La deformation des roches de la croute est etudiee par une approche theorique et experimentale. L'etude experimentale porte sur trois roches: le marbre de Carrare, le granite d'Oshima et le calcaire de Solnhoffen. Les mecanismes de deformation de ces roches sont etudies en regime fragile et ductile. En regime fragile, la rupture est atteinte pour des densites de fissure superieures ou egales a 1. La transition fragile-ductile est caracterisee par une competition entre les mecanismes de la dilatance (fissuration) et ceux de la compaction (effondrements de pores, etc. ). L'etude theorique est fondee sur le modele de milieux fissures non-interactif de Kachanov [1993]. Ce modele permet de quantifier la fissuration et la saturation d'une roche en combinant des mesures de vitesses P et S a haute frequence (en laboratoire) en regime sec et sature. En couplant ce modele a la poroelasticite, il est possible de predire la dispersion des vitesses de propagation elastique en regime sature.
Effects of thermal crack damage on the rupture processes of a fine‐grained granite were investigated under triaxial stress, under water (wet) and argon gas (dry) saturated conditions, and at room temperature. Thermal cracking was introduced by slowly heating and cooling two samples of La Peyratte granite up to 700°C, which were compared to two intact specimens. For each rock sample, a hydrostatic test was first carried up to 90 MPa effective pressure (5 MPa constant pore pressure). The samples were then deformed to failure at a constant strain rate of 2.10 −6 s −1 , at 30 MPa effective pressure. Our results show that (1) permeability of heat‐treated specimens was 4–5 orders of magnitude larger than that of intact specimens at low effective mean pressure; (2) nevertheless, at our experimental conditions (2.10 −6 s −1 ), thermal cracking had no significant influence on the brittle strength; (3) similarly, no obvious water weakening effect was observed; (4) however, with increasing stress, elastic anisotropy appeared at lower differential stress in heat‐treated specimens than in intact ones, but close to failure, the magnitude of P wave anisotropy was approximately the same for both types of specimens; (5) acoustic emission hypocenter locations and P wave velocity anisotropy in the basal plane demonstrate that strain localization started right at the onset of dilatancy for heat‐treated specimens, later in the intact specimens; and (6) inverting wave velocities for crack density, we show that failure was reached for vertical crack densities of 0.35 for dry specimens and possibly 0.5 for water‐saturated specimens.
<p>&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160; Anomalously high seismic P- to S-wave velocity ratios (Vp/Vs) have been observed in subduction zones, in locations where varieties of earthquakes and slips are expected to occur. From qualitative laboratory knowledge of rocks Poisson&#8217;s ratio, these results were interpreted as evidence of near-lithostatic pore fluid pressure. Because most laboratory data did not document such high Vp/Vs values, these were further linked to additional constrains of anisotropy or the dominance of minerals of very high intrinsic Vp/Vs, e.g. mafic rocks.However, does high Vp/Vs necessarily imply anisotropy and/or mafic composition?</p><p>&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160; Recently, the measuring frequency (f) was shown to play a major role on rocks&#8217; resulting Poisson&#8217;s ratio, so that usual laboratory results (at f = 1 MHz) might not directly transfer to field ones (at f = 1 Hz). From this consideration, we investigate Vp/Vs of a variety of crustal rocks in the elastic regime relevant at the field scale, the undrained elastic regime.Accounting for rocks dispersive properties, this work aims to show that:</p><ul><li>In the laboratory, in isotropic rocks, one might attain Vp/Vs values as high as the anomalous ones observed in subduction zones.</li> <li>No mineralogical control is needed for such high Vp/Vs values, which could be consistent with the inherent mineral variability in different settings across the globe.</li> <li>High pore fluid pressure is a major parameter, but not alone: such high values cannot be achieved without very high degree of micro-fracturing of the rock, opened by high fluid pressures, an information of potential importance to understand those seismogenic zones.</li> </ul>
Basaltic rocks are the main component of the oceanic upper crust, thus of potential interest for water and geothermal resources, storage of CO 2 and volcanic edifice stability. In this work, we investigated experimentally the mechanical behavior and the failure modes of a porous basalt, with an initial connected porosity of 18%. Results were acquired under triaxial compression experiments at confining pressure in the range of 25–200 MPa on water saturated samples. In addition, a purely hydrostatic test was also performed to reach the pore collapse critical pressure P*. During hydrostatic loading, our results show that the permeability is highly pressure dependent, which suggests that the permeability is mainly controlled by pre‐existing cracks. When the sample is deformed at pressure higher than the pore collapse pressure P*, some very small dilatancy develops due to microcracking, and an increase in permeability is observed. Under triaxial loading, two modes of deformation can be highlighted. At low confining pressure (Pc < 50 MPa), the samples are brittle and shear localization occurs. For confining pressure > 50 MPa, the stress‐strain curves are characterized by strain hardening and volumetric compaction. Stress drops are also observed, suggesting that compaction may be localized. The presence of compaction bands is confirmed by our microstructure analysis. In addition, the mechanical data allows us to plot the full yield surface for this porous basalt, which follows an elliptic cap as previously observed in high porosity sandstones and limestones.
In order to address geological processes at great depths, rock deformation should ideally be tested at high pressure (> 0.5 GPa) and high temperature (> 300 °C). However, because of the low stress resolution of current solid-pressure-medium apparatuses, high-resolution measurements are today restricted to low-pressure deformation experiments in the gas-pressure-medium apparatus. A new generation of solid-medium piston-cylinder ("Griggs-type") apparatus is here described. Able to perform high-pressure deformation experiments up to 5 GPa and designed to adapt an internal load cell, such a new apparatus offers the potential to establish a technological basis for high-pressure rheology. This paper provides video-based detailed documentation of the procedure (using the "conventional" solid-salt assembly) to perform high-pressure, high-temperature experiments with the newly designed Griggs-type apparatus. A representative result of a Carrara marble sample deformed at 700 °C, 1.5 GPa and 10-5 s-1 with the new press is also given. The related stress-time curve illustrates all steps of a Griggs-type experiment, from increasing pressure and temperature to sample quenching when deformation is stopped. Together with future developments, the critical steps and limitations of the Griggs apparatus are then discussed.
Decades of seismological observations have highlighted the variability of foreshock occurrence prior to natural earthquakes, making thus difficult to track how earthquakes start. Here, we report on three stick-slip experiments performed on cylindrical samples of Indian metagabbro under upper crustal stress conditions (30-60 $MPa$). Acoustic emission activity (AE) was continuously recorded by 8 calibrated acoustic sensors during the experiments, and the seismological parameters (moment magnitude, corner frequency and stress-drop) of the detected AEs were estimated. The scaling law between moment magnitude and corner frequency that characterizes natural earthquakes also applies to the detected AEs ($-8.8 \leq Mw \leq -7 $. Precursory AE activity is systemically detected during the pre-failure period, increases towards failure and is found to be driven by along fault slip velocity. Consistently for all three experiments, the stacked AE sequences follow an inverse power law of the time to failure. Decreasing fault strength heterogeneity promotes, on average, AEs migration towards zones where stick-slip events initiate. Overall, the seismic component of the pre-failure phase differs by several orders of magnitude from the aseismic component. Our observations suggest that, in this particular experimental setting, precursory AE activity is predominantly triggered by the larger nucleation phase of the upcoming stick-slip event which is an almost fully aseismic process.
[1] We study the influence of metamorphic dehydration reactions on the stability of slip in a one-dimensional, spring-slider model. The equations that govern the evolution of the velocity of sliding block and of pore pressure and temperature inside the slip zone are deduced from the mass and energy balance of the multiphases saturated medium and from the kinetics of the dehydration reaction. Such reactions induce two competing effects: a direct increase in pore pressure because they release fluid and a limit in temperature increase because part of the frictional heat is absorbed in the endothermic reactions. The effect of the chemical reaction on the stability of stationary slip is studied. Dehydration reactions increase the critical stiffness at which the system becomes unstable. Depending on the sign of the perturbations, it is shown that dehydration reactions can either (1) trigger a catastrophic increase of pore pressure at quasi-constant temperature leading to vanishing effective stress or (2) trigger an arrest of the fault. Numerical simulations demonstrate the crucial role of initial pore pressure and temperature in the slip zone prior to the onset of the chemical reaction on the subsequent evolution of the system. For highly pressurized fault zones, in which the creep motion of the fault is stable in absence of dehydration reactions, the onset of the reaction can trigger transient slip events induced by chemical pressurization. The magnitude of such events appears to be proportional to the reaction progress. We conclude that metamorphic dehydration reactions strongly modify the nucleation of unstable slip and are a possible origin for slow slip events in subduction zones.
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