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| Titel |
The viscous and frictional strength of faults in experiment and nature |
| VerfasserIn |
Renée Heilbronner, Matěj Peč, Holger Stünitz |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250105227
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| Publikation (Nr.) |
 EGU/EGU2015-4698.pdf |
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| Zusammenfassung |
| In an extended study of one of the authors (PhD thesis of Matej Pec), the deformational
behaviour of granitoid fault rocks was explored using a Griggs solid medium deformation
apparatus and a range of temperatures (T = 300Ë C to 600Ë C), confining pressures (Pc = 0.5
to 1.5 GPa) and strain rates (dγ/dt≈ 10-3 to 10-5 s-1). Layers of crushed and sieved
material (1 mm thick) were deformed between alumina forcing blocks (on a 45°pre-cut) to
finite shear strains of up to γ = 5.
The deformation within the fault rock layers is one of plane shear accompanied by
considerable thinning. To a certain extent, extrusion occurs parallel to the displacement
direction but not transverse to it. The fault rock material does not deform homogeneously,
rather the microstructure develops from an initial Riedel fracture set into an SC’ fabric at
higher strains. Progressive comminution leads to strain partitioning with a microstructure that
is characterized by a few survivor grains surrounded by a fine grained mantle (of the same
mineral composition as the survivor grains) and an evolving network of slip zones consisting
- at first - of nano-crystalline, partially amorphous and - at higher strains - of completely
amorphous material.
The slip zones are approx. 10 μm thick - they are the site of highly localized shear strain.
They form a percolating network from one end of the shear zone to the other and must be
considerably weaker than the surrounding material as evidenced by turbulent flow
structures and the occasional formation of apophyses. Yet, the fault rock layers
as a whole support high shear stresses (Ï ~ 570 – 1600 MPa) and even in the
presence of a fully connected network of slip zones (forming up to 20 vol% of the total
fault rock material), they continue to deform at more or less steady state stress
levels.
Within the range of experimental conditions, the flow stress sustained by the fault
rock depends clearly on confining pressure (indicating a frictional components of
flow) and temperature (indicating a viscous components) but only weakly on strain
rate. Whether the fault rocks are comparatively strong or weak depends on which
criterion is use to describe strength. For example, our experiments show that the flow
stresses increase for increasing confining pressure. At the same time, the friction
coefficient (Ï / Ïăn) decreases. In other words: with respect to the sustained flow
stress, faults are ’Pc-strengthening’ with respect to the friction coefficient, they are
’Pc-weakening’.
To extrapolate our experimental data to nature and to compare them to friction
experiments, we present our results in terms of equivalent viscosity describing the
deformation of a thin volume of material, and in terms of the friction coefficient describing
the displacement along a ’thick’ fault surface. We present a simple conceptual model for
the temporal and spatial evolution of the geometry or topology of the weak slip
zones, and the interplay between viscous and brittle behavior of faults at all scales. |
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