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