Jean-Pierre Gratier

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

Experimental pressure solution-deposition on quartz grains: the crucial effect of the nature of the fluid

Abstract Experimental deformation by pressure solution was performed on an aggregate of small grains subjected to deviatoric stress (50 MPa) for a long time (several weeks or months) at relatively high temperature and pressure in contact with various fluids (air, water, 0.1 to 1 N NaOH for quartz, water and 5% NH 4 Cl for calcite). The change in shape of the grains by solution—deposition depended; on the duration of the experiment (with the same fluid) and on the concentration of the solid in solution (with the same duration but various fluids). Significant shape changes were obtained for quartz grains, but only with both long duration and very good solvents (1 N NaOH). By comparison with previously obtained results on the change of shape of fluid inclusions (where the kinetics of dissolution was the rate controlling process), the limiting process of the deformation of the quartz grains was inferred to be the rate of diffusion along grain boundaries saturated with trapped fluid.

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.

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.

Compatibility constraints on folded and faulted strata and calculation of total displacement using computational restoration (UNFOLD program)

Abstract A balanced-surface method is proposed that allows one to test the reliability of the interpretation of the structural geometry of folded and faulted strata. It also estimates both the finite total displacement field linked to the folding and faulting processes and the finite displacement field linked to the folding. The method describes a thin competent folded and faulted sedimentary layer using rigid (triangular) elements, their sizes depending on the curvature of the surface. The elements are laid flat (and are automatically fit) to form a horizontal surface, which represents the initial state of the layer. The degree of compatibility (given by different indicators) tests the reliability of the geometric fitting of the layer. If folding and faulting occur without bed stretching (or if this change is known and introduced as a parameter in the code) a plausible interpretation can be perfectly retrodeformed, just as a folded and torn sheet of paper may be smoothed with an iron. The method has been applied to two natural examples in oil-field regions using three- or two-dimensional depth-migrated seismic data. The main results reveal in general that the petroleum companys interpretations of the data were nonoptimal. A careful reinterpretation of the seismic data was necessary to obtain balanced folded and faulted surfaces. The estimation of the finite displacement fields revealed the compatibility between fold and fault deformation, and also the strike-slip movement or rotation associated with the deformation.

Restoration and balance of a folded and faulted surface by best-fitting of finite elements: principle and applications

Abstract A computer program is presented which allows us to test the restoration of a folded and faulted thin competent layer and then to balance this surface. The balance of such a surface is useful both to constrain the three-dimensional shape of the folds and the geometry of the limits of the faults. If a part of the surface is fixed the restoration can also give the finite displacement field linked to the deformation of the layer. The principle of the method is given and its accuracy is tested for the restoration of an experimentally folded sheet of paper. Finally the applicability to the restoration of natural structures is discussed.

Self‐organization during reactive fluid flow in a porous medium

When a reactive fluid circulates inside a porous medium it can dissolve some minerals if equilibrium is not reached and modiy the porosity and permeability. The positive feedback between fluid transport and mineral dissolution lead to complex reaction front morphologies such as fingers. Our study is carried out with two objectives: 1) to evaluate experimentally these processes at a decimeter scale, 2) to compare the experiment to a numerical model of water-rock interaction. The experiment consists of a two-dimensional porous medium that allows for the dissolution of halite under an imposed fluid flow. The numerical code used solves the equations of reaction and transport in a porous medium. Both experiment and numerical simulation indicate the development of an instability whose propagation rate depends on the rate of water injection raised to a 2/3 power.

Deformation pattern in a heterogeneous material: Folded and cleaved sedimentary cover immediately overlying a crystalline basement (Oisans, French Alps)

Abstract Particularly well exposed structures in folded and cleaved sedimentary cover immediately overlying a crystalline basement have been studied. Chemical analysis (X.R.F. and microprobe) reveal pressure solution process and give the possibilities of measurement of mass transfer. Study of fluid inclusion veins has determined the temperature pressure conditions: thermal effect of the basement and decrease of temperature and pressure with the age of various synkinematic veins. Characteristic examples of the behaviour of a heterogeneous material during coaxial and non-coaxial deformation are shown: 1. (1) Successive different asymmetrical folds, various cleavages and fractures appear in a shear zone parallel to the main fabric with variations of thickness and rock behaviour. 2. (2) Evolution of cleavage in such a shear zone (with or without slipping) is linked to the relations between the rotation of contraction direction and the rate of the cleavage process. 3. (3) Fold axes changed from the horizontal y direction to the vertical (or E—W transversal to the crystalline massif) X direction, with increase of the (X/Z) and (X/Y) ratios (obtained by fossils and reconstructed fold shape). This strain is always heterogeneous and the most deformed zone frequently evolves to discontinuities with slip. 4. (4) Indentation exists on all scale: from hard object (100 μ, with parenthesis form of pressure solution cleavage apparent on map distribution of various element) to basement block (with variation of strain value in the indented cover). A model of the evolution of the deformation of sedimentary cover immediately overlying a crystalline basement is given in conclusion.