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| Titel |
Frictional-viscous transition in simulated calcite fault gouge sheared at 550°C |
| VerfasserIn |
Berend Antonie Verberne, André R. Niemeijer, Johannes H. P. de Bresser, Christopher J. Spiers |
| 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 |
250107268
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| Publikation (Nr.) |
 EGU/EGU2015-10069.pdf |
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| Zusammenfassung |
| Understanding the strength and slip stability of fault rocks throughout the frictional-viscous transition is important for understanding the mechanisms controlling seismogenesis. We report ring shear experiments on simulated calcite fault gouge (median grain size ~20 μm), consisting of i) tests performed at an effective normal stress (σneff) of 50 MPa at constant sliding velocities (v) of 0.1 and 100 μm/s, and ii) σneff-stepping tests at constant v = 0.1 and 10 μm/s, using sequentially increased σneff values within the range from 30 to 140 MPa. Each test was performed wet at a fixed temperature of 550°C, and sheared gouges recovered from constant σneff tests were used for microstructure analyses. Our results show that samples sheared at σneff = 50 MPa, at v = 0.1 μm/s, have a steady-state shear strength (τss) of ~46 MPa, and are characterized by a microstructure consisting of 10 to 30 µm sized elongated grains embedded in a matrix of ~1 to 5 μm sized polygonal grains. By contrast, samples sheared at σneff = 50 MPa at v = 100 μm/s, showed τss ≈ 38 MPa, and a microstructure consisting of a matrix of 1 to 40 μm sized angular to sub-rounded grains cut by a sharply-defined, 30 to 40 µm wide, boundary-parallel shear band composed of ~0.3 to 1 μm sized polygonal grains, characterized by strong, uniform, birefringence colours, and uniform extinction, suggestive of a crystallographic preferred orientation (CPO). Plots of shear strength against σneff, showed a clear deviation from linearity for σneff-stepping tests conducted at 0.1 µm/s, whereas for those conducted at 10 µm/s, the τ- σneff dependence was well-described with a straight line. Our results suggest that, in experiments conducted at low slip rates (v = 0.1 μm/s), the shear strain was accommodated by pervasive, frictional-viscous flow, whereas for samples sheared at high rates (v > 10 μm/s), this occurred via localized, frictional slip. Using a recent microphysical model involving a competition between dilatant granular flow vs. creep-controlled compaction, we show that the latter may correspond with a regime of potentially seismogenic, velocity weakening slip, as opposed to stable, velocity strengthening shear for tests performed at v = 0.1 μm/s. |
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