Preferred Orientation Patterns Research Articles

Effect of strain geometry on the petrophysical properties of plastically deformed aggregates: experiments on Solnhofen limestone

Abstract Specimens of Solnhofen limestone were deformed under conditions where calcite deforms plastically using four experimental configurations: extension, torsion, direct shear and axisymmetric shortening. All experiments were run on dry specimens at the same temperature (600 °C), confining pressure (200 MPa) and comparable strain rates ( c. 10 −4 s −1 ). The different experimental settings and the heterogeneity of deformation within some of the specimens provided a large range of strain geometries. They allowed locally imposed strain geometries to be related to the crystallographic preferred orientation (CPO) patterns of calcite and the orientation of the shape fabric of calcite grains. CPO in calcite was measured using electron back-scattered diffraction (EBSD) in scanning electron microscopy. The development of CPO during deformation under the dominance of intracrystalline plasticity contains information about the strain geometry accumulated in rocks in 3D, although in nature the strain geometry can be modified by dynamic recrystallization that was not seen in the experiments. The different CPO patterns have a significant effect on the velocity structure of the deformed aggregates. Seismic properties inferred from CPO show that the orientation of the fastest V p wave aligns with principal strain directions that are not equivalent in different strain geometries.

The formation mechanism of garnet porphyroblast in quartz schist, Namche Barwa, east Himalaya

The quartz schist in the Indus-Yarlung Tsangpo suture zone, from Namche Barwa, east Himalaya undergone extensive mylonization with well developed, foliation and lineation; its S-C fabric, "σ" and "δ" type porphyroblasts and asymmetrical folds indicate the northwestward normal slip ductile shear deformation. The electron backscattered diffraction (EBSD) data, lattice preferred orientation (LPO) patterns for quartzes in the matrix foliation (or external foliation, S) and inclusion-trails in the rim of the garnet porphyroblasts (S) both show a top-to-NW (normal) shear sense. While LPO patterns for quartzes of the inclusion-trails in the garnet porphyroblast cores (S) show an opposite shear sense (top-to-SE). The garnet compositional zonation indicates a growth zoning characteristic from energy dispersive spectroscopy (EDS) analysis. By connecting the inclusion-trails in garnet cores, an initial foliation trace with asymmetrical fold shape can be obtained. It suggests that these garnet porphyroblasts with snov, ball structure formed on the asymmetrical folds by the rotation of external shear sense rather than the rotation of garnet porphyroblasts. Gamet porphyroblast with snowball structure proposed that the rocks from the lndus Yarlung Tsangpo suture zone west side of the Namche Barwa syntaxis have undergone southeastward thrusting and later followed by northwestward normal slipping.

The earliest mantle fabrics formed during subduction zone infancy

Harzburgites obtained from the oldest crust–mantle section in the Philippine Sea plate (∼52 Ma) along the landward slope of the southern Izu–Ogasawara Trench, preserve mantle fabrics formed during the infancy of the subduction zone; that is during the initial stages of Pacific plate subduction beneath the Philippine Sea plate. The harzburgites have relatively fresh primary minerals despite of their heavy serpentinizations, and show inequigranular interlobate textures, and crystal preferred orientation patterns in olivine (001)[100] and Opx (100)[001]. The harzburgites have the characteristics of residual peridotites, whereas the dunites, obtained from the same location as the harzburgites, provide evidence for the earliest stages of arc volcanism during the inception of subduction. We propose that the (001)[100] olivine patterns began forming in immature fore-arc mantle with an increase in slab-derived hydrous fluids during the initial stages of subduction in in situ oceanic island arc.

The effects of Dissolution–Precipitation Creep on quartz fabrics within the Purgatory Conglomerate, Rhode Island

Quartz Crystallographic Preferred Orientation (CPO) patterns are most commonly a result of deformation by dislocation creep. We investigated whether Dissolution–Precipitation Creep (DPC), a process that occurs at lower differential stresses and temperatures, may result in CPO in quartz. The Purgatory Conglomerate is part of the SE Narragansett basin where strain intensity increases from west to east and is associated with top-to-the-west transport and folding during the Alleghanian orogeny. Within the Purgatory Conglomerate, DPC led to quartz dissolution along cobble surfaces perpendicular to the shortening direction, and quartz precipitation in overgrowths at the ends of the cobbles (strain shadows), parallel to the maximum extension direction. Quartz c-axis orientations as revealed by Electron Backscatter Diffraction (EBSD) methods were random in all analyzed domains within the cobbles and strain shadows irrespective of the intensity of strain or metamorphic grade of the sample. Quartz dissolution probably occurred exclusively along the cobbles' margins, leaving the remaining grains unaffected by DPC. The fact that quartz precipitated in random orientations may indicate that the strain shadows were regions of little or no differential stress.

Application of GIS methods to retrieve orientation patterns from imagery; a case study from Beds I and II, Olduvai Gorge (Tanzania)

The role of natural processes in the formation of the Bed I and II sites at Olduvai Gorge (Tanzania) remains the subject of much debate, five decades after their original excavation. Preferred orientation of items is indicative of water disturbance, but compass bearings were not available in Mary Leakey's (1971) study. Using GIS methods, we have vectorized maps from 1960s excavations at Olduvai, and applied a range of statistical techniques to calculate items strike. The GIS analysis suggests strongly preferred orientation patterns in Bed I, and more heterogeneous angular histograms in Bed II. Two complementary lines of evidence support these results. Firstly, modelling of potential mapping errors provides a large interval of confidence for preferred orientation patterns. Second, the GIS study was extended to photographs of the earlier excavations and to maps from recent fieldwork at Olduvai; both yielded patterned arrangement of lithics and bones, and are consistent with results based on the analysis of the 1960s maps, i.e. a large number of the Bed I and II assemblages are preferentially oriented. These results highlight the potential of GIS applications to the analysis of imagery in Stone Age studies, and bear important implications for the understanding of the role of natural agents in site formation processes at Olduvai.

Anisotropic poroelasticity and the response of faulted rock to changes in pore-fluid pressure

Abstract The Law of Effective Stress has found wide application in structural geology, rock mechanics and petroleum geology. The commonly used form of this law relies on an assumption of isotropic porosity. The porosity in and around fluid-saturated fault zones is likely to be dominated by tectonically induced cracks of various shapes and sizes. Previously published field and laboratory data show that these cracks occur in distinct patterns of preferred orientation, and that these patterns vary around the fault zone. This paper uses the more general form of the Law of Effective Stress which incorporates anisotropic poroelasticity to model the geomechanical response of fault zones surrounded by patterns of oriented cracks. Predictions of fault stability in response to fluid pressure changes are shown to depend on both the nature (or symmetry) of the crack pattern and the orientation of the crack patterns with respect to the in situ stress. More complete data on the porosity of natural fault zones will enable more accurate predictions of fault stability in the subsurface.

Strain partitioning in the mid-crust of a transpressional shear zone system: Insights from the Homestake and Slide Lake shear zones, central Colorado

Kinematic analysis and field mapping of the Homestake shear zone (HSZ) and Slide Lake shear zone (SLSZ) in central Colorado may provide insight into the interaction between subvertical and low-angle shear zones in the middle crust. The northeast-striking, steeply dipping HSZ comprises a ∼10-km-wide set of anastomosing ductile shear zones and pseudotachylyte-bearing faults. Approximately 4 km south of the HSZ, north–northeast-striking, shallowly dipping mylonites of the SLSZ form three 1–10-m-thick splays. Oblique stretching lineations and shear sense in both shear zones record components of dip-slip (top-up-to-the-northwest and top-down-to-the-southeast) and dextral strike-slip movement during mylonite development. Quartz and feldspar deformation mechanisms and quartz [c] axis lattice preferred orientation (LPO) patterns suggest deformation temperatures ranging from ∼280–500 °C in the HSZ to ∼280–600 °C in the SLSZ. Quartz [c] axis LPOs suggest plane strain general shear across the shear system. Based on the relative timing of fabric development, compatible kinematics and similar deformation temperatures in the SLSZ and the HSZ, we propose that both shear zones formed during strain localization and partitioning within a transpressional shear zone system that involved subvertical shuffling in the mid-crust at 1.4 Ga.

Nanometer Scale Microstructure and Microtexture of Biological Materials Revealed by High Spatial Resolution (15 to 5 kV) EBSD

High resolution EBSD analysis was carried out under specific experimental conditions (15 to 5 kV) on the skeleton of the modern carbonate brachiopod Gryphus vitreus and resolved nano- to microscale preferred crystallographic orientation patterns undetcted so far. As biologic superstructures are formed by controlled nanoparticle assembly it is essential to resolve their internal structure and texture with the highest possible spatial resolution. Low kV EBSD (15 kV and at 5 kV) provides the required resolution. We observe in the investigated carbonate skeletons a strongly interlocking microstructure of concave/convex grains. The interface topology of the interdigitating structure reaches below the micrometer scale. Individual grains reach sizes up to 20 µm (or even more) in one dimension. They show a mosaic spread of several degrees such that they must be addressed as mesocrystals. Even though the shell consists of three different microstructures with completely different crystal morphologies and grain boundary topologies the crystallographic texture of the three layers is similar. This indicates that distinct control mechanisms prevail when the shell is formed.

Subduction related antigorite CPO patterns from forearc mantle in the Sanbagawa belt, southwest Japan

Antigorite (Atg) is stable throughout large parts of the wedge mantle of most subduction zones. Atg shows strong acoustic anisotropy and crystallographic preferred orientation (CPO) patterns of this mineral may contribute significantly to seismic anisotropy in convergent margins. Atg CPO patterns from the Higashi-Akaishi (HA) forearc mantle body of southwest Japan adds to the data set suggesting the most common Atg CPO pattern has a c-axis perpendicular to the foliation and a b-axis parallel to the stretching lineation. Statistical analysis using the eigenvector method of Atg CPO from two mutually perpendicular directions in the same sample ( YZ-section and XZ-section) shows no significant differences implying sample preparation has no significant affect on the resulting Atg CPO. Reuss (uniform stress) averages of anisotropy for the Higashi-Akaishi samples are approximately treble the values for Voigt (uniform strain) averages. When comparing calculated anisotropy of hydrated mantle peridotite samples—such as the Higashi-Akaishi unit—with observed S-wave delay times in convergent margins, the appropriate averaging method needs to be considered.

Deformation in the lowermost mantle: From polycrystal plasticity to seismic anisotropy

In the deep earth, deformation occurs at many scales: large-scale convection produces subduction of slabs and upwelling of plumes in the mantle. At the high temperature/high pressure conditions, strain is accommodated through crystal plasticity, either by diffusion or the movement of dislocations. Slip causes crystal rotations and thus produces a characteristic pattern of crystal preferred orientation and corresponding anisotropy of physical properties at the macroscopic scale. In this study we use polycrystal plasticity, with experimentally derived deformation mechanisms for perovskite, post-perovskite and magnesiowuestite, to predict texture development along streamlines in a 2D geodynamic convection model of the lowermost mantle. Strong preferred orientation develops during subduction and upwelling, while during spreading along the core–mantle boundary the orientation pattern is relatively stable. From preferred crystal orientation and single crystal elastic properties, bulk elastic properties can be calculated and compared with seismic observations. Post-perovskite with predominant (001) slip and magnesiowuestite with {110} and {111} slip produce anisotropy patterns which are consistent with observed anisotropy, i.e. fast S-waves polarized parallel to the core–mantle boundary and anti-correlation between P and S-wave anisotropies. In contrast, perovskite with dominant (001) slip and two post-perovskite models with dominant slip on (010) and (100) produce anisotropy patterns which are inconsistent with seismic observations.

Obliteration of olivine crystallographic preferred orientation patterns in subduction-related antigorite-bearing mantle peridotite: an example from the Higashi–Akaishi body, SW Japan

Abstract Large parts of the mantle wedge near subduction boundaries are likely to be hydrated and contain antigorite. This mineral is acoustically highly anisotropic and potentially has a strong influence on seismic properties of the wedge. The Higashi–Akaishi body of SW Japan is an exhumed sliver of partially serpentinized forearc mantle, ideal for studying the effects of antigorite on the development of tectonic fabrics in the mantle. Samples with less than 1% antigorite show strong B-type olivine crystallographic preferred orientation (CPO) patterns. In contrast, samples with >10% antigorite deformed during the same tectonic event show much weaker olivine CPO patterns lacking the flow-normal a -axis concentration. These microstructural data suggest that the development of antigorite during deformation weakens olivine CPO due to phase boundary slip and associated rigid-body rotation of olivine grains. Antigorite and similar sheet silicates are likely to be present to some extent in the mantle wedge of all convergent margins. Our results suggest that even if this amount is only a few percent, strong olivine CPO is unlikely to develop and any pre-existing CPO is likely to be destroyed. Under these conditions, olivine CPO is unlikely to contribute significantly to seismic anisotropy in the mantle wedge.

Natural annealing of dynamically recrystallised quartzite fabrics: Example from the Cévennes, SE French Massif Central

Quartzite samples from the SW-Cévennes (French Massif Central) have been investigated in order to reveal the effects of annealing on a previously dynamically recrystallised quartz fabric. The studied quartzite is interlayered with turbiditic micaschist series, and after regional deformation the whole sequence was intruded by the St-Guiral granodiorite at 1–3 kbar and up to 650–700 °C. Recorded crystallographic preferred orientation (CPO) patterns of the dynamically recrystallised quartzite are classified as type I with a monoclinic symmetry and type II with an orthorhombic symmetry. Type I is related to simple shear-dominated deformation and type II to apparent constriction. These two fabric types are consistent with structural observation that a constrictive and coaxial deformation regime controlled the formation of non-cylindrical and refolded folds. In the metamorphic aureole, which produced essentially static annealing, CPO patterns intensify slightly and grain size increases towards the granodiorite until the temperature at which inversion from low- to high quartz is triggered. This inversion caused a volume increase and subsequent intergranular stresses activated slip systems fitting the temperature and water content. This study shows that annealing of dynamically recrystallised quartz crystals can activate previously inactive slip systems, reactivate slip systems and amplify older CPO fabrics. This conclusion identifies limits for the use of annealed quartz fabrics in reconstructing structural histories when earlier dynamic recrystallisation has occured.

Role of extrusion of the Rand and Sierra de Salinas schists in Late Cretaceous extension and rotation of the southern Sierra Nevada and vicinity

The Rand and Sierra de Salinas schists of southern California were underplated beneath the southern Sierra Nevada batholith and adjacent Mojave-Salinia region along a shallow segment of the subducting Farallon plate in Late Cretaceous time. Various mechanisms, including return flow, isostatically driven uplift, upper plate normal faulting, erosion, or some combination thereof, have been proposed for the exhumation of the schist. We supplement existing kinematic data with new vorticity and strain analysis to characterize deformation in the Rand and Sierra de Salinas schists. These data indicate that the schist was transported to the SSW from deep to shallow crustal levels along a mylonitic contact (the Rand fault and Salinas shear zone) with upper plate assemblages. Crystallographic preferred orientation patterns in deformed quartzites reveal a decreasing simple shear component with increasing structural depth, suggesting a pure shear dominated westward flow within the subduction channel and localized simple shear along the upper channel boundary. The resulting flow type within the channel is that of general shear extrusion. Integration of these observations with published geochronologic, thermochronometric, thermobarometric, and paleomagnetic studies reveals a temporal relationship between schist unroofing and upper crustal extension and rotation. We present a model whereby trench-directed channelized extrusion of the underplated schist triggered gravitational collapse and clockwise rotation of the upper plate.

Microstructure of Horseshoe Nails Using Neutron Diffraction

Neutron diffraction allows nondestructive testing of the bulk microstructure of mechanical components. The microstructures of horseshoe nails made through three different processes have been explored as a function of position along the nail. Despite all nails being made of similar plain low-carbon steel and being process annealed after manufacture, the microstructures are far from the same. Nails made from strip, using a cold forging stamping process, show narrower diffraction peaks indicating a narrower distribution of lattice parameters and also show diffraction peak intensity ratios closer to those expected for unstrained steel. Thus, the distribution of the orientation of grains in these nails is closer to that of undistorted steel compared to nails made through the other two processes considered—one a drawing from wire and the other a combination of rolling and cold forging. The blades of the drawn nails showed little preferred orientation but the converse was true in the heads. Differing patterns of preferred orientation suggest that the various manufacturing approaches result in substantially different mechanical advantages for the three types of nails, a result in accord with mechanical testing.

Deformation of lower-mantle ferropericlase (Mg,Fe)O across the electronic spin transition

Recent high-pressure studies have shown that an electronic spin transition of iron in ferropericlase, an expected major phase of Earth's lower mantle, results in changes in its properties, including density, incompress- ibility, radiative thermal conductivity, electrical conduc- tivity, and sound velocities. To understand the rheology of ferropericlase across the spin transition, we have used in situ radial X-ray diffraction techniques to examine ferro- periclase, (Mg0.83,Fe0.17)O, deformed non-hydrostatically in a diamond cell up to 81 GPa at room temperature. Compared with recent quasi-hydrostatic studies, the range of the spin transition is shifted by approximately 20 GPa as a result of the presence of large differential stress in the sample. We also observed a reduction in incompressibility and in the unit cell volume of 3% across the spin transition. Our radial X-ray diffraction results show that the {0 0 1} texture is the dominant lattice preferred orientation in ferropericlase across the spin transition and in the low-spin state. Viscoplastic self-consistent polycrystal plasticity simulations suggest that this preferred orientation pattern is produced by {1 1 0}\1-10( slip. Analyzing our radial X-ray diffraction patterns using lattice strain theory, we evaluated the lattice d-spacings of ferropericlase and Mo as a function of the w angle between the compression direc- tion and the diffracting plane normal. These analyses give the ratio between the uniaxial stress component (t) and the shear modulus (G) under constant stress condition, which represents a proxy for the supported differential stress and elastic strength. This ratio in the mixed-spin and low-spin states is lower than what is expected from previous studies of high-spin ferropericlase, indicating that the spin transi- tion results in a reduced differential stress and elastic strength along with the volume reduction. The influence of the spin transition on the differential stress and strength of ferropericlase is expected to be less dominant across the wide spin transition zone at high pressure-temperature conditions relevant to the lower mantle.

Dauphiné twinning and texture memory in polycrystalline quartz. Part 3: texture memory during phase transformation

Samples of quartz-bearing rocks were heated above the a (trigonal)-b (hexagonal) phase transformation of quartz (625-950� C) to explore changes in preferred orientation patterns. Textures were measured both in situ and ex situ with neutron, synchrotron X-ray and electron backscatter diffraction. The trigonal-hexagonal phase transformation does not change the orientation of c- and a-axes, but positive and negative rhombs become equal in the hexagonal b-phase. In naturally deformed quartzites measured by neutron diffraction a perfect texture memory was observed, i.e. crystals returned to the same trigonal orientation they started from, with no evidence of twin boundaries. Samples measured by electron back-scattered diffraction on surfaces show considerable twinning and memory loss after the phase transformation. In experi- mentally deformed quartz rocks, where twinning was induced mechanically before heating, the orientation memory is lost. A mechanical model can explain the memory loss but so far it does not account for the persis- tence of the memory in quartzites. Stresses imposed by neighboring grains remain a likely cause of texture mem- ory in this mineral with a very high elastic anisotropy. If stresses are imposed experimentally the internal stresses are released during the phase transformation and the material returns to its original state prior to deformation. Similarly, on surfaces there are no tractions and thus tex- ture memory is partially lost.

Large strain shearing of halite: Experimental and theoretical evidence for dynamic texture changes

We report results from torsion experiments on polycrystalline halite (NaCl) to shear strains γ = 8 and observe a very complex texture evolution. The same behavior was reproduced with polycrystal plasticity simulations, suggesting that we capture the underlying mechanisms. While crystal shapes gradually rotate into the shear plane, crystal orientations change continuously and dynamic texture patterns evolve with increasing shear. This is highly significant for ultra-large deformation, as for example implied from geodynamic modeling for the deep earth, where seismic anisotropy patterns may develop and locally disappear again as material is deformed during convection. The study also suggests that caution is required when interpreting deformation mechanisms from simple shear preferred orientation patterns.

U-stage and EBSD technique as complementary methods

In the last years, the electronic microscopy has become an important tool to textural analyses of mineral aggregates. The electron backscattering diffraction (EBSD) has been used to determine preferred orientation patterns quickly in areas statistically representative. However, when the grain boundary geometry and their orientations are considered, there is not a way to determine the grain boundary orientation individually for EBSD, what is possible using U-stage mounted on a petrographic microscope. The geometry and grain boundary orientation pattern constitute important microstructural and textural parameters. They are directly related to deformational processes occurred during evolution and stabilization of mineral aggregates, as well as their physical properties. Therefore, when the microfabric is characterized is necessary, besides a complete characterization of crystallographic preferred orientation, to have individual control of geometry and grain boundary segments orientation. In this sense, the use of U-stage, although there are restrictions, is fundamental. This method allows the determination of grain boundary plane orientation, individually, while the EBSD permit a global analyze related to misorientation studies between those grains that share a boundary plane. Therefore, the methodology to characterization of grain boundary orientation patterns is presented. The referred methods are applied as complementary techniques, what allows the characterization of 5 degrees of freedom necessary to determine a boundary surface: the grain boundary plane orientation and the distribution of angle/axes misorientation pair. The data were obtained in quartz aggregates, which came from banded iron formation, in Quadrilátero Ferrífero (MG)

Crystallographic preferred orientation of akimotoite and seismic anisotropy of Tonga slab

The mineral akimotoite, ilmenite-structured MgSiO(3), exists at the bottom of the Earth's mantle transition zone and within the uppermost lower mantle, especially under low-temperature conditions. Akimotoite is thought to be a major constituent of the harzburgite layer of subducting slabs, and the most anisotropic mineral in the mantle transition zone. It has been predicted that if akimotoite crystals are preferentially oriented by plastic deformation, a cold subducted slab would be extremely anisotropic. However, there have been no studies of crystallographic preferred orientations and very few reports of plastic deformation experiments for MgSiO(3) ilmenite. Here we present plastic deformation experiments on polycrystalline akimotoite, which were conducted at confining pressures of 20-22 GPa and temperatures of 1,000-1,300 degrees C. We found a change in crystallographic preferred orientation pattern of akimotoite with temperature, where the c-axis maximum parallel to the compression direction develops at high temperature, whereas the c axes are preferentially oriented parallel to the shear direction or perpendicular to the compression direction at lower temperature. The previously reported difference in compressional-wave seismic anisotropy between the northern and southern segments of the Tonga slab at depths of the mantle transition zone can conceivably be attributed to the difference in the crystallographic preferred orientation pattern of akimotoite at varying temperature within the slab.

Strain localization in direct shear experiments on Solnhofen limestone at high temperature – Effects of transpression

Some features of natural shear zones formed under non-coaxial strain geometries, including some effects of transpression, can be simulated in the laboratory by using the direct shear experimental configuration. Slices of ∼1 mm thick Solnhofen limestone were deformed in direct shear between two stronger forcing blocks of cores of Tennessee sandstone pre-cut at 45° to the cylinder axis. Experiments were run dry at 600 °C, 200 MPa confining pressure and bulk shear strain rates of ∼5 × 10 −3 s −1, at which conditions Solnhofen limestone deformed by dislocation creep with a stress exponent of 4.7. When loaded, strain concentrates in the limestone band, producing non-coaxial deformation as one pre-cut block slides past the other. The orientation and intensity of the shape fabric developed in calcite grains indicate that strain is heterogeneous across the specimen, with the formation of two high-strain shear bands close to the limestone–sandstone interface, separated by a central zone of low strain. Crystallographic preferred orientation patterns in the calcite grains measured by electron backscatter diffraction are consistent with a switch in deformation geometry from flattening-dominated in the middle of the specimen towards shear-dominated in the high-strain bands. From tests on thin slices of the same material compressed axisymmetrically (without shearing) normal to the layer, heterogeneous thinning of the slice develops, from a maximum in the centre of the slice to zero at the edges. The formation of the paired shear zones observed in the sheared experiments is interpreted in terms of superposed strain fields, with shearing in the centre of the slice being inhibited by the strain hardening that accompanies the higher flattening strain in the centre of the specimen.