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| Titel |
Slickenline development during fault slip depends on temperature under hydrothermal conditions |
| VerfasserIn |
Virginia Toy, André Niemeijer, Luiz Morales, Richard Wirth, Francois Renard |
| 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 |
250128660
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| Publikation (Nr.) |
 EGU/EGU2016-8667.pdf |
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| Zusammenfassung |
| Polished silica discs embedded in synthetic gouge were deformed under hydrothermal
conditions to examine i) if there is any relationship between slickenline morphology
and P, T, or shear strain, ii) what processes and deformation mechanisms control
slickenline development, and iii) the implications of slickenline development for
fault rheology. Experiments were performed using the hydrothermal rotary shear
apparatus at the HPT laboratory, Utrecht University at 100 or 450˚ C, σneff = 100
MPa, Pf = 100 MPa to shear strains in the range 2.02< γ <8.25. The mechanical
records show slip hardening and slip weakening at low and high temperatures,
respectively.
Examination of recovered samples on the scanning electron microscope reveals that the
disc surfaces are decorated by fine gouge, sometimes arranged in trails, pits, and scratch
marks. In the discs deformed at 100˚ C the orientation of these slip vector-parallel
features varies across a single disc surface. Additionally, only the discs deformed at
450˚ C and best developed on higher strain samples, have smooth grooves that
extend in one direction over the whole disc surface. White light interferometry
confirms a cross sectional wavelength <7μm and amplitude <0.5μm. These grooves
are slickenlines developed parallel to the direction of relative shear of the gouge
and the disc surfaces. In some places, the parts of the grooves below the original
disc surface have surfaces decorated by scattered rounded beads of silica ∼100nm
diameter (composition confirmed by EDS). Conversely, close examination of pits
in the 100˚ C experiments reveals they contain angular particles ranging <2μm
diameter.
A TEM foil cut perpendicular to the disc surface and inferred slip direction
cross-sectioned the undulating surface and the rounded particles. There are no significant
systematic dislocation arrays in the adjacent disc quartz, but microfractures are sporadically
present. Additionally, amorphous silica fills the space between the rounded quartz beads. We
infer that dislocation processes were not important in deformation of the sample surface.
Instead, we propose that at both temperatures brittle failure generated microfractures and
micro-comminution occurred where gouge particles impacted the disc surfaces. Furthermore,
at the higher temperatures, amorphous silica formed on the disc surface. We calculate that
200 nm diameter crystalline quartz particle could precipitate at 450OC in only 250 second,
well within the timeframe of the experiments but that at 100˚ C, precipitation takes 8
years.
We therefore suggest formation of silica beads by growth (precipitation) from
amorphous silica facilitates slip weakening, smoothing of the fault surface parallel to
the slip vector, development of undulations perpendicular to the slip vector, and
maintenance of a constant slip direction. This mechanism is likely to be effective
on a seismic timescale only at elevated (greenschist or greater) facies conditions. |
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