H.-R. Wenk

Calcite textures: examples from nappes with strain-path partitioning

Abstract Strain-path partitioning (partitioning of deformation into components of pure and simple shear) in two Alpine nappes is qualitatively indicated by a variation in orientation of foliation, microkinematic criteria, quartz textures, and a comparison with strain-path partitioning in theoretical nappe models. The degree of non-coaxial deformation relative to the total deformation is quantified by using the asymmetry of textures in calcite tectonites. The texture interpretation is based on both polycrystal plasticity models and experimental data. Calcite tectonites in the Graz-Paleozoic thrust sheet (Austria) and the Morcles nappe (Switzerland) were deformed in a strain path with both pure and simple shear components. Texture asymmetry supports the strain path derived from geological observations. As an exception, a high-strain ultramylonite deformed nearly exclusively in pure shear. It is suggested that this represents a softening instability since deformation in pure shear is energetically more favorable than in simple shear. The incompatibilities caused by heterogeneous pure shear are accommodated by detachments and heterogeneous simple shear adjacent to the pure shear zone.

Deformation of dolomite single crystals from 20–800° C

AbstractDolomite single crystals of six different crystallographic orientations were tested in compression under confining pressure at temperatures of 20 to 800° C. The chosen orientations favoured slip or twinning on particular systems. The deformed crystals were analysed by optical and high voltage transmission electron microscopy to determine activated deformation systems, dislocation behaviour, etc., and to assist in interpreting stress-strain data. It is shown that slip on c≡(0001) and on

Structure, formation, and decomposition of APB's in calcic plagioclase

f \equiv (\bar 1012)

Dauphiné twinning in deformed quartzites: Implications of an in situ TEM study of the α-β phase transformation

), and twinning on f between 300 and 600° C, are the principal modes of deformation. At low temperatures there is considerable cataclasis, and shear-fracturing must be counted as a significant deformation mechanism. The effects of climb become apparent at temperatures ≳600° C.A pronounced increase in strength with testing temperature shown by some orientations of the crystals is largely associated with c slip, but f twinning also shows similar although weaker tendencies. The yield stress for f slip decreases markedly with temperature. Values of critical resolved shear stress are obtained for c and f slip, and for f twinning. Major and minor slip systems, dislocation and twin configurations generated by deformation under different regimes are documented and illustrated. Some of the characteristics of the deformation systems are attributed to the details of atomic displacements. In particular, the increase in strength with temperature for c slip is explained by friction of CO32−groups during dislocation movement, which is unique for c slip. This friction increases with thermal vibration, expansion and rotation of the CO32−groups.

Texture analysis of polycrystalline silicon films

The paper compares pole figure determinations with neutron diffraction and X-ray diffraction on experimentally deformed carbonate rocks, a coarse grained marble and fine grained limestone. Neutron diffraction enables determination of complete pole figures on a single spherical specimen and is advantageous for coarse grained materials. Results obtained with both diffraction techniques agree satisfactorily, although uncertainties introduced by counting statistics are more serious for neutrons than for X-rays. Continuous detectors add new possibilities which are still little explored.

Elastic properties of polycrystals with trigonal crystal and orthorhombic specimen symmetry

In calcic plagioclase (Ca, Na) [(Al, Si) AlSi2O8] Al-Si ordering produces superstructures with periodic and non periodic antiphase boundaries (APBs). Crystals growing at high temperature close to the melting point and cooling fairly rapidly order by nucleation of ordered domains which grow, resulting in an irregular pattern of curved APBs (b plagioclase). A modulated structure with periodic APBs forms by continuous ordering at large undercooling below the critical ordering temperature (e plagioclase). During annealing APBs are eliminated by pairwise recombination of adjacent APBs to reduce strain energy along the boundaries thereby transforming nonstable e plagioclase into stable b plagioclase without change in chemical composition. This process is often accompanied by a chemical phase separation with APBs providing favorable surfaces for diffusion. Transformations are documented by transmission electron microscopy (TEM) micrographs illustrating the variation in morphology of APB patterns in igneous and metamorphic plagioclase. They are in agreement with Korekawa et al.s (1978) model of intermediate plagioclase which relies on periodic stacking of basic units rather than wavelike modulations. The paper includes observations of a new type of satellite in Stillwater bytownite (‘h’ satellites) which are due to fine lamellar exsolution.

Textures of laser ablated superconducting thin films of YBa sub 2 Cu sub 3 O sub 7 minus. delta. as a function of deposition temperature

A Middle Devonian suprasubduction zone ophiolite, the Careón Unit (northwest Spain), displays amphibolite‐facies ductile deformation fabrics related to the onset of the Rheic Ocean closure. Two different fabrics, an early high‐T foliation and a subsequent lower‐T foliation, each of which characterized by distinct deformation mechanisms, have been identified in two distinct crustal‐scale shear zones of the same ophiolitic thrust sheet. Combined quantitative texture analysis by electron backscattered diffraction and time‐of‐flight neutron diffraction, were carried out on the shear zones and correlated with micro‐ and macrostructural data. The results indicate that the regional lineation and shear zone kinematics (east‐west, top‐to‐the‐east) represent fabrics developed essentially during the intraoceanic subduction of the Rheic Ocean, and their orientation may be considered a reference vector for convergence models in this part of the Variscan belt.

Atomic resolution microscopy of carbonates. Interpretation of contrast

The occurrence of Dauphiné twinning in deformed quartzites has been investigated by means of hotstage high voltage transmission electron microscopy (TEM). In-situ observations show that Dauphiné twins created during the α-β phase transition interact strongly with dislocation substructures, with the result that some twins persist at temperatures many degrees below the phase transition temperature, and probably to room temperature. Attempts were made to establish whether Dauphiné twinning was responsible for the differences in preferred orientations between positive and negative rhombs, reported by Tullis and Tullis (1972) in quartzites experimentally loaded below the yield stress at various temperatures. The results were inconclusive. We could not identify any Dauphiné twinning in the samples, even in regions where there were concentrations of dislocations.