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Strain Localization and Ductile Failure in Feldspar Rocks Georg Dresen and Erik Rybacki GFZ German Research Center of Geosciences. 25 km. 250 m. Shear Zones Cutting Through Lower Crust. SE Madagaskar Quartzofeldspathic Granulites 700°C-800°C ~ 600-800 MPa. Cap de Creus

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Strain Localization and Ductile Failurein Feldspar RocksGeorg Dresen and Erik Rybacki

GFZGerman Research Center of Geosciences


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


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


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


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


Localization and failure

.

.

.

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


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



Pores, Cavities in TEM BF

1150 °C, g ~ 3.5

1150 °C, g ~ 3.5


SiO2 Glassin Shear Bands (FIB STEM)

1µm

1µm

Glass

SiO2

200 nm

1 µm


  • 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


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)



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


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

-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


Effect of Feldspar Water Content on

Flow Regimes and Viscosity

„dry“

„wet“


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


PLB

Byerlee‘s Law


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