM)
1µm 1µm
1 µm 200 nm
SiO2
Glass
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
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)
Monkman – Grant (1956) Relation
Melt-enhanced grain boundary sliding and cavitationin qtz-fsp mylonites
800-900°C900 - 1000 MPaMelt 2-4 vol%
500 µm
Kfs
Qtz-Pl Melt
200 µm
Zavada et al., JGR, 112, 2007
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)
Stress (MPa)
20 40 60 100 200 400
Log
Str
ain
Rat
e (s
-1)
-3.0
-4.0
-5.0
• 1200°C
• 1120°C
3
1
1.3 2.1 2.9Log 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 TechniquesAxial
Load
Pressure Vessel
Sample
RT
QfdA OH
mn exp2
Effect of Feldspar Water Content on
Flow Regimes and Viscosity
„wet“
„dry“
19
Sh
ear
Zon
eIn-plane slip-induced shear stress vs depth
1/
/2
Zz
ZuG
11410 s
Rupture depth Z =15 km
Uniform slip u = 5 m
Shear Modulus G = 30 GPa
Okada, 1992
Montesi, 2004
20
Bye
rlee
‘s L
aw
PLB