Janos L. Urai

Deformation-related recrystallization processes

Abstract Recrystallization is a common microstructural transformation that occurs during deformation, metamorphism and diagenesis of rocks. Studies on minerals and rock analogues have demonstrated that a wide range of recrystallization mechanisms can occur. The range of mechanisms is related to the various ways in which two basic processes, grain boundary migration and new grain boundary formation combine to transform the microstructure. Two recent papers (Drury et al., 1985; Urai et al., 1986) have proposed different schemes for the description of recrystallization mechanisms. The purpose of this paper is to provide a unified framework for the description of mechanisms. Recrystallization mechanisms are divided into three main types; rotation mechanisms which principally involve the formation of new grain boundaries; migration mechanisms which principally involve grain boundary migration; and general mechanisms which involve both basic processes. A further distinction is made on the basis of the continuity of the microstructural transformation with respect to time. Each of the three main types of mechanism can be divided into a number of sub-types depending on whether the processes of grain boundary migration, new grain boundary formation and new grain formation occur in a discontinuous or continuous manner with respect to time. As the terms continuous and discontinuous have been used in the metallurgical literature to signify the spatial continuity of the microstructural transformation, the terms discontinuai and continual are used to refer to the temporal continuity of the transformation. It is recommended that the following aspects should be specified, if possible, in a general description of recrystallization mechanisms: 1. (1) How do the basic processes combine to transform the microstructure. 2. (2) If new grain development occurs, what is the development mechanism, and does new grain formation occur in a continual or discontinuai manner. 3. (3) If grain boundary migration is involved in the transformation, what is the migration mechanism (i.e. fast solute escape migration, slow solute loaded migration, fluid assisted migration, etc.), and is migration a continual or discontinuai process. The application of the unified scheme is illustrated by reviewing studies that have provided detailed information on the recrystallization mechanisms involved. The complicating effects of solid solution impurities, dispersed second phase particles and grain boundary fluid films are also considered and it is demonstrated that variations in content of these types of impurity can significantly effect the types of recrystallization that occur in a given material.

The swelling of clays: molecular simulations of the hydration of montmorillonite.

The swelling of clay minerals on contact with an aqueous solution can produce strong adverse effects in the exploration and production of gas and oil. Molecular dynamics and Monte Carlo simulations were used to study the mechanism of swelling of sodium-montmorillonite. The simulations showed that the abundant clay mineral has four stable states at basal spacings of 9.7, 12.0, 15.5, and 18.3 angstroms, respectively. The amount of swelling and the locations of the stable states of sodium-montmorillonite are in good quantitative agreement with the experimental data.

Weakening of rock salt by water during long-term creep

The rheological properties of rock salt are of fundamental importance in predicting the long-term evolution of salt-based radioactive waste repositories and strategic storage caverns, and in modelling the formation of salt diapirs and associated oil traps1,2. The short-term, high-stress rheology of rock salt is well known from laboratory experiments; however, extrapolation to appropriately low stresses fails to predict the rapid flow seen in certain natural structures. Furthermore, experiments have failed to reproduce the recrystallized microstructure of naturally deformed salt. Here we report experiments indicating that the above discrepancies can be explained by taking into account the influence of trace amounts of brine. Trace brine is always present in natural salt but sometimes escapes during experiments. Our tests on dry dilated salt show more or less conventional dislocation creep behaviour, but brine-bearing samples show marked weakening at low strain rates. This is associated with dynamic recrystallization and a change of deformation mechanism to solution transfer creep. Because natural rock salt always contains some brine, these results cast substantial doubt on the validity of presently accepted dislocation creep laws for predicting the long-term rheological behaviour of salt in nature.

Kinematics of crystal growth in syntectonic fibrous veins

Abstract Detailed observations of a set of fibrous antitaxial calcite veins in a slate reveal that some of the calcite fibres do not connect material markers on both sides of the vein and can therefore not have tracked the full opening trajectory during vein growth. This calls for a better understanding of the mechanisms of fibre formation and reliable criteria to test the tracking hypothesis. Based on surface roughness characteristics of the vein wall we develop a simple model for shape-growth of crystals in a crack-seal environment which can account for both tracking and non-tracking behaviours, and propose a set of ‘tracking criteria’ for antitaxial veins. Finally we discuss ways by which the model can be tested against natural examples.

Water assisted dynamic recrystallization and weakening in polycrystalline bischofite

Urai. J.L.. 1983. Water assisted dynamic recrystallization and weakening in polycrystalline bischofite.

Top-seal leakage through faults and fractures: the role of mudrock properties

Abstract Mudrocks are effective top seals for hydrocarbon accumulations because they possess very low permeabilities, high capillary entry pressures, and are often laterally continuous basin-wide. For leakage through the seal to take place, an additional mechanism must provide enhanced permeability in mudrocks. Tectonically induced, dilatant faulting and fracturing in brittle rocks is such a mechanism. The effectiveness of mudrocks as seals may be compromised by a number of other factors, such as tectonic fault displacements in excess of the seal thickness; tensile fracturing under extreme fluid pressure conditions; and leakage via a network of juxtaposed thin leaky beds across sub-seismic faults within the seal. Before hydrocarbon trap integrity analysis, seismic interpretations should honour the fundamental geometry of the trap as closely as possible because top seals can be reliably appraised only when basic geometries are accurately determined. Fluid pressure is a proven risk in many exploration provinces, in terms of mudrock top-seal leakage via opening mode fractures. This natural hydrofracturing can take place if buoyancy pressures, combined with fluid overpressures, exceed the minimum in situ horizontal stress, plus the tensile strenght of the seal, an occurrence that leads to mechanical failure of the seal rock. The rheology of a seal is another important factor as it determines the failure mode, i.e. whether the rocks are ductile and remain sealing after deformation or whether they deform in a brittle manner to create permeable leak paths. Several techniques have been developed to predict rock rheology; these rely on uplift, sonic velocity and clay content data. Burial curves can be used to determine overconsolidation of mudrocks. Sonic velocity data can be used to estimate unconfined compressive strength in mudrocks. The most direct method utilizes relationships between mudrock friction angle, swelling clay content and mudrock surface area to determine ductility. The effect on trap integrity of sub-seismic faults is also quantifiable because advances in structural geological technology now permit predictions of the numbers of sub-seismic faults in a trap. This information, combined with detailed top-seal stratigraphic data, provides the power to screen traps for the risk of top-seal leakage via sub-seismic fault juxtapositions within the seal. A simple strategy incorporating these factors and linking them to other structural and stratigraphic information can contribute to reducing uncertainty about top-seal leakage caused by tectonic deformation.

Development of crystal morphology during unitaxial growth in a progressively widening vein: II. Numerical simulations of the evolution of antitaxial fibrous veins

The development of fibrous morphology and capability of fibres for tracking the opening trajectory were investigated using numerical simulations of a natural antitaxial fibrous vein. Starting from a non-unique best case, variation of fracture opening velocity, grain size, wall roughness, growth anisotropy and crystal growth velocity shows that these parameters differ in importance for crystal morphology and tracking capability. Fibrous veins can be simulated using crack–seal opening of the fracture. Grain boundaries track the opening trajectory if the wall roughness is high, opening increments are small and crystals touch the wall before the next crack increment starts.

Alpine deformation on Naxos (Greece)

Abstract A detailed structural study of Naxos (Attic-Cycladic massif, Greece) reveals two major deformation events. The first one is associated with large scale thrusting and high-pressure-low-temperature metamorphism during an early Alpine subduction episode. The second event occurred during continental extension and the associated development of localized thermal domes, where lower crustal rocks were brought into contact with upper crustal units along a major shallow dipping shear zone. We agree with a model of Naxos as a Cordilleran type Metamorphic Core Complex. However, our observations show that the sense of shear was ‘upper plate moving North’ during the second event, calling for a reinterpretation of existing tectonic models of the Cyclades.

Microstructural observations on natural syntectonic fibrous veins: implications for the growth process

Abstract Syntectonic antitaxial and ataxial fibrous veins were investigated using SEM, microprobe, cathodoluminescence (CL) and optical microscopy. In antitaxial calcite veins, fibres and surrounding selvage grew simultaneously, with similar growth rates of crystallographically differently oriented grains. New material precipitated at the vein margin in antitaxial and bi-mineralic ataxial microstructures. Bridges of country rock material formed during vein growth in an initial en-echelon vein system. In our antitaxial and bi-mineralic ataxial samples, the spacing of solid inclusions does not reflect individual crack-seal openings.

Vorticity and strain analysis using Mohr diagrams

Abstract Fabric elements in naturally deformed rocks are usually of a highly variable nature, and measurements contain a high degree of uncertainty. Calculation of general deformation parameters such as finite strain, volume change or the vorticity number of the flow can be difficult with such data. We present an application of the Mohr diagram for stretch which can be used with poorly constrained data on stretch and rotation of lines to construct the best fit to the position gradient tensor; this tensor describes all deformation parameters. The method has been tested on a slate specimen, yielding a kinematic vorticity number of 0.8 ± 0.1.