This paper presents an experimental study of the deformation of Beaucaire marl subjected to drained triaxial tests under a wide range of confining pressure. Several methods of observation have been ...used: X-ray computed tomography, scanning electronic microscopy, quantitative image analysis of thin sections and mercury porosimetry. The analysis at the grain scale shows the existence of various deformation modes depending on the mean stress level. These modes are classified with reference to the brittle and ductile regimes. In specimens deformed under low confining pressure, macro-cracks are localised in a band more contracting than the rest of the specimen. In specimens deformed under moderate confining pressure, a series of micro-scale shear bands characterised by a grain re-orientation towards the direction of macro-shear is detected in the zones of localised deformation. The material in between the micro-scale shear bands is contracting more than the rest of the specimen in some cases; in other cases, a schistosity is detected also in the rest of the specimen outside the localisation zones. In the specimen deformed under high confining pressure, no localisation occurs, and a schistosity is observable throughout the specimen.
The brittle structures of a Hercynian granite (La Borne, French Massif Central) observed at several scales, from regional to microscopic, are presented and interpreted on mechanical terms. Emphasis ...is placed on strike slip faults, joints, and cracks related to incipient shear fracturing during horizontal compression. Three compressive tectonic phases have been identified according to their brittle structures and characterised by the burial depth at the time of their generation.
The two first phases (H1, H2) are Hercynian and occurred while the granite was deeply buried (∼5 to 3 km depth) whereas the last phase (P) is Pyrenean and occurred at very low depth. The geometric organizations of cracks, joints and faults are clearly similar at various scales from about 10 μm to several m.
These field observations are strong arguments for the occurrence of shear structures at microscopic to macroscopic scales during tectonic events. Following this inference, it is proposed that, at least in the case of Hercynian phases, cracks, joints and macroscopic strike slip faults have been generated by the same mechanical process under high overburden weight. Therefore, incipient fractures seem to be generated as shearing structures and to evolve by coalescence with neighbouring ones with the same direction.
•La Borne granite brittle structures are characterized at different scales.•The observations are deciphered according to three compressive tectonic phases.•For each phase the geometric organisation of cracks, joints and faults are similar.•The burial depth controls the deformation style.•For large overburden depth incipient discontinuity occurs in shear mode at any scale.
In this study, we describe the characteristics of tectonic stylolites and related veins affecting a low-porosity micritic limestone (Jurassic carbonates, Les Matelles, South of France) in order to ...unravel the conditions of initiation and interaction between pressure-solution and fracturing in such rock. Field description, various petrographic and microstructural investigations (cathodoluminescence, SEM imaging, EBSD analysis), and petrophysical/geochemical analyses (Hg porosimetry, XRD, EPMA) are used. We document that pressure-solution initiates at micropores and propagates along calcite grain contacts, connecting surrounding stylolite micro-segments, and progressively concentrates insoluble material such as clays and siliceous particles. The dissolved material is evacuated to the veins where the newly-formed porous space is progressively filled by calcite cement. These deformation processes are strictly restricted to the stylolitic interface and veins, as no modification of porosity or grain deformation is detected in the neighboring host rock. This is due to the low-permeability of the surrounding host rock impeding the evacuation of dissolved material and fluids through interstitial porosity around the pressure-solution zone, leading to overpressure and veins formation. The water release and microporosity caused by diagenesis of the clay fraction (smectite-illite transformation) are discussed as key diagenetic processes instigating the conditions of pressure-solution initiation, then tectonic stylolite formation in low-porosity limestones.
•The pressure-solution process is restricted to the stylolitic interface.•Stylolites propagate along grain contact and concentrate insoluble material.•Soluble material is evacuated to the vein formed by hydraulic fracturing.•The material transfer from stylolite to vein is a closed system.•Clay diagenesis releasing water and porosity allows the pressure-solution initiation.
In most rock mechanical testing devices, the texture developed during deformation is only observed after the failure, and the detailed mapping of cracks and pore structure evolution, just before or ...after rupture, remains speculative. Computerized X-ray tomography (CT X-ray) offers a way to observe ‘in vivo’ some characteristics of a tested sample in the course of its deformation. Therefore we developed an X-ray transparent tri-axial cell that was included in a medical purpose X-ray scanner with a volumetric resolution of the order of 1mm3. This technique is described and applied to a porous calcarenite, which is regularly scanned for porosity evolution during mechanical testing. The experiment is performed at low confining pressure (2–10MPa). During testing, an axial load is applied until failure and is then progressively decreased. In these experiments, the peak stress is preceded by phase of diffuse dilatancy. After failure localization, the axial stress is unloaded until the stress reaches isotropy. During this unloading phase, the previous diffuse dilatancy is lost due to an elastic relaxation. Then, the stress is completely relaxed. At this moment the dilatancy increases again and the residual dilatancy is larger than that reached at the peak stress. When observed “post mortem” at microscopic scale, the dilatant structures are comprised of cataclastic deformation bands and cracks in some larger crystals.
► A porous calcarenite is tested at 3 confining pressures until failure. Then it is progressively unloaded. ► A special cell transparent to X-ray allows to map in vivo its porosity évolution. ► The elastic and inelastic contraction and dilation phases are followed in détail. ► After unloading, the residual dilation is larger that the peak stress déformation.
We present an experimental study of relations between hydraulic properties of clayey rocks and their deformation at bulk and micro scale. The experimentation is based on triaxial deformation of ...kaolinitic core samples, on permeability measurements, and on several methods of analysis of the microstructure. To produce the samples, remoulded kaolinite material is consolidated by axial loading up to 20
MPa in an oedometric cell. Then, in a triaxial cell, an undrained stress path is applied in order to induce shearing damage in the sample. The permeability of the sample is measured by fluid flow across the sample. Before and after these hydro-mechanical tests, the evolution and the homogeneity of the pore space and moreover the texture of the material are analysed by X-ray tomography, environmental scanning electron microscopy and thermic neutron diffraction. Contrary to expectations, the permeability at the sample scale does not vary significantly, although quite large deformation is reached. The final deformation is heterogeneous, with well-developed shear bands. It appears that the development of compacting and dilating zones generates a heterogeneous permeability field, which results in a negligible variation of the bulk permeability at the sample scale.
Les marbres du Languedoc et des Pyrénées sont localisés dans les montagnes, Pyrénées et montagne Noire, car leur genèse est associée à la surrection de ces chaînes de montagne. La diversité des ...marbres est liée au grand nombre de roches différentes engagées dans le processus de surrection et à la complexité des évènements qui interviennent alors. Les marbres peuvent être classés en calcaires, calcaires épi-métamorphiques : marbres griottes, marbres s.s., brèches et roches diverses dont les granites. La variété des dessins et des couleurs de ces marbres est corrélée à leur histoire géologique. L’origine des couleurs est étudiée par des analyses chimiques.
Until now, observations of mechanically and thermally induced microcracks in rocks could only be carried out by indirect measurements or destructive observations on samples brought back to ...atmospheric pressure conditions. A special triaxial test cell was designed in order to perform direct observations during loading. The use of a cell in tomography apparatus involves new devices: (1) a movable horizontal load frame around a scanner; and (2) a test cell transparent to X-rays, able to withstand up to 28 MPa maximum confining pressure and temperatures of up to 180 °C. Volumetric strains are compared with radiological density measurements. The first processed X-ray images locating microcracks during propagation are also presented. Mineralogical effects on the crack location can be demonstrated. Strain inferred from CT density measurement is clearly correlated with the strain usually measured by a strain gauge. Different phases of mechanical behaviour are described: contracted phase and failure by macrocrack formation. The principal results obtained with this tool are the description of the porosity formation and macrocracking. Results show two principal factors localizing the porosity. First, the diffused porosity volume is controlled by mineralogical parameters, quartz and plagioclase grains, and boundaries of biotite grains during the thermal and mechanical loading. Second, macrocracking begins at the perimeter of the central section of core and grows towards the sample/piston interface. It seems that the first macrocracking is not located in the high-porosity zone formed during the loading phase, but in a relatively low-porosity zone.
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