strain localization and ductile failure in feldspar rocks georg dresen and erik rybacki gfz german...

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