• Toy, V. G.
• Wirth, R.
• Mitchell, Thomas

Abstract

The development of at least partially ';amorphous' and/or ';nanocrystalline' materials within fault principal slip zones has been shown to reduce frictional shear resistance during fault slip. Thus it is proposed generation of these materials facilitates shear localization and possibly even seismic slip. The generation of such materials has been demonstrated experimentally, both in high velocity friction experiments at ambient conditions (e.g. silica gels reported by Goldsby &amp; Tullis, 2002: GRL 29, 1844; Di Toro et al., 2004: Nature 427, 436), and very low velocity shear experiments at higher temperatures and confining pressures (e.g. apparent pseudotachylytes reported by Pec et al, 2012: EPSL 355-356, 299). They have also been reported in natural fault zones (e.g. natural silica gel from the Corona Fault described by Kirkpatrick et al., in press: Geology). These materials commonly comprise some proportion of randomly-oriented nanocrystals embedded in a non-crystalline matrix that displays no TEM diffraction contrast or lattice fringes. Proposed generation mechanisms include: irradiation damage, deformation, application of pressure, and chemical reactions. In particular, Pec et al., (2012) proposed that micro-comminution processes precede the generation of lattice defects. In this study we show that partially-amorphous silica material can be generated experimentally on a saw-cut surface in novaculite during shear at ~8 x 10<SUP>-4</SUP>m/s, in a Griggs apparatus under P<SUB>conf</SUB> ~0.5 GPa, T = 450 and 600°C. The material comprises angular nanocrystals ranging from 2-10 nm diameter in a entirely non-crystalline matrix,has variable density that increases with decreasing proportion of nanocrystal remnants, suggesting it is a partially compacted nanopowder. This material is restricted to a zone &lt;50 μm wide between the sawcut sliders. We infer an origin by micro-comminution, wherein repeated microfracturing results in formation of a very high proportion of non-crystalline surfaces. The transition to intact wall rock is relatively sharp, although localized zones of ultracataclastic wall rock are preserved in dilational structures. This shows the nanopowder is significantly weaker than the surrounding wall rock and effectively localizes shear once formed. The dislocation structure of the surrounding quartz grains was examined for indications on the type of defects that might be precursors to micro-comminution during nanopowder generation. Within nearby surrounding grains we observe &lt;50 nm wide planar defects, indexed by HR-TEM to lie within (0001). Within these defects the crystal lattice is disordered compared to the surrounding grain. The defects truncate or interrupt diffraction contrast bands but do not necessarily displace these bands laterally (ie. they have not accommodated shear displacement). Importantly, we note wall rock dislocation densities are not significantly higher around these defects, immediately adjacent to the sheared surface, or even within the few nanocrystals large enough to resolve within the nanopowder. We suggest these planar defects, which might otherwise be described as ';deformation bands', are the first form of damage within the quartz grains, and that they represent a precursor to the fracture surfaces that would have divided the nanopowder grains....

Topics

• density
• impedance spectroscopy
• surface
• amorphous
• grain
• experiment
• transmission electron microscopy
• dislocation
• crystalline lattice