Mirror-like Surfaces (MSs) are ultra-polished fault surfaces widespread in carbonate seismic terrains, but their formation process is still debated. We deformed gouge samples from exposed fault surfaces hosted in bituminous dolostone rocks in a rotary shear apparatus (SHIVA) at seismic slip rates (1 m/s). By changing the water availability (water-pressurised and room-humidity conditions) and the organic matter/dolomite content (> 35%, dark gouge DG; < 30% bright gouge BG) we investigated the mechanical behaviour leading to MSs formation in fault gouges. We run tests at 15 MPa effective normal stress, 2 MPa confinement and 1 MPa pore pressure for the water-pressurised experiments and a total displacement of 0.13 m. Mirror-like fault surfaces were obtained in all successful experiments; mirrors were more developed under room-humidity conditions. Bituminous dolostones under room-humidity conditions had a slip neutral behaviour with a low friction (0.3). Bituminous dolostones under water-pressurised conditions showed a slip weakening behaviour with an initial peak effective friction μp = 0.65, followed by a drop to effective friction μss DG than in BG (i.e., μss of 0.25 vs 0.28). Future work will focus on the microstructural analysis of the experimental products and the investigation of the slip behaviour of bituminous dolostones at sub-seismic slip rates for a complete study of the slip behaviour spectra.
This publication results from work conducted under the national open access action at SHIVA (Slow to High Velocity Apparatus) - HP-HT laboratory of experimental Volcanology and Geophysics (INGV, Roma 1 section) supported by WP3 ILGE - MEET project, PNRR - EU Next Generation Europe program, MUR grant number D53C22001400005.
Phyllosilicate-bearing faults are characterized by an anastomosing foliation with intervening hard clasts and are believed to be long-term weak structures. Here, I present results of sliding experiments on gouges of 80 wt% quartz and 20 wt% muscovite, sheared under hydrothermal conditions at constant velocity. The results show that significant strengthening occurs over a narrow range of sliding velocities (0.03-1* m-6/s). At the lowest velocity investigated, weakness is achieved after a considerable sliding distance of over 20 mm with friction reaching a value of 0.3. Microstructural observations and the application of existing models point to the operation of frictional-viscous flow (FVF), through the serial operation of frictional sliding over a weak foliation and pressure solution of intervening clasts, resulting in low frictional strength and pronounced velocity-strengthening. At higher velocities, grain size reduction becomes dominant in a localized zone, which results in disruption of the foliation and the cessation of the FVF mechanism. In natural settings, earthquakes originating elsewhere on the fault would be rapidly arrested when encountering a foliated part of the fault deforming via FVF. Furthermore, pulses of elevated slip velocity would lead to grain size reduction which would destroy the foliation and cause a long-term strengthening of the fault.
The largest magnitude earthquakes nucleate at depths near the base of the seismogenic zone, near the transition from velocity weakening frictional slip to velocity strengthening ductile flow. However, the mechanisms controlling this transition, and relevant to earthquake nucleation, remain poorly understood. Here we present data from experiments investigating the effect of slip rate on the mechanical properties and microstructure development of simulated calcite fault gouge sheared at ~550°C, close to the transition from (unstable) velocity weakening to (stable) velocity strengthening behaviour, reported by Verberne et al. (2015).
We conducted experiments at a constant effective normal stress (σneff) of 50 MPa, as well as σneff-stepping tests employing 20 MPa ≤ σneff ≤ 140 MPa, at constant sliding velocities (v) of 0.1, 1, 10, or 100 µm/s. Samples sheared at v ≥ 1 µm/s showed a microstructure characterized by a single, 30 to 40 μm wide boundary shear, as well as a linear correlation of shear strength (τ) with σneff. Remarkably, electron backscatter diffraction mapping of polygonal shear band grains demonstrated a crystallographic preferred orientation. By contrast, samples sheared at 0.1 µm/s showed a microstructure characterized by homogeneous deformation and plastic flow, as well as a flattening-off of the τ-σneff curve. Our results point to a strain rate dependent frictional-to-viscous transition in simulated calcite fault gouge, and have important implications for the processes controlling earthquake nucleation at the base of the seismogenic zone.
The Alpine Fault, New Zealand, is a major plate-bounding fault that accommodates 65–75% of the total relative motion between the Australian and Pacific plates.
Here we present data on the hydrothermal frictional properties of Alpine Fault rocks that surround the principal slip zones (PSZ) of the Alpine Fault and those comprising the PSZ itself. The samples were retrieved from relatively shallow depths during phase 1 of the Deep Fault Drilling Project (DFDP-1) at Gaunt Creek. Simulated fault gouges were sheared at temperatures of 25, 150, 300, 450, and 600°C in order to determine the friction coefficient as well as the velocity dependence of friction.
Friction remains more or less constant with changes in temperature, but a transition from velocity-strengthening behavior to velocity-weakening behavior occurs at a temperature of T = 150°C. The transition depends on the absolute value of sliding velocity as well as temperature, with the velocity-weakening region restricted to higher velocity for higher temperatures.Friction was substantially lower for low-velocity shearing (V<0.3 μm/s) at 600°C, but no transition to normal stress independence was observed. In the framework of rate-and-state friction, earthquake nucleation is most likely at an intermediate temperature of T = 300°C.
The velocity-strengthening nature of the Alpine Fault rocks at higher temperatures may pose a barrier for rupture propagation to deeper levels, limiting the possible depth extent of large earthquakes. Our results highlight the importance of strain rate in controlling frictional behavior under conditions spanning the classical brittle-plastic transition for quartzofeldspathic compositions.
The data is provided in a .zip folder with 33 subfolders for 33 samples. Detailed information about the files in these subdfolders as well as sensors used, conversions and data specifications is given in the explanatory file Niemeijer-2017-DFDP-explanation-of-folder-structure-and-file-list.pdf.