Brittle Deformation Regime Research Articles

Reactivated fracture-controlled uranium mineralization: An example from NNE-SSW Kamaguttapalle–Kammapalle tract, Kadapa district, Andhra Pradesh, India

The Cuddapah Basin (CB) and its basement can be considered as an appropriate uranium metallogenic province. The basement complex of CB in the Eastern Dharwar Craton (EDC), India, has been the target for structurally controlled uranium exploration since the early 1980s. Most of the brittle-ductile deformation induced reactivated narrow linear fracture zones often exhibit significant surface and sub-surface uranium occurrence. There are about 10 fracture zones associated with uranium mineralization. Amongst them, the Kamaguttapalle–Kammapalle fracture zone (KKF) in the south of the CB is quite interesting from an exploration viewpoint with a suitable geologic setting in space and time frame. Geomorphic, structural and petro-mineralogic exploration guides imply the possibility of economic uranium concentration along the NNE-SSW trend where tectonic reactivation took place over pre-existing ductile shear zone. Progressive exhumation and transition from ductile to brittle deformation regime led to the development of dominant strike slip tectonics with an associated riedel fracture system, which are subsidiary coeval shear fractures used in depicting sense of movement. Intense illitization and hematitization are often observed. Quartz reefs and veins along the NNE-SSW trend cut the preexisting mylonites at places. About 9 stages of fracturing is observed out of which 3 stages are related to uranium mineralization. All the 9 deformations are related to strike slip tectonics which in turn is part of Grenville orogeny. Paragenetic sequence and mode of occurrences of uranium phases deciphers about the importance of low to moderate level epi‑meso type hydrothermal (100°–250 °C) fluids as carrier of U. Reactivated fractures and quartz veination also act as controlling factors of mineralization. Strain analysis points to positive and negative flower structures at different segments of the fracture zone. Based on the litho-structural attributes it is established that maximum compression (σ1) acted along the NNW-SSE direction. Fractured quartz veins provide well defined permeability and trapping facility by restricting transverse fluid movement as a mechanical barrier. The presented model gives an idea on role of strike-slip tectonics in uranium mineralization for narrowing down the target zone for sub-surface exploration.

Genesis of the Heneshk IOCG deposit, Iran: magnetite mineral chemistry and sulfur isotope

The Heneshk Fe ± Cu deposit, a metamorphosed IOCG (Iron Oxide Copper Gold) is situated in Sanandaj-Sirjan Zone, SW Iran. Mineralization occurs as two stratabound ‘U’ shape ore bodies, hosted by a meta-dolomite member of a metamorphic complex. The ore and gangue minerals comprise magnetite, chalcopyrite, hematite, martite, mushketovite, pyrite, barite, calcite, dolomite, and quartz. The successive mineral assemblage formed in the five paragenetic stages (I to V) during an ongoing deformation regime from the plastic to the brittle deformation regime. The mineralization stages comprise stage I, III, and IV. Stage I–III and IV formed in the plastic and the brittle deformation regime by the metamorphic processes in a shear zone, respectively. The concomitant alteration products of these stages comprise sodic, Fe, K, hydrolytic and silicification. The sodic and Fe alterations occur as a pre-mineralization stage and each mineralization stage is accompanied by K and hydrolytic alterations. A late barite-calcite-quartz assemblage (stage V) overprinted on the previous stages (I to IV). Stage I mineralization formed at a depth of ~10 km and temperature of ~300 °C (a high pressure-low temperature environment). The partitioning and substitution of the Mg, Al, Ti, Ni, Cr, Mn, Si, and Zn elements increased in this relatively higher pressure environment cause of the highest concentrations of the Al, Ti, Ni, Cr, and Zn, in comparison with metamorphic magnetite. The mineral chemistry of the Heneshk magnetite (average elemental concentrations) is in accordance with the metamorphic magnetite and can consider as a pure magnetite with the low elemental concentration. Also, the $${{\rm{\delta }}^{34}}{S_{{H_2}S}}$$ Heneshk mineralized fluid ranges between 39.8 to 40.6‰, indicating the contribution of the paleo-evaporates. The pleo-evaporates leached from the older strata during metamorphism, and their precipitation in the form of the sulfide minerals occurs through the TSR (Thermochemical Sulfate Reduction) mechanism.

Earthquake induced soft sediment deformation structures in the Paleoproterozoic Vempalle Formation (Cuddapah basin, India)

Soft sediment deformation structures (SSDS) are preserved in a homogenous dolomudstone lithology of crinkly laminated and heterolithic facies of the Paleoproterozoic Vempalle Formation in the Cuddapah Basin. This basin was formed on the eroded basement rocks of the Eastern Dharwar Craton, India. Deformation structures preserved in this succession include combination of breccia and folds, intrastratal faults, cracks, and dikes belonging mostly to a brittle deformation regime associated with ductile imprints. Based on the observed SSDS, their lateral homogeneity and traceability, draping by undeformed strata, proximity of faults as well as apparent lack of storm signatures and gravity induced mass movement, these fine-grained deformed beds can be tentatively ascribed to a large to intermediate depth earthquake with Richter magnitude scale 4 and above, generated during reactivation of basement faults owing to plume related mantle activity. The occurrence of SSDS in the Vempalle Formation emphasizes the role of downwarping along pre-existing planes of weakness in the Archean basement in the evolution of the Cuddapah Basin, analogous to subsidence in present-day continental margin.

Three‐Dimensional Seismic Structure of the Mid‐Atlantic Ridge: An Investigation of Tectonic, Magmatic, and Hydrothermal Processes in the Rainbow Area

AbstractTo test models of tectonic, magmatic, and hydrothermal processes along slow‐spreading mid‐ocean ridges, we analyzed seismic refraction data from the Mid‐Atlantic Ridge INtegrated Experiments at Rainbow (MARINER) seismic and geophysical mapping experiment. Centered at the Rainbow area of the Mid‐Atlantic Ridge (36°14'N), this study examines a section of ridge with volcanically active segments and a relatively amagmatic ridge offset that hosts the ultramafic Rainbow massif and its high‐temperature hydrothermal vent field. Tomographic images of the crust and upper mantle show segment‐scale variations in crustal structure, thickness, and the crust‐mantle transition, which forms a vertical gradient rather than a sharp boundary. There is little definitive evidence for large regions of sustained high temperatures and melt in the lower crust or upper mantle along the ridge axes, suggesting that melts rising from the mantle intrude as small intermittent magma bodies at crustal and subcrustal levels. The images reveal large rotated crustal blocks, which extend to mantle depths in some places, corresponding to off‐axis normal fault locations. Low velocities cap the Rainbow massif, suggesting an extensive near‐surface alteration zone due to low‐temperature fluid‐rock reactions. Within the interior of the massif, seismic images suggest a mixture of peridotite and gabbroic intrusions, with little serpentinization. Here diffuse microearthquake activity indicates a brittle deformation regime supporting a broad network of cracks. Beneath the Rainbow hydrothermal vent field, fluid circulation is largely driven by the heat of small cooling melt bodies intruded into the base of the massif and channeled by the crack network and shallow faults.

TOURMALINITE VEINS AND BRECCIAS FROM THE SYMVOLON-KAVALA PLUTON, NORTHERN GREECE: PETROGENETIC PRELIMINARY RESULTS

A previously undescribed tourmalinite rock consisting of tourmaline-rich veins and breccias occurs within the Lower Miocene Symvolon-Kavala granodiorite pluton, which intrudes the Lower Tectonic Unit of the Rhodope Metamorphic Province. Tourmaline, usually with crypto- to micro-crystalline grain size, is abundant within the matrix of breccias, also forming along with quartz and host rock clasts massive veins and injections in the fractures of the pluton. Field relations indicate that the tourmaline veins and breccias most likely formed in a brittle deformation regime, overprinting the previously developed foliation of the pluton. In addition, mineralogical and textural observations indicate that the precipitation of tourmaline was a result of cooling of B-rich hydrothermal fluids, implying for a possible magmatic-hydrothermal origin for the studied tourmalinite.

Fracture-Induced Anisotropy of the Stress–Strain Response of Shale at Multiple Scales

Author(s): Xu, H; Busetti, S; Arson, C | Abstract: This paper investigates deformation and stiffness anisotropy induced by damage propagation in a rock brittle deformation regime. Specifically, a finite-element-based Continuum damage mechanics model is used to capture sample size effects and the influence of intrinsic anisotropy on the stress-strain response of shale. The differential stress-induced damage (DSID) model previously proposed by the authors is calibrated against triaxial compression tests performed on North Dakota Bakken shale samples. Laboratory tests simulated with the FEM reproduce deformation and damage localization phenomena and capture the increase of boundary effects expected in larger samples. Simulations performed for various initial states of damage are used to investigate the role of the dominant fabric anisotropy of the rock: bedding planes in shale are modeled by a smeared damage zone with the DSID model and by a discrete crack plane. The continuum approach successfully captures the development of microcrack propagation and energy dissipation at the early stage of the strain hardening process observed in triaxial compression tests. Additionally, using initial anisotropic damage can effectively account for various types of mechanical anisotropy in shale.

An experimental study on creep of partially molten granulite under high temperature and wet conditions

Samples of natural granulite were deformed in a gas medium apparatus to evaluate the flow strength of the lower crust. The sample consists of ∼52vol% plagioclase, ∼40vol% pyroxene, ∼3vol% quartz, ∼5vol% magnetite and ilmenite. Water content was ∼0.17±0.05wt% in the deformed samples. 40 creep tests were performed on 13 samples at 300MPa confining pressure, temperatures of 900–1200°C, and strain rates between 3.13×10−6 and 5×10−5/s, resulting in axial stresses of 12–764MPa and the total strain up to 7.8–20.5%.At low temperatures of 900–1000°C, the microstructural observations show that the granulite samples were deformed in semi-brittle deformation regime, mainly by dislocation glide and intragranular microcracking. At medium temperatures (MT) of 1050–1100°C, deformation was observed to be dominated by grain boundary migration recrystallization, corresponding to stress exponent nMT of 5.7±0.1, activation energies QMT of 525±34kJ/mol, logAMT of 1.3. At high temperatures (HT) of 1125–1150°C, the samples was deformed mainly by grain boundary migration recrystallization accommodated by partial melting and metamorphic reactions characterized by neo-crystallization of fine-grained olivine, with nHT of 4.8±0.1, QHT of 1392±63kJ/mol, and logAHT of 37.5. Partial melting at high temperatures of 1125–1200°C, which induces grain boundaries slip and enhances diffusion, has a significant weakening effect on the rheology of granulite, with an estimated strain rate enhancement by 5 times at melt fraction of ∼2vol%. Reaction from pyroxene to olivine may affect the flow law parameters and deformation mechanism. Based on our data, a wet and cool continental lower crust may still be in brittle deformation regime, whereas a hot lower crust may likely have a weak layer with plastic deformation.

Layered crustal anisotropy around the San Andreas Fault near Parkfield, California

AbstractThe rheology of the Earth's crust controls the long‐term and short‐term strength and stability of plate boundary faults and depends on the architecture and physical properties of crustal materials. In this paper we examine the seismic structure and anisotropy of the crust around the San Andreas Fault (SAF) near Parkfield, California, using teleseismic receiver functions. These data indicate that the crust is characterized by spatially variable and strongly anisotropic upper and middle crustal layers, with a Moho at ∼35 km depth. The upper layer is ∼5–10 km thick and is characterized by strong (≥30%) anisotropy with a slow axis of hexagonal symmetry, where the plane of fast velocity has a strike parallel to that of the SAF and a dip of ∼40∘. We interpret this layer as pervasive fluid‐filled microcracks within the brittle deformation regime. The ∼10–15 km thick midcrustal layer is also characterized by a weak axis of hexagonal symmetry with ≥20% anisotropy, but the dip direction of the plane of fast velocity is reversed. The midcrustal anisotropic layer is more prominent to the northeast of the San Andreas Fault. We interpret the mid crustal anisotropic layer as fossilized fabric within fluid‐rich foliated mica schists. When combined with various other geophysical observations, our results suggest that fault creep behavior around Parkfield is favored by intrinsically weak and overpressured crustal fabric.

Rate Dependency in Conical Ice Indenter Failure

Research of the properties of the material ice and ice-structure interaction are important for design of ships and offshore structures, because resource extraction, transportation links and tourism move north into the Arctic. Experimental benchmark tests have been conducted within the Sustainable Technology for Polar Ships and Structures (STePS2) group project at the Memorial University of Newfoundland (MUN) in Canada to examine the rate dependency of conical ice indenter deformation and failure behaviour in the ductile, ductile-to-brittle and brittle ice deformation regime. The final objective was to develop a baseline curve and a database indicating rate dependent changes in peak load and failure mechanism of ice cones crushing into a steel structure. Additionally temperature and crushing depth effects were examined. The paper summarizes the results, which correlate well with other available data and extends the database for conical ice test specimens.

The Geounri shear zone in the Paleozoic Taebaeksan Basin of Korea: Tectonic implications

The Songrim and Daebo orogenies represent two major Phanerozoic tectonic events that are well-preserved across much of the present-day Korean Peninsula. The Songrim orogeny corresponds to the Late Permian–Triassic collision of the North and South China cratons whereas the Jurassic Daebo tectonic event represents a thin-skinned contractional deformation in a continental arc setting. It is well-established that the Songrim orogeny left a strong imprint on the geological record preserved in the middle and northern parts of the Korean Peninsula (e.g., the Pyeongnam Basin and the Imjingang belt) with only a minor impact on the geology of southern Korea (e.g., the Okcheon and Taebaeksan basins). It was the Daebo tectonic event, however, which generated most of the deformational structures observed within the Okcheon and Taebaeksan basins. The Deokpori thrust in the Taebaeksan Basin is a significant fault structure that formed during the Daebo tectonic event; no other regional structures related to the Songrim orogeny have been found in the Taebaeksan Basin. In the vicinity of the Deokpori thrust, we have identified in this study a previously undocumented reverse-slip shear zone, which is named the Geounri shear zone. Microfabrics observed in phyllonite and marble mylonite samples suggest that this shear zone was developed in a plastic deformation regime at ∼400 °C, in contrast with the nearby Deokpori thrust, which formed in a brittle deformation regime. A geochronological analysis of muscovite isolated from phyllonite samples of the Geounri shear zone, yielded two 40Ar/39Ar age spectra with a combined weighted mean age of 209 ± 5 (2σ) Ma. Thermal modeling of the Ar data, combined with structural interpretations, suggests that this muscovite 40Ar/39Ar age represents a robust lower age limit for the timing of the Geounri shearing event, therefore linking formation of the shear zone with deformational events caused by the Songrim orogeny. These results imply that structural features caused by ‘Songrim deformation’ were developed at higher temperatures than structures caused by ‘Daebo deformation’ as found within the Taebaeksan Basin.

Mass transfer and porosity evolution during low temperature water–rock interaction in gneisses of the simano nappe: Arvigo, Val Calanca, Swiss Alps

Late Alpine fissures and fractures in amphibolite-facies basement gneisses at Arvigo (Val Calanca, Swiss Alps) show distinct cm-sized reaction selvages parallel to the fracture walls that composed of subgreenschist facies assemblages produced by the interaction of water present in the fracture porosity with the old high-grade gneiss assemblages. The process of selvage or reaction-vein formation occurred in the brittle deformation regime and at temperatures characteristic of, first the prehnite-pumpellyite facies and then later of the zeolite facies. The vein formation occurred during uplift and cooling at very late stages of the Alpine orogeny. The reaction veins are composed of a selvage of altered gneiss on both sides of the central fracture and a central zone with fissure minerals that have been growing in the open fracture pore space. The central zone of the Arvigo veins contains an early assemblage with epidote, prehnite and chlorite and a late succession sequence of various species of zeolite. The veins of the Arvigo quarry are convincing evidence that fracture fluids in gneiss and granite have the potential to precipitate Ca–zeolite. This is an important find because many fluids recovered from deep continental drill holes and from geothermal energy exploration are found to be oversaturated in respect to a number of Ca–zeolite species. Vein formation during late uplift and cooling of the Alps occurred at continuously decreasing T and at hydrostatic pressure: (1) coexisting prehnite/epidote records temperatures of 330–380°C, (2) chlorite formation at temperature of 333 ± 32°C and (3) formation of zeolites <250°C. In the selvages the prime reaction is the replacement of plagioclase by albite along a sharp reaction front that separates the selvage from unaltered gneiss. In addition to albitisation, chloritisation of biotite is the second important reaction in the alteration process. The reactions release components for the formation of Ca–Al silicates. The water–rock interaction is associated with a depletion of Al, Si, Ca, Fe and K in the altered wall rock. The overall reaction is associated with an increase in porosity of up to 14.2 ± 2.2% in the selvage zone (altered wall rock), caused by the volume decrease during albitisation and the removal of biotite. The propagation of the sharp reaction front through the gneiss matrix occurred via a dissolution-reprecipitation mechanism. Zeolite formation is tied to the plagioclase alteration reaction in the rock matrix, which releases components for zeolite formation to a CO2-poor aqueous liquid.

Cretaceous-Eocene compression in the central Southern Alps (N Italy) inferred from 40Ar/39Ar dating of pseudotachylytes along regional thrust faults

The integration of structural analyses with 40Ar/39Ar dating of fault-related pseudotachylytes provides time constraints for the reconstruction of the Alpine evolution of the central portion of the South Alpine orogenic wedge. In the northern sector of the belt a Variscan basement is stacked southward on the Permian to Mesozoic cover along regional faults (Orobic and Porcile thrusts). Fault zones, slightly postdating a first folding event of Alpine age, experienced a complex evolution through the ductile and brittle deformation regime, showing greenschist facies mylonites overprinted by a penetrative cataclastic deformation. Generation of fault-related pseudotachylyte veins marks the onset of brittle conditions, lasting up to the youngest episodes of fault activity. 40Ar/39Ar dating of the pseudotachylyte matrix of 9 samples give two separated age clusters: Late Cretaceous (80–68 Ma) and latest Palaeocene to Middle Eocene (55–43 Ma). These new data provide evidence that the pre-Adamello evolution of the central Southern Alps was characterised by the superposition of different tectonic events accompanying the exhumation of the deepest part of the belt through the brittle–ductile transition. The oldest pseudotachylyte ages demonstrate that south-verging regional thrusting in the central Southern Alps was already active during the Late Cretaceous, concurrently with the development of a synorogenic foredeep basin where the Upper Cretaceous Lombardian Flysch was deposited. Our reconstruction provides direct evidence for an important Cretaceous “EoAlpine” orogenic event which was nearly coeval to the HP metamorphism affecting the Austroalpine units of the eastern Alps. In our view, the Upper Cretaceous Southern Alps can be interpreted as the south-verging retrobelt, which formed during subduction phenomena active to the north in the Austroalpine realm.

The brittle deformation regime of water-saturated siliceous sandstones

SUMMARY We present here new experimental data on the mechanical behaviour of water-saturated Bentheim and Fontainebleau sandstones deformed in the brittle failure regime. Bentheim sandstone samples were stressed at room temperature and subjected to confining pressures PC ranging from 12 to 120 MPa and pore pressures PP ranging from 1 to 70 MPa. For all samples the evolution of the volumetric strain first shows compaction eventually reversing to dilation of the pore volume when approaching the peak stress. All samples failed localized on a single shear zone. Critical stresses that is onset of dilatancy and peak stress, can be uniquely defined as a function of the effective pressure Peff = PC – PP. The failure curve parameters, when fitted with the Hoek–Brown criterion or a parabolic envelope, are consistent with previous values obtained on similar material. When compared to data obtained on dry samples of the same rock, the present data show no notable effect of the presence of water on the critical stress levels. The same conclusion holds for the stress–strain curves when dry and water-saturated experiments under equivalent effective confining pressures are compared. When compared to previously published data obtained on various quartzose rocks, which show a quite variable water-weakening effect, our results lead to the conclusion that the quasi-exclusive presence of quartz grains bonded together by a quartzose cement combined with the absence of clayey minerals may explain the absence of this effect in Bentheim sandstone. Moreover, this conclusion is supported by complementary results obtained by Fontainebleau sandstone with similar microstructural and compositional characteristics. A previously developed micromechanical model based on the interaction of pore cracks has been used and modified to take into account the presence of water. When compared to the experimental data, the results of the model show good agreement for low to intermediate effective confining pressures. Critical stresses as well as deformation curves are well reproduced as was the case for the dry rock provided that effective pressure is introduced. At higher effective pressures, a mix of mechanisms including shear-enhanced compaction may explain the progressive discrepancy between the modelled and the experimental curves when approaching the apex of the failure envelope.

Ore controls in the Charters Towers goldfield, NE Australia: Constraints from geological, geophysical and numerical analyses

The approach taken in this paper, namely synthesising a wealth of previous information with new data and a genetic model, in combination with integrated numerical analyses, led to new insights into the geological controls on the localisation of auriferous veins and residual prospectivity of the Charters Towers goldfield, NE Australia. The method also has implications for the assessment of other “mature” goldfields worldwide. Despite a number of different ore controls having operated within the Charters Towers goldfield, the controlling factors can be linked to a single genetic model for orogenic, granitoid-hosted lode-gold mineralisation in a brittle deformation regime (D 4) of NE–SW to NNE–SSW shortening, under conditions of supralithostatic fluid pressure and low stress difference. Spatial autocorrelation results suggest district-scale alignment of the auriferous veins parallel to and overlapping with the ESE–WNW- to E–W-striking Charters Towers–Ravenswood lineament, a major crustal boundary in the basement to the Ravenswood batholith. At the camp-scale, auriferous veins have abundance and proximity relationships with NW–SE-, NNW–SSE-, NE–SW- and ENE–WSW-oriented lineaments, suggesting that structures that controlled gold deposition in one camp did not necessarily control mineralisation in other camps. Fractal dimensions obtained with the box-counting method range from 1.02 to 1.10, whereas veins in the Charters Towers City camp are characterised by a significantly higher fractal dimension of 1.28. This discrepancy may be taken to imply that most or all outcropping and near-surface deposits within the Charters Towers City camp have been found and that new discoveries are more likely to occur at greater levels of depth, or outside the boundaries of this camp. The new understanding has implications for the assessment of the residual prospectivity of the Charters Towers goldfield, where large areas of prospective rock types and structures (e.g., approximately 40% of the Charters Towers–Ravenswood lineament) are hidden under cover. This parameter space was inaccessible to the historic prospectors and has received relatively little attention from recent explorers. The following steps are suggested for the development of a targeting strategy for lode-gold exploration in areas of the goldfield under cover: (1) identify from geological and geophysical data the ENE–WSW (± 15°) and NNW–SSE (± 15°) striking structures and geological boundaries within a 20-km-wide corridor parallel to and centred upon the Charters Towers–Ravenswood lineament, the potential control on gold deposit distribution at the regional- to district-scale, (2) interpret from geological and geophysical data the distribution of pre-Middle Devonian granitoids within these areas that are the preferred host rocks of the payable gold deposits, (3) deduce from geophysical data the ENE–WSW (± 15°) and NNW–SSE (± 15°) striking structures that cut or bound the intrusions identified in step 2, (4) locate segments along the structures identified in step 3 that deviate most from the geometry of a straight line (e.g., potential bends or splays) and/or intersect other structures or geological contacts, or both, and (5) define and rank potential targets within the prospective areas identified in step 4 and systematically test the best ones.

A pore crack model for the mechanical behaviour of porous granular rocks in the brittle deformation regime

Abstract A model is developed for predicting and simulating the mechanical behaviour and failure mode of brittle porous granular rocks loaded in compression. It is based on a fracture mechanics analysis applied to cracks emanating from the surface of cylindrical pores in two-dimensional, which is well suited to the microstructure of such rocks. It is also consistent with the usual experimental procedure used for biaxial compression tests since the numerical scheme is implemented under the assumption of imposed axial strain. The model takes into account interactions between neighbouring cracks, which grow when their stress intensity factor reaches the fracture toughness of the rock. The simulation of crack growth from cylindrical holes, associated with a failure criterion based on the coalescence of interacting cracks, allows one to calculate the critical stress at rupture and to derive theoretical stress–strain curves. The present model is then used to compare theoretical results with laboratory data obtained on four sandstones with porosity ranging between 13% and 25.5% which were deformed under conventional triaxial compression conditions at confining pressures between 0 and 35 MPa. The comparison shows that by using a small number of parameters (pore size, pore density, fracture toughness, and elastic moduli), the model is able to predict the rock behaviour during the compression tests and the stress level at rupture in a quite accurate way, when the microstructural parameters introduced are consistent with the observations.

A Model for the Mechanical Behaviour of Bentheim Sandstone in the Brittle Regime

The mechanical behaviour of Bentheim sandstone, a homogeneous quartz-rich sandstone with porosity of 22.8%, was investigated by triaxial compression tests conducted on dry samples. At confining pressures up to 35 MPa, the failure mode was characterized by a typical brittle deformation regime, as the samples showed dilatancy and failed by strain softening and brittle faulting. Previous studies have shown that the mechanical behaviour and failure mode of brittle porous granular rocks are governed by the time-dependent growth of microcracks. We analyse this process using the “Pore Crack Model” based on fracture mechanics analysis. It is consistent with the microstructure of porous granular rocks since it considers the growth of axial cracks from cylindrical holes in two dimensions. These cracks grow when their stress intensity factors reach the subcritical crack growth limit. Interaction between neighbouring cracks is introduced by calculating the stress intensity factor as the sum of two terms: a component for an isolated crack and an interaction term computed using the method of successive approximations. It depends on crack length, pore radius, pore density, and applied stresses. The simulation of crack growth from cylindrical holes, associated with a failure criterion based on the coalescence of interacting cracks, is used to compare the theoretical stress at the onset of dilatancy and at macroscopic rupture to the experimental determined values. Our approach gives theoretical results in good agreement with experimental data when microstructural parameters consistent with observations are introduced

Effect of step-loading history and related grain-boundary fatigue in freshwater columnar ice in the brittle deformation regime

Across-column biaxial compression tests at- 10o C on laboratory-grown freshwater ice samples in the brittle deformation regime have indicated a large influence of the step loading history on both the failure mechanism and the ice's ultimate strength. Samples loaded to failure with multiple load increments showed homogeneous cracking throughout the sample volume, in contrast with the failure observed in a single step where failure was predominantly through localized damage onto a diagonal shear fault The failure strength was also found to be consistently higher for the incrementally loaded samples. This increase in the cracking activity and the compressive strength is attributed to local inelastic events of grain-boundary sliding, the occurrence of which was shown previously by Picu and Gupta (1995, Acta metall. mater., 43, 3791, 3798), even in the brittle deformation regime. Because of the creep deformation at the junctions of the sliding boundaries, the boundaries appear to be strengthened on subsequent reloading. This allows loading of other grains and homogeneous cracking throughout.

Transmission electron microscope analysis of the deformation of chromites from ophiolites

TEM investigations of natural chromites from New Caledonia and Turkey show contrasting dislocation structures which relate to the ore structures and to the deformation history. Some chromites appear to have been deformed in the ductile field ; dislocation substructures are typical of steady state dislocation creep. Other chromites show many fractures filled with matrix minerals. In these latter chromites one observes a large density of very small particles of diopside (typically 0.1x1 µm2 cross section) which pin the dislocations. It is suggested that these particle-dislocation interactions have caused a strong hardening and are responsible of the observed brittle deformation regime (pull-apart). Fractures have occurred on the surfaces which contained the largest particles. A rough estimation of the crystallization temperature of these latter chromites (Tc ⋍ 1200 ± 100 °C) is deduced from the spacing between the mismatch dislocations surrounding the diopside inclusions which exhibit epitaxial relations with the chromite matrix.