Shape Preferred Orientation Research Articles

In situ high-pressure and high-temperature X-ray microtomographic imaging during large deformation: A new technique for studying mechanical behavior of multiphase composites

We have examined the microstructural evolution of a two-phase composite (olivine + Fe-Ni-S) during large shear deformation, using a newly developed high-pressure X-ray tomography microscope. Two samples were examined: a load-bearing framework–type texture, where the alloy phase (Fe-Ni-S) was present as isolated spherical inclusions, and an interconnected network–type texture, where the alloy phase was concentrated along the silicate grain boundaries and tended to form an interconnected network. The samples, both containing ∼10 vol% alloy inclusions, were compressed to 6 GPa, followed by shear deformation at temperatures up to 800 K. Shear strains were introduced by twisting the samples at high pressure and high temperature. At each imposed shear strain, samples were cooled to ambient temperature and tomographic images collected. The three-dimensional tomographic images were analyzed for textural evolution. We found that in both samples, Fe-Ni-S, which is the weaker phase in the composite, underwent significant deformation. The resulting lens-shaped alloy phase is subparallel to the shear plane and has a laminated, highly anisotropic interconnected weak layer texture. Scanning electron microscopy showed that many alloy inclusions became film-like, with thicknesses <1 μm, suggesting that Fe-Ni-S was highly mobile under nonhydrostatic stress, migrated into silicate grain boundaries, and propagated in a manner similar to melt inclusions in a deforming solid matrix. The grain size of the silicate matrix was significantly reduced under large strain deformation. The strong shape-preferred orientation thus developed can profoundly influence a composite's bulk elastic and rheological properties. High-pressure–high temperature tomography not only provides quantitative observations on textural evolution, but also can be compared with simulation results to derive more rigorous models of the mechanical properties of composite materials relevant to Earth's deep mantle.

Biases in three-dimensional vorticity analysis using porphyroclast system: limits and application to natural examples

Abstract A description of the systematic errors associated with the measurement of the vorticity number from poryhroclasts in natural systems is presented and discussed. We show that strong biases and systematic errors could derive both from some erroneous physical (i.e. no slip across clast/matrix boundary, homogeneity within the matrix) as well as geometrical assumptions (i.e. the radius ratio and angular measurements carried out in two dimensions on outcrop surfaces and thin section). By comparing natural datasets of porphyroclast shape preferred orientation (SPO) with different theoretical curves plots, we suggest that at least one of the Jeffery physical assumptions can be tested when applying vorticity techniques. The comparison of different possible sources of systematic errors indicates that, for medium-to-low vorticity numbers ( W m &lt;0.8), vorticity data are strongly biased and that a minimum systematic error of 0.2 should be taken into account. Finally, we use data from natural shear zones from the Southern Variscan Belt in Sardinia to test and discuss the starting assumptions of the Jeffery model.

Characterization of microstructures and interpretation of flow mechanisms in naturally deformed, fine-grained anhydrite by means of EBSD analysis

Abstract Anhydrite-rich layers within foreland fold and thrust belts are frequently observed to be the weakest horizon of the sequence. Characterizing the microstructure of anhydrite is therefore important for interpreting the larger-scale deformation history of these rocks. Two microstructures from naturally deformed, fine-grained (&lt;15 µm mean grain size) anhydrite samples from the Triassic Evaporites of the Umbria–Marche Apennines, Italy were analysed using electron backscatter diffraction (EBSD). Microstructural observations, misorientation analysis and crystallographic preferred orientation (CPO) determination were carried out on these samples. Both samples have a CPO characterized by alignment of 〈001〉 and distribution of 〈100〉 and 〈010〉 on a great circle normal to this. This anhydrite 〈001〉 ‘fibre texture’ has not been described before. Microstructure A is characterized by a moderate to weak CPO and a weak shape preferred orientation at 55° to 70° from the trace of the 〈001〉 maximum. Low-angle boundaries are revealed by misorientation analysis. A change in grain size from c . 10 to c . 7 µm corresponds to reduction in strength of CPO and reduction in the number of low-angle grain boundaries. Microstructure B is characterized by a very strong CPO. The orientation of the CPO changes between different microstructural domains. The 〈001〉 maximum is always perpendicular to the trace of a strong grain elongation and high-angle grain boundaries have misorientations close to 〈001〉, suggesting that the CPO is geometrically controlled: anhydrite grains are platy with 〈001〉 short axes. The origin of the CPO is therefore unclear but it need not relate to dislocation creep deformation. Whether or not CPO relates to dislocation creep, both samples have a high number of lower-angle grain boundaries and internal grain distortions with 〈010〉 and 〈001〉 misorientation axes. These are indicative of dislocation activity and the data are best explained by slip on either (100)[010] (dominant) and (001)[100] or a combination of these. Neither of these slip systems has been recognized before. Both microstructures are interpreted to have undergone dynamic recrystallization, and the weakening of the CPO with decreasing grain size in microstructure A is suggested to be indicative of a grain-boundary sliding mechanism becoming active. Comparison with experimental data shows that creep mechanisms involving dislocations at the observed grain sizes require the differential stress magnitudes driving deformation to be greater than c . 100 MPa.

The emplacement mechanisms and growth styles of the Guposhan-Huashan batholith in western Nanling Range, South China

The Guposhan-Huashan batholith was emplaced during transition from Tethysian tectonic regime to Pacific tectonic regime in the Middle-Late Jurassic. The regional EW-striking Yishan-Quannan basement deep fault zone, associated with the Tethysian regime, is thought likely to control the formation of the batholith. The development of the batholith was, however, achieved when the Pacific tectonic extensional regime dominated. The batholith consists of seven individual intrusive units that were emplaced subsequently over a 15 m.y. span. Deformation structures and strain analysis of the intrusive units and their country rocks suggest the existence of active and passive emplacement mechanisms. For example, shape-preferred orientation of dioritic enclaves and megacrystic K-spars within earlier units and the development of rim synforms resulted from active mechanisms. It is estimated that active expansion and structural propagation each contributed approximately 27%, and stoping accounted for 46% of the space for the entire batholith. The active emplacement mechanisms produced earlier units at deeper levels of the crust and followed by passive emplacement mechanism at shallower crustal levels when the tectonic regime changed from compression to extension. Each intrusive unit shows a different growth mode. Niumiao and Yangmeishan intrusions show a polarized lateral growth mode, the main-phase Lisong and Wanggao intrusions show either a central growth mode or an eccentric growth mode, and the youngest Baishuidai and Huamei units have a polarized multi-point central growth mode. Both Guposhan and Huashan plutons show a concentric growth zoning pattern with the former built in an outward growth style and the latter an inward growth style. The well-preserved circular shape of both plutons is indicative of relatively stationary magma feeders, suggesting that any considerable horizontal displacement in the region was unlikely even if the area was in an intracontinental strike-slip tectonic setting during the last emplacement phases of the batholith.

When an ‘inverse’ fabric is not inverse: an integrated AMS‐SPO study in MORB‐like dykes

Terra Nova, 23, 49–55, 2011AbstractMagma flow direction is recorded in the macroscopic mineral petrofabric and magnetic fabric of the MORB‐like Rooi Rand dyke swarm. The bulk magnetic fabric is sub‐parallel to the dyke plane, resulting from preferred orientation of grains during magma flow. This is referred to as type‐A fabric. A study of mineral shape preferred orientation reveals an additional fabric, which is orthogonal to the average dyke orientation in 30% of the data. This fabric is carried by both plagioclase and opaque grains and would typically be interpreted as ‘inverse’. It is referred to here as type‐B fabric. However, because it is carried by the shape preferred orientation of macroscopic grains and is in most cases not coaxial to the anisotropy of magnetic susceptibility fabric, it is interpreted as having been acquired during increased grain interaction during late‐stage magma flow associated with decreasing magma pressure.

Bowing of dimensional granitic stones

Bowing is a well-known phenomenon seen in marbles used as building veneers. This form of rock weathering occurs as a result of external factors such as temperature, humidity, the system for anchoring the marble slabs or the panel dimensions. Under the same external conditions, many factors will determine the degree of deformation including petrography, thermal properties and residual locked stresses. The usual way to solve the problem of bowed marble slabs is to replace them with other materials, such as granites, in which the deformation still exists but is less common. In this study, eight ornamental granites with different mineralogy, grain size, grain shape, porosity and fabric were tested in a laboratory to assess their susceptibility to bowing. Three slabs of granite, each cut with a different orientation, were studied under different conditions of temperature (90 and 120°C) and water saturation (dry and wet) to investigate the influence of these factors together with that of anisotropy. At 90°C, only the granite with the coarsest grain size and low porosity exhibited deformation under wet conditions. At 120°C and wet conditions, three of the granites showed evident signs of bowing. Again, the granite with the coarsest grain size was the most deformed. It was concluded that the wide grain size distribution influences microcracking more than other expected factors, such as the quartz content of the rock. Also, mineral shape-preferred orientation and porosity play an important role in the bowing of the studied granites.

Effects of lattice preferred orientation and retrogression on seismic properties of eclogite

We report here lattice preferred orientations (LPOs) and seismic properties of eclogites from the Sulu (苏鲁) UHP terrane. Our results show strong fabrics in omphacite and amphibole, and approximately random fabrics in garnet with or without strong shape preferred orientations (SPOs). Dislocation creep is likely to be responsible for the observed omphacite fabrics that vary with geometry and orientation of finite strain ellipsoid. Weak garnet LPOs suggest that garnet did not accommodate plastic strain or was not deformed by dislocation creep with a dominant slip system. The calculated seismic properties of eclogites and their component minerals show a strong correlation with their LPOs. Seismic anisotropies are mostly induced by omphacite component in fresh eclogites and by amphibole component in retrograded eclogites, respectively. Retrogression of omphacite to amphibole and quartz will increase seismic anisotropies but decreases seismic velocities of eclogite. Garnet component in- creases the seismic velocities but decreases seismic anisotropies of eclogite. Comparison of the calcu- lated and the measured seismic properties of eclogites suggests that both methods resolve comparable results with some discrepancies. Compositional layering can play a very important role in determining the seismic properties of eclogites in addition to LPO.

Influence of shape fabric and crystal texture on marble degradation phenomena: simulations

Microstructure-based finite element simulations were used to study the influence of grain shape fabric and crystal texture on thermoelastic responses related to marble degradation phenomena. Calcite was used as an illustrative example for studying extremes of shape preferred orientation (SPO) in shape fabric and lattice preferred orientation (LPO) in crystal texture. Three SPOs were analyzed: equiaxed grains, elongated grains, and a mixture of equiaxed and elongated grains. Three LPOs were considered: a random orientation distribution function and two degrees of strong directional crystal texture. Finally, the correlation between the direction of the LPO with respect to that of the SPO was examined. Results show that certain combinations of SPO, LPO, and their directional relationship have significant influence on the thermomechanical behavior of marble. For instance, while there is no major dependence of the elastic strain energy density and the maximum principal stress on SPO for randomly textured microstructures, there is a strong synergy between LPO and its directional relationship with respect to the SPO direction. Microcracking precursors, elastic strain energy density, and maximum principal stress, decrease when the crystalline c-axes have fiber texture perpendicular to the SPO direction, but increase significantly when the c-axes have fiber texture parallel to the SPO direction. Moreover, the microstructural variability increases dramatically for these latter configurations. In general, the influence of LPO was as expected, namely, the strain energy density and the maximum principal stress decreased with more crystal texture, apart from for the exception noted above. Spatial variations of these precursors indicated regions in the microstructure with a propensity for microcracking. Unexpectedly, important variables were the microstructural standard deviations of the spatial distributions of the microcracking indicators. These microstructural standard deviations were as large as or larger than the variables themselves. The elastic misfit-strain contributions to the coefficients of thermal expansion were also calculated, but their dependence was as expected.

Smectite swelling in the Miura–Boso accretionary prism: Possible cause for incipient décollement zone formation

An analysis of the mineralogy of the Miura–Boso accretionary prism, which consists of off-scraped upper Miocene strata exposed on the southern parts of the Miura and Boso peninsulas in Central Japan, revealed homogeneous mineral composition throughout all levels of silt layers hosting deformation structures associated with sediment accretion. XRD analyses showed that the sediment consists of randomly ordered mixed layer illite–smectite (I–S) with low illite content (> 40 wt.%), quartz (30 wt.%), illite (10–20 wt.%), plagioclase (10 wt.%), kaolinite (+ chlorite) (5 wt.%), and calcite (0–10 wt.%). The observed mineralogical assemblage indicates that location of the décollement formation was not constrained by mechanical heterogeneities associated with mineralogical variations, but was controlled by spatial variations in effective pressure. Within the over-consolidated sediment located above the incipient décollement zone, the occurrence of abundant smectite with a shape-preferred orientation gave rise to smectite swelling, producing pressures in the order of ~ 3.0 MPa. The combined effect of high swelling pressure and high pore pressure resulted in an overpressured zone at the base of the over-consolidated zone, facilitating the development of a décollement zone within the prism.

Lattice-preferred orientations of late-Variscan granitoids derived from neutron diffraction data: implications for magma emplacement mechanisms

The lattice-preferred orientation (LPOs) of two late-Variscan granitoids, the Meissen monzonite and the Podlesí dyke granite, were determined from high-resolution time-of-flight neutron diffraction patterns gained at the diffractometer SKAT in Dubna, Russia. The results demonstrate that the method is suitable for the LPO analysis of polyphase, relatively coarse-grained (0.1–6 mm) rocks. The Meissen monzonite has a prominent shape-preferred orientation (SPO) of the non-equidimensional minerals feldspar, mica and amphibole, whereas SPO of the Podlesí granite is unapparent at the hand-specimen scale. The neutron diffraction data revealed distinct LPOs in both granitoids. The LPO of the non-equidimensional minerals feldspar, mica and amphibole developed mainly during magmatic flow. In the case of the Meissen monzonite, the magmatic flow was superimposed by regional shear tectonics, which, however, had no significant effect on the LPOs. In both samples, quartz shows a weak but distinct LPO, which is atypical for plastic deformation and different in the syn-kinematic Meissen monzonite and the post-kinematic Podlesí granite. We suggest that, first of all, the quartz LPO of the Meissen monzonite is the result of oriented growth in an anisotropic stress field. The quartz LPO of the Podlesí granite, which more or less resembles a deformational LPO in the flattening field of the local strain field, developed during magmatic flow, whereby the rhombohedral faces of the quartz crystals adhered to the (010) faces of aligned albite and to the (001) faces of zinnwaldite. Due to shape anisotropy of their attachments, the quartz crystals were passively aligned by magmatic flow. Thus, magmatic flow and oriented crystal growth are the major LPO-forming processes in both granitoids. For the Meissen monzonite, the solid-state flow was too weak to cause significant crystallographic re-orientation of the minerals aligned by magmatic flow. Finally, the significance of our results for the evaluation of the regional tectonic environment during magma emplacement is discussed. The discussion on the regional implications of the more methodologically oriented results provides the basis for future, more regionally aimed studies in view of the fabric characteristics of such plutons and their developing mechanisms.

Anisotropic stratification beneath Africa from joint inversion of SKS and P receiver functions

The analysis of rock anisotropy revealed by seismic waves provides fundamental constraints on stress‐strain field in the lithosphere and asthenosphere. Nevertheless, the anisotropic models resolved for the crust and the upper mantle using seismic waves sometimes show substantial discrepancies depending on the type of data analyzed. In particular, at several permanent stations located in Africa, previous studies revealed that the observations of SKS splitting are accounted for by models with a single and homogeneous anisotropic layer whereas 3‐D tomographic models derived from surface waves exhibit clear anisotropic stratification. Here we tackle the issue of depth‐dependent anisotropy by performing joint inversion of receiver functions (RF) and SKS waveforms at four permanent broadband stations along the East African Rift System (EARS) and also on the Congo Craton. For three out of the four stations studied, stratified models allow for the best fit of the data. The vertical variations in the anisotropic pattern show interesting correlations with changes in the thermomechanical state of the mantle associated with the lithosphere‐asthenosphere transition and with the presence of hot mantle beneath the Afar region and beneath the EARS branches that surround the Tanzanian Craton. Our interpretation is consistent with the conclusion of earlier studies that suggest that beneath individual stations, multiple sources of anisotropy, chiefly olivine lattice preferred orientation and melt pocket shape preferred orientation in our case, exist at different depths. Our study further emphasizes that multiple layers of anisotropy must often be considered to obtain realistic models of the crust and upper mantle.

Magma flow and feeder chamber location inferred from magnetic fabrics in jotunitic dykes (Rogaland anorthosite province, SW Norway)

The ca. 930–920 Ma Rogaland anorthosite province of Southwest Norway is cross cutted by up to several-km-long dykes of the jotunite kindred. The fabric of three major jotunitic dykes from this igneous complex (the Vetteland, Varberg and Lomland dykes) is investigated here, using measurements of the anisotropy of magnetic susceptibility (AMS) made along transversal sampling sections. The petrofabric in the three studied dykes has an overall magmatic character and it is shown, through image analysis, that the AMS, which is controlled by the shape-preferred orientation of low-Ti titanomagnetite grains, can be used as a proxy for its orientation. The average magnetic fabrics, taken as flow markers, give valuable information on the geometry of the dykes and enable to locate the feeder magma chambers. It appears that each dyke is connected to its own chamber, in agreement with the conclusions of published geochemical studies. A model of emplacement along a set, probably two conjugate sets, of active shear zones is proposed. This model takes into account, in particular, the local occurrence of asymmetrical distributions of magnetic fabrics.

Radial anisotropy along the Tethyan margin

SUMMARY We estimate radial anisotropy along the Tethyan margin by jointly fitting regional S and Love waveform trains and fundamental-mode Love-wave group velocities. About 3600 wave trains with S and Love waves and 5700 Love-wave group velocity dispersion curves are jointly inverted for SH-velocity perturbations from a pre-existing, 3-D SV-velocity model. These perturbations are predominantly positive (SH faster than SV) and consistent with PREM, but our model also shows significant lateral variation in radial anisotropy that appears to be correlated with tectonic environment. SH waves travel faster than SV wave beneath backarc basins, oceans and orogenic belts such as the Tyrrhenian and Pannonian basins, the Ionian Sea, the Alps, the Apennines, the Dinarides and the Caucasus. The Algero-Provencal basin, however, is underlain by faster SV velocity. Faster SV velocity of radial anisotropy is also detected within cratons such as the East European platform and the Arabian shield. Beneath hotspots we detect a change in radial anisotropic polarity with depth, which may be caused by transition between the lattice-preferred orientation from horizontal deformation in the asthenosphere and the shape-preferred orientation from vertically oriented melt channels in the lithosphere. We also find significant portion of radial anisotropy within subducting slabs depends on the slab's dip angle.

Sequential deformation from serpentinite mylonite to metasomatic rocks along the Sashu Fault, SW Japan

The deformation of serpentinites along the Sashu Fault, Saganoseki Peninsula, SW Japan, involved both mylonitization and brecciation. The brecciation was accompanied by metasomatism, and the metasomatized rocks record additional ductile deformation. Serpentinite mylonite is composed mainly of antigorite, with minor magnetite and Cr-spinel. Foliation and lineation in this rock type are defined by the shape preferred orientation of antigorite and the alignment of fine-grained magnetite and carbonate minerals. The antigorite also shows a lattice preferred orientation (LPO), as measured with a universal stage, with c axes oriented near-perpendicular to the foliation and b axes oriented parallel to the lineation. Brecciation is concentrated in the reaction zone along the boundary between serpentinites and surrounding country rocks. Talc and chlorite schists developed in this zone as a result of associated metasomatism and deformation; consequently, they contain composite planar fabrics and drag folds. The breccias themselves are composed of serpentinite fragments cut by veins of calcite and talc. Analyses of fluid inclusions in calcite from the breccia indicate that brecciation occurred at 200–300 °C at depths of 3.8–11.8 km. Metasomatic reactions, particularly the formation of talc, contributed to weakening of this serpentinite-bearing fault zone.

The Farsund intrusion (SW Norway): A marker of late-Sveconorwegian (Grenvillian) tectonism emplaced along a newly defined major shear zone

The ca. 930 Ma Farsund intrusion (SW Norway) belongs to a series of 0.99–0.92 Ga post-collisional plutons from the Sveconorwegian (Grenvillian) orogen. It is made of two rock facies (charnockite and quartz mangerite, and subordinate quartz monzonite and quartz monzodiorite) that show mingling relationships. As shown elsewhere, these two facies belong, respectively, to the two suites of A-type affinity recognized in the Sveconorwegian post-collisional magmatism of Southern Norway, namely the AMC and HBG suites. A structural study of the Farsund intrusion, based on the anisotropy of magnetic susceptibility (AMS) technique, is presented here. The AMS is controlled by the shape-preferred orientation of low-Ti titanomagnetite grains and it can be used as a proxy for the global petrofabric. The AMS data, when combined with micro- to macrostructural observations, unfold the occurrence of a steeply dipping shear zone, straddling the NE border of the pluton and characterized by a likely strike-slip component of shearing. This high-strain zone is roughly coincident with the boundary between the outcrop domains of the AMC and HBG suites, and was formed or, more probably, reactivated during the gravitational collapse of the Sveconorwegian orogen, in an extensional (possibly transtensional) tectonic regime. It would have controlled the ascent and emplacement of the Farsund intrusion, and materializes as well the structural weakness that would have channelled the magmas of the neighbouring and coeval Rogaland anorthosite province. It is also suggested that vertical, gravity-driven movements were recorded in the Farsund intrusion and its close surroundings. They would have been induced in a very hot environment, akin to that prevailing in the Precambrian ultra-hot orogens, but linked in the present case to the emplacement of anorthosites and penecontemporaneous igneous bodies, including the Farsund intrusion.

Orientation-related deformation mechanisms of naturally deformed amphibole in amphibolite mylonites from the Diancang Shan, SW Yunnan, China

Sheared amphibolite rocks from Diancang Shan high-grade metamorphic complex along the Ailao Shan-Red River shear zone, southwestern Yunnan, China, show typical mylonitic microstructures. The mylonites are characterized by porphyroclastic microstructures and the ultramylonites are highly lineated with alternating amphibole- and quartzofeldspathic domains. Microstructural analysis and P/T estimation suggest that the amphibole grains in the mylonitic rocks are deformed and dynamically recrystallized at amphibolite facies. In the mylonitic amphibolites, there are two types of amphibole porphyroclasts, i.e. type I “hard” and type II “soft” porphyroclasts. They have their [001] crystallographic orientations subnormal and sub-parallel to the stretching lineation of the rocks, respectively. The two types of porphyroclasts show distinct deformation microstructures and sub-microstructures formed by various deformation mechanisms, which contribute in different ways to the generation of the fine-grained matrix. Shape preferred orientation analysis, misorientation analysis of the two types of porphyroclasts and new fine grains around them further prove the generation of the fine grains in matrix from the type II porphyroclasts. The type I “hard” porphyroclasts are deformed mainly by mechanical rotation, work hardening and intragranular microfracturing. In contrast, the deformation of the type II “soft” porphyroclasts is mainly attributed to crystalline plasticity, i.e. twinning, dislocation creep and dynamic recrystallization. During the deformation of the type II porphyroclasts, the (100) [001] slip system plays a dominant role during deformation and grain size reduction of amphibole. Twinning along the active (100) slip system, in combination with dislocation creep (gliding and climbing) governs the nucleation of subgrains and formation of dynamically recrystallized fine grains, a process here named Twinning Nucleation Recrystallization.

Petrographic, biological, and chemical techniques used to characterize two tombs in the Protestant Cemetery of Rome (Italy)

In this multidisciplinary contribution, several diagnostic tests were carried out in order to characterize the stone materials, forms of alteration, and protective products applied in the past to two monumental tombs located in the Protestant Cemetery of Rome (Italy). The Protestant Cemetery is a very important historic site, and has been included in the List of 100 Most Endangered Sites in the World since 2005. In this work, two of its tombs were studied: those of Karl (or Charles) Brjullov, a Russian painter who lived in the first half of the nineteenth century, and of Lady Elisa Temple, wife of the artist Sir Grenville Temple. The tombs are both made of white marble and travertine, and the same forms of alteration and degradation, such as blackish biological patinas, black crusts, and chromatic alterations, were found on both monuments. Petrographic analysis of the different lithotypes made it possible to determine textural characteristics, evaluate the state of preservation, and formulate some hypotheses about their provenance by means of oxygen and carbon isotopic ratios, and evaluation of maximum grain size (MGS) and shape preferred orientation (SPO) of calcite grains. Laboratory culture analysis identified autotrophic species and, in some cases, black patinas caused by fungal species were found. Lastly, Fourier transform infrared spectroscopy (FT-IR) revealed that some synthetic protective products had been used in previous, undocumented restoration processes on some portions of both graves.

Brittle plus plastic deformation of gypsum aggregates experimentally deformed in torsion to high strains: quantitative microstructural and texture analysis from optical and diffraction data

Abstract This contribution presents a quantitative microstructural analysis of a polycrystalline aggregate of gypsum, deformed in torsion ( T =70–90 °C) at γ (shear strain) ranging from 0 to 4.82. Quantitative microstructural analysis is used to compare the evolution of microstructures observed by optical microscope with those obtained from analysis of X-ray and neutron diffraction data. This analysis shows that during experimental deformation, gypsum accommodated strain by brittle and plastic deformation mechanisms, developing Riedel-like microfaults with plastic foliations and crystallographic preferred orientation (CPO). The relations of microstructures show that with increasing strain, the Riedel systems start from R planes with an angle of ≈30° to the Imposed Shear Plane. This angle decreases (5°–15°) when strain increases, and Y planes develop. Quantitative texture analysis (QTA) shows that S-foliations start developing at low γ and maintain their orientation up to high γ, and that the most active slip system is the (010) along normal to (100) and the [001]-axis. Shape preferred orientation (SPO) of gypsum does not coincide with the theoretical orientation as it does not decrease with increasing strain. This discrepancy is explained by the role of the brittle shear planes that impose a back rotation to gypsum. No brittle to plastic transition occurs. But both plastic and brittle structures contemporaneously accommodate and localize strain.

Cretaceous metamorphism in the Austroalpine Matsch Unit (Eastern Alps): The interrelation between deformation and chemical equilibration processes

In the polymetamorphic Austroalpine Matsch Unit (European Eastern Alps) Cretaceous upper greenschist facies metamorphism overprinted Variscan and Permian magmatic and metamorphic assemblages. Mineral compositional and (micro-)structural data of metapelites and metapegmatites document different mechanisms of interrelated deformation and (re-)equilibration during Cretaceous overprinting: i) Microfractures in relic garnet represented pathways for material transport, and thus established material exchange between intragranular domains and the matrix. Major element equilibration by fast diffusion along microfractures contrasts with limited volume diffusion in adjacent host garnet. ii) Syn-tectonic breakdown of staurolite initially to paragonite, then chloritoid allows correlating reaction progress with the formation of different fracture sets. iii) Syn-tectonic mineral growth with shape-preferred orientation in foliation domains contrasts with radial growth in microlithons and strain shadows of the mylonitic foliation. iv) Syn-tectonic unmixing of pre-existing oligoclase (an14–16) produced fine-grained aggregates of two supposedly coexisting plagioclase-phases (an3–6 and an20–25) in strain shadows of the oligoclase-clasts. v) Pre-existing deformation-induced heterogeneities in the spatial distribution of phases and their preferred orientation influence the kinetics of phase equilibration. Understanding the mechanisms of the mutual interrelation between deformation and phase equilibration is a prerequisite for deducing PT-constraints from strained metamorphic rocks. New garnet—whole rock Sm-Nd data from metapegmatites indicate their emplacement at 263–280 Ma and provide an important age constraint on the interrelated deformation and re-equilibration processes.

Microfabric of folded quartz veins in metagreywackes: dislocation creep and subgrain rotation at high stress

AbstractThe microfabrics of folded quartz veins in fine‐grained high pressure–low temperature metamorphic greywackes of the Franciscan Subduction Complex at Pacheco Pass, California, were investigated by optical microscopy, scanning electron microscopy including electron backscatter diffraction, and transmission electron microscopy. The foliated host metagreywacke is deformed by dissolution–precipitation creep, as indicated by the shape preferred orientation of mica and clastic quartz without any signs of crystal‐plastic deformation. The absence of crystal‐plastic deformation of clastic quartz suggests that the flow stress in the host metagreywacke remained below a few tens of MPa at temperatures of 250–300 °C. In contrast, the microfabric of the folded quartz veins indicates deformation by dislocation creep accompanied by subgrain rotation recrystallization. For the small recrystallized grain size of ∼8 ± 6 μm, paleopiezometers indicate differential stresses of a few hundred MPa. The stress concentration in the single phase quartz vein is interpreted to be due to its higher effective viscosity compared to the fine‐grained host metagreywacke deforming by dissolution–precipitation creep. The fold shape suggests a viscosity contrast of one to two orders of magnitude. Deformation by dissolution–precipitation creep is expected to be a continuous process. The same must hold for folding of the vein and deformation of the vein quartz by dislocation creep. The microfabric suggests dynamic recrystallization predominantly by subgrain rotation and only minor strain‐induced grain boundary migration, which requires low contrasts in dislocation density across high‐angle grain boundaries to be maintained during climb‐controlled creep at high differential stress. The record of quartz in these continuously deformed veins is characteristic and different from the record in metamorphic rocks exhumed in seismically active regions, where high‐stress deformation at similar temperatures is episodic and related to the seismic cycle.