LOW FRICTIONAL STRENGTH OF PHYLLOSILICATE-BEARING MIXTURES: MECHANICAL DATA AND MICROSTRUCTURAL OBSERVATIONS

It has long been suggested that the depth extent of the seismogenic zone is controlled by the temperature at which minerals can deform plastically, e.g. for quartz at a temperature of about 350 ºC. This transition temperature is based on experiments typically performed on low porosity rocks. However, exposures of exhumed faults show that these are typically filled with fine-powdered materials, i.e. fault gouge, which can have significant porosity. In addition, fluid-rock interaction is typically ubiquitous in faults and fault zones, for instance leading to the production of phyllosilicates from the breakdown of feldspars. It is widely accepted that the presence of a weak through-going foliation can accommodate slip at a low driving stress.

Here, we present results from friction experiments performed at elevated temperatures and varying slip velocities on a mixture of 80/20 quartz/muscovite as well as on natural samples of the Alpine Fault, New Zealand. These experiments show that low sliding velocities can result in low friction due to the development of a foliation. At the same time, localized grain size reduction occurs along the boundary with the hard Ni-alloy piston, which becomes more abundant with increasing slip velocity. Since the strength of the mixtures increases drastically with increasing slip velocity, we interpret the strength of this localized zone to be higher than that of the matrix, at least at low slip velocity. At the same time, when driving the sample at a typical experimental velocity and at 600 ºC, the sample experienced an almost complete stress drop which we interpret to be due to localized pressurization from phyllosilicate dehydroxylation.

Microstructural analyses show a through-going boundary-parallel fractured zone with the occasional in-filling of very fine-grained material. This material consists of intact small quartz grains (1-5 mm) surrounded by very small phyllosilicate platelets (long axis < 500 nm) and a diffuse mass of what appears to be poorly crystalline to amorphous material. Our results demonstrate the crucial combined effect of driving velocity (i.e. the operation of time-dependent processes) with the presence of phyllosilicates on both the mechanical results and the evolution of the microstructure.