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| Titel |
Brittle-to-viscous behaviour of quartz gouge in shear experiments |
| VerfasserIn |
Bettina Richter, Holger Stünitz, Renée Heilbronner |
| Konferenz |
EGU General Assembly 2016
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250132552
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| Publikation (Nr.) |
 EGU/EGU2016-13069.pdf |
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| Zusammenfassung |
| In order to study the microstructure development across the brittle-viscous transition and to
derive the corresponding flow laws, we performed shear experiments on quartz gouge in a
Griggs-type deformation apparatus. The starting material is a crushed quartz single crystal
(sieved grain size <100 μm) with 0.2 wt% water added. The experiments were conducted at
temperatures between 500 ˚ C and 1000 ˚ C at confining pressures of 0.5 GPa, 1.0 GPa or
1.5 GPa. Four strain-rate-stepping experiments were conducted at strain rates between ∼2.5 x
10−6 s−1 and ∼2.5 x 10−4 s−1. Other experiments were conducted at constant strain
rates of ∼2.5 x 10−6 s−1, ∼2.5 x 10−5 s−1, ∼2.5 x 10−4 s−1 and ∼2.5 x 10−3
s−1.
At high confining pressure, the strength of the samples decreases with increasing
temperature for all strain rates. The largest decrease occurred between 650 ˚ C and 700 ˚ C
at shear strain rates of ∼2.5 x 10−5 s−1. At the same time, the pressure dependence
of strength is positive for T ≤ 650 ˚ C while an inverse pressure dependence is
observed at T > 650 ˚ C. For T < 700 ˚ C, the friction coefficient decreases slightly
with increasing temperature, from 700-1000 ˚ C it shows a strong temperature
dependence.
Between 650 ˚ C and 700 ˚ C at shear strain rates of ∼2.5 x 10−5 s−1 a change in the
deformation process occurs from one dominated by cataclastic flow to one dominated by
crystal plasticity. The microstructure reveals a less abrupt transition in terms of operating
processes, because brittle and viscous processes are equally active around 650 ˚ C. With
increasing temperature the volume fraction of recrystallised grains increases, and at 900 ˚ C -
1000 ˚ C recrystallisation is nearly complete at strains of γ ∼ 3. The crystallographic
preferred orientation of the c-axis evolves from a random distribution at low temperatures
towards two peripheral maxima at intermediate temperatures. At high temperatures the c-axis
show a single Y-maximum.
At high temperature, the stress exponent is n = 2.1 ± 0.2. The activation energy Q is
193 ± 12 kJ/mol at strain rates of 10−5 s−1, at faster strain rates the activation
energy drops down to Q = 119 ± 12 kJ/mol. This small stress exponent at high
temperatures indicates a combination of deformation processes (diffusion in very
fine grained material and dislocation creep in coarser grained material). At lower
temperatures the n-value is significantly higher (n ∼ 8) indicating the beginning of the
power-law-breakdown.
Interestingly, the mechanical data indicate a sharp transition between brittle- and
viscous-dominated regimes around 650 ˚ C to 700 ˚ C, while the microstructure reveals a
smoother transition over a wider temperature range. The relatively low stress exponent of
n ∼ 2 clearly suggests the activation of diffusion creep deformation processes. |
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