Geoffrey E. Lloyd

MISORIENTATION ANALYSIS AND THE FORMATION AND ORIENTATION OF SUBGRAIN AND GRAIN BOUNDARIES

In contrast to the behaviour of individual grains, both inter- and intra-granular boundaries within rocks have received much less attention. However, many geological processes, particularly during deformation (e.g., yielding, dislocation creep, recrystallisation, superplasticity and various fracture mechanisms), and petrophysical properties depend to some extent on the nature of boundaries present in a rock. In this contribution, we consider the role of intergranular and intragranular crystal boundaries. A precise characterisation of such boundaries depends on defining the crystallographic and dimensional orientations of the boundary and the misorientation between the adjacent regions (i.e. grains, subgrains, etc.) separated by the boundary. Although several theoretical descriptions of boundary configuration are available, practical precision is lacking and approximations are necessary. We describe two specific approximations for boundary formation and orientation obtained using the SEM electron channelling technique. The first is a geometrical interpretation of electron channelling patterns (ECP) in terms of the likely formation and orientation of the intervening boundary. The second considers the misorientation between adjacent regions across a boundary. This involves a model which assumes a simple geometrical relationship between crystal slip systems responsible for the rotation and misorientation between adjacent regions, and the formation and orientation of the resulting boundary. These approximations are capable of: (a) identifying trends in the dispersion of crystallographic directions during deformation; (b) identifying active slip systems; (c) calculating the relative Schmid Factors for each crystal slip system (and therefore the most likely system to be activated); (d) modelling synthetic misorientations and predicting the crystal slip systems and boundary configurations to be expected; and (e) comparing real data with synthetic models. Our analyses are illustrated via natural examples of dynamic recrystallisation in quartzite and a theoretical simulation of the behaviour of an individual quartz grain during deformation.

A stress-transfer model for the development of extension fracture boudinage

Abstract The term boudinage is used to describe a wide variety of extensional structures in deformed rocks. This paper is mainly concerned with boudinage resulting from through-layer extension fractures followed by separation of the layer segments, thus forming boudins with more or less rectangular cross-sections. In principle, this process is similar to the break up of fibres in fibre-reinforced composite materials extended parallel to the fibre direction. Both processes are controlled by the transfer of stress from the matrix to the fibre (or layer) and a mathematical model for fibre-matrix stress transfer (the ‘fibre-loading’ model) is well established. We have used this as a basis for developing a stress transfer model for boudinage. The only difference in the basic mathematical formulation results from geometric differences between the two systems; the geometric expressions in the fibre-loading model have, therefore, been rederived for the layer-matrix case. Stress-transfer theory predicts that the tensile stress in a layer segment rises from a minimum at the end of a segment to a maximum at the centre. This behaviour, which is clearly shown by finite-element models of boudinage structure, suggests that extension fracture boudinage develops by successive ‘mid-point’ fracturing. According to stress-transfer theory, the process will continue until a layer is reduced to segments (boudins) all of which are shorter than some critical length (for which the tensile fracture strength of the layer is equal to the tensile stress at the mid-point). In practice, successive fracturing will be influenced by two other factors: (1) in nature the controlling material properties (tensile fracture strength, elastic moduli) will not be single-valued but will have a distribution reflecting local variations in lithology and microstructure and (2) major pre-deformation flaws may be present in a layer which will control the ‘starting length’ of layer segments. These factors are incorporated with the stress-transfer theory into a statistical (Monte Carlo) model for extension fracture boudinage which results in a prediction of boudin aspect ratios. The predicted distribution compares very closely with the observed distribution of 91 quartzite boudins within Lower Carboniferous slates at Tintagel, Cornwall. The stress-transfer model implies that boudin-defining fractures occur sequentially so that inter-boudin gap lengths will be unequal. Strain estimates based on boudinage structure will vary according to which part and how much of a layer is sampled. A much improved strain estimate is possible based on sequentially closing the inter-boudin gaps. The stress-transfer theory also leads to the possibility of estimating palaeostress from boudinage structure and is the only model available which predicts an aspect ratio distribution of boudins formed by extension fracture. Other than our own, we know of no published data on boudin aspect ratio distributions. Hence, further elaboration of the model is not possible until more field data is available. We hope that our work will encourage the systematic measurement of boudinage as well as the development of alternative models.

Dynamic recrystallization of quartz under greenschist conditions

The SEM electron channelling technique has been used to investigate the dynamic recrystallization of three quartz grains representative of optical textures observed in quartzite deformed under greenschist facies conditions. The degree of recrystallization has been determined at 10–20, ∼50 and 100%, respectively. We demonstrate that dynamic recrystallization is a sequential process involving subgrain-grain rotation and grainboundary migration. It initiates via subgrain polygonization on a single crystal slip system, and continues via subgrain and neoblast rotation on several slip systems. In the early stages of recrystallization ( < 50%), the orientation relationships between subgrains-neoblasts and parent grains are systematic and are related to the dominantly active crystal slip systems. Beyond ∼ 50% recrystallization, orientation relationships are less systematic and this is attributed to an increase in the activity of grain-boundary migration. However, dynamic recrystallization is potentially a cyclical process with new subgrains forming within migrational neoblasts due to continued deformation. The three sequences represent different stages in the same continuous, time- or strain-dependent, dynamic recrystallization history. They therefore combine to give a generalized dynamic recrystallization history for greenschist facies quartz grains at low to moderate strains. Porphyroclasts have similar recrystallization histories because the original bimodal grain size distribution leads to strain partitioning, allowing the porphyroclasts to rotate into weak orientations with the subsequent activation of weak crystal slip systems.

Deformation mechanisms accommodating faulting of quartzite under upper crustal conditions

Abstract Analysis of the deformation microstructures associated with a high-level fault in quartzite (Skiag Bridge, Assynt, NW Scotland) reveals a complex variation in the deformation mechanisms active during faulting. The different mechanisms have been identified using an integrated study involving optical, cathodoluminescence and electron (both SEM and TEM) microscopy. The specific mechanisms identified include: intragranular cleavage fracture (types 1, 2 and 3), brittle intergranular fracture (types 1 and 2), low-temperature ductile fracture, diffusion mass transfer and low-temperature crystal plasticity. Fracturing dominates the deformation (faulting), initially via intragranular extension cleavage fractures due to stress concentrations at grain contacts (although many of these may be healed by quasi-simultaneous diffusive mass transfer processes). These eventually link and are then exploited as shear fractures, leading to the development of microbreccia-cataclasite zones which define a three-dimensional fracture array. Quasi-simultaneous diffusive mass transfer processes may heal these through-going fractures. Continued fault zone deformation involves the development of a damage (‘wake’) zone along the displacement zone borders where low-temperature plasticity and subsequent low-temperature ductile fracture processes aid the expansion of the fault zone. This study emphasizes that the evolution of the Skiag Bridge fault zone has involved three main categories of deformation mechanisms: fracture, crystal plasticity and diffusion mass transfer. The interrelationship between these categories, and the transition between individual fracture mechanisms, are significant aspects of this evolution. The examples presented demonstrate the complex interrelationships which exist between a group of deformation mechanisms and emphasize the potential importance of low-temperature plasticity and low-temperature ductile fracture processes during faulting under upper crustal conditions.

Boudinage structure: some new interpretations based on elastic-plastic finite element simulations

Abstract An elastic-plastic finite element method, based on the Prandtl-Reuss equations of plastic flow and involving equivalent stresses and strains, is used to study boudinage structure. Our choice of data for the simulations was guided by published stress-strain curves for marble (matrix) and quartzite (boudin), the essential parameters being yield stress and rock ‘hardness’ (defined by the slope of the stress-strain curve). All models assume an initial fracture and slight separation and therefore only simulate post-fracture behaviour. The simulations suggest that boudin shape is determined by boudin hardness; maximum stresses are concentrated in the corners which therefore shows the most shape modification. Matrix hardness determines the amount of boudin separation. Direct comparison with natural examples is restricted to boudins suffering no significant pre-fracture plastic deformation (i.e. rectangular- and barrel-shaped boudins), although other types are likely to have the characteristics of barrel and pinch-and-swell styles. The simulations do not consider the nature and timing of boudin-defining fractures but these are important in determining the style of boudinage which ultimately develops. Some mechanical problems associated with the infilling of inter-boudin gaps by ductile rock matrix are discussed and two models proposed. The first, based on yielding fracture mechanics, is used to explain boudins with wedge-shaped (or otherwise nonmatching) ends. The second, a hydraulic model, is proposed to account for gaps between rectangular boudins that are filled by ductile rock matrix.

The role of basement reactivation in continental deformation

The papers in this thematic set arise from a discussion meeting sponsored by the Tectonic Studies Group and held at Burlington House on 6-7 March 1996. About 170 people heard 19 formal oral presentations and studied 17 posters of which 15 were briefly introduced with five minute talks. Sixteen of these contributions are published in this thematic set. Pre-existing heterogeneities in the continental lithosphere are thought to influence markedly its response to subsequent deformation. A major reason for this thinking is the long held view (e.g. Hills 1956) that fault zones are inherently weak. Virtually all regions of continental lithosphere are riddled with faults and shear zones. Thus any subsequent deformation might be expected to utilize these pre-existing weak zones (as reviewed by White et al. 1986). This type of reactivation, where faulting activity is localized onto pre-existing faults, can be contrasted with more general cases of deformation reworking broader volumes of lithosphere. There need not be complete reactivation of individual faults and shear zones. The distinction is nicely illustrated by the concepts of basin inversion, a topic that has been well aired in recent years (e.g. Cooper & Williams 1989;Coward 1994). In many sedimentary basins there is evidence of contractional reactivation of segments of originally extensional faults (e.g. Letouzey 1990). Yet for the contrasting tectonic setting of collisional mountain belts, recent work has emphasized that, in general, thrusts do not reactivate pre-existing extensional faults (e.g. Gillchrist et al. 1987). However, the sites of almost all the shortened continental

Individual orientation measurements in quartz polycrystals: advantages and limitations for texture and petrophysical property determinations

Individual orientation determination of quartz grains by electron channelling in principle gives the complete orientation. However, in routine analysis the noise level in electron channelling patterns (ECPs) does not permit the determination of handedness of a quartz grain in a polycrystal. In practice, all quartz grains are arbitrarily indexed as right-handed. Hence, Dauphine twins can be identified, but not Brazil twins. This practice also means that only the centrosymmetric petrophysical properties can be determined from texture measurements. These include most geologically relevant properties (e.g. thermal conductivity, thermal expansion and elasticity). However, other properties (e.g. piezoelectricity) which are not centrosymmetric cannot be calculated from such texture measurements. Some texture-forming processes (e.g. dislocation glide) can also be considered to be centrosymmetric in quartz, whereas others (e.g. grain boundary migration) may not be. The method of quantitative texture analysis from individual measurements is briefly recalled. As an example, 382 grains from Tongue quartzite are used to illustrate the advantages of texture analysis from ECPs. The orientation distribution function (ODF) is calculated from ECPs and X-ray pole figures of the same sample. The agreement is found to be good between the two methods, proving that ECPs can be used for quantitative analysis. The methods used in local texture analysis and the definitions of the various misorientation distribution functions (MODFs) are given. Data collected from a traverse of a quartzo-feldspathic shear zone in Lewisian gneiss (Torridon ‘quartzite’) are used to illustrate local texture analysis. Examples from a region of shear strain of about one are given of core and mantle subgrains and Dauphine twins. Dispersion trails of the crystallographic axes within a single grain show an apparent rotation about the intermediate structural axis Y. Detailed analysis of the subgrain misorientation axes in specimen and crystallographic co-ordinates show an important scatter, implying that the subgrains resulted from local incompatibility strains rather than specimen-scale kinematics. The method of calculation of physical properties from individual orientation measurements is given for second-and fourth-order tensors. Using the texture data from Tongue quartzite we have calculated thermal conductivity, thermal expansion and seismic velocities. All these properties are extremely anisotropic in quartz. However, it is emphasized that the presence of a second phase on grain boundaries (e. g. water, graphite) may completely alter a physical property (e.g. electric) and render the values calculated from texture measurements inappropriate.

Microstructural and crystal fabric evolution during shear zone formation

Abstract The microstructures and crystal fabrics associated with the development of an amphibolite facies quartzo-feldspathic mylonitic shear zone (Torridon, NW Scotland) have been investigated using SEM electron channelling. Our results illustrate a variety of microstructures and fabrics which attest to a complex shear zone deformation history. Microstructural variation is particularly pronounced at low shear strains: significant intragranular deformation occurs via a domino-faulting style process, whilst mechanical incompatibilities between individual grains result in characteristic grain boundary deformation accommodation microstructures. A sudden reduction in grain size defines the transition to medium shear strains, but many of the boundaries inherited from the original and low shear strain regions can still be recognized and define distinctive bands oriented at low angles to the shear zone margin. Grains within these bands have somewhat steeper preferred dimensional orientations. These domains persist into the high shear strain mylonitic region, where they are oriented subparallel to the shear zone margin and consist of sub-20 μm grains. The microstructures suggest that the principal deformation mechanism was intracrystalline plasticity (with contributions from grain size reduction via dynamic recrystallization, grain boundary migration and grain boundary sliding). Crystal fabrics measured from the shear zone vary with position depending on the shear strain involved, and are consistent with the operation of several crystal slip systems (e.g. prism, basal, rhomb and acute rhomb planes) in a consistent direction (probably parallel to a and/or m ). They also reveal the presence of Dauphine twinning and suggest that this may be a significant process in quartz deformation. A single crystal fabric evolution path linking the shear zone margin fabric with the mylonitic fabric was not observed. Rather, the mylonitic fabric reflects the instantaneous fabric which developed at a particular location for a particular shear strain and original parental grain orientation. The mature shear zone therefore consists of a series of deformed original grains stacked on top of each other in a manner which preserves original grain boundaries and intragranular features which develop during shear zone evolution. The stability of some microstructures to higher shear strains, the exploitation of others at lower shear strains, and a continuously evolving crystal fabric, mean that the strain gradient observed across many shear zones is unlikely to be equivalent to a time gradient.

Microstructural evolution in a mylonitic quartz simple shear zone: the significant roles of dauphine twinning and misorientation

Abstract SEM/EBSD-based orientation and misorientation analyses are described for a lower amphibolite facies simple shear zone (Torridon, NW Scotland). It is shown that as well as conventional crystal-slip processes (i.e. basal-a, prism-a, rhomb-a and negative second order rhomb-a slip), dauphine twinning also plays a role in both microstructural and petrofabric evolution. Twinning assists in the initial grain size comminution processes, including dynamic recrystallization, from originally coarse wall rock grains to a typical mylonitic microstructure in the centre of the shear zone. Subsequently, twinning helps to accommodate high shear strains in the mylonite whilst maintaining a stable microstructure and constant ‘single crystal’ petrofabric. The role of dauphine twinning appears to be to allow efficient switching between relatively ‘soft’ and relatively ‘hard’ slip directions that possibly exploit a distinction between negative and positive crystal forms. Misorientation analysis emphasizes the relationships between crystal-slip systems and grain boundary network, including dauphine twin planes, and suggests that the mylonitic microstructure contains preferred orientations of both tilt and twist boundaries that help to explain shear zone microstructural evolution and stability.

Grain boundary contact effects during faulting of quartzite: an SEM/EBSD analysis

Abstract During low-temperature faulting of Cambrian quartzite (Assynt, NW Scotland), stress concentrations develop at grain contacts either at the onset of deformation, prior to the establishment of a through-going fault plane, or within the damage zone remote from the main displacement segment. Such concentrations contribute to the development of intragranular microfractures, cataclastic microstructures and fault rocks. This contribution considers the progressive deformation sequence that precedes microfracturing and cataclasis. The complexity of this deformation is revealed by scanning electron microscope (SEM) electron backscattered diffraction (EBSD). Dauphine twinning is a widespread feature associated with grain contact stress concentration and forms distinctive EBSD microstructures. Automatic SEM/EBSD analysis reveals that whilst initial indentation causes dauphine twinning of many grains, continued indentation results in the formation of an arcuate array of subgrains via low temperature plasticity and/or microcracking, which overprint the dauphine twins. These observations are consistent with transmission electron microscopic analysis of quartz crystals used for microhardness indentation tests, which reveal that indentation causes an intensely deformed region to develop, comprising a high density of microfractures and a submicron scale ‘blocky’ microstructure that accommodates any ‘plastic’ deformation. Deformation mechanisms and associated microstructures develop sequentially with progressive indentation and may provide sites of microfracture nucleation via low-temperature ductile fracture. The new microstructures assist diffusive mass transfer (DMT) processes by the formation of a cellular or subgrain array that represents a reduction of several orders of magnitude in apparent grain size and hence in diffusion path length. Concomitantly, associated microfracturing perturbs local thermodynamic equilibrium, leading to enhanced DMT, crack healing and cementation overgrowths. Together, these processes form the aseismic creep and sealing components of fault zone development.