-Accurate description of the topography of active faults surfaces represents an 23 important geophysical issue because this topography is strongly related to the stress 24 distribution along fault planes, and therefore to processes implicated in earthquake nucleation, 25 propagation, and arrest. 26Up to know, due to technical limitations, studies of natural fault roughness either performed 27 using laboratory or field profilometers, were obtained mainly from 1D profiles. With the 28 recent development of Light Detection And Ranging (LIDAR) apparatus, it is now possible to 29 measure accurately the 3D topography of rough surfaces with a comparable resolution in all 30 directions, both at field and laboratory scales. In the present study, we have investigated the 31 scaling properties including possible anisotropy properties of several outcrops of two natural 32 fault surfaces (Vuache strike-slip fault, France, and Magnola normal fault, Italy) in 33 limestones. At the field scale, digital elevation models of the fault roughness were obtained 34 over surfaces of 0.25 m 2 to 600 m 2 with a height resolution ranging from 0.5 mm to 20 mm. 35At the laboratory scale, the 3D geometry was measured on two slip planes, using a laser 36 profilometer with a spatial resolution of 20 μm and a height resolution less than 1 μm. 37

[1] 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.

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[1] A kinetic study including dissolution process and diffusion of the dissolved molecules for a stressed solid in contact with its solution is analyzed. We estimate a condition fixing the prevailing dissipation mechanism and analyze it with a linear stability criterion. This criterion depends on which process is limiting the rate of dissipation: dissolution at the solid-liquid interface or diffusion in the fluid. For definiteness we focus on recent experiments on various salts, which have shown that grooves, oriented perpendicular to the main compressive stress, develop on the free surfaces of crystals. We provide the characteristic length scale and the time scale for the development of this stress-induced roughening of solid surfaces, which are consistent with the experiments. Finally, we estimate these parameters for relevant geological conditions.

Magnesium carbonate production at the industrial scale is a realistic option to reduce the industrial emissions of CO2. Ultrabasic rocks and/or alkaline mine waste provide magnesium sources and are widely available in the Earth's crust. Here, we investigated the aqueous carbonation of magnesium hydroxide under moderate temperature (25-90°C) and pressure (initial pressure of CO2=50 bar) using NaOH as the CO2 sequestering agent. From time-resolved Raman measurements, we demonstrate that the aqueous carbonation of magnesium hydroxide can be an effective engineered method to trap CO2 into a solid material and produce large amounts of magnesite MgCO3 (6 kg/m 3 h), or hydromagnesite Mg5(CO3)4(OH)2.4H2O (120 kg/m 3 h) at 90°C or nesquehonite MgCO3.3H2O (40 kg/m 3 h) at 25°C. Higher production rates were measured for nesquehonite (at 25°C) and hydromagnesite (at 60 and 90°C). However, only the magnesite produced at 90°C ensures a permanent CO2 storage because this mineral is the most stable Mg carbonate under Earth surface conditions, and it could be co-used as construction material in roadbeds, bricks with fireretarding property and granular fill. The use of specific organic additives can reduce the reaction temperature to precipitate magnesite. For example, ferric EDTA (ethylenediaminetetraacetic acid) reduces the temperature from 90 to 60°C. However, more time is required to complete magnesite precipitation reaction at this lower temperature (15h at 90°C and 7 days at 60°C). These results suggests that functionalized organic groups can reduce the energetic barriers during mineral nucleation.

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