Localized folding of viscoelastic layers

Naturally occurring fold systems are typically irregular. Although such systems may sometimes be approximated by a periodic geometry, in reality they are commonly aperiodic. Ord (1994) has proposed that naturally occurring fold systems may display spatial chaos in their geometry. Previous work has indicated that linear theories for the formation of fold systems, such as those developed by Biot (1965), result in strictly periodic geometries. In this paper the development of spatially chaotic geometries is explored for a thin compressed elastic layer embedded in a viscoelastic medium which shows elastic softening. In particular, it is shown that spatially localized forms of buckling can develop and the evolution of these systems in the time domain is presented. A nonlinear partial differential equation, fourth order in a spatial variable and first order in time, is found to govern the evolution. A related nonlinear fourth-order ordinary differential equation governs an initial elastic phase of folding. The latter equation belongs to a class with spatially chaotic solutions. The paper reviews the implications of localization in the geological framework, and draws some tentative conclusions about the development of spatial chaos. Crudely arrived-at, yet plausible, evolutionary time plots under the constraint of constant applied end displacement are presented. Emphasis throughout is on phenomenology, rather than underlying mathematics or numerics.     

Diffusion and partitioning of magnesium in quartz grain boundaries

Experiments were conducted to study Mg diffusion in quartz grain boundaries. A detector particle method was used to study grain-boundary diffusion because Mg was confined exclusively to the grain boundaries. Diffusion couples were assembled by placing a MgF₂ disk against a disk of quartzite, which was placed against a disk of quartzite that contained fayalite (Fe₂SiO₄) ‘detector particles.’ During diffusion experiments, Mg diffused along the grain boundaries of the central quartzite toward the fayalite quartzite where it was incorporated into fayalite detector particles. The only pathway for transport from the diffusant source to fayalite detector particles was through the grain boundaries in the central quartzite. The cross-sectional area of the grain boundaries that delivered Mg to the fayalite detector particles was determined from scanning electron microscope images. The Mg contents of the fayalite detector particles were used to calculate the mass of Mg that fluxed through the grain boundaries. During the diffusion experiments, pyroxene crystallites nucleated and grew in the central quartzites from Mg and Fe that was transported along quartz grain boundaries. The Mg contents of the crystallites vary linearly throughout the quartzites, suggesting that steady-state transport conditions were rapidly established in the quartz grain boundaries. Magnesium concentrations in the pyroxene crystallites are proportional to concentration gradients in the grain boundaries of the central quartzite. Grain-boundary fluxes and linear concentration gradients were used to calculate diffusion of Mg in grain boundaries of the central quartzite component in the diffusion couples.     

Deformation and transformation in experimentally shock-loaded quartz

Single crystals of quartz, shock-loaded along the a axis to pressures of 22 Gpa, 24 GPa, 26 GPa and 30 GPa were examined by high-voltage transmission electron microscopy (TEM), scanning electron microscopy (SEM) and X-ray diffraction. Asymmetric broadenings of X-ray lines indicate spatial inhomogeneity of shock effects. X-ray streaking angles in the reciprocal lattice planes h0 \(\bar h\) l, 0k \(\bar k\) l and hki0 indicate a slight tilting deformation by rotation about [00.1] in (0001). TEM reveals glass lamellae which are mostly in (01 \(\bar 1\) 2) orientation, and are correlated with optical planar elements and with surface steps seen in SEM. No dislocations are found. There are (0001) lamellar features, probably Brazil twins. The (01 \(\bar 1\) 2) glass lamellae develop directly from bands of quartz in which intense deformation has produced a fine-scale lamellar to blocky structure, possibly also originating by twinning. Relics of crystalline structure are found in almost completely vitrified lamellae. Stishovite occurs in heavily deformed parts of the 22 GPa and 24 GPa specimens, in patches of densified glass distinct from the sharply bounded lamellae. The nucleationless, pervasive transformation of lamellae to glass, with preservation of their sharp boundaries, is attributed to defect coalescence analogous to vitrification by radiation damage (metamictization). Some patchy glass may be due to melting.     

Large scale transposition by folding in northern Norway

A spectacular example of transposition, in the sense ofSander (1911), is described from a large (2–3 km²) area of continuous outcrop in northern Norway. Three generations of folds transpose an earlier layering into its persent orientation. It is shown that the volume problems associated with isoclinal folding are largely accommodated by disharmony of the folds. The large scale structure is considered and its relationship to regional structure is discussed.     

Exoelectron spectroscopy of traps in surface layers of phenakite and quartz

The possibilities of exoelectron spectroscopy to investigate defects in dielectrics are demonstrated for phenakite Be₂SiO₄, its structural analogs Zn₂ SiO₄, Be₂GeO₄, solid solutions Be₂Si_1?x Ge _x O₄ (x=0÷1) and α-quartz. Emission maxima at 330 and 670 K in phenakite have been found to be due to [GeO₄]^5? andE' centers, respectively. Structural disturbances in the silicon and oxygen positions have been shown to control the exoemission activity of the crystals. Radiation induced decrease of exoemission activity connected with generation ofE' centers by neutron irradiation has been discovered. The energy level scheme of active centers in the subsurface region of Be₂SiO₄ has been established.     

Morphoscopy and morphometry of quartz grains from loess and buried soil layers

A complex method is suggested for the study of surfaces of sand grains. With this method, middle- and late Pleistocene loess-palaeosol sequences of the Russian Plain, Dnieper Basin, were studied. Surface texture of grains pointed to a dry and cold climate for their formation. During deposition of loess layers predominantly aeolian and cryogenic processes formed the grain surfaces. During interglacial and interstadial periods chemical weathering dominated.     

Stress reorientation during folding

Intermediate principal stress, ₂, is, for mechanical reasons, taken to be parallel to the statistical direction of fold axes and traces of thrust faults during evolution of fold and thrust belts. Regionally, maximum principal stress, ₁, and least, ₃, may be taken to be the directions of maximum shortening and maximum thickening of the section, respectively. Where crystalline basement is not involved in the deformation, maximum shortening is manifestly parallel to the top of the basement, or subhorizontal, and ₃ is, therefore, subvertical. While this stress system is grossly adequate on the scale of the fold and thrust belt, it fails locally, particularly in late stages of deformation. Sinuosity develops on all scales within the belt as deformation progresses. Individual fold axes tend to be straight lines in incipient stages of folding, as shown by unrolling folds, but commonly develop with increasing curvature during deformation. The curvature resulting during deformation is a measure of extension parallel to the axial direction, if the ends of the fold are fixed points. Thus, ₂ progressively decreases. With marked sinuosity, stress parellel to the axial direction can be reduced at a given depth below the magnitude of the weight of the overburden, originally ₃. Intermediate and least principal stresses switch position, and strike-slip faulting is favored where the deformational response is failure by shear fracture. The percent axial extension can be expressed in an equation that compares the final arcuate length with the original length. With a knowledge of the physical properties of the rock, the time in the evolution of the structure at which he stresses switch can be predicted, as well as the subsequent structural response.

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Zusammenfassung Die Richtung des intermediären Hauptstresses, ₂, wird aus mechanischen Gründen als parallel zu der statistischen Richtung der Faltenachsen und der Spuren der Überschiebungsflächen angenommen während der Entwicklung von Faltungs- und Überschiebungszonen. Regional können die Richtungen des maximalen Haupstresses, ₁, und des minimalen Hauptstresses, ₃, als Richtungen der größten Verkürzung, respektive der größten Verdickung des Querschnittes betrachtet werden. Wo der kristalline Untergrund nicht in den Deformationsvorgang einbezogen wird, ist die Richtung der maximalen Verkürzung offenbar parallel zur Kristallinoberfläche oder subhorizontal und ₃ somit subvertikal orientiert. Währenddessen im großen Maßstab diese Zuordnung der Hauptstreßrichtungen zu einer ganzen Faltungs- und Überschiebungszone vorgenommen werden kann, versagt sie im lokalen Bereich, vor allem in späten Phasen der Deformation. Bei fortschreitender Deformation entwickeln sich im Deformationsgürtel Bogenformen in verschiedenem Maßstab. Individuelle Faltenachsen neigen dazu, sich an geraden Linien auszubilden in frühen Stadien der Faltung, wie dies die Abwicklung von Falten zeigt. Sie entwickeln sich aber im allgemeinen während der weiteren Deformation mit zunehmend gebogener Achsenrichtung. Die resultierende Kurvatur ist ein Maß der Dehnung parallel zur Achsenrichtung, wenn die seitlichen Endpunkte der Falte Fixpunkte darstellen. In dieser Weise nimmt der Betrag von ₂ fortschreitend ab. Bei ausgeprägter Bogenform kann der Streß parallel zur Richtung der Faltenachse in einer bestimmten Tiefe reduziert werden bis zu einem Betrag, der unterhalb des Ausmaßes der Überlast, also ursprünglich ₃, liegt. Die Richtungen des intermediären und des kleinsten Hauptstresses wechseln ihre Positionen, und Blattverschiebungen werden begünstigt, wo die Deformation Scherbrüche erzeugt. Das Ausmaß der axialen Dehnung kann durch eine Gleichung ausgedrückt werden, welche die Länge des endgültigen Faltenbogens mit der ursprünglichen Länge der Falte verknüpft. Mit der Kenntnis der physikalischen Eigenschaften des Gesteins können sowohl der Zeitpunkt in der Entwicklung der Faltenstruktur, zu welchem die Hauptstreßrichtungen ihre Positionen wechseln, als auch die nachfolgende strukturelle Entwicklung bestimmt werden.

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Résumé La contrainte principale intermédiaire, ₂, est, pour des raisons mécaniques, considérée comme étant parallèle à la direction statistique des axes du pli et des traces de chevauchement durant l'évolution du pli et des zones de chevauchement. Régionalement, on peut supposer que les contraintes principales maximum, ₁, et minimum, ₃, suivent respectivement les directions du raccourcissement maximum et de l'épaississement maximum du profil. Là, où le soubassement cristallin n'est pas entrainé dans la déformation, le raccourcissement maximum est manifestement parallèle à la surface du soubassement, ou subhorizontal, et ₃ est, par conséquent, subverticale. Tant que l'échelle de ce système de contraintes correspond à peu près à celle du pli et de la zone de chevauchement, il change de direction localement, spécialement dans les derniers stades de la déformation. Une sinuosité se développe à toutes les échelles à l'intérieur de la zone, tandis que la déformation progresse. Les axes individuels du pli ont tendance à devenir des lignes droites dans les stades embryonnaires de la déformation, comme le montre le déroulement des plis, mais ordinairement ils se développent avec une courbure croissante pendant la déformation. La courbure qui en résulte durant la déformation est une mesure de l'extension parallèle à la direction axiale. Ainsi ₂ décroit progressivement. Avec une sinuosité prononcée, la contrainte parallèle à la direction axiale peut se réduire, à une profordeur donné au dessous du poids de la surcharge, originellement ₃. Les contraintes principales maximum et minimum échangent leur position, et la composante horizontale du rejet de la faille est alors favorisée là où la réaction à la déformation devient négligeable par suite de fracture de cisaillement. Le pourcentage de l'extension axiale peut s'exprimer par une équation qui compare la longueur de la courbe finale à la longueur originelle. Avec la connaissance des propriétés physiques des roches, on peut prévoir, le moment où les contraintes s'échangent durant l'évolution de la structure, ainsi que la réaction structurale qui en résulte.

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Deformation partitioning during transpression in response to Early Devonian oblique convergence,northern Appalachian orogen,USA

Transpressive deformation was distributed heterogeneously within the Central Maine belt shear zone system, which formed in response to Early Devonian oblique convergence during the Acadian orogeny in the northern Appalachians. ‘Straight’ belts are characterized by tight folds, S>L fabrics and sub-parallel form lines, and asymmetric structures that together indicate dextral–SE-side-up kinematics. In contrast, intervening zones between ‘straight’ belts are characterized by open folds and L≫S fabrics. Within both types of zone, metasedimentary rocks have fabrics defined by the same minerals at the same metamorphic grade, including a penetrative, moderately to steeply NE-plunging mineral lineation. Thus, we interpret accumulation of plastic deformation and regional metamorphic (re-) crystallization to have been synchronous across the Central Maine belt shear zone system. Discordance between inclusion trails in regionally developed porphyroblasts of garnet and staurolite and matrix fabrics in ‘straight’ belt rocks records shortening by tightening of folds and greater reorientation of matrix fabrics with respect to porphyroblasts. Kinematic partitioning of flow was responsible for the contrasting states of finite deformation recorded in the Central Maine belt shear zone system. Perturbations in the flow were caused by serially developed thrust-ramp anticlines in the stratigraphic succession immediately above the Avalon-like basement, at which décollement of the shear zone system was initially rooted. General shear deformation at the ramps involved strain softening with an enhanced component of noncoaxial flow. In contrast, deformation during extrusion in the intervening zones involved strain hardening with a greater component of coaxial flow. Part of the thickening stratigraphic succession exceeded T_solidus, reflected by the occurrence of migmatites and granites. The latter were partly sourced from the underlying Avalon-like basement that was involved in the deformation and melting.     

Deformation of experimentally shock-loaded quartz powders: X-ray line broadening studies

Quartz powders (mean grain size: 22 μm) were pressed into sample discs of different green densities (ρ₀=1.65; 1.85 and 2.05 g/cm³) and subjected to shock pressure between 1.5 and 17.0 GPa. Peak shock pressures were determined by the impedance method using the Hugoniot curves of steel and quartz powders. Fourier techniques were used to analyse the line broadening of 5 X-ray reflections in each case. The Fourier coefficients were subjected to a Rothman-Cohen correction before further processing. The coherent domain size decreases abruptly from several thousand Ångström in the unshocked state to about 800 Å at 1.5 GPa, and reaches a constant mean value of about 200 to 300 Å at dynamic pressures of about 4 GPa. At very high dynamic pressures (?15 GPa) there is further fragmentation to very small domain size. There is no systematic correlation between sample density and coherent domain size produced by the shock event. A more or less linear dependency exists between microstrain and pressure for each starting density of the quartz powder. The observation that microstrain decreases with increasing starting density could be explained by the fact that increasingly larger portions of the input energy are consumed to create fresh surfaces by comminution. Consequently, less energy will be available for strain strengthening. The stored energies in the shocked quartz powders are of the same order as the surface energy. Shock-treated quartz should therefore be suitable for accelerating any activated process such as sintering.     

Deformation partitioning and porphyroblast rotation in meta-morphic rocks: a radical reinterpretation

Abstract Most porphyroblasts never rotate during ductile deformation, provided they do not internally deform during subsequent events, with the exception of relatively uncommon but spectacular examples of spiralling garnets. Instead, the surrounding foliation rotates and reactivates due to partitioning of the deformation around the porphyroblast. Consequently, porphyroblasts commonly preserve the orientation of early foliations and stretching lineations within strain shadows or inclusion trails, even where these structures have been rotated or obliterated in the matrix due to subsequent deformation. These relationships can be readily used to help develop an understanding of the processes of foliation development and they demonstrate the prominent role of reactivation of old foliations during subsequent deformation. They can also be used to determine the deformation history, as porphyroblasts only rotate when the deformation cannot partition and involves progressive shearing with no combined bulk shortening component.     

Manganese partitioning during hydrous melting of peridotite

Manganese contents and the iron/manganese ratio of igneous rocks have been used as a method of probing the heterogeneity in the Earth’s mantle during melting of peridotite and pyroxenite lithologies. Most previous work has assumed that changes in these parameters require differences in either source lithology or composition based on experiments indicating that manganese is slightly incompatible during melting and that the iron/manganese ratio is fixed by the presence of olivine. However, the presence of volatiles in the mantle drives melting at lower temperatures and with different compositions than in volatile-free systems, and thus the partitioning of Fe and Mn may in fact vary. We have produced silicate liquids in equilibrium with a peridotite assemblage under hydrous conditions at 3 GPa that show that Mn can also be unexpectedly compatible in garnet at 1375 °C and that Mn partitioning between solids and liquids can be strongly affected by temperature and liquid composition. The compatibility of Mn in garnet provides a mechanism for large variations of Mn contents and the Fe/Mn ratio in silicate melts that solely involves melting of mantle peridotite with only small compositional changes. Correlations between Mn variations and other indices indicative of melting in the presence of garnet may provide a means of more completely understanding the role of garnet at high pressures in peridotite melting.     

不同地层条件下复合土钉墙的变形分析及承载机理的研究

复合土钉墙是一种新型的支护技术,不同于传统的土钉支护.本文将复合土钉墙支护技术分别应用于饱和黄土地层和粉喷桩复合地层,并通过有限元模拟计算,对其在不同地层条件和不同开挖阶段的变形及其承载机理进行了分析研究,可以看出,复合土钉墙变形随地层条件的不同而不同,其承载机理也与传统土钉墙有较大差异.这些结论将为以后设计复合土钉墙和进行基坑开挖过程中事故的防范提供重要参考.     

Trace element diffusion and incorporation in quartz during heating experiments

Heating of quartz crystals in order to study melt and high-temperature fluid inclusions is a common practice to constrain major physical and chemical parameters of magmatic and hydrothermal processes. Diffusion and modification of trace element content in quartz and its hosted melt inclusions have been investigated through step-heating experiments of both matrix-free quartz crystals and quartz crystals associated with sulfides and other minerals using a Linkam TS1500 stage. Magmatic and hydrothermal quartz were successively analyzed after each heating step for Cu, Al, and Ti using electron probe micro-analyzer. After the last heating step, quartz crystals and their hosted melt inclusions were analyzed by laser ablation inductively coupled plasma mass spectrometry and compared to unheated samples. Heated samples reveal modification of Cu, Li, Na, and B contents in quartz and modification of Cu, Li, Ag, and K concentrations in melt inclusions. Our results show that different mechanisms of Cu, Li, and Na incorporation occur in magmatic and hydrothermal quartz. Heated magmatic quartz records only small, up to a few ppm, enrichment in Cu and Na, mostly substituting for Li. By contrast, heated hydrothermal quartz shows enrichment up to several hundreds of ppm in Cu, Li, and Na, which substitute for originally present H. This study reveals that the composition of both quartz and its hosted melt inclusions may be significantly modified upon heating experiments, leading to erroneous quantification of elemental concentrations. In addition, each quartz crystal also becomes significantly enriched in Cu in the sub-surface layer during heating. We propose that sub-surface Cu enrichment is a direct indication of Cu diffusion in quartz externally sourced from both the surrounding sulfides as well as the copper pins belonging to the heating device. Our study shows that the chemical compositions of both heated quartz and its hosted inclusions must be interpreted with great caution to avoid misleading geological interpretations.     

Exhumation during crustal folding in the Namche-Barwa syntaxis

Geological observation in the eastern end of the Himalaya shows that the Asia/India Suture is folded. Metamorphic rocks derived from India occur structurally below the suture, in the core of a regional antiform. Isotopic and fission track dating establish cooling-exhumation of rocks from c.30 km depth within the last 4 Myr. We argue that exhumation is caused by ～ 10 mm yr^-1 erosion coeval with crustal scale folding.     

Variation of interlayer slip in space and time during flexural folding

This paper describes the results of an experiment which was performed with a Plasticine model to investigate the progressive evolution of flexural-slip folds. The model analyses the relationship between bulk shortening and the amount of flexural slip on fold limbs and in hinge zones. In the light of the experimental results, the theoretical relationship between limb dip and angular shear strain proposed by Ramsay (1967, p. 393) needs modifying to take into account the effect of hinge dilation and limb thinning at large deformations.     

Implications of trace element composition of syntaxial quartz cements for the geochemical conditions during quartz precipitation in sandstones

A petrographic investigation revealed polyphase quartz cementation in the Finefrau Sandstone (Upper Carboniferous, Western Germany) and the Solling Sandstone (Lower Triassic, Central Germany). Three different cements could be distinguished in each sandstone based on their cathodoluminescence and trace element composition. The first quartz generation is suggested to have been formed during eogenesis due to dissolution and replacement of feldspar. The mesogenetic paragenesis comprises two generations of quartz and illite, which are accompanied by albite in the Solling Sandstone. Sharp luminescence zoning in quartz overgrowths points to distinct episodes of cementation in both sandstones. Significant amounts of Al, Li and H and traces of Ge and B have been detected in the quartz overgrowths. The Al‐content of the quartz cements in the Finefrau Sandstones exceeds that in the quartz cements in the Solling Sandstone by a factor of five. It is suggested that this compositional variation reflects the conditions in the pore‐water, such as temperature and pH. The Al‐concentration is generally correlated to the Li‐content with the exception of the latest quartz generation in the Finefrau Sandstones which is also most enriched in trace elements. The ratio of Li/Al varies between 0·11 and 0·25 in the two sandstones. The Li/H‐ratio, which ranges from 0·12 to 0·3, is controlled by the activity ratio of Li and H in the pore fluid. Clay minerals are the most important source for Li and high salinities favour the mobilization of Li during diagenesis. Thus, a relatively low salinity and low pH are responsible for the low Li/H‐ratio in the Finefrau Sandstone, while high salinity and neutral to alkaline pH results in a high Li/H‐ratio for the Solling Sandstone. The Ge‐contents are generally near the average of detrital quartz and indicate that pressure dissolution is a major source for quartz cementation. Different chemical compositions of distinct quartz generations indicate changes in the physico‐chemical conditions and point to mobilization of silica from different sources (for example, pressure solution and clay mineral transformations).     

Deformation and ultrafine dynamic recrystallization of quartz in pseudotachylyte-bearing brittle faults: A matter of a few seconds

Tectonic pseudotachylytes, i.e. quenched friction-induced silicate melts, record coseismic slip along faults and are mainly reported from the brittle crust in association with cataclasites. In this study, we document the occurrence of recrystallization of quartz to ultrafine-grained (grain size 1–2 μm) aggregates along microshear zones (50–150 μm thick) in the host rock adjacent to pseudotachylytes from two different faults within quartzite (Schneeberg Normal Fault Zone, Eastern Alps), and tonalite (Adamello fault, Southern Alps) in the brittle crust. The transition from the host quartz to microshear zone interior includes: (i) formation of high dislocation densities; (ii) fine (0.3–0.5 μm) polygonization to subgrains defined by disordered to well-ordered dislocation walls; (iii) development of a mosaic aggregate of dislocation-free new grains. The crystallographic preferred orientation (CPO) of quartz towards the microshear zone shows a progressive misorientation from the host grain, by subgrain rotation recrystallization, to a nearly random CPO possibly related to grain boundary sliding. These ultrafine aggregates appear to be typically associated with pseudotachylytes in nature. We refer the crystal plastic deformation of quartz accompanied by dramatic grain size refinement to the coseismic stages of fault slip due to high differential stress and temperature transients induced by frictional heating. Microshear zones localized on precursory fractures developed during the stages of earthquake rupture propagation and the very initial stages of fault slip. Thermal models indicate that the process of recrystallization, including recovery processes, occurred in a time lapse of a few tens of seconds.     

Thermoluminescence spectra of igneous quartz and hydrothermal vein quartz

Variations in thermoluminescence spectra are reported for four types of geological quartz examined with a new spectrometer featuring dual imaging photon detectors that separately and simultaneously detect (1) uv-blue (200–450 nm) and (2) blue to near infrared (400–800 nm) emission. Samples show striking differences which appear to be characteristic of their geological origin. Volcanic quartz phenocrysts from acid volcanics show red thermoluminescence (TL) emission bands centered at 620–630 nm that are 100 times more intense than similar bands in other quartz, while a violet emission at 420–435 nm was observed exclusively in igneous quartz (volcanic and granitic). A broad emission band centered at 560–580 nm was observed only in quartz formed hydrothermally. Massive quartz from Li-rich pegmatite bodies shows narrow, intense 470 nm emission bands at 230° C apparently related to Al and to Ge defects detected with electron paramagnetic resonance (EPR), and emission bands at 330 and 280 nm, possibly related to recombination at oxygen vacancies. The common 380 nm emission band of quartz was observed in both volcanic and granitic quartz, but was not detected in either the pegmatitic or the hydrothermal vein quartz. Observed spectral variation is identified as a potential source of error in luminescence dating.     

Elastic behavior of quartz during stress induced dauphiné twinning

Suitably oriented and cut strain gauged crystals of synthetic α quartz have been subjected to uniaxial compressive stress with the aid of a stiff, 2MN press. Complete axial and lateral strain versus stress curves were obtained during stress-induced Dauphiné twinning and eventual complete volumetric transition to the alternate structural state. The alternate twinned state as revealed by observed stress-strain curves, is attained by either a continuous domain wall movement or by a sudden intricate composite transition process after reaching a critical coercive stress. Lateral strain versus stress curves indicate that contraction occurs in two perpendicular directions during compression in two of the three orientations studied, and that a change in sign of the lateral strains usually accompanies the final transition to the alternate structural state. The order and the sign of observed lateral and axial strains before and after transformation, is confirmed by theory. The movement of twin domain walls is explained in terms of ferrobielasticity. The initial development of stripe-shaped Dauphiné twins requires stress concentrations between the contact surfaces of a crystal and the platens of the press.