Nicolas Brantut

Fast slip with inhibited temperature rise due to mineral dehydration: Evidence from experiments on gypsum

Anomalously low heat flow around active faults has been a recurrent subject of debate over past decades. We present a series of high-velocity friction experiments on gypsum rock cylinders showing that the temperature of the simulated fault plane is efficiently buffered due to large-scale endothermic dehydration reaction. The tests were performed at 1 MPa normal stress and a velocity of 1.3 m s −1 , while measuring the temperature close to the sliding surface and the relative humidity around the sample. The temperature close to the sliding surface is remarkably stable at ∼100 °C during the dehydration reaction of gypsum. Microstructural and X-ray diffraction investigations show that dehydration occurs at the very beginning of the test, and progresses into the bulk as slip increases. In the hottest parts of the sample, anhydrite crystal growth is observed. The half-thickness of the dehydrated layer ranges from 160 μm at 2 m slip to 5 mm at 68 m slip. Thermodynamic estimates of the energy needed for the dehydration to occur yield values ranging from 10% to 50% of the total mechanical work input. The temperature plateau is thus well explained by the energy sink due to the dehydration reaction and the phase change from liquid water into steam. We suggest that similar endothermic reactions can efficiently buffer the temperature of fault zones during an earthquake. This is a way to explain the low heat flow around active faults and the apparent scarcity of frictional melts in nature.

Mechanisms of time-dependent deformation in porous limestone

We performed triaxial deformation experiments on a water-saturated porous limestone under constant strain rate and constant stress (creep) conditions. The tests were conducted at room temperature and at low effective pressures Peff=10 and Peff=20 MPa, in a regime where the rock is nominally brittle when tested at a constant strain rate of 10−5 s−1. Under these conditions and at constant stress, the phenomenon of brittle creep occurs. At Peff=10 MPa, brittle creep follows similar trends as those observed in other rock types (e.g., sandstones and granites): only small strains are accumulated before failure, and damage accumulation with increasing strain (as monitored by P wave speeds measurements during the tests) is not strongly dependent on the applied stresses. At Peff=20 MPa, brittle creep is also macroscopically observed, but when the creep strain rate is lower than ≈10−7 s−1, we observe that (1) much larger strains are accumulated, (2) less damage is accumulated with increasing strain, and (3) the deformation tends to be more compactant. These observations can be understood by considering that another deformation mechanism, different from crack growth, is active at low strain rates. We explore this possibility by constructing a deformation mechanism map that includes both subcritical crack growth and pressure solution creep processes; the increasing contribution of pressure solution creep at low strain rates is consistent with our observations.

Rate‐ and strain‐dependent brittle deformation of rocks

We develop a unifying framework to quantify rate-dependent deformation in the brittle field and establish links between the microscale time-dependent crack growth processes and the macroscopically observed rate dependency. Triaxial deformation experiments have been performed under both constant strain rate and constant stress (creep) conditions on three types of sandstone. The measured relative evolution of P wave speeds as a function of inelastic axial strain is similar for both types of test, despite differences in strain rate of up to 3 orders of magnitude. This similarity indicates that there exists a direct, time-independent link between the microstructural state (as reflected by the variations in P wave speed) and the inelastic axial strain. Comparison of applied stresses between constant strain rate and creep experiments as a function of inelastic strain indicates that creep deformation requires less mechanical work to bring the sample to failure. This energy deficit corresponds to a stress deficit, which can be related to a deficit in energy release rate of the microcracks. We establish empirically that the creep strain rate is given by e∝exp(ΔQ/σ∗), where ΔQ is the stress deficit (negative) and σ∗ is an activation stress. This empirical exponential relation between creep strain rate and stress deficit is analogous to rate-and-state friction law. We develop a micromechanical approach based on fracture mechanics to determine the evolution of an effective stress intensity factor at crack tips during creep deformation and estimate the activation volume of the stress corrosion reaction responsible for brittle creep.

Fracture toughness anisotropy in shale

The use of hydraulic fracturing to recover shale-gas has focused attention on the fundamental fracture properties of gas-bearing shales, but there remains a paucity of available experimental data on their mechanical and physical properties. Such shales are strongly anisotropic, so that their fracture propagation trajectories depend on the interaction between their anisotropic mechanical properties and the anisotropic in-situ stress field in the shallow crust. Here we report fracture toughness measurements on Mancos shale determined in all three principal fracture orientations; Divider, Short-Transverse and Arrester, using a modified Short-Rod methodology. Experimental results for a range of other sedimentary and carbonate rocks are also reported for comparison purposes. Significant anisotropy is observed in shale fracture toughness measurements at ambient conditions, with values, as high as 0.72MPam1/2 where the crack plane is normal to the bedding, and values as low as 0.21MPam1/2 where the crack plane is parallel to the bedding. For cracks propagating non-parallel to bedding, we observe a tendency for deviation towards the bedding-parallel orientation. Applying a maximum energy release rate criterion, we determined the conditions under which such deviations are more or less likely to occur under more generalized mixed-mode loading conditions. We find for Mancos shale that the fracture should deviate towards the plane with lowest toughness regardless of the loading conditions.

Strain Localization and Slip Instability in a Strain-Rate Hardening, Chemically Weakening Material

The stability of steady slip and homogeneous shear is studied for rate-hardening materials undergoing chemical reactions that produce weaker materials (reaction-weakening process), in drained conditions. In a spring- slider configuration, a linear perturbation analysis provides analytical expressions of the critical stiffness below which unstable slip occurs. In the framework of a frictional constitutive law, numerical tests are performed to study the effects of a nonlinear reaction kinetics on the evolution of the instability. Slip instabilities can be stopped at relatively small slip rates (only a few orders of magnitude higher than the forcing velocity) when the reactant is fully depleted. The stability analysis of homogeneous shear provides an independent estimate of the thickness of the shear localization zone due to the reaction weakening, which can be as low as 0.1 m in the case of lizardite dehydration. The potential effect of thermo-chemical pore fluid pressurization during dehydration is discussed, and shown to be negligible compared to the reaction-weakening effect. We finally argue that the slip instabilities originating from the reaction-weakening process could be a plausible candidate for intermediate depth earthquakes in subduction zones.

Dynamic weakening and amorphization in serpentinite during laboratory earthquakes

The mechanical properties of serpentinites are key factors in our understanding of the dynamics of earthquake ruptures in subduction zones, especially intermediate-depth earthquakes. Here, we performed shear rupture experiments on natural antigorite serpentinite, which showed that friction reaches near-zero values during spontaneous dynamic rupture propagation. Rapid coseismic slip (>1 m/s), although it occurs over short distances (<1 mm), induces significant overheating of microscale asperities along the sliding surface, sufficient to produce surface amorphization and likely some melting. Antigorite dehydration occurs in the fault walls, which leaves a partially amorphized material. The water generated potentially contributes to the production of a low-viscosity pressurized melt, explaining the near-zero dynamic friction levels observed in some events. The rapid and dramatic dynamic weakening in serpentinite might be a key process facilitating the propagation of earthquakes at intermediate depths in subduction zones.

Time‐dependent recovery of microcrack damage and seismic wave speeds in deformed limestone

Limestone samples were deformed up to 5% inelastic axial strain at an effective confining pressure Peff=50 MPa in the cataclastic flow regime and subsequently maintained under constant static stress conditions (either isostatic of triaxial) for extended periods of time while elastic wave speeds and permeability were continuously monitored. During deformation, both seismic wave speeds and permeability decrease with increasing strain, due to the growth of subvertical microcracks and inelastic porosity reduction. During the static hold period under water-saturated conditions, the seismic wave speeds recovered gradually, typically by around 5% (relative to their initial value) after 2 days, while permeability remained constant. The recovery in wave speed increases with increasing confining pressure but decreases with increasing applied differential stress. The recovery is markedly lower when the samples are saturated with an inert fluid as opposed to water. The evolution in wave speed is interpreted quantitatively in terms of microcrack density, which shows that the post-deformation recovery is associated with a decrease in effective microcrack length, typically of the order to 10% after 2 days. The proposed mechanism for the observed damage recovery is microcrack closure due to a combination of backsliding on wing cracks driven by time-dependent friction and closure due to pressure solution at contacts between propping particles or asperities and microcrack walls. The recovery rates observed in the experiments, and the proposed underlying mechanisms, are compatible with seismological observations of seismic wave speed recovery along faults following earthquakes.

Time-dependent compaction band formation in sandstone

Compaction bands in sandstone are laterally extensive planar deformation features that are characterized by lower porosity and permeability than the surrounding host rock. As a result, this form of localization has important implications for both strain partitioning and fluid flow in the Earths upper crust. To better understand the time dependency of compaction band growth, we performed triaxial deformation experiments on water-saturated Bleurswiller sandstone (initial porosity = 0.24) under constant stress (creep) conditions in the compactant regime. Our experiments show that inelastic strain accumulates at a constant stress in the compactant regime, manifest as compaction bands. While creep in the dilatant regime is characterized by an increase in porosity and, ultimately, an acceleration in axial strain rate to shear failure, compaction creep is characterized by a reduction in porosity and a gradual deceleration in axial strain rate. The global decrease in the rates of axial strain, acoustic emission energy, and porosity change during creep compaction is punctuated at intervals by higher rate excursions, interpreted as the formation of compaction bands. The growth rate of compaction bands formed during creep is lower as the applied differential stress, and hence, background creep strain rate, is decreased. However, the inelastic strain associated with the growth of a compaction band remains constant over strain rates spanning several orders of magnitude (from 10−8 to 10−5 s−1). We find that despite the large differences in strain rate and growth rate (from both creep and constant strain rate experiments), the characteristics (geometry and thickness) of the compaction bands remain essentially the same. Several lines of evidence, notably the similarity between the differential stress dependence of creep strain rate in the dilatant and compactant regimes, suggest that as for dilatant creep, subcritical stress corrosion cracking is the mechanism responsible for compactant creep in our experiments. Our study highlights that stress corrosion is an important mechanism in the time-dependent porosity loss, subsidence, and permeability reduction of sandstone reservoirs.

Strain localization driven by thermal decomposition during seismic shear

Field and laboratory observations show that shear deformation is often extremely localized at seismic slip rates, with a typical deforming zone width on the order of a few tens of microns. This extreme localization can be understood in terms of thermally driven weakening mechanisms. A zone of initially high strain rate will experience more shear heating and thus weaken faster, making it more likely to accommodate subsequent deformation. Fault zones often contain thermally unstable minerals such as clays or carbonates, which devolatilize at the high temperatures attained during seismic slip. In this paper, we investigate how these thermal decomposition reactions drive strain localization when coupled to a model for thermal pressurization of in situ groundwater. Building on Rice et al. (2014), we use a linear stability analysis to predict a localized zone thickness that depends on a combination of hydraulic, frictional, and thermochemical properties of the deforming fault rock. Numerical simulations show that the onset of thermal decomposition drives additional strain localization when compared with thermal pressurization alone and predict localized zone thicknesses of ∼7 and ∼13 μm for lizardite and calcite, respectively. Finally we show how thermal diffusion and the endothermic reaction combine to limit the peak temperature of the fault and that the pore fluid released by the reaction provides additional weakening of ∼20–40% of the initial strength.

Compaction creep of sands due to time‐dependent grain failure: Effects of chemical environment, applied stress, and grain size

Time-dependent brittle creep plays a role in controlling compaction of sands and sandstones under upper crustal conditions, influencing phenomena such as production-induced reservoir compaction, surface subsidence, and induced seismicity. Brittle creep also plays a role in determining the mechanical behavior of gouge-rich faults. We performed uniaxial creep experiments on sand to investigate the effects of chemical environment (dry versus solution flooded), grain size (d=196-378 mu m), and applied effective stress (sigma(a) up to 30MPa), at room temperature conditions favoring grain-scale brittle processes. Creep measurements were complemented with acoustic emission (AE) detection and microstructural analysis to characterize the main creep mechanism. Wet samples showed much higher creep strains than dry-tested samples. AE event counts showed a direct relation between grain failure and creep strain, with higher AE rates occurring in the wet samples. Therefore, we inferred that time-dependent deformation was dominated by subcritical crack growth, resulting in grain failure accompanied by intergranular sliding rearrangements, and that crack growth in the presence of chemically active fluids was controlled by stress corrosion. The sensitivity of the compaction rate of the sands to d and sigma(a) can be expressed as (epsilon) over bar proportional to d(i)sigma(j)(a) where i approximate to 6 and j approximate to 21 under dry conditions and i approximate to 9 and j approximate to 15 under wet conditions. Our results were compared to a simple model based on Hertzian contact theory, linear elastic fracture mechanics, and subcritical crack growth. This model showed agreement between the observed stress and grain size sensitivities of creep, within a factor of 2.