Intergranular Pressure Solution Research Articles

Unstable Sliding of Plagioclase Gouge and Deformation Mechanisms Under Hydrothermal Conditions With Effective Normal Stresses of 100–300 MPa

AbstractPlagioclase feldspar is a major mineral in mafic crustal rocks. To better understand the deformation mechanism of plagioclase feldspar during frictional faulting, we conducted shearing experiments on simulated plagioclase gouge in a wide range of effective normal stress of 100–300 MPa, pore‐water pressure of 30–100 MPa, and temperatures ranging from 100°C to 600°C. The coefficient of friction is found to range from 0.65 to 0.74 across the entire temperature range, showing no significant thermal weakening process. Except for a case at 200°C with an effective normal stress of 300 MPa, the frictional sliding is velocity weakening over the whole temperature range, showing a steady‐state rate dependence (a−b) ranging from −0.5 × 10−3 to −8.6 × 10−3. This property facilitates nucleation of unstable slips in frictional faulting. Above 200°C, the direct rate effect parameter (a) and the evolution effect parameter (b) of friction increase with temperature up to a threshold of 400°C or 500°C, depending on the effective normal stress. This thermal enhancement suggests thermally activated creep at contact junctions governed by intergranular pressure solution, as evidenced by microstructural signatures indicating the prevalence of very fine precipitates formed at the surfaces of gouge particles as a result of pressure solution. In frictional sliding of plagioclase, a low effective normal stress of 100 MPa corresponds to a higher degree of velocity weakening and tends to facilitate seismic slip rather than slow slips, whereas the high effective normal stress of 300 MPa corresponds to a minor velocity weakening which may cause slow‐slip events in faults of limited size.

Frictional Properties of Natural Granite Fault Gouge Under Hydrothermal Conditions: A Case Study of Strike‐Slip Fault From Anninghe Fault Zone, Southeastern Tibetan Plateau

AbstractThe Anninghe Fault (ANHF) is a major left‐lateral strike‐slip fault in southwestern China and one of the main seismogenic fault zones with a history of strong earthquakes. To understand the frictional properties of natural granitic gouges from the principal slip zone, we conducted hydrothermal friction experiments using both saw‐cut and ring shear methods. These experiments were performed at temperatures (T) of 25–600°C, pore pressures (Pf) of zero (dry), 30 and 100 MPa, sliding velocities (V) of 0.01–100 μm/s and effective normal stresses () of 68, 100, and 200 MPa. The (apparent) friction coefficient is low (μ < 0.5) at high T (600°C), high Pf (100 MPa) and low V (<1 μm/s); but high (μ > 0.6) under all other T, Pf and V conditions. Under high Pf, the velocity dependence of friction, (a‐b), displays three regimes with increasing temperature, from positive below ∼100°C to negative at 100–300°C (at V = 1–3 μm/s) or else 100–450°C (at V = 30–100 μm/s), becoming positive again above 300–450°C. At low Pf, the negative (a‐b) expands to the range ∼300–600°C. Microstructural observations and microphysical interpretation imply that the frictional weakening and transitions in (a‐b) are related to competition between dilatant granular flow and deformation of the fine‐grained gouge by intergranular pressure solution accompanied by healing phenomena (leading to cavitation‐creep‐like behavior). Our results provide a possible explanation for the distribution of earthquakes at different depths in the continental crust, in particular for the depth range of the seismogenic zone between 4 and 24 km along the ANHF.

Controls of normal diagenesis on poroperm parameters in red beds: examples in Miocene Muddy Creek Formation, Mesquite basin, USA and upper Devonian Old Red Sandstone, Orcadian basin, Scotland

General diagenetic patterns in Miocene Muddy Creek Formation and upper Devonian Old Red Sandstone (ORS) can be characterized as follows: i) early diagenesis characterized by the formation of early hematite, carbonate, and clay cement; ii) burial diagenesis followed by the formation of quartz and feldspar overgrowths, poikilotopic calcite and pore-filling clays; iii) late diagenesis characterized by the formation of late hematite replacing previous poikilotopic carbonate cement. Normal diagenesis has a significant impact on poroperm parameters as indicated by the destruction of pore spaces from cementation and intergranular pressure solution. Early cementation in unburied sandstones of the Muddy Creek Formation reduces sample porosities to 2÷20% of the total rock volume. Cementation destroyed the original porosity of studied sandstones through pore occlusion due to the formation of equant and meniscus calcite cement. Point-count data indicate that the intergranular cement of studied samples ranges between 22 and 44%; in contrast, the intergranular porosities range from 2÷20% of the total rock volume. These consequently indicate that the intergranular volumes that are considered to represent the original porosities of studied samples, ranged from 35÷50% of the total rock volume. Completely pore-occluding poikilotopic calcite cement in the upper Old Red Sandstone reduces poroperm values to as low as 5% porosity and 0.003 mD permeability. Late reddening is caused by replacive hematite cement in the calcite. In addition, normal diagenesis also has an impact on porosity enhancement due to dissolution in the studied red beds. This improves porosity and permeability by as much as 14% and 254 mD in the upper ORS.

Surface microstructures developed on polished quartz crystals embedded in wet quartz sand compacted under hydrothermal conditions

Intergranular pressure solution plays a key role as a deformation mechanism during diagenesis and in fault sealing and healing. Here, we present microstructural observations following experiments conducted on quartz aggregates under conditions known to favor pressure solution. We conducted two long term experiments in which a quartz crystal with polished faces of known crystallographic orientation was embedded in a matrix of randomly oriented quartz sand grains. For about two months an effective axial stress of 15 MPa was applied in one experiment, and an effective confining pressure of 28 MPa in the second. Loading occurred at 350 °C in the presence of a silica-saturated aqueous solution. In the first experiment, quartz sand grains in contact with polished quartz prism (overline10{1 }0) faces became ubiquitously truncated against these faces, without indenting or pitting them. By contrast, numerous sand-grain-shaped pits formed in polished pyramidal (17overline{6 }3) and (overline{4 }134) crystal faces in the second experiment. In addition, four-leaved and (in some cases) three-leafed clover-shaped zones of precipitation formed on these prism faces, in a consistent orientation and pattern around individual pits. The microstructures observed in both experiments were interpreted as evidence for the operation of intergranular pressure solution. The dependence of the observed indentation/truncation microstructures on crystal face orientation can be explained by crystallographic control of stress-induced quartz dissolution kinetics, in line with previously published experimental and petrographic data, or possibly by an effect of contact orientation on the stress-induced driving force for pressure solution. This should be investigated in future experiments, providing data and microstructures which enable further mechanism-based analysis of deformation by pressure solution and the effect of crystallographic control on its kinetics in quartz-rich sands and sandstones.

Petrography, Diagenesis and Paragenesis of Wadi Halfa Oolitic Ironstone Formation, North Sudan

Utilizing petrography, diagenesis and paragenesis are the major step to study the sediments of Wadi Halfa Oolitic Ironstone Formation for their origin and economic potential. Petrographical analysis of Wadi Halfa Oolitic Ironstone Formation were carried out depending on the investigation of six vertical lithofacies profiles from the region of Wadi Halfa. The petrographic classification of sandstone samples of Wadi Halfa Oolitic Ironstone Formation according to mineral composition (quartz, lithic fragments, and feldspars) are classified as quartzarenite 66.7%, sub lithic arenite 20.9% and subarkose 12.5%. The provenance of the sandstone revealed that all sample were derived mainly from interior stable craton. The stable craton offered good environments for detrital constitution to be recycled and transported for relatively long-distance. The present study adopted an intergranular pressure solution (IGV) diagram for IGV values more than 50 percent for all studied samples indicating that more of their original porosity has been destroyed cementation rather than by compaction processes for most samples.

Velocity Weakening of Simulated Augite Gouge at Hydrothermal Conditions: Implications for Frictional Slip of Pyroxene‐Bearing Mafic Lower Crust

AbstractTo understand the velocity‐weakening behavior of pyroxene as a major constituent in mafic rocks under hydrothermal conditions, we employed augite (clinopyroxene, (Ca0.577Na0.110Mn0.004Mg0.886Fe2+0.193Fe3+0.037AlVI0.173Ti0.019)[AlIV0.137Si1.863O6]) as simulated gouge sample material to run velocity stepping sliding tests at temperatures of 101–607 °C, confining pressure of 127 MPa with 30 MPa pore pressure and axial loading rates of 0.04–1.0 μm/s. The friction coefficient was found to range from 0.71 to 0.73 with no systematic temperature dependence. Velocity‐weakening behavior occurred at temperatures above 203 °C, with low velocity dependence or (b‐a) value around 0.0012, corresponding to b/a values of 1.15–1.18. Inferred b values in the rate and state friction laws exhibited an increasing trend with temperature increase from ~0.0056 to ~0.0098 up to 403 °C, indicating activation of Arrhenius‐type creep at the frictional contacts. Microstructural observations on deformed samples show ubiquitous precipitated particles suggesting intergranular pressure solution, and no evidence for deformation by crystalline plasticity was observed. This fact, together with the comparison between experimental data and model prediction, suggests that the process of intergranular pressure solution at frictional contacts may have governed the evolution effect for augite. The minor velocity weakening of augite combined with previous data on plagioclase is consistent with the modest velocity weakening seen in mafic rocks, such as quartz‐free oceanic basalt. Such a minor velocity‐weakening property tends to favor slow slip events on the downdip portion of the subduction interface below currently locked seismogenic zones, such as in southwestern Japan, northern Cascadia, and Mexico.

Overpressure development in sedimentary basins induced by chemo-mechanical compaction of sandstones

Abstract Mechanisms of overpressure build-up resulting from chemically-induced compaction are investigated by considering that intergranular pressure-solution, controlled by temperature and effective stresses, is the dominant process of chemical deformation in sandstones. The numerical simulations are performed using a thermo-poro-mechanical tool based on the finite element method specifically devised to deal with sedimentary basin modeling. At the material level, purely mechanical and chemo-mechanical deformations are respectively addressed by means of plastic and viscoplastic constitutive components. Porosity data of the Middle Jurassic Garn Formation from the Haltenbanken area of the Mid-Norwegian Continental Shelf are taken as reference for calibration of the sandstone model to be used on synthetic cases of a siliciclastic basin in oedometric conditions. Two situations are proposed involving different depositional sequences of sandy and shaly sediments with the aim to assess the permeability effect in the numerical model. The results have shown that early overpressure development in low permeability formations preserves sandstone porosities by reducing effective stresses and thus retarding pressure-solution compaction, whereas higher effective stresses associated with permeable depositional environments may lead to important chemo-mechanical deformation, resulting in low porosity sandstones and significant overpressure generation at later stages of basin history. An additional case is finally analyzed to investigate the consequences of pressure-solution inhibition due to diagenetic grain coating in a sandstone reservoir. The simulation resulted in porosity preservation and lower overpressure values, showing that a sedimentary basin submitted to an important level of chemo-mechanical compaction can present substantially higher overpressure distribution than basins where this phenomenon did not take place. The paper highlights the importance of integrating stresses, fluid pressure and temperature to represent the porous material evolution in order to describe the geological state of sedimentary basins.

The permeability of stylolite-bearing limestone

Stylolites are planar features that form due to intergranular pressure solution. Due to their planar geometry and relative abundance in limestone reservoirs, their impact on regional fluid flow has attracted considerable interest. We present laboratory permeability data that show that stylolites can be considered as conduits for flow in the stylolite-bearing limestones measured. A combination of analysis techniques shows that this is due to a zone that surrounds these stylolites that is more porous and contains larger pores than the host rock. Our data also show that the water permeability of a sample containing a stylolite parallel to fluid flow is typically lower than its permeability to gas, explained here as a result of the expansion of minor amounts of clay found in the stylolite, and that, due to their microstructural similarities, tectonic and sedimentary stylolites affect sample permeability similarly. Finally, we show that the permeability anisotropy that develops in the rock mass due to the presence of sedimentary stylolites makes it appear as though the stylolites are acting as barriers to fluid flow, and may explain the discrepancy between laboratory measurements and field-scale observations. This approach can provide estimates for the equivalent permeability, and permeability anisotropy, for stylolite-bearing limestone reservoirs worldwide.

Investigating Compaction by Intergranular Pressure Solution Using the Discrete Element Method.

Intergranular pressure solution creep is an important deformation mechanism in the Earth's crust. The phenomenon has been frequently studied and several analytical models have been proposed that describe its constitutive behavior. These models require assumptions regarding the geometry of the aggregate and the grain size distribution in order to solve for the contact stresses and often neglect shear tractions. Furthermore, analytical models tend to overestimate experimental compaction rates at low porosities, an observation for which the underlying mechanisms remain to be elucidated. Here we present a conceptually simple, 3‐D discrete element method (DEM) approach for simulating intergranular pressure solution creep that explicitly models individual grains, relaxing many of the assumptions that are required by analytical models. The DEM model is validated against experiments by direct comparison of macroscopic sample compaction rates. Furthermore, the sensitivity of the overall DEM compaction rate to the grain size and applied stress is tested. The effects of the interparticle friction and of a distributed grain size on macroscopic strain rates are subsequently investigated. Overall, we find that the DEM model is capable of reproducing realistic compaction behavior, and that the strain rates produced by the model are in good agreement with uniaxial compaction experiments. Characteristic features, such as the dependence of the strain rate on grain size and applied stress, as predicted by analytical models, are also observed in the simulations. DEM results show that interparticle friction and a distributed grain size affect the compaction rates by less than half an order of magnitude.

A constitutive model for mechanical and chemo‐mechanical compaction in sedimentary basins and finite element analysis

SummaryCompaction and associated fluid flow are fundamental processes in sedimentary basin deformation. Purely mechanical compaction originates mainly from pore fluid expulsion and rearrangement of solid particles during burial, while chemo‐mechanical compaction results from Intergranular Pressure‐Solution (IPS) and represents a major mechanism of deformation in sedimentary basins during diagenesis. The aim of the present contribution is to provide a comprehensive 3D framework for constitutive and numerical modeling of purely mechanical and chemo‐mechanical compaction in sedimentary basins. Extending the concepts that have been previously proposed for the modeling of purely mechanical compaction in finite poroplasticity, deformation by IPS is addressed herein by means of additional viscoplastic terms in the state equations of the porous material. The finite element model integrates the poroplastic and poroviscoplastic components of deformation at large strains. The corresponding implementation allows for numerical simulation of sediments accretion/erosion periods by progressive activation/deactivation of the gravity forces within a fictitious closed material system. Validation of the numerical approach is assessed by means of comparison with closed‐form solutions derived in the context of a simplified compaction model. The last part of the paper presents the results of numerical basin simulation performed in one dimensional setting, demonstrating the ability of the modeling to capture the main features in elastoplastic and viscoplastic compaction. Copyright © 2016 John Wiley & Sons, Ltd.

The effects of rock heterogeneity on compaction localization in porous carbonates

Recent field-based studies document the presence of bed-parallel compaction bands within the Oligocene-Miocene carbonates of Bolognano Formation exposed at the Majella Mountain of central Italy. These compaction bands are interpreted as burial-related structures, which accommodate volumetric strain by means of grain rotation/sliding, grain crushing, intergranular pressure solution and pore collapse.In order to constrain the pressure conditions at which these compaction bands formed, and investigate the role exerted by rock heterogeneity (grain and pore size and cement amount) on compaction localization, we carried out a suite of triaxial compression experiments, under dry conditions and room temperature on representative host rock samples of the Bolognano Formation. The experiments were performed at confining pressures that are proxy of those experienced by the rock during burial (5–35 MPa). Cylinders were cored out from a sample of the carbonate lithofacies most commonly affected by natural compaction bands. Natural structures were sampled and compared to the laboratory ones.During the experiments, the samples displayed shear-enhanced compaction and strain hardening associated with various patterns of strain localization. The brittle–ductile transition occurred at 12.5 MPa whereas compaction bands nucleated at 25 MPa confining pressure. A positive correlation between confining pressure and the angle formed by the deformation bands and the major principal stress axis was documented. Additional experiments were performed at 25 MPa on specimens cored oblique (parallel and at 45°) to the bedding. Detailed microstructural analyses, performed on pristine and deformed rocks by using optical microscopy, scanning electron microscopy and X-ray computed microtomography techniques, showed that grain crushing and mechanical twinning are the dominant deformation processes in the laboratory structures. Conversely, pressure solution appears to be dominant in the natural compaction bands. Experimental results highlight the strong influence exerted by bedding-parallel rock heterogeneity on both orientation and kinematics of deformation bands in the studied carbonates.

The influence of water and supercritical CO2 on the failure behavior of chalk

Abstract Reduction of compressive strength by injection of water into chalk is a well-known mechanism responsible for increased compaction in chalk reservoirs. This raises the question of whether such effects might be enhanced in the context of long-term storage of CO 2 or of CO 2 injection for enhanced oil and gas recovery (EOR/EGR) purposes. Therefore, data regarding the effect of supercritical CO 2 on the mechanical behavior of chalk are needed. The effect of supercritical CO 2 on the short-term failure behavior of wet chalk was accordingly investigated by means of conventional triaxial deformation experiments, performed on Maastrichtian chalk cores under dry conditions, in the presence of saturated chalk solution and using CO 2 -saturated solution at temperatures simulating reservoir conditions (20–80 °C) and effective confining pressures up to 7 MPa. Increasing temperature from 20 to 80 °C did not show any significant effects on the strength of the dry samples. Addition of aqueous solution to the samples led to drastic weakening of the chalk, the effect being more pronounced at high effective confining pressures (Peff > 3 MPa). Addition of 10 MPa supercritical CO 2 to wet samples did not produce any significant additional effect in comparison with the wet samples. All samples showed a yield strength envelope characterized by shear failure at low effective mean stresses giving way to a compaction cap at high mean stresses. The weakening effect of aqueous solution was explained in terms of a reduction in frictional resistance of the material, due to water-enhanced grain-contact cracking, and perhaps pressure solution, with a possible contribution by disjoining pressure effects caused by water adsorption. While CO 2 does not seem to reduce short-term failure strength of wet chalk, processes such as intergranular pressure solution have to be considered for assessing mechanical stability of chalk in the context of long-term CO 2 storage or EOR/EGR operations.

Reactive transport at stressed grain contact and creep compaction of quartz sand

A kinetic model is developed to investigate intergranular pressure solution. This model couples stress‐induced dissolution at grain contacts, diffusion through grain boundaries, and precipitation in pore spaces. The rate‐controlling processes are evaluated according to the dimensionless concentrations at grain contacts and in the pore fluid. Constrained by the experimental results of creep compaction of quartz sands, calculations suggest that the equilibrium concentration at stressed grain contacts (ceqb) does not exceed 1 order of magnitude higher than the hydrostatic equilibrium concentration (ceq) at the initial stages of creep compaction. However, ceqb decays rapidly with increasing compaction and becomes close to ceq after several percent strain. The diffusivity at grain contacts is 1–2 orders of magnitude lower than the diffusivity in pore fluid. The rate‐controlling process is related to grain size and strain. An increase in grain size shifts the systems toward the diffusion‐controlled regime, while an increase in strain shifts the systems from dissolution‐controlled regime toward either diffusion‐controlled regime (larger grains) or precipitation‐controlled regime (smaller grains). Intergranular pressure solution appears to be very sensitive to pore‐fluid chemistry. Even a slight supersaturation in the pore fluid could prevent the diffusion along grain contacts. This may explain why the strength of intergranular pressure solution varies widely in natural sandstones.

Effects of interfacial energy on compaction creep by intergranular pressure solution: Theory versus experiments on a rock analog (NaNO3)

Pressure solution plays an important role in compaction and lithification of sediments and fault gouges, but the effects of interfacial energy on this process are generally neglected. Here, microphysical models for densification of solid/liquid systems by pressure solution are derived, accounting for interfacial energy besides stress‐related driving forces. They predict that densification by pressure solution creep will slow down at very fine grain sizes, where opposing interfacial energy driving forces become important compared to applied stress, and will come to a halt below a certain “yield” stress. To test the models, uniaxial compaction creep experiments were performed at ambient conditions on granular NaNO3 aggregates (d = 8–250 μm, σeff = 0.0062–4.9 MPa). Though no significant creep occurred in dry or oil‐flooded material, rapid, grain‐size sensitive creep occurred in the presence of saturated NaNO3 solution. At high effective stresses (σeff > 0.025 MPa), wet‐compacted, coarser‐grained (d > 20 μm) samples showed compaction behavior roughly consistent with diffusion‐controlled pressure solution involving negligible interfacial energy effects. Creep rates in this regime imply an effective grain boundary diffusivity product of 5.7 × 10−19 m3/s. At low effective stress (σeff < 0.025 MPa), finer‐grained samples (d < 20 μm) showed a decrease in strain rate with decreasing grain size, reflecting a growing influence of interfacial energy‐related driving forces. This demonstrates a yield stress effect, broadly consistent with the predictions of the models incorporating interfacial energy and imply that compaction by pressure solution can be strongly inhibited in very fine‐grained materials, such as nanogouge in seismogenic faults.

Influence of the volume loss of framework grains on the quantitative analysis of diagenetic modification of the original intergranular porosity

The evaluation of the original intergranular porosity loss due to compaction (COPL) and cementation (CEPL) is an important tool for understanding the contribution of diagenetic processes to porosity reduction and permeability retention of sandstones and carbonate grainstones. The previous approach for computing CEPL and COPL assumed that the volume of framework grains remained constant during diagenesis. However, such assumption will introduce error in COPL and CEPL if diagenetic processes reduced the volume of framework grains. New equations are presented that account for such situations. Comparably, the previous approach underestimates COPL and the compaction index, but over-estimates CEPL in the case of volume loss of framework grains by dissolution or intergranular pressure solution (DIPL). The deviation of CEPL and COPL caused by DIPL rises with the increase in the intergranular volume and DIPL. Analysis of three published case studies suggests that DIPL values of just a few percentages will significantly impact COPL and CEPL values. The effects of DIPL can be greater than the effects of uncertainty in original porosity estimates and uncertainty in point count data.

Creep of garnet in eclogite: Mechanisms and implications

The rheological properties of subducting rocks represent a fundamental parameter in the dynamics of subduction zones. Making robust predictions about these properties and the general strength evolution of subducting plates is hampered by a relatively restricted understanding of the mechanisms by which rocks deform at high- and ultrahigh pressure. This uncertainty is relates to the discrepancy between experimental and field-based observations of eclogite deformation and, in particular, the role of garnet. To further investigate this important aspect, we performed a textural and micro-structural investigation, applying optical microscopy, element mapping, and electron backscatter diffraction on deformed garnet polycrystals from an eclogite mylonite. The results were compared to those from a study of undeformed polycrystals that formed from a supercooled frictional melt at HP conditions. The mylonites' polycrystals are flattened parallel to the main high-pressure foliation in the rock and individual grains were shortened by an average 15%. Although dislocation creep is commonly presumed to dominate garnet straining in eclogites, no record of this mechanism was found. Instead, the garnet grains have dissolution surfaces indicative of deformation by intergranular pressure solution. The observations provide compelling evidence for the role of fluids and syn-tectonic porosity in the weakening of garnet: a supposedly rigid eclogite component. Such weakening represents a crucial step in the fundamental feedback loop between fluid ingress, metamorphism and near-instantaneous competence loss in rocks undergoing deep subduction.

Principal Slip Zones in Limestone: Microstructural Characterization and Implications for the Seismic Cycle (Tre Monti Fault, Central Apennines, Italy)

Earthquakes in central Italy, and in other areas worldwide, often nucleate within and rupture through carbonates in the upper crust. During individual earthquake ruptures, most fault displacement is thought to be accommodated by thin principal slip zones. This study presents detailed microstructural observations of the slip zones of the seismically active Tre Monti normal fault zone. All of the slip zones cut limestone, and geological constraints indicate exhumation from \2 km depth, where ambient tempera- tures are � 100C. Scanning electron microscope observations suggest that the slip zones are composed of 100% calcite. The slip zones of secondary faults in the damage zone contain protocata- clastic and cataclastic fabrics that are cross-cut by systematic fracture networks and stylolite dissolution surfaces. The slip zone of the principal fault has much more microstructural complexity, and contains a 2-10 mm thick ultracataclasite that lies immediately beneath the principal slip surface. The ultracataclasite itself is internally zoned; 200-300 lm-thick ultracataclastic sub-layers record extreme localization of slip. Syn-tectonic calcite vein net- works spatially associated with the sub-layers suggest fluid involvement in faulting. The ultracataclastic sub-layers preserve compelling microstructural evidence of fluidization, and also con- tain peculiar rounded grains consisting of a central (often angular) clast wrapped by a laminated outer cortex of ultra-fine-grained calcite. These ''clast-cortex grains'' closely resemble those pro- duced during layer fluidization in other settings, including the basal detachments of catastrophic landslides and saturated high-velocity friction experiments on clay-bearing gouges. An overprinting foliation is present in the slip zone of the principal fault, and electron backscatter diffraction analyses indicate the presence of a weak calcite crystallographic preferred orientation (CPO) in the fine-grained matrix. The calcite c-axes are systematically inclined in the direction of shear. We suggest that fluidization of ultra- cataclastic sub-layers and formation of clast-cortex grains within the principal slip zone occurred at high strain rates during propa- gation of seismic ruptures whereas development of an overprinting CPO occurred by intergranular pressure solution during post-seis- mic creep. Further work is required to document the range of microstructures in localized slip zones that cross-cut different lithologies, and to compare natural slip zone microstructures with those produced in controlled deformation experiments.

Effects of pore fluid flow and chemistry on compaction creep of calcite by pressure solution at 150°C

Uni-axial compaction creep experiments were performed on crushed limestone and analytical grade calcite powders at 150°C, a pore fluid pressure of 20 MPa, and effective axial stresses of 30 and 40 MPa. Previous experiments have shown that compaction under these conditions is dominated by intergranular pressure solution (IPS). The aim of the present tests was to determine the inter-relationship between pore fluid chemistry, compaction rate and the rate-controlling process of IPS. Intermittent flow-through runs conducted using CaCO3 solution showed no effect on creep rate at low strains (<4–5%) but a major acceleration at high strains (5–10%). Measurements of the Ca concentration present in fluid samples revealed the build-up of a high super-saturation of CaCO3 during compaction under zero flow conditions, especially at high strains. Active flow-through led to a drop in Ca concentration, which corresponded with creep acceleration. Addition of NaCl to the pore fluid, at a concentration of 0.5 m, increased the creep rate of the analytical grade calcite samples roughly in proportion to the enhancement of calcite solubility. Addition of Mg2+ and to the pore fluid, in concentrations of 0.05 and 0.001 m, respectively, caused major retardation of compaction creep. Integrating our mechanical, flow-through and chemical data points strongly to diffusion-controlled IPS being the dominant deformation mechanism in the calcite-water system under closed-system (zero flow) conditions at low strains (<4–5%), giving way to precipitation control at higher strains. Our fluid composition data suggest that this transition is because of accumulation of impurities in the pore fluid. As Mg2+ and phosphate ions are common in natural pore fluids, it is likely that retarded precipitation will be the rate-limiting step of IPS in carbonates in nature. To quantify diagenetic compaction and porosity-permeability reduction rates by IPS in carbonates needs to account for this.

Estimation of the damage of a porous limestone from continuous (P- and S-) wave velocity measurements under uniaxial loading and different hydrous conditions

SUMMARY The damage of a porous rock (Euville oolitic limestone) was studied through uniaxial stresscyclingtests.Anexperimentaldevice,allowingthesimultaneousandcontinuousmeasurement of strains (in two perpendicular directions) and five elastic wave hree P waves and two S waves) velocities in two different directions under fully controlled hydrous conditions, was developed for the work presented in this paper. Hence, the damage was monitored in a really precise and continuous way through the evolution of dynamic and static elastic moduli. The evolutions of wave velocities and elastic moduli, which reproduce very remarkably the shape of the stress–strains curves, showed that the limestone, initially isotropic, became progressively anisotropic during uniaxial loading due to microcrack damage. Indeed, even if the creation of microcracks is probably scattered and isotropic before the coalescence of microcracks,asshowninpaststudies,pre-existingmicrocracksandpenny-shapedporeswhich are perpendicular (or almost perpendicular) to the uniaxial stress direction closed, whereas axially-oriented microcracks opened. The anisotropy of the damage is completely reversible but some of the damage is irreversible. VP(90◦), which cannot record the opening of these microcracks, started to decrease just before the macroscopic failure of the sample and can detect, therefore, very precisely the macroscopic failure of the material. The influence of water on the strength and deformation of the Euville limestone was analysed by considering both the hydromechanical and physio-chemical (‘Rehbinder effect’, intergranular pressure solution and subcritical cracking) effects of water.

Compaction creep of wet granular calcite by pressure solution at 28°C to 150°C

Uniaxial compaction experiments have been carried out on wet calcite powders prepared from milled limestone, analytical grade calcite, and superpure calcite. The tests were performed at 28°C–150°C, effective stresses of 20–47 MPa, and a pore pressure of 20 MPa, using presaturated CaCO3 solution as the pore fluid. Sample grain sizes ranged from 12 to 86 μm. The aim was to determine if creep occurs by intergranular pressure solution (IPS) under these conditions and to identify the rate‐controlling process. The wet samples showed significant creep, while dry and oil‐flooded samples did not. Wet samples were also characterized by microstructures such as sutured grain contacts, grain indentations, and overgrowths, suggesting the operation of IPS. The behavior of the wet samples at low strains (&lt;4–5%) agreed well with the predictions of standard models for diffusion‐controlled pressure solution. However, at higher strains (5–10%), creep slowed down compared with the model predictions. Transient flow‐through tests performed in this regime led to an acceleration of creep, consistent with models for precipitation‐controlled IPS, and demonstrated an increase in the impurity content of the pore fluid toward higher strains. Since some of these impurities (notably Mg2+) retard precipitation in calcite, we suggest that creep occurred by diffusion‐controlled IPS at low strain, giving way to precipitation control at larger strains as impurities accumulated. Fitting of the diffusion‐controlled IPS model to the low strain data yielded values of the grain boundary diffusion product DS in calcite of ∼10−19 m3 s−1 at 150°C.