François Renard

Pressure solution in sandstones: influence of clays and dependence on temperature and stress

Abstract The enhancement of dissolution of quartz under the influence of clays has been recognized in sandstones for many years. It is well known that a grain of quartz in contact with a clay flake dissolves faster than when in contact with another grain of quartz. This phenomenon promotes silica transfer during the diagenesis of sandstones and is responsible of deformation and porosity variations. Here we make an attempt to explain the process of this rock deformation using a pressure solution mechanism. The model of water film diffusion assumes that matter is dissolved inside the contact between two grains. The resulting solutes are transported to the pore fluid through diffusion along an adsorbed water film. Between two micas, this trapped film is thicker than between two grains of quartz. As a consequence diffusion is easier and the rate of pressure solution faster. Experiments on pressure solution show that diffusion controls the mechanism at great depth whereas a model based on natural mica indentation indicates that kinetics is the limiting process through the precipitation rate of quartz at low depth, thus temperature is a crucial parameter. There should be a transition between thermally controlled rate and diffusion limited evolution.

Kinetics of crack-sealing, intergranular pressure solution, and compaction around active faults

Geological evidence indicates that fluids play a key role during the seismic cycle. After an earthquake, fractures are open in the fault and in the surroundings rocks. With time, during the interseismic period, the permeability of the fault and the country rocks tends to decrease by gouge compaction and fracture healing and sealing. Dissolution along stylolite seams provides the matter that fills the fractures, whereas intergranular pressure solution is responsible for gouge compaction. If these processes are fast enough during the seismic cycle, they can modify the creep properties of the fault. Based on field observations and experimental data, we model the porosity decrease by pressure solution processes around an active fault after an earthquake. We arrive at plausible rates of fracture sealing that are comparable to the recurrence time for earthquakes. We also study the sensitivity of these rates to various parameters such as grain size, fracture spacing, and the coeAcient of diAusion along grain

Enhanced pressure solution creep rates induced by clay particles: Experimental evidence in salt aggregates

Pressure solution is responsible for mechano-chemical compaction of sediments in the upper crust (2–10 km). This process also controls porosity variations in a fault gouge after an earthquake. We present experimental results from chemical compaction of aggregates of halite mixed with clays. It is shown that clay particles (1–5 microns) greatly enhance the deformation by pressure solution in salt aggregates (100–200 micron), the strain rates being 50% to 200% faster in samples containing 10% clays than for clay-free samples. Even the presence of 1% clay increases the strain rate significantly. We propose that clay particles enhance pressure solution creep because these microscopic minerals are trapped within the salt particle contacts where they allow faster diffusion of solutes from the particle contacts to the pore space and inhibit grain boundary formation.

The Role of Pressure Solution Creep in the Ductility of the Earth’s Upper Crust

Abstract The aim of this review is to characterize the role of pressure solution creep in the ductility of the Earth’s upper crust and to describe how this creep mechanism competes and interacts with other deformation mechanisms. Pressure solution creep is a major mechanism of ductile deformation of the upper crust, accommodating basin compaction, folding, shear zone development, and fault creep and interseismic healing. However, its kinetics is strongly dependent on the composition of the rocks (mainly the presence of phyllosilicates minerals that activate pressure solution) and on its interaction with fracturing and healing processes (that activate and slow down pressure solution, respectively). The present review combines three approaches: natural observations, theoretical developments, and laboratory experiments. Natural observations can be used to identify the pressure solution markers necessary to evaluate creep law parameters, such as the nature of the material, the temperature and stress conditions, or the geometry of mass transfer domains. Theoretical developments help to investigate the thermodynamics and kinetics of the processes and to build theoretical creep laws. Laboratory experiments are implemented in order to test the models and to measure creep law parameters such as driving forces and kinetic coefficients. Finally, applications are discussed for the modeling of sedimentary basin compaction and fault creep. The sensitivity of the models to time is given particular attention: viscous versus plastic rheology during sediment compaction; steady state versus non-steady state behavior of fault and shear zones. The conclusions discuss recent advances for modeling pressure solution creep and the main questions that remain to be solved.

Aseismic sliding of active faults by pressure solution creep: Evidence from the San Andreas Fault Observatory at Depth

Active faults in the upper crust can either slide steadily by aseismic creep, or abruptly causing earthquakes. Creep relaxes the stress and prevents large earthquakes from occurring. Identifying the mechanisms controlling creep, and their evolution with time and depth, represents a major challenge for predicting the behavior of active faults. Based on microstructural studies of rock samples collected from the San Andreas Fault Observatory at Depth (California), we propose that pressure solution creep, a pervasive deformation mechanism, can account for aseismic creep. Experimental data on minerals such as quartz and calcite are used to demonstrate that such creep mechanism can accommodate the documented 20 mm/yr aseismic displacement rate of the San Andreas fault creeping zone. We show how the interaction between fracturing and sealing controls the pressure solution rate, and discuss how such a stress-driven mass transfer process is localized along some segments of the fault.

How pressure solution creep and fracturing processes interact in the upper crust to make it behave in both a brittle and viscous manner

Abstract The upper crust has been described as being dominated by brittle deformation along faults, or ductile where folds and cleavage have developed. These two mechanical behaviors are explained by two different mechanisms of deformation: (i) fracture; and (ii) fluid-enhanced deformation (e.g. pressure solution). These two mechanisms operate at two time scales: fast for brittle deformation, slow for pressure solution. Natural observations of relationships between pressure solution and fractures in sandstones, or indented pebbles, and experimental results of pressure solution with an indenter technique indicate that both mechanisms can interact: fracture development increases the kinetics of the pressure solution process, pressure solution relaxes the stress between fracturing events. A simple model of brittle–ductile deformation, applied to indented limestone pebbles, shows that cycles of slow deformation can alternate with short-time fracture.

WATER FILMS AT GRAIN-GRAIN CONTACTS : DEBYE-HUCKEL, OSMOTIC MODEL OF STRESS, SALINITY, AND MINERALOGY DEPENDENCE

Abstract Water film diffusion is one of the mechanisms proposed to explain the deformation of rocks by pressure-solution during geological processes in the upper crust. This mechanism assumes that matter is dissolved inside the contact between two grains. The resulting solutes are transported in the pore fluid through diffusion in an adsorbed water film. The main problem of this theory is that it requires the presence of a water film that is believed to be stable under large deviatoric stresses inside the contact between two grains. In this paper, we show that the electrically charged surface of a mineral can attract counter-ions from the pore and, by the related change of osmotic pressure, keep water within the contact. This is due to the counter ions in the water film that increase the salinity in the film relative to that in the pore. This lowers the free energy of water in the contact zone to a degree that balances the increase in free energy of water due to the elevated pressure in the film. These notions are made more precise by combining the theory of the Debye-Huckle double layer with equations of osmotic pressure. The resulting D-H/O theory predicts the dependence of the water film thickness on stress across the contact, composition of the pore fluid, and the identity of the minerals involved.

Three‐dimensional roughness of stylolites in limestones

Stylolites are dynamic roughly planar surfaces formed by pressure solution of blocks of rocks into each other. The three-dimensional geometry of 12 bedding-parallel stylolites in several limestones was measured using laser and mechanical profilometers, and statistical characteristics of the surfaces were calculated. All the stylolites analyzed turn out to have self-affine fractal roughness with a well-characterized crossover length scale separating two self-affine regimes. Strikingly, this characteristic length scale falls within a very narrow range for all the stylolites studied, regardless of the microstructure sizes. To explain the data, we propose a continuous phenomenological model that accounts for the development of the measured roughness from an initially flat surface. The model postulates that the complex interface morphology is the result of competition between the long-range elastic redistribution of local stress fluctuations, which roughen the surface, and surface tension forces along the interface, which smooth it. The model accounts for the geometrical variability of stylolite surfaces and predicts the dependence of the crossover length scale on the mechanical properties of the rock.

Growth of stylolite teeth patterns depending on normal stress and finite compaction

Abstract Stylolites are spectacular rough dissolution surfaces that are found in many rock types. They are formed during a slow irreversible deformation in sedimentary rocks and therefore participate to the dissipation of tectonic stresses in the Earths upper crust. Despite many studies, their genesis is still debated, particularly the time scales of their formation and the relationship between this time and their morphology. We developed a new discrete simulation technique to explore the dynamic growth of the stylolite roughness, starting from an initially flat dissolution surface. We demonstrate that the typical steep stylolite teeth geometry can accurately be modelled and reproduce natural patterns. The growth of the roughness takes place in two successive time regimes: i) an initial non-linear increase in roughness amplitude that follows a power-law in time up to ii) a critical time where the roughness amplitude saturates and stays constant. We also find two different spatial scaling regimes. At small spatial scales, surface energy is dominant and the growth of the roughness amplitude follows a power-law in time with an exponent of 0.5 and reaches an early saturation. Conversely, at large spatial scales, elastic energy is dominant and the growth follows a power-law in time with an exponent of 0.8. In this elastic regime, the roughness does not saturate within the given simulation time. Our findings show that a stylolites roughness amplitude only captures a very small part of the actual compaction that a rock experienced. Moreover the memory of the compaction history may be lost once the roughness growth saturates. We also show that the stylolite teeth geometry tracks the main compressive stress direction. If we rotate the external main compressive stress direction, the teeth are always tracking the new direction. Finally, we present a model that explains why teeth geometries form and grow non-linearly with time, why they are relatively stable and why their geometry is strongly deterministic while their location is random.

Stress Drop during Earthquakes: Effect of Fault Roughness Scaling

We propose that a controlling parameter of static stress drop during an earthquake is related to the scaling properties of the fault-surface topography. Using high resolution laser distance meters, we have accurately measured the roughness scaling properties of two fault surfaces in different geological settings (the French Alps and Nevada). The data show that fault-surface topography is scale dependent and may be accurately described by a self-affine geometry with a slight anisotropy characterized by two extreme roughness exponents ( H R ), H ||=0.6 in the direction of slip and H ⊥=0.8 perpendicular to slip. Disregarding plastic processes like rock fragmentation and focusing on elastic deformation of the topography, which is the dominant mode at large scales, the stress drop is proportional to the deformation, which is a spatial derivative of the slip. The evolution of stress-drop fluctuations on the fault plane can be derived directly from the self-affine property of the fault surface, with the length scale ( λ ) as std Δσ ( λ )∝ λ H R -1. Assuming no characteristic length scale in fault roughness and a rupture cascade model, we show that as the rupture grows, the average stress drop, and its variability should decrease with increasing source dimension. That is for the average stress drop Δσ ( r )∝ r H R -1, where r is the radius of a circular rupture. This result is a direct consequence of the elastic squeeze of fault asperities that induces the largest spatial fluctuations of the shear strength before and after the earthquake at local (small) scales with peculiar spatial correlations.