Preferred Mineral Orientation Research Articles

An Investigation on the Fabric Type Dependency of the Crack Damage Thresholds in Brittle Rocks

Fabric-guided micro-fracturing phenomenon in brittle rocks and its effect on crack damage thresholds remains subject to continuing research. The available fabric in rocks can act as a motivator for nucleation and/or extension and interaction of micro-fractures in a preferred orientation, or as a suppressor for growth of micro-cracks in a given direction by different mechanisms such as compliance (stiffness contrast) or preferred orientation of minerals and their boundaries. While anisotropy of brittle rocks in terms of their mechanical strengths can play a significant role in the stability of underground openings, the understanding of the dependency of crack initiation (CI) and crack propagation (CD) thresholds on the available fabric in rocks can improve predictions of the extension and density of micro-fracturing in different directions in the walls of underground openings. To better understand the fabric-guided micro-fracturing phenomenon, and also to study the effect of fabric types available in brittle rocks on their anisotropic behaviour, four types of brittle rocks with different types of fabric are investigated in terms of crack damage anisotropy in this paper. The rocks that are chosen for this study are limestone from the Cobourg Formation, Queenston shale, Olkiluoto mica gneiss and Lac du Bonnet granite. For each rock type, CI and CD thresholds are identified from the unconfined compressive strength testing data. The mechanical behaviour of the four rock types are investigated at each damage stress level and the contributing factors to the isotropic or anisotropic behaviour of the rocks at different crack damage thresholds are discussed.

Petrophysical properties of a granite-protomylonite-ultramylonite sequence: insight from the Monte Grighini shear zone, central Sardinia, Italy

The Monte Grighini intrusive complex (central-west Sardinia) has been affected by a 1.5 km wide late-Variscan shear zone; microstructures and deformation fabrics indicate that high strain domains resulted in sharp transitions among protomylonite to ultramylonite. A representative geological cross section of the shear zone has been sampled, as well as samples from outside the mylonitic belt for use as slightly deformed protolith reference samples. Physical and mechanical analyses were performed on each group in order to evaluate a possible correlation between petrophysical properties (density, porosity and mechanical strength) and the degree of mylonitic deformation. Ultramylonites are generally characterized by a higher solid-phase density (2.82–2.85 g/cm3) than less deformed protomylonites and mylonites (2.73–2.79 g/cm3), testifying to the formation of high-density minerals with an increase in deformation. Conversely, bulk density tends to decrease as deformation increases (from 2.52–2.57 g/cm3 to 2.47–2.48 g/cm3) due to the total porosity increase occurring during mylonitization. The mean values of total porosity range from 6.33 to 9.27 % in protomylonites and mylonites and from 12.01 to 12.85 % in ultramylonites. The relatively high value of porosity in more deformed ultramylonites is the result of the sum of closed micropores and open pores produced by different processes. Mechanical strength measurements show a noticeable anisotropy, with higher strength values measured applying the load perpendicular to foliation (Z axis) and lower values obtained parallel to it (X,Y axes). In the Monte Grighini mylonite the anisotropy is due to the distribution of pores and to the preferred orientation of new minerals, as highlighted by the correlation between scalar physical properties (solid, real and bulk densities, porosity) and vector properties (resistance to punching or uniaxial compressive strengths). The results show that the degree of deformation of mylonitic rocks can be characterized by petrophysical properties, in order to shed light on the tectonic processes experienced at depth.

Carbon dioxide sealing capacity: Textural or compositional controls? A case study from the Oklahoma Panhandle

One of the challenges confronting carbon dioxide capture and sequestration (CCS) in geologic media over extended periods of time is determining the caprock sealing capacity. If the pressure of supercritical carbon dioxide injected in the repository overcomes the caprock sealing capacity, leaking of may enter other porous formations, compromising the storage formation, or even may go back to the atmosphere, and thus the process of sequestration becomes futile. Carbon dioxide sealing capacity is controlled by two groups of parameters: (1) texture (e.g., the pore-throat size, distribution, geometry, and sorting; median grain size, porosity, degree of bioturbation, specific surface area, preferred orientation of matrix clay minerals, orientation, and aspect of ratio of organic particles) and (2) composition (mineralogical content, proportion of soft, deformable mineral grains to rigid grains, organic matter content, carbonate content, silt content, cementation, ductility, compaction, and ash content). The primary goal of this study was to investigate several parameters listed above and to estimate their respective contributions to sealing capacity to better understand its role in shale and carbonates. To assess the effect of textural and compositional properties on maximum retention column height, we collected 30 representative core samples from caprock formations in three counties (Cimarron, Texas, and Beaver) in the Oklahoma Panhandle. The study area was chosen because it hosts three depleted gas fields with a storage capacity of more than 35 million bbl and is situated at a crossroad leading to some significant stationary sources from North Texas, South Kansas, and northern Oklahoma. We used mercury injection porosimetry, scanning electron microscopy (SEM), Sedigraph energy dispersive spectra (EDS), x-ray diffraction (XRD), Brunauer–Emmett–Teller-specific surface area, and total organic carbon (TOC) measurements to assess textural and compositional properties of collected samples. The range of column height for the samples used in this study is between 0.2 and 1358 m (0.66 and 4455 ft). The average column height is 351 m (1152 ft). The depth interval approximately 1400 m (4593 ft) could reach relatively high values of column height, up to 1200 m (3937 ft). The above-mentioned interval is composed of mainly Cherokee and Morrowan Formations (shale seals). Principal component analysis (PCA) was carried out to infer the possible relationships between textural and compositional parameters. Generally, composition of our samples (shales vs. carbonates and sandstones) indicates a relatively stronger control on caprock sealing capacity, although individual mineral makeup of shale samples seems not correlated with retention column heights. In the same time, many textural parameters play a significant role in determining the sealing capacity of carbonate caprocks.

Clay mineral formation and fabric development in the DFDP-1B borehole, central Alpine Fault, New Zealand

Clay minerals are increasingly recognised as important controls on the state and mechanical behaviour of fault systems in the upper crust. Samples retrieved by shallow drilling from two principal slip zones within the central Alpine Fault, South Island, New Zealand, offer an excellent opportunity to investigate clay formation and fluid–rock interaction in an active fault zone. Two shallow boreholes, DFDP-1A (100.6 m deep) and DFDP-1B (151.4 m) were drilled in Phase 1 of the Deep Fault Drilling Project (DFDP-1) in 2011. We provide a mineralogical and textural analysis of clays in fault gouge extracted from the Alpine Fault. Newly formed smectitic clays are observed solely in the narrow zones of fault gouge in drill core, indicating that localised mineral reactions are restricted to the fault zone. The weak preferred orientation of the clay minerals in the fault gouge indicates minimal strain-driven modification of rock fabrics. While limited in extent, our results support observations from surface outcrops and faults systems elsewhere regarding the key role of clays in fault zones and emphasise the need for future, deeper drilling into the Alpine Fault in order to understand correlative mineralogies and fabrics as a function of higher temperature and pressure conditions.

Magnetic fabric and petrology of Miocene sub-volcanic sills and dykes emplaced into the SW Flysch Belt of the West Carpathians (S Moravia, Czech Republic) and their volcanological and tectonic implications

Magnetic fabrics were investigated in the sub-volcanic bodies occurring within the SW Flysch Belt of the Western Carpathians. Magnetite or low-Ti titanomagnetite is the principal carrier of the anisotropy of magnetic susceptibility (AMS) in these rocks, though various Fe–Ti oxides were also identified in minor amounts. The magnetic fabric in almost all sills and dykes is conformable to the shapes of the bodies and no doubt originated through more or less free magma flow that requires relatively open voids to be injected, that likely originated in an extensive tectonic regime. In an ankaramite sill, where the rock creates typical hexagonal columns, the magnetic fabric is related rather to the column axes than to the overall shape of the sill. The magnetic fabric formation in this sill is controlled by the post-emplacement process of the creation of the columns. At some studied localities, signs of post-magmatic Middle Miocene tectonic deformation are observed in the field, the youngest known in the Flysch Belt. As the magnetic fabric is unrelated to those signs, we conclude that these young tectonic movements did not penetrate ductilely in the volume of the volcanic body. It seems that the strain was focused to thin zones along the faults, since it did not change the preferred orientation of magnetic minerals in the rock volume substantially.

Preferred mineral orientation of a chloritoid-bearing slate in relation to its magnetic fabric

A regional analysis of the anisotropy of the magnetic susceptibility on low-grade metamorphic, chloritoid-bearing slates of the Paleozoic in Central Armorica (Brittany, France) revealed very high values for the degree of anisotropy (up to 1.43). Nonetheless, high-field torque magnetometry indicates that the magnetic fabric is dominantly paramagnetic. Chloritoid's intrinsic degree of anisotropy of 1.47 ± 0.06, suggests that chloritoid-bearing slates can have a high degree of anisotropy without the need of invoking a significant contribution of strongly anisotropic ferromagnetic (s.l.) minerals. To validate this assumption we performed a texture analysis on a representative sample of the chloritoid-bearing slates using hard X-ray synchrotron diffraction. The preferred orientation patterns of both muscovite and chloritoid are extremely strong (∼38.6 m.r.d. for muscovite, 20.9 m.r.d. for chloritoid) and display roughly axial symmetry about the minimum magnetic susceptibility axis, indeed suggesting that chloritoid may have a profound impact on the magnetic fabric of chloritoid-bearing rocks. However, modeling the anisotropy of magnetic susceptibility by averaging single crystal properties indicates that the CPO of chloritoid only partially explains the slate's anisotropy.

Crustal anisotropy in a subduction zone forearc: Northern Cascadia

AbstractS‐wave splitting analyses using low‐frequency earthquake templates at three‐component stations across southern Vancouver Island and northern Washington indicate the presence of a heterogeneous distribution of crustal anisotropy in the North American plate. For southern Vancouver Island, we investigate contributions to anisotropy from the Leech River Complex, a terrane composed of strongly foliated phyllites and schists with steeply dipping foliations striking east‐west. Fast directions across mainland southern Vancouver Island are subparallel to the dominant Leech River Complex foliation direction. East‐to‐west increases in delay times and small‐scale azimuthal variations in fast directions indicate heterogeneous anisotropy. We test azimuthally anisotropic Leech River Complex models constrained by previous geological and seismic reflection studies, through forward modeling using 3‐D spectral element method simulations. The preferred model of a north/northeast shallowly dipping wedge of Leech River Complex material with varying orientation of anisotropy terminating at midcrustal levels explains the splitting observations at a majority of southern Vancouver Island stations. For stations where anisotropic Leech River Complex models do not recreate observations, fast directions are subparallel to local estimates of maximum compressive horizontal stress, suggesting that fluid‐filled cracks could be a source of anisotropy. We assert that the Leech River Complex is the primary source of crustal anisotropy beneath southern Vancouver Island, not cracks as suggested by prior studies. Fast directions at stations on northern Washington exhibit variations with azimuth and incidence angle suggesting complex anisotropy interpreted as due to a combination of cracks and preferred mineral orientation of metamorphosed slates of the Olympic core rocks. These slates may also underlay stations on southern Vancouver Island and represent another source of anisotropy.

Microstructural evolution of an incipient fault zone in Opalinus Clay: Insights from an optical and electron microscopic study of ion-beam polished samples from the Main Fault in the Mt-Terri Underground Research Laboratory

Slickensided shear surfaces are ubiquitous in many fault zones. However the internal structure, the micromechanics and the evolution of these structures are not fully understood and the contributions of crystal plasticity, grain-boundary sliding, microfracturing, solution-precipitation and mineral transformation under different conditions are subject of debate.We studied well- preserved core samples from the Main Fault, an up to 3 m wide zone of approximately 10 m offset in the Mont Terri Underground Research Laboratory (CH), a site to evaluate long-term safety of radioactive waste disposal. The drill core breaks easily along many slickensided shear surfaces indicating reverse slip, which form an anastomosing network connected by branch lines.Broad ion beam polishing and scanning electron microscopy shows that the slickensides are invariably revealed by fracture of the drill core along a few μm thick shear zone, which acts as a crack guide for fracturing the samples. In this zone, a complex set of processes is inferred, leading to extreme localization of strain, development of strong particle preferred orientation, the formation of nanoparticles, and local precipitation of calcite veins in releasing sections. In lenses between shear zones, homogeneous gouge is formed with a well-developed oblique foliation and removal of calcite grains by pressure solution. We infer that with progressive deformation, the number and density of slickensided shear surfaces increases, generating tectonically derived scaly clay and more homogeneous gouge. In all deformed elements of the Main Fault, porosity is much smaller than in the undeformed Opalinus Clay. An interesting observation is the almost complete absence of cataclastic microstructures. Transmission electron microscopy (TEM) of focused ion beam lamellae of this micron-wide shear zone shows a strong preferred orientation of clay minerals, including nano-sized illite particles. In TEM, the shear zones envelop hard particles and confirm an almost complete loss of porosity compared to the protolith.We propose that inter- and transgranular microcracking, pressure solution, clay neoformation, crystal plasticity and grain boundary sliding are important micro-scale processes during the early stages of faulting in Opalinus Clay and thus need to be considered in extrapolating laboratory results to long-term mechanical behavior.

Regional variations of the shear-wave polarization anisotropy in the crust and mantle wedge beneath the Tohoku district

We investigated the regional variations of the shear-wave polarization anisotropy in the upper crust, lower crust, and mantle wedge beneath the Tohoku district in the northeast Japan arc. The shear-wave splitting parameters (fast polarization direction (FPD) and split time) for the upper and lower crusts are estimated by shear-wave splitting analysis of the Ps phases, which are generated by the conversion of a P wave into an S wave at the Conrad and Moho discontinuity interfaces. The splitting parameters for the mantle wedge are determined by splitting analysis of the slab Ps phase converted at the oceanic crust on the subducting Pacific slab and the direct S waves from deep focus earthquakes located just below seismic stations. The Ps phases are identified on the P-wave receiver functions constructed from the teleseismic records. To accurately determine the splitting parameters for the lower crust from the Moho Ps phase, the Ps phase should be corrected for the shear-wave splitting effect in the upper crust. Similarly, to estimate the mantle wedge anisotropy, the direct S wave and slab Ps phase must be corrected for the splitting effect in the crust. The anisotropies of the crust and mantle wedge are estimated from the corrected Ps phases and direct S waves. The anisotropy of the upper crust exhibits a regional variation where FPD of the split shear wave is predominant in the N–S direction in the Pacific coast area and in the E–W direction in the rest of the Tohoku district. The split time is less than 0.2 s. The upper crustal anisotropy is attributed to the alignment of vertical cracks induced in the upper crust by tectonic stress. In the lower crust, FPD is predominant in the E–W direction with a split time similar to that in the upper crust, and the anisotropy of the lower crust is due to the lattice preferred orientation of rock-forming minerals. In the mantle wedge, FPD is predominant in the N–S direction (trench-parallel) in the fore-arc side of the volcanic front, but in the E–W direction (trench-perpendicular) in the back-arc side. The split time in the mantle wedge is similar to that of the upper crust. The most likely cause for the trench-perpendicular anisotropy in the back-arc mantle wedge is the lattice preferred orientation of A-type olivine along flows of the secondary mantle convection. For trench-parallel anisotropy in the fore-arc mantle wedge, we considered two possible causes: the preferred orientation of B-type olivine along the secondary mantle convection and that of dry olivine along the trench-parallel flow produced by the along-strike dip variation of the Pacific slab. However, we could not determine which one is the origin.

3D velocity distribution of P- and S-waves in a biotite gneiss, measured in oil as the pressure medium: Comparison with velocity measurements in a multi-anvil pressure apparatus and with texture-based calculated data

Ultrasonic measurements of the 3D velocity distribution of P- and S-waves were performed on a spherical sample of a biotite gneiss from the Outokumpu scientific drill hole. Measurements were done at room temperature and pressures up to 400 and 70MPa, respectively, in a pressure vessel with oil as a pressure medium. A modified transducer/sample assembly and the installation of a new mechanical system allowed simultaneous measurements of P- and S-wave velocities in 132 independent directions of the sphere on a net in steps of 15°. Proper signals for P- and S-waves could be recorded by coating the sample surface with a high-viscosity shear wave gel and by temporal point contacting of the transmitter and receiver transducers with the sample surface during the measurements. The 3D seismic measurements revealed a strong foliation-related directional dependence (anisotropy) of P- and S-wave velocities, which is confirmed by measurements in a multi-anvil apparatus on a cube-shaped specimen of the same rock. Both experimental approaches show a marked pressure sensitivity of P- and S-wave velocities and velocity anisotropies. With increasing pressure, P- and S-wave velocities increase non-linearly due to progressive closure of micro-cracks. The reverse is true for velocity anisotropy. 3D velocity calculations based on neutron diffraction measurements of crystallographic preferred orientation (CPO) of major minerals show that the intrinsic bulk anisotropy is basically caused by the CPO of biotite constituting about 23vol.% of the rock. Including the shape of biotite grains and oriented low-aspect ratio microcracks into the modelling increases bulk anisotropy. An important finding from this study is that the measurements on the sample sphere and on the sample cube displayed distinct differences, particularly in shear wave velocities. It is assumed that the differences are due to the different geometries of the samples and the configuration of the transducer–sample assembly. On the spherical sample with a point source and point receiver the first break at the velocity arrival time is suggested to define group velocities in most cases, whereas the receivers in the multi-anvil apparatus recorded the flat part of the wavefront, that is, phase velocities.

Occurrence of Tephra / Volcanic Tuff in the Tertiary Sediments of Himachal Himalaya from Tileli Area, Mandi District, H.P.: Implication for Stratigraphy and Uranium Mineralization

Abstract: Presence of Early Tertiary pyroclastic material (tephra) has been documented petrographically, for the first time, in the Mandi-Bilaspur Sector from Tileli area, Dharamsala basin of Himachal Pradesh. The tephra is reported from the red shale, identified as tuffaceous siltstone belonging to lower Dharamsala Formation that lies above the uraniferous sandstone body and occurs as thin layers of over 300m along the strike, close to the contact of lower and upper Dharamsala formations. The tuffaceous material shows crude but preferred orientation of minerals like biotite, muscovite, chlorite, clay, hematite and specularite. Various features indicating presence of tephra are, glass shards altered to clay but retaining ‘U’ shaped outline, spindle-shaped hematite with preferred orientation, spherical to sub-spherical clay and altered Feoxide rich balls, clay groundmass with flow pattern, flaky minerals in association with clast depicting asymmetrical ramp structure. A zone of approximately 300 m length containing tuffaceous material has been established at Tileli overlying the uraniferous sandstone body. Identification of tephra at Tileli has significant implications as it enabled in demarcating the boundary between the upper and lower Dharamsala formations in central part of the basin in Bilaspur-Mandi Sector of HP Himalaya and also in guiding the uranium exploration programme in the lower Dharamsala Formation.

Seismic anisotropy in cracked crystalline rock from Outokumpu, Finland

SUMMARY Lattice and shape preferred orientation of minerals, along with aligned fractures and microcracks, is expected to cause significant seismic velocity anisotropy in crystalline rocks. As seismic surveys in hard rock environments become more common, quantifying and accounting for this anisotropy in seismic processing becomes increasingly important. Outokumpu, Finland is the site of a historic base metal mine and is a classical ore province known for its Cu-CoZn sulphide deposits. The 2.5 km deep ICDP borehole shows the lithology in the area of the Outokumpu 2006 2D seismic survey to be primarily composed of a biotiterich schist. Three walk-away VSP profiles were used to quantify the tilted orthorhombic in-situ anisotropy. Laboratory measurements of the qS1, qS2 and qP waves along the axial directions and in select off-axis directions at confining pressures from 10-200 MPa, and effective medium modelling were used to further inform the seismic anisotropy of the schist. Strong anisotropy is observed both in-situ and in the laboratory measurements. A 3D velocity distribution is calculated from modelling of these results.

Modified Jeffery model: Influence of particle concentration on mineral fabric in moderately concentrated suspensions

AbstractA modified Jeffery model involving particle interactions allows the simulation of preferred orientation of crystals and the anisotropy of magnetic susceptibility in magmatic flow with up to 40% of rigid particle fraction. Numerical simulations are compared to analogue and natural fabrics. A study of the role of particle concentration shows that the increasing percentage of crystal fraction in dilute suspensions leads to progressive attenuation of the fabric intensity and cyclicity. At high strains in simple shear flow, the lineation converges to a constant value close to the shear plane. With increasing particle concentration, the fabric diverges from the shear direction reflecting the effect of increased crystal tiling. The model can be applied to quantify mineral preferred orientations of crystal mushes in magmatic fabric studies.

The control of mineral constituents and textural characteristics on the petrophysical & mechanical (PM) properties of different rocks of the Himalaya

Petrophysical and mechanical (PM) properties of a rock are primarily controlled by its mineral constituents and various textural parameters. Similar rock types in different tectonic setups may be characterized by different mineralogical compositions and textural properties because they might have suffered different deformational processes. This paper presents a study on the mineralogical and textural control on various PM properties in quartzites, granitoids, gneisses, metabasics and dolomite collected from different tectonic set ups. The inter relationships among mineral constituents, various textural parameters, Unconfined Compressive Strength (UCS), Schmidt hammer rebound (R-) value, seismic wave (both P- and S-waves) velocity and their attenuation characteristics were obtained using regression analyses. It has been concluded that there exists no relation between UCS and seismic wave velocity in quartzite, granitoids and gneisses, moderate positive relationship in metabasics and strong negative relationship in dolomite. It has further been noted that in quartzite and dolomite (monominerelic rocks), grain size dominantly control the seismic wave velocity as the relatively coarser grained samples have higher seismic wave velocity, whereas UCS is strongly controlled by grain size as well as the quantity and the orientation of the weak minerals like muscovite, biotite, calcite etc. In gneisses and granitic gneisses, preferred orientation of minerals especially the mica minerals, although in present small quantity is the main factor controlling UCS and seismic velocity. Both these properties increase with the increase in intensity of preferred alignment of minerals, whereas in metabasics it is the presence of amphibole+pyroxene minerals/feldspar that dominantly controls the seismic wave velocity.

Contribution of crystallographic preferred orientation to seismic anisotropy across a surface analog of the continental Moho at Cabo Ortegal, Spain

The lithostratigraphic sequence of the Upper Allochthon of the Cabo Ortegal complex in northwestern Spain provides an excel lent analog for the direct study of petrophysical properties of the continental Moho transition. The various lithologies present were sampled for velocity measurements on minicores and determination of seismic veloc ities through microstructural analyses, with emphasis on the crustal part of the sequence. Here, we present data from electron backscatter diffraction (EBSD) to determine the crystallographic preferred orientation of all major rock-forming minerals. The orientation data are then combined with the elastic properties, densities, and modal fractions of the mineral phases to calculate P-wave and S-wave velocities. Calculated velocities coincide very well with direct measurements made on minicores of the same samples in the high-pressure laboratory. The threedimensional velocity patterns show the signifi cant contribution of the crystallographic preferred orientation of certain constituent minerals on the bulk properties of the rock. The bulk anisotropic signal is dominated by highly anisotropic phases that are susceptible to both shape and crystal preferred orientation. Factors of particular importance are small modal fractions of micas in gneisses, and amphiboles and clinopyroxenes in eclogites and high-pressure granulites. In the lower-crustal rocks, the direction of maximum P-wave velocities, although contained in the foliation plane, does not coincide with the orientation of the mineral and stretching lineation and therefore cannot be used as an indicator of the main strain direction in rocks. The maximum seismic birefringence is often contained within the foliation plane but very rarely coincides with the lineation. Our data set also illustrates the strong effect of the breakdown reactions of clinopyroxene and the appearance of plagioclase on the petrophysical properties of mafi c lower-crustal rocks. While it does not enhance anisotropy, it does produce a drop in velocity and as a consequence enhances the refl ectivity of the contact between lower-crustal rocks and ultra mafi cs from the mantle.

Mineral Preferred Orientation and Microstructure in the Posidonia Shale in Relation to Different Degrees of Thermal Maturity

AbstractThe thermal maturity of samples of the Posidonia Shale collected from the Hils Syncline, northern Germany, varies significantly as a function of location indicating variations in local history. Synchrotron X-ray diffraction was used to document the composition and the preferred orientation of four samples of the Posidonia Shale with different degrees of maturity (0.68–1.45%, Ro) to determine possible effects on diagenesis and preferred orientation. Overall, the degree of preferred orientation of all clay minerals (illite-smectite, illite-mica, and kaolinite) and in all samples is similar, with (001) pole figure maxima ranging from 3.7 to 6.3 multiples of a random distribution (m.r.d.). Calcite displays weak preferred orientation, with c axes perpendicular to the bedding plane (1.1–1.3 m.r.d.). Other constituent phases such as quartz, feldspars, and pyrite have a random orientation distribution. The difference in thermal history, which causes significant changes in the maturity of organic matter, influenced the preferred orientation of clay minerals only marginally as most of the alignment seems to have evolved early in their history. Synchrotron X-ray microtomography was used to characterize the three-dimensional microstructure of a high-maturity sample. Low-density features, including porosity, fractures, and kerogen, were observed to be elongated and aligned roughly parallel to the bedding plane. The volume of low-density features was estimated to be ~7 vol.%, consistent with previous petrophysical measurements of porosity of 8–10 vol.%. Transmission electron microscopy analysis of samples with different degrees of maturity (0.74%Ro and 1.45%Ro) was used to document microstructures at the nanoscale as well as the presence of kerogen. In the high-maturity sample, pores were less abundant while minerals were more deformed as shown by fractured calcite and by kinked and folded illite. Some of the porosity was aligned with clay platelets.

Relationships between pore space anisotropy and anisotropy of physical properties of silicoclastic rocks from the Corbières–Minervois fold-and-thrust-belt (north-east Pyrenees, France)

We present a study integrating petrophysical measurements along a general N–S section of the Corbières–Minervois fold and thrust belt (NE Pyrenees, France) in order to study the petrofabric of weakly deformed sediments. We focus on the relationship between the evolution of deformation measured with the anisotropy of various physical properties (anisotropy of magnetic susceptibility (AMS), anisotropy of P-waves velocity (APV) and anisotropy of electrical conductivity (AEC)) and the distribution of the porous space. The origin of AMS fabric is interpreted as recording a layer parallel shortening oriented N145, steadily throughout the studied section with local gradient associated with folding as sites close to the forelimbs. AEC is in good agreement with AMS results and records the same shortening direction. The most probable source of anisotropy for AMS and AEC is the preferred orientation of phyllosilicate minerals. APV, which reflects the distribution of porous space, is more complex and is controlled by the tectonic style of the magnetic fabric. The porosity is a flat-lying-bedding porosity, where the magnetic fabric is poorly evolved and it becomes a flat-lying-pressure-solution-cleavage porosity where the magnetic fabric is tectonic. At regional scale, we show that changes in anisotropy of petrophysical properties are controlled by individual structures, like regional fold or thrust fault and occurred mainly before folding as a consequence of a well-expressed NNE–SSW layer parallel shortening. The folds and thrust faults are developed subsequently from inherited E–W basement faults. This explains why the structural grain of the sedimentary cover, expressed notably by the magnetic lineation, is oblique to the major structures of the cross-section: the Alaric Anticline and the North-Mouthoumet Fault.

The fabric of consolidation in Gulf of Mexico mudstones

How the micro-scale fabric of clay-rich mudstone evolves during consolidation in early burial is critical to how they are interpreted in the deeper portions of sedimentary basins. Core samples from the Integrated Ocean Drilling Program Expedition 308, Ursa Basin, Gulf of Mexico, covering seafloor to 600 meters below sea floor (mbsf) are ideal for studying the micro-scale fabric of mudstones. Mudstones of consistent composition and grain size decrease in porosity from 80% at the seafloor to 37% at 600 mbsf .Argon-ion milling produces flat surfaces to image this pore evolution over a vertical effective stress range of 0.25 (71 mbsf) to 4.05MPa (597 mbsf). With increasing burial, pores become elongated, mean pore size decreases, and there is preferential loss of the largest pores. There is a small increase in clay mineral preferred orientation as recorded by high resolution X-ray goniometry with burial.

Mesoproterozoic syntectonic garnet within Belt Supergroup metamorphic tectonites: Evidence of Grenville-age metamorphism and deformation along northwest Laurentia

Northern Idaho contains Belt-Purcell Supergroup equivalent metamorphic tectonites that underwent two regional deformational and metamorphic events during the Mesoproterozoic. Garnet-bearing pelitic schists from the Snow Peak area of northern Idaho yield Lu–Hf garnet-whole rock ages of 1085±2Ma, 1198±79Ma, 1207±8Ma, 1255±28Ma, and 1314±2Ma. Garnet from one sample, collected from the Clarkia area, was micro-drilled to obtain separate core and rim material that produced ages of 1347±10Ma and 1102±47Ma. The core versus rim ages from the micro-drilled sample along with the textural and spatial evidence of the other Lu–Hf garnet ages indicate two metamorphic garnet growth events at ~1330Ma (M1) and ~1080Ma (M2) with the intermediate ages representing mixed ages. Some garnet likely nucleated and grew M1 garnet cores that were later overgrown by younger M2 garnet rims. Most garnet throughout the Clarkia and Snow Peak areas are syntectonic with a regional penetrative deformational fabric, preserved as a strong preferred orientation of metamorphic matrix minerals (e.g., muscovite and biotite). The syntectonic garnets are interpreted to represent one regional, coeval metamorphic and deformation event at ~1080Ma, which overlaps in time with the Grenville Orogeny. The older ~1330Ma ages may represent an extension of the East Kootenay Orogeny described in western Canada. These deformational and metamorphic events indicate that western Laurentia (North America) was tectonically active in the Mesoproterozoic and during the assembly of the supercontinent Rodinia.

Origin and behavior of clay minerals in the Bogd fault gouge, Mongolia

We analyzed twelve fault gouge samples from the Bogd fault in south-western Mongolia to understand the origin and behavior of clay minerals. The investigation relies on x-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high energy synchrotron x-ray diffraction methods to investigate microstructure and preferred orientation. Smectite (montmorillonite), illite-smectite mixed layers, illite-mica and kaolinite are the major clay components, in addition to quartz and feldspars, which are present in all samples. The observations suggest that the protoliths and the fault rocks were highly altered by fluids. The fluid-rock interactions allow clay minerals to form, as well as alter feldspars to precipitate kaolinite and montmorillonite. Thus, newly formed clay minerals are heterogeneously distributed in the fault zone. The decrease of montmorillonite component of some of the highly deformed samples also suggests that dehydration processes during deformation were leading to illite precipitation. Based on synchrotron x-ray diffraction data, the degree of preferred orientation of constituent clay minerals is weak, with maxima for (001) ranging from 1.3 to 2.6 multiples of a random distribution (m.r.d). Co-existing quartz and feldspars have random orientation distributions. Microstructure and texture observations of the gouges from the foliated microscopic zone, alternating with micrometric isotropic clay-rich area, also indicate that the Bogd fault experienced brittle and ductile deformation episodes. The clay minerals may contribute to a slip weakening behavior of the fault.