Mark Jessell

Grain-boundary migration microstructures in a naturally deformed quartzite

Abstract In this study several grain-scale microstructures are presented that are thought to demonstrate the migration direction of once-mobile grain boundaries in a naturally deformed quartzite. An analysis is presented of the sense of migration of the boundaries, and the characteristics of the patterns of relative grain growth and shrinkage. Grain-boundary migration seems to be correlated with the relative crystallographic orientations of neighbouring grains for the quartz—quartz grain boundaries, and the pattern of preferred grain growth is roughly symmetrical about the mica foliation plane.

Grain boundary migration and fabric development in experimentally deformed octachloropropane

Abstract In this study in-situ observations of a deforming aggregate of the hexagonal material octachloropropane have been analysed. Calculations of micro-strains and measurement of c -axis orientations have enabled the processes influencing fabric development to be distinguished, and the importance of dynamic recrystallization to be assessed. It was found that, in this material, inter-grain strain contrasts could be large, and the effect of grain boundary migration was to modify the fabric in a measurable way. A simple model for the driving force for grain boundary migration based on dislocation density contrasts, as controlled by intra-grain strains and grain orientations, is proposed and tested, with an 80% success rate for the mobile grain boundaries studied.

Three-dimensional geological modelling of potential-field data

This paper reviews different ways potential-field modelling systems represent the 3D structure of the Earths crust. 3D structures may be represented by discrete objects, voxels, surfaces or as a kinematic history and each technique provides the user with a different path through the model-building process and different abilities to honour the available geological and geophysical observations. At present, no one system is capable of supporting the range of uses for which potential-field data are collected, and one emphasis for future development is the ability to translate between the different representations that are used.

Elle: the numerical simulation of metamorphic and deformation microstructures

Abstract We present a generalised framework for the numerical simulation of the evolution of rock microstructures during deformation and metamorphism. This approach is based upon a data structure that describes a polycrystalline material using a two-dimensional network of nodes and connecting boundaries that allows micro-processes to be analysed at a range of scales. The nodes may possess attributes of position, topology and chemistry; and polygonal domains defined by these nodes may possess attributes of mineralogy, rheology and lattice orientation. We represent the complex behaviour of deforming and metamorphosing rocks at the grain scale as the interaction of a set of locally-defined driving forces and micro-processes, calculated for small time steps. A central program controls the evolution of extrinsic variables such as temperature and defines the history of deformation or metamorphic processes. The central program then passes the data structure to distinct process algorithms, which interact with this data structure, both by using it to determine the local values of driving forces, and by altering the attributes to simulate the progress of the process. We outline the function of the different aspects of the modelling task, provide simple examples showing porphyroblast growth during deformation and static grain growth, and describe the data structure that we have developed which enables us to handle multi-process simulations.

Bedding parallel veins and their relationship to folding

Abstract Laminated bedding parallel veins hosted in turbiditic sandstone shale sequences from central Victoria, Australia, consist of stacked, millimetre thick, sub-parallel sheets of quartz separated by micaceous layers, wall rock slivers and pressure solution seams. They have very high length to thickness ratios, are laterally continuous over tens to hundreds of metres, and have relatively uniform thickness compared to other vein types. They are intimately associated with and folded by chevron folds, and the quartz grain shape elongation lineation is commonly orthogonal to mesoscopic and macroscopic fold hinge lines. The bedding parallel veins have two morphological forms. Type I are thin (commonly 5–10 cm) laminated veins which have complex microstructures dominated by phyllosilicate inclusion surfaces, related to oblique opening along bedding with varying rates of deposition (opening) relative to shear displacement (slip) along the bedding surfaces. More common are Type II, thicker (generally

A simulation of the temperature dependence of quartz fabrics

Abstract This paper presents the predictions of a hybrid model of fabric development in quartzites that combines aspects of previous models based on lattice rotations and dynamic recrystallization. The input parameters for the model have been systematically varied in order to investigate changes in deformation fabrics due to increasing temperature, and predictions of crystallographic preferred orientations and grain shape fabrics are made. The resulting fabrics reflect the combination of both deformation processes over the range of the parameters used. The fabrics evolve continuously with progressive deformation, and produce crossed girdles, single girdles and point maxima for a single deformation geometry. The results suggest that the coupling of dynamic recrystallization with lattice rotations can actually produce a larger variation in crystallographic preferred orientations with temperature than is predicted by using a model that only considers lattice rotations. Using this model point maxima develop even though a multiple slip model has been assumed.

Experimental simulation of the formation of fibrous veins by localised dissolution-precipitation creep

Abstract Fibrous veins are generally interpreted in terms of the crack-seal mechanism. Several aspects of fibrous veins (fibrous structure, curved fibres, symmetry of antitaxial veins) are however better explained by vein formation without fracturing. Mass transfer to such veins would be by diffusional transport rather than by fluid flow through the veins. Deformation by dissolution-precipitation creep can provide the driving force for the necessary mass transfer. Veins form when mass transfer is heterogeneous and precipitation is localised. Experiments were performed which enforced a chemical potential gradient, acting as the driving force for diffusional mass transfer. These experiments resulted in fibrous growths in aggregates of soluble salts (NaCl and KCl) saturated with brine. The experimental results support the theory that fibrous veins may form without fracturing and that rather than providing evidence for major fluid pathways, fibrous veins may instead represent localised precipitation during diffusional material transfer.

Simulation of fabric development in recrystallizing aggregates—II. Example model runs

Abstract This study investigates the role of the coupling of dynamic recrystallization and lattice rotations in fabric development. A new two-dimensional computer simulation of polycrystalline deformation is used, which combines homogeneous straining, internal lattice rotations and dynamic recrystallization processes. Five example runs of the simulation are described here, which compare the progressive development of fabrics resulting from different recrystallization regimes and different straining geometries. It is found that these fabrics can evolve significantly with progressive strain from one strong fabric pattern to another. The effect of recrystallization is not only to create point maxima concentrations of c-axes, but also to modify and create girdle distributions. Correlations are made between the fabrics generated by this model and previously reported quartz and ice fabrics.

Simulation of fabric development in recrystallizing aggregates—I. Description of the model

Abstract A new computer simulation of the development of grain shape and crystallographic preferred orientations in dynamically recrystallizing aggregates is presented. This model is based on a uniform array of points, each of which represents a small area of crystal. This model combines homogeneous strains, simplified versions of the lattice rotations predicted by Taylor-Bishop-Hill theory, mobile grain boundaries and the nucleation of subgrains. It allows the progressive development of the fabrics and microstructures to be followed. The limitations of the simplifying assumptions used in this model are discussed.

Dominance of microstructural processes and their effect on microstructural development: insights from numerical modelling of dynamic recrystallization

Abstract The influence of the dominance of different processes on the microstructural development of a quartzite was investigated using the numerical model ‘ELLE’. Dynamic recrystallization of a polycrystalline aggregate was simulated by the concurrent operation of viscous deformation, lattice rotation, subgrain formation, rotational recrystallization, nucleation of new grains from strongly strained grains and recovery. The different observed microstructural characteristics depend on the relative rates at which grain boundary migration, subgrain formation, recrystallization by rotation and nucleation affect the microstructure. Observed sizes of recrystallized grains are significantly influenced by these different relative rates of processes. These rates are determined by parameters that mainly depend on temperature, fluid absence or presence, shear stress and strain rate. Therefore, the specific conditions at which deformation took place have to be taken into account if recrystallized grain sizes are used for palaeopiezometry. Comparison and combination of our results with experimental data and observations in natural examples provide the possibility of interpreting microstructures quantitatively in terms of temperature and shear strain rate.