Strong Shape Preferred Orientation Research Articles

Resurfacing of deeply buried oceanic crust in Naga Hills Ophiolite, North-East India: Petrofabric, microstructure and seismic properties

We have studied the microstructure, texture and seismic anisotropy of a foliated blueschist-eclogite rock in the Naga Hills Ophiolite (NHO) complex, North-East India. The rock is composed essentially of omphacite, sodic amphibole (glaucophane), sodic-calcic amphibole (barroisite), phengite, and garnet, and represents oceanic crust that experienced Neo-Tethyan subduction and metamorphism before it resurfaced and accreted in the ophiolite. Omphacite and glaucophane show weak shape preferred orientation (SPO), while phengite displays a strong SPO. The Crystallographic Preferred Orientation (CPO) of omphacite is characterized by the [001]-axes gridle within the foliation, and the (010)-poles concentrated sub-perpendicular to the foliation. For glaucophane, the [001] axis aligns parallel to lineation and the [100] axis and (110) pole plunge perpendicular to foliation. These CPOs correspond to SL-type fabrics, related to plane strain deformation geometry. Based on our analysis, we propose that the observed texture had developed from plastic deformation through dislocation creep. Employing the textural data, we have presented the seismic anisotropies of the individual minerals, blueschist-eclogite domains and discussed their contributions in the bulk-rock anisotropy. The calculated bulk-rock seismic anisotropies, AVP and AVS, of the rock are 12.8% and 8.1%, respectively, which correlate with the observed seismic anisotropy of the subducting oceanic crust.

Slate – A new record for crystal preferred orientation

This report describes microstructures and preferred orientation of a Devonian slate from the Belgian Ardennes (La Fortelle quarry). The sample, composed largely of muscovite, chlorite, and quartz, displays extraordinary texture strength of muscovite, exceeding 100 multiples of random distribution (m.r.d.) for (001) pole figures and thus setting a new record for preferred orientation in rocks. Quantitative texture analysis was performed with the Rietveld method on synchrotron X-ray diffraction images. Interestingly, quartz also displays strong shape preferred orientation (SPO) as revealed by X-ray microtomography and SEM imaging but largely random crystallographic preferred orientation (CPO). Microstructural evidence suggests that this fine-grained rock formed by recrystallization of shale under stress, with growth of phyllosilicates and dissolution and growth of quartz.

Microstructural and rheological evolution of calcite mylonites during shear zone thinning: Constraints from the Mount Irene shear zone, Fiordland, New Zealand

Layers of calc-mylonite in the Mount Irene shear zone, Fiordland, New Zealand, show substantial variations in thickness due to deflection of the shear zone boundaries around wall rock asperities. In relatively thick parts (c. 2.6 m) of the shear zone, calcite porphyroclasts are internally strained, contain abundant subgrain boundaries and have a strong shape preferred orientation (SPO) and crystallographic preferred orientation (CPO), suggesting that deformation occurred mainly by dislocation creep involving subgrain-rotation recrystallization. In relatively thin parts (c. 1.5 m) of the shear zone, aggregates of fine-grained recrystallized calcite surrounding flattened porphyroclasts have a weak SPO and CPO, and contain polygonal calcite grains with low degrees of internal misorientation. The recrystallized aggregates also contain microstructures (e.g. grain quadruple junctions, randomized misorientation axes) similar to those reported for neighbor-switching processes during grain-boundary sliding. Comparison of subgrain sizes in the porphyroclasts to published grain-size differential-stress relationships indicates that stresses and strain rates were substantially higher in relatively thin parts of the shear zone. The primary microstructural response to higher stresses and strain rates was an increase in the amount of recrystallization to produce aggregates that deformed by grain-boundary sliding. However, even after the development of interconnected networks of recrystallized grains, dislocation creep by subgrain-rotation recrystallization continued to occur within porphyroclasts. This behavior suggests that the bulk rheology of shear zones undergoing thinning and thickening can be controlled by concomitant grain-size insensitive and grain-size sensitive mechanisms. Overall, our observations show that shear zone thickness variations at constant P-T can result in highly variable stresses and strain rates, which in turn modifies microstructure, deformation mechanism and shear zone rheology.

Physical analysis of an Antarctic ice core-towards an integration of micro- and macrodynamics of polar ice.

Microstructures from deep ice cores reflect the dynamic conditions of the drill location as well as the thermodynamic history of the drill site and catchment area in great detail. Ice core parameters (crystal lattice-preferred orientation (LPO), grain size, grain shape), mesostructures (visual stratigraphy) as well as borehole deformation were measured in a deep ice core drilled at Kohnen Station, Dronning Maud Land (DML), Antarctica. These observations are used to characterize the local dynamic setting and its rheological as well as microstructural effects at the EDML ice core drilling site (European Project for Ice Coring in Antarctica in DML). The results suggest a division of the core into five distinct sections, interpreted as the effects of changing deformation boundary conditions from triaxial deformation with horizontal extension to bedrock-parallel shear. Region 1 (uppermost approx. 450 m depth) with still small macroscopic strain is dominated by compression of bubbles and strong strain and recrystallization localization. Region 2 (approx. 450–1700 m depth) shows a girdle-type LPO with the girdle plane being perpendicular to grain elongations, which indicates triaxial deformation with dominating horizontal extension. In this region (approx. 1000 m depth), the first subtle traces of shear deformation are observed in the shape-preferred orientation (SPO) by inclination of the grain elongation. Region 3 (approx. 1700–2030 m depth) represents a transitional regime between triaxial deformation and dominance of shear, which becomes apparent in the progression of the girdle to a single maximum LPO and increasing obliqueness of grain elongations. The fully developed single maximum LPO in region 4 (approx. 2030–2385 m depth) is an indicator of shear dominance. Region 5 (below approx. 2385 m depth) is marked by signs of strong shear, such as strong SPO values of grain elongation and strong kink folding of visual layers. The details of structural observations are compared with results from a numerical ice sheet model (PISM, isotropic) for comparison of strain rate trends predicted from the large-scale geometry of the ice sheet and borehole logging data. This comparison confirms the segmentation into these depth regions and in turn provides a wider view of the ice sheet.This article is part of the themed issue ‘Microdynamics of ice’.

Precipitation of rutile needles in garnet from sillimanite-bearing pelitic granulite from the Khondalite Belt, North China Craton

Pelitic granulite from the Khondalite Belt, North China Craton, is composed of garnet, sillimanite, perthite, biotite and quartz. Oriented rutile needles in garnet are reported for the first time from the granulite. The rutile needles have been identified by optical microscopy and confirmed by in situ Raman spectroscopy, where they show the characteristic bands at 446–448 and 610 cm−1. Because of their very strong shape preferred orientation in 3–4 easily observable directions, these rutile needles are interpreted to have formed by precipitation during exhumation and cooling following peak metamorphism. The ternary feldspar geothermometer gives a peak metamorphic temperature of ~ 980 °C at 10 kbar. The presence of oriented rutile needles in garnet is suggested to be a valuable indicator of ultrahigh-temperature (UHT) metamorphism in the Khondalite Belt and therefore likely in other UHT terrains.

Amphibole fabric formation during diffusion creep and the rheology of shear zones

Research Article| June 01, 2014 Amphibole fabric formation during diffusion creep and the rheology of shear zones A.J. Getsinger; A.J. Getsinger * Department of Geological Sciences, Brown University, Box 1846, 324 Brook Street, Providence, Rhode Island 02912, USA *E-mails: Amanda_Getsinger@brown.edu; Greg_Hirth@brown.edu. Search for other works by this author on: GSW Google Scholar G. Hirth G. Hirth * Department of Geological Sciences, Brown University, Box 1846, 324 Brook Street, Providence, Rhode Island 02912, USA *E-mails: Amanda_Getsinger@brown.edu; Greg_Hirth@brown.edu. Search for other works by this author on: GSW Google Scholar Geology (2014) 42 (6): 535–538. https://doi.org/10.1130/G35327.1 Article history received: 24 Nov 2013 rev-recd: 19 Mar 2014 accepted: 21 Mar 2014 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation A.J. Getsinger, G. Hirth; Amphibole fabric formation during diffusion creep and the rheology of shear zones. Geology 2014;; 42 (6): 535–538. doi: https://doi.org/10.1130/G35327.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract We conducted experiments to examine the effect of amphibole-forming hydration reactions on mineral fabric development and the strength of mafic rocks. Both hydrostatic and general shear deformation experiments were conducted on powdered basalt with added water at lower continental crust conditions (800 °C, 1 GPa). Amphibole that formed under hydrostatic conditions exhibits a random lattice-preferred orientation (LPO). In contrast, amphibole formed during deformation exhibits both a strong shape-preferred orientation (SPO) and LPO with a [001] maximum aligned subparallel to the shear direction. Plagioclase in both hydrostatic and deformed samples shows a very weak to random LPO. At low effective strain rates (10−5 s−1 to 10−6 s−1), the stress exponent is ≈1–1.5, consistent with deformation accommodated by diffusion creep. The correlation of the SPO and LPO coupled with the rheological evidence for diffusion creep indicates that the amphibole fabric results from oriented grain growth and rigid grain rotation during deformation. The experimentally produced fabrics are strikingly similar to those observed in amphibolite-grade natural shear zones, supporting interpretations that such rocks deform by diffusion creep. In addition, the rheology of the fine-grained experimental amphibolite is comparable to that predicted using flow laws for wet anorthite. Thus, both our experiments and field analyses indicate that wet plagioclase rheology provides a good constraint on the strength of hydrated lower crust. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

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.

Dilatant plasticity in high-strain experiments on calcite–muscovite aggregates

Torsion experiments were performed on synthetic aggregates of calcite with a 50% volume of muscovite. The tests were performed at 627–727 °C with a confining pressure of 300 MPa at constant shear strain rates of 3 × 10 −5–3 × 10 −4 s −1 on cylindrical samples with the starting foliation parallel and perpendicular to the cylinder axis. Both the foliation parallel and the foliation perpendicular experiments show similar stress–strain patterns, with an initial hardening stage followed by significant strain weakening (>60%) before a catastrophic rupture. Microstructural analysis shows that in low-strain experiments calcite grains are intensely twinned while muscovite grains appear slightly bent and kinked. Higher strains promote a segregation of the two phases with calcite forming thin layers of fine, dynamically recrystallized grains, which act as localized shear bands, while muscovite grains keep their original size and rotate assuming a strong shape preferred orientation. This strain localization of the calcite from an initially homogeneous rock produced catastrophic failure at moderate bulk shear strains (γ ∼ 3). Localization of the strain first involved ductile deformation to produce a new calcite layering with fine dynamically recrystallized grains along which cavities nucleated. The orientation and kinematics of the cavities are comparable to R1 Riedel structures. All experiments on calcite–muscovite mixtures resulted in heterogeneous strain. In these torsion experiments chemical changes and crystallization of new phases (anorthite and kalsilite) are observed at 627 °C. Whereas, samples hot pressed or deformed in compression at 670 °C did not show such reactions or any localization. The effect of stress-field geometry and pore pressure upon mineral reactions is discussed. It is concluded that deformation-induced heterogeneous phase distributions caused local strength differences initiating strain localization in the calcite–muscovite mixtures, eventually leading to plastic failure.

The effect of oriented microcracks and crystallographic and shape preferred orientation on bulk elastic anisotropy of a foliated biotite gneiss from Outokumpu

Elastic anisotropy is an important property of crustal and mantle rocks. This study investigates the contribution of oriented microcracks and crystallographic (LPO) and shape preferred orientation (SPO) to the bulk elastic anisotropy of a strongly foliated biotite gneiss, using different methodologies. The rock is felsic in composition (about 70 vol.% SiO 2) and made up by about 40 vol.% quartz, 37 vol.% plagioclase and 23 vol.% biotite. Measurements of compressional (Vp) and shear wave (Vs) velocities on a sample cube in the three foliation-related structural directions (up to 600 MPa) and of the 3D P-wave velocity distribution on a sample sphere (up to 200 MPa) revealed a strong pressure sensitivity of Vp, Vs and P-wave anisotropy in the low pressure range. A major contribution to bulk anisotropy is from biotite. Importantly, intercrystalline and intracrystalline cracks are closely linked to the morphologic sheet plane (001) of the biotite minerals, leading to very high anisotropy at low pressure. Above about 150 MPa the effect of cracks is almost eliminated, due to progressive closure of microcracks. The residual (pressure-independent) part of velocity anisotropy is mainly caused by the strong alignment of the platy biotite minerals, displaying a strong SPO and LPO. Calculation of the 3D velocity distribution based on neutron diffraction texture measurements of biotite, quartz, and plagioclase and their single-crystal properties give evidence for an important contribution of the biotite LPO to the intrinsic velocity anisotropy, confirming the experimental findings that maximum and minimum velocities and shear wave splitting are closely related to foliation. Comparison of the LPO-based calculated anisotropy (about 8%) with measured intrinsic anisotropy (about 15% at 600 MPa) give hints for a major contribution of SPO to the bulk anisotropy of the rock.

Experimental Investigation of Eclogite Rheology and Its Fabrics at High Temperature and Pressure

AbstractEclogite plays an important role in mantle convection and geodynamics in subduction zones. An improved understanding of processes in the deeper levels of subduction zones and collision belts requires information on eclogite rheology. However, the deformation processes and associated fabrics in eclogite are not well understood. Incompatible views of deformation mechanism have been proposed for both garnet and omphacite. We present here deformation behaviour of eclogite at temperatures of 1027–1427 °C, confining pressures of 2.5–3.5 GPa, and strain rates of 1 × 10−5 s−1 to 5 × 10−4 s−1. We obtained a power‐law creep for the high temperature and pressure deformation of a ‘dry’ eclogite (50 vol.% garnet, 40% omphacite and 10% quartz) with A = 103.3 ± 1.0, n = 3.5 ± 0.4, ΔE =403 ± 30 KJ mol−1 and ΔV = 27.2 cm3 mol−1. The two principal minerals of eclogite have greatly different strengths. Progressive increase of garnet results in a smooth increase in strength. Analysis by electron back‐scattered diffraction shows that: (1) garnet displays pole figures with near random distributions of misorientation angle under both dry and wet conditions; (2) omphacite shows pronounced lattice preferred orientations (LPOs), suggesting a dominant dislocation creep mechanism. Further investigation into the water effects on eclogite show: (3) water content does not influence the style of omphacite fabric but increases slightly the fabric strength; (4) grain boundary processes dominate the deformation of garnet under high water fugacity or high shear‐strain conditions, yielding a random LPO similar to that of non‐deforming garnet, despite the strong shape preferred orientation (SPO) observed. {110} [001] slip may dominate the deformation of rutile. Quartz displays complicated and inconsistent LPOs in eclogite. These results are remarkably similar to observations from deformed eclogites in nature.

Deformation and recrystallization mechanisms in mylonitic shear zones in naturally deformed extrusive Eocene–Oligocene rocksalt from Eyvanekey plateau and Garmsar hills (central Iran)

Microstructural processes in mylonitic shear zones from extrusions of Eocene–Oligocene rocksalts from Garmsar hills and Eyvanekey plateau (central Iran) are described and analyzed. The halite samples were studied by transmitted light microscopy of gamma-irradiated thin sections, subgrain size palaeopiezometry of polished and chemically etched samples and texture measurements by electron backscatter diffraction (EBSD). The studied shear zones are thin (<5 cm), subhorizontal and bedding parallel. The protomylonite comprises 2–6 mm sized grains, occasionally rich in primary fluid inclusions indicative of their primary non-recrystallized state. Abundant, well-developed subgrains suggest that the protomylonite was deformed mainly by dislocation processes. Elongated subgrains at grain edges point to recrystallization by fluid-assisted grain boundary migration. Recrystallized, strain-free grains are common. The material in the mylonitic zones is fine grained (∼0.6 mm) with strong shape-preferred orientation. Microstructures such as oriented fibrous overgrowths and growth banding (observed in gamma-irradiated sections) suggest that the principal deformation mechanism was solution-precipitation creep (non-conservative grain boundary migration and grain boundary sliding accommodated by solution-precipitation). Crystal fabrics measured by EBSD show a weak crystallographic preferred orientation that is consistent with solution-precipitation accommodated grain boundary sliding. Using published flow laws, the strain rate in the fine-grained mylonites is about 10 −10 l/s, which is in a good agreement with earlier in situ measurements on glacier salt flow rate.

Introduction: linked processes of metamorphism and deformation

The interaction of deformation and metamorphism within the crust results in textures, structures and mineral growth that provide a record of tectonic activity as well as the progressive metamorphic and structural development of the crust. Analysis of these textures and structures is central to deciphering and evaluating the progressive nature of their development. Such rocks are commonly interpreted from the viewpoint of either structural or metamorphic processes; however, integration of these two perspectives has proved to be a powerful tool in enhancing our understanding of the structural development of metamorphic rocks at all scales. In recent decades, research into the linkages between metamorphism and deformation has increasingly involved the integration of textural, chemical and structural data. The present volume arose from a topical session titled Understanding Coupled Metamorphic and Deformational Processes: Advances in Integrated Textural, Chemical and Microstructural Analysis held at the 2003 annual meeting of the Geological Society of America, Seattle, USA. We were pleased that the session attracted 21 contributions from 48 geologists representing seven countries. The breadth of topics covered by the presentations, and the interest expressed by both participants and attendees encouraged us to assemble a set of thematic papers related to the topical session. The result is the present special issue of the Journal of Metamorphic Geology. Whitmeyer & Wintsch document the linkages between stress, metamorphic reactions, grain shapepreferred orientation (SPO) and reactivation. They describe the reactivation of an amphibolite facies ultramylonite along discrete greenschist facies mylonitic shear bands. A random SPO within the ultramylonite created a texturally hardened rock that displaced strain into adjacent weaker foliated gneiss. Later replacement reactions within the ultramylonite were concentrated at areas of high stress and resulted in a chemical and mechanical softening of the rock via the development of a SPO of muscovite, chlorite and quartz. Reactivation of the ultramylonite was localized along discrete mylonitic shear bands at the sites of these reactions and accompanying SPO development. The relationship between strain and SPO development is also explored by Stallard et al. who measured the 3D shape, size and orientation of quartz grains sampled along two transects of increasing metamorphic grade in the Otago Schist, New Zealand. Quartz SPO was initiated by dissolution–precipitation creep at sub-greenschist facies conditions, and was triggered by strain accompanying fold development. Detrital grains became blade-shaped, and were an important component of the metamorphic foliation. A second fold event, accompanied by crenulation cleavage development and resulting segregation of the rock mass, initiated a change in both the texture of the quartz SPO and deformation mechanisms. Previously discrete and aligned detrital quartz grains were replaced by aggregates and layers of dynamically recrystallized quartz grains of reduced aspect ratio and alignment. Pressure solution then affected the margins of quartz-rich layers rather than individual grains. The authors emphasize the contribution of quartz SPO to foliation development and the significance of fold events as a control on SPO formation and development. In a companion paper that analyses the 3D shape, size and orientation of white mica grains from the same samples, Stallard & Shelley describe the initiation and development of white mica SPO with increasing metamorphic grade. The character of white mica SPO is determined early in the deformation and metamorphic history. The onset of penetrative strain metamorphism is marked by the rapid elimination of poorly oriented large clastic micas in favour of numerous new smaller metamorphic grains of contrasting composition, higher aspect ratio and a strong SPO. The SPO results from the progressive competitive anisotropic growth of blade-shaped grains so that the fastest growth directions and blade lengths tend to parallel the extension direction during deformation. The competitive nature of mica growth is indicated by the progressive increase in size and resultant decrease in number of metamorphic mica with increasing grade, from c. 1000 relatively small mica grains per square millimetre of thin section at lower grades, to c. 100 relatively large grains per square millimetre in higher-grade samples. Selverstone describes how changes in fluid composition correlate with changes in mechanical behaviour of graphitic schists during decompression. Interlayered graphitic and non-graphitic schists from the Tauern Window, Eastern Alps, record different mechanical behaviour during extensional exhumation. Graphitic schists contain brittle extensional structures oriented J. metamorphic Geol., 2005, 23, 409–410 doi:10.1111/j.1525-1314.2005.00593.x

The initiation and development of metamorphic foliation in the Otago Schist, Part 2: evidence from quartz grain‐shape data

AbstractShape, size and orientation measurements of quartz grains sampled along two transects that cross zones of increasing metamorphic grade in the Otago Schist, New Zealand, reveal the role of quartz in the progressive development of metamorphic foliation. Sedimentary compaction and diagenesis contributed little to the formation of a shape‐preferred orientation (SPO) within the analysed samples. Metamorphic foliation was initiated at sub‐greenschist facies conditions as part of a composite S1‐bedding structure parallel to the axial planes of tight to isoclinal F1 folds. An important component of this foliation is a pronounced quartz SPO that formed dominantly by the effect of dissolution–precipitation creep on detrital grains in association with F1 strain. With increasing grade, the following trends are evident from the SPO data: (i) a progressive increase in the aspect ratio of grains in sections parallel to lineation, and the development of blade‐shaped grains; (ii) the early development of a strong shape preferred orientation so that blade lengths define the linear aspect of the foliation (lineation) and the intermediate axes of the blades define a partial girdle about the lineation; (iii) a slight thinning and reduction in volume of grains in the one transect; and (iv) an actual increase in thickness and volume in the survivor grains of the second transect. The highest‐grade samples, within the chlorite zone of the greenschist facies, record segregation into quartz‐ and mica‐rich layers. This segregation resulted largely from F2 crenulation and marks a key change in the distribution, deformation and SPO of the quartz grains. The contribution of quartz SPO to defining the foliation lessens as the previously discrete and aligned detrital quartz grains are replaced by aggregates and layers of dynamically recrystallized quartz grains of reduced aspect ratio and reduced alignment. Pressure solution now affects the margins of quartz‐rich layers rather than individual grains. In higher‐grade samples, therefore, the rock structure is characterized increasingly by segregation layering parallel to a foliation defined predominantly by mica SPO.

Are isolated stable rigid clasts in shear zones equivalent to voids?

Particles in shear enclose important information about a rock's past and can potentially be used to decipher the kinematic history and mechanical behavior of a certain outcrop or region. Isolated rigid clasts in shear zones often exhibit systematic inclinations with respect to the shear-plane at small angles, tending towards the instantaneous stretching direction of the shear zone. This shape preferred orientation cannot be easily explained by any of the analytical theories used in geology. It was recently recognized that a weak mantle surrounding the clast or a slipping clast–matrix interface might be responsible for the development of the observed inclinations. Physical considerations lead us to conjecture that such mantled, rigid clasts can be effectively treated as voids that are not allowed to change their shape. The resulting equivalent void conjecture agrees well with numerical and field data and has the following important geological implications. (i) Clasts in shear zones can have stable positions in simple shear without the requirement of an additional pure shear component. (ii) The stable orientation can be approached either syn- or antithetically; hence, the clast can rotate against the applied shear sense. (iii) The strain needed to develop a strong shape preferred orientation is small ( γ≈1) and therefore evaluations based on other theories may overestimate strain by orders of magnitude. (iv) The reconstruction of far-field shear flow conditions and kinematic vorticity analysis must be modified to incorporate these new findings.

Analogue modelling of the influence of shape and particle/matrix interface lubrication on the rotational behaviour of rigid particles in simple shear

The rotational behaviour of a rigid particle embedded in a linear viscous matrix undergoing cylindrical simple shear (Couette) flow was studied in 2D rock-analogue experiments. The influence of particle shape (elliptical vs. monoclinic), aspect ratio and the nature of the matrix/particle interface (lubricated vs. unlubricated) was investigated. Both matrix (PDMS) and lubricant (liquid soap) were linear viscous, with a viscosity ratio of ca. 10 4. Without lubricant, the rotational behaviour of all particles closely approximates the Jeffery theory. Lubricated monoclinic particles with the long diagonal initially parallel to the shear direction show back rotation and approach a stable position. Lubricated elliptical particles initially parallel to the shear direction also show back rotation but only transiently stabilize. Weak planar zones in the matrix adjacent to unlubricated elliptical particles do not induce backward rotation. In general for elliptical particles, rotation rate as a function of orientation depends on axial ratio and thickness of the lubricant mantle. For thick mantles (initially >10% of the volume of the particle), rotation rates are faster than Jeffery theory. For very thin mantles they are markedly slower compared with thick mantles, particularly when the long axis is nearly parallel to the shear direction. Rotation rates are never strictly zero, so true stabilization does not occur. However, for more elongate particles (axial ratio=6) rotation rates are so slow that a very strong shape preferred orientation would develop in a lubricated elliptical particle population. In experiments, the volume of lubricant is constant and the thickness adjacent to the long side of the particle progressively decreases with increasing strain. In natural examples of porphyroclast systems, the weak mantle continually develops by recrystallization and/or cataclasis of the rigid clast core and a steady state between production and thinning could be attained, potentially leading to true stabilization for particles with a high axial ratio.

Strain-insensitive preferred orientation of porphyroclasts in Mont Mary mylonites

The shape preferred orientation (SPO) of porphyroclasts was determined in high temperature mylonites. The porphyroclasts approach rhomboidal (sillimanite) or elliptical (garnet, plagioclase, sillimanite) shapes, and exhibit aspect ratios (R) as high as 11. Particles with R>3 are dominantly rhomboidal. The long axis of the best-fit ellipse defines a very strong SPO inclined at 5–10° to the mylonitic foliation, in an antithetic sense with respect to the shear direction. This angle is independent of R. The inclination of the long sides of rhomboidal sillimanite increases from 10 to 20° with decreasing R. In contrast, the short sides have a constant orientation of 15 to 17° irrespective of R and are parallel to extensional crenulation cleavage. Low aspect ratio (mainly elliptical) objects show low intensity SPO close to the shear plane. The two SPOs appear strain-insensitive. In the case of R<3, the SPO is caused by the combined effect of the slower rotation rate of elongate objects with long axes close to the shear plane and the continuous supply, by synkinematic fracturing and grain size refinement, of new porphyroclast populations. In the case of R>3, a stable position is acquired. This is not explained by any of the current theoretical and experimental models of SPO.

Microfabric study on the deformational and thermal history of the Alpi Apuane marbles (Carrara marbles), Italy

Marbles from different geometrical and structural positions within the Alpi Apuane metamorphic complex show a large variability in microfabric types as indicated by microstructure, c-axis orientation and temperature analysis. Statically recrystallized samples showing a granoblastic microstructure and polygonal grain boundaries are characterized by a grain size variation from east to west from 80–100 μm to 250–300 μm. This is correlated with an equilibration calcite-dolomite temperature from 360–380°C to 420°–430°C. Two kinds of dynamically recrystallized microstructures have been investigated: a first one exhibiting coarse grains (150–200 μm) with lobate grain boundaries and a strong shape preferred orientation and a second one characterized by a smaller grain size (40–50 μm) and predominantly straight grain boundaries. These microstructural types, associated with localized post-thermal peak shear zones and meter- to kilometer-scale folds, are interpreted as related to high strain and high temperature crystal plastic deformation mechanisms (dislocation creep) associated with predominant grain boundary migration (type-B1) or subgrain-rotation recrystallization (type-B2). These differences in dynamically recrystallized microstructures are related to equilibration temperatures higher in type-B1 (390°C) than in type-B2 (370–340°C). We have been able to relate the development of the different microfabric types to the successive stages of deformation of the Alpi Apuane metamorphic complex.

Eclogite-facies shear zones—deep crustal reflectors?

Strongly foliated eclogite-facies rocks in 30–150 m thick shear zones of Caledonian age occur within a Grenvillian garnet granulite-facies gabbro-anorthosite terrain in the Bergen Arcs of Norway. The predominant eclogite-facies mineral assemblages in the shear zones are omphacite + garnet + zoisite + kyanite in gabbroic anorthosite and omphacite + garnet in gabbro. Eclogite-facies rocks in shear zones are generally fine-grained; alternating omphacite/garnet- and kyanite/clinozoisite-rich layers define gneissic layering. A strong shape preferred orientation of omphacite, kyanite, and white mica (phengitic muscovite and/or paragonite) define the foliation. The anorthositic eclogites show omphacite b-axis maxima approximately normal to the foliation and c-axis girdles within the foliation plane. P-wave velocities ( V p ) determined at confining pressures to 600 MPa for samples from eclogite-facies shear zones range from 8.3 to 8.5 km s −1 and anisotropy ranges from 1 to 7%. The few samples with more pronounced anisotropy tend to be approximately transversely isotropic with minimum velocities for propagation directions normal to foliation and maximum velocities for propagation directions parallel to foliation. The fast propagation direction lies within the c-axis girdles (parallel to foliation) and the slow propagation direction is parallel to the b-axis concentration (normal to foliation) in samples for which omphacite crystallographic preferred orientation was determined. V p for the granulite-facies protoliths average about 7.5 km s −1. High calculated reflection coefficients for these shear zones, 0.04–0.14, indicate that they are excellent candidates for deep crustal reflectors in portions of crust that experienced high-pressure conditions but escaped thermal reactivation.