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| Titel |
Frictional processes in smectite-rich gouges sheared at slow to high slip rates |
| VerfasserIn |
Stefano Aretusini, Silvia Mittempergher, Alessandro Gualtieri, Giulio Di Toro |
| 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 |
250109735
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| Publikation (Nr.) |
 EGU/EGU2015-9672.pdf |
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| Zusammenfassung |
| The slipping zones of shallow sections of megathrusts and of large landslides are often
smectite-rich (e.g., montmorillonite type). Consequently, similar "frictional" processes
operating at high slip rates (> 1 m/s) might be responsible of the large slips estimated in
megathrust (50 m for the 2011 Tohoku Mw 9.1 earthquake) and measured in large landslides
(500 m for the 1963 Vajont slide, Italy). At present, only rotary shear apparatuses can
reproduce simultaneously the large slips and slip rates of these events. Noteworthy, the
frictional processes proposed so far (thermal and thermochemical pressurization, etc.) remain
rather obscure.
Here we present preliminary results obtained with the ROtary Shear Apparatus
(ROSA) installed at Padua University. Thirty-one experiments were performed at
ambient conditions on pure end-members of (1) smectite-rich standard powders
(STx-1b: ~68 wt% Ca-montmorillonite, ~30 wt% opal-CT and ~2 wt% quartz), (2)
quartz powders (qtz) and (3) on 80:20 = Stx-1b:qtz mixtures. The gouges were
sandwiched between two (1) hollow (25/15 mm external/internal diameter) or (2)
solid (25 mm in diameter) stainless-steel made cylinders and confined by inner and
outer Teflon rings (only outer for solid cylinders). Gouges were sheared at a normal
stress of 5 MPa, slip rates V from 300 μm/s to 1.5 m/s and total slip of 3 m. The
deformed gouges were investigated with quantitative (Rietveld method with internal
standard) X-ray powder diffraction (XRPD) and Scanning Electron Microscopy
(SEM).
In the smectite-rich standard endmember, (1) for 300 μm/s ≈¤ V ≈¤ 0.1 m/s, initial
friction coefficient (μi) was 0.6±0.05 whereas the steady-state friction coefficient (μss) was
velocity and slip strengthening (μss 0.85±0.05), (2) for 0.1 m/s < V < 0.3 m/s, velocity
and slip neutral (μi = μss = 0.62±0.08) and (3) for V > 0.8 m/s, velocity and slip
weakening (μi = 0.7±0.1 and μss = 0.25±0.05). In the 80:20 Stx-1b:qtz mixtures, (1) for
300 μm/s ≈¤ V ≈¤ 0.1 m/s, μi ranged was 0.7±0.05 and increased with slip
to μss = 0.77±0.02 (slip-strengthening behavior), (2) for V = 0.1 m/s velocity
and slip neutral (μi = μss = 0.77±0.02) and (3) for V ≈¥ 0.3 m/s the friction
coefficient was velocity and slip weakening with μss = 0.32±0.02 for V = 1.5
m/s.
The Rietveld analysis of the smectite-rich standard endmember showed (1) the
insensitivity of the amount of the amorphous fraction with frictional work and (2) the shift
and broadening of both the (001) and (110) peaks of Ca-montmorillonite with increasing
frictional work (i.e., product of shear stress with slip, here from 5.2 Jm-2 to 11.8 Jm-2).
Instead, mineralogical and lattice changes were unrelated to the frictional work
rate (i.e., product of shear stress with slip rate). Strain localization in the gouge
layer was observed for V ≈¥ 0.3 m/s (SEM investigations); for V < 0.3 m/s,
strain was distributed and the gouge layer pervasively foliated. We conclude that the
degree of amorphization of the sheared gouges was not responsible of the measured
frictional weakening; instead, weakening was concomitant to strain localization. |
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