Abstract We present the first detailed stratigraphic and structural geological map of offshore north Cornwall and Devon, SW England, based on freely available bathymetric data. Although the bathymetry is often spectacular, revealing fold and fault structures exposed on the seabed at a range of scales and with high resolution, interpretation is not as straightforward as it might appear and depends critically upon both accuracy and knowledge of the onshore geology. Unfortunately, onshore stratigraphic controls are limited and restricted to several thin ‘named shales’ whose coastal outcrops are not always well constrained. In addition, the structure is markedly non-cylindrical on local to regional scales, making seaward projection problematic, while the impact of early thrusting on the stratigraphy has often been previously neglected. We therefore developed a workflow to handle the problems we encountered, including: recognition of vertical to horizontal and 3D to 2D projections; variations in bathymetric data characteristics; prediction of expected seabed outcrop geometries based on coastal structures; incorporation of non-cylindrical effects; and problems with quantitative GIS terrain profiles and structural measurements that result in an absence of such measurements. The geological map produced should therefore be viewed as a step forward, but as forming a base for further detailed bathymetric mapping.

Abstract Mapping on the microstructural scale can contribute significantly to conventional field and larger scale mapping and understanding of spatial, temporal and process-oriented relationships. Here, electron backscattered diffraction (EBSD)-based microstructural maps are presented of subgrain and Dauphiné twin boundaries in undeformed sedimentary quartzite. ‘Plane matching’ analysis permits determination of the complete orientation of boundaries. A workflow is presented to facilitate the necessary crystallographic calculations. The resulting maps indicate: (1) boundary plane rotation angle/axis pairs, including tilt–twist components; (2) boundary migration vectors; and (3) conventional EBSD misorientation angle/axis pairs. Subgrain boundaries are general with small misorientations and boundary plane normal directions sub-parallel to grain boundary stress concentrations; rotation axes are oriented sub-parallel to the bedding dip (017°/10°E). Most exhibit bi-direction boundary migration vectors parallel to bedding normal. The EBSD misorientation analysis results are different as they only recognize the parallelism of adjacent crystal lattices. The differences are especially apparent for Dauphiné twin boundaries. Maps are presented of twin boundaries using matched plane analysis, including explanation for lateral twin migration. Driving forces to move twin boundaries are also estimated by mapping variations in Young's modulus between parents and twins; differences are significant, indicating that the Young's modulus and driving forces do not need to be large, explaining the propensity for twinning in many quartzites.

Abstract Two models to explain the progressive deformation of syntectonic quartz veins are derived from conventional theories for simple and pure shears. The simple-shear model is based on reorientation and changes in length of linear vein elements, and predicts initial orientations of veins for imposed shear strains, elongations and strain ratios. The pure-shear model considers changes in length of lines variably orientated relative to the maximum compression direction, and yields estimates of elongation strains and strain ratios. Expectations of both models are different, as illustrated by analysis of quartz veins from the Rhoscolyn Anticline, Anglesey, NW Wales. The simple-shear model recognizes three distinct initial orientations, which predict different strains across the fold; the pure-shear model suggests veins were initially sub-parallel to the principal compression direction and predicts effectively constant strains across the fold. In addition, both models predict different patterns of fold vergence: for simple shear, vergence depends on magnitude and direction of shearing and may exhibit complex patterns; for pure shear, vergence patterns are predicted to be essentially constant. In general, the predictions of either model are critically dependent on the origin of the veins, particularly relative to the formation of the Rhoscolyn Anticline.

Abstract This paper presents the background for the calculation of anisotropic piezoelectric properties of single crystals and the graphical display of the results in two or three dimensions, and the calculation of the aggregate properties from constituent crystals and the texture of the aggregate in a coherent manner. The texture data can be obtained from a wide range of sources, including pole figure diffraction and single orientation measurements (electron backscattered diffraction, electron channelling pattern, Laue Pattern, optical microscope universal-stage). We consider the elastic wave propagation in piezoelectric crystals as an example of the interaction of electrical (2nd rank tensor), piezoelectric (3rd rank tensor) and elastic properties (4th rank tensor). In particular, we give explicit formulae for the calculation of the Voigt averaged tensor from individual orientations or from an orientation distribution function. For the latter we consider numerical integration and an approach based on the expansion into spherical harmonics. We illustrate the methods using single crystals, polycrystalline quartz measured using electron channelling patterns and ideal Curie limiting groups applied to quartz aggregates. This paper also serves as a reference paper for the mathematical tensor capabilities of the texture analysis software MTEX .

Abstract Samples of the Acebuches metabasites (SW Spain), deformed under low-pressure/medium-to-high temperature metamorphic conditions, have been analysed via electron backscattered diffraction (EBSD) to obtain their plagioclase crystal lattice preferred orientations (LPO). Plagioclases from the highest temperature amphibolites show moderate LPO and a good correlation between 180° misorientation angles and both the crystal and the kinematic coordinate systems, which is attributed to dislocation glide accommodated by mechanical albite+pericline twinning. Plagioclases from medium-temperature amphibolites exhibit well-developed LPO, suggesting that dislocation creep was active during plagioclase deformation. Plagioclases from the more intensively deformed mafic schists exhibit weak LPO, indicating the activity of LPO-destroying deformation mechanisms. Evidence points to grain-boundary sliding accompanied by limited fracturing. The observed LPO are characterized by the alignment of [100] parallel to the kinematic X -direction. This association suggests that [100] was the preferential slip direction during dislocation creep of plagioclase, with (010) and/or (001) appearing to have acted as the dominant slip planes. The observed plagioclase LPO is combined with hornblende LPO to define the seismic fabric of the Acebuches metabasites. In samples with strong plagioclase LPO, the resulting seismic fabrics are highly influenced by this phase.

Abstract We have applied transmission electron microscopy (TEM) analyses coupled with viscoplastic self-consistent (VPSC) numerical modelling to identify the active slip systems and to better understand the crystal preferred orientation (CPO) development of the Torridon quartz mylonite (NW Scotland). TEM analyses showed evidence of activation of 1/3〈 a 〉{π′}, 1/3〈 a 〉{ z } and possible 〈 a 〉( c ) slip systems, as well as dislocation climb and dynamic recrystallization. All the CPOs generated by VPSC models share common characteristics with the Torridon quartz mylonite, but only Models 2 and 3 reproduce the [ c ]-axes maxima at low angle (<20°) to the foliation pole along the YZ plane, as observed in the mylonite. In Model 2, this concentration only occurs at γ≥2.6, whereas in Model 3 this maxima occurs at lower shear strains. The models that start with a previous preferred orientation acquire very strong CPOs after small-imposed strains, followed by the rapid rotation of the fabric in relation to the new imposed finite strain axes. The combined activation of 〈 a 〉{π′}, 〈 a 〉{ z } and possibly 〈 a 〉( c ) slip systems, as demonstrated by TEM analyses, suggests that the VPSC model that best predicts CPO development in the Torridon quartz mylonite is Model 2, where the critical resolved shear stress (CRSS) of 〈 a 〉{π/π′} is assumed to be slightly stronger than 〈 a 〉( c ).

Abstract The mylonitic Cambrian quartzites, Moine Thrust Zone, NW Scotland, have long been used to study microstructural and petrofabric evolution and to develop understanding of grain-scale processes accommodating large-scale displacements. Today, structural geology is entering a new age of understanding of the basic processes involved in microstructural evolution due to the emergence of novel instrumental techniques and theoretical models. It seems apposite therefore to re-evaluate the microstructure of one example of this classic quartz mylonite from the Stack of Glencoul, Assynt, using arguably the most important of these new techniques, electron backscattered diffraction (EBSD). The three-dimensional (3D) microstructure and petrofabric of this rock was analysed using EBSD, to: 1) corroborate previous optical and X-ray texture goniometry measurements; 2) investigate the potential for sampling and/or tectonic sectioning bias that may be introduced inadvertently into any petrofabric analysis; and 3) predict its seismic properties. It is found that microstructures do differ between orthogonal structural sections, leading to variations in strengths of different components in the overall petrofabric that might impact on seismic properties. The results emphasize the true 3D nature of microstructures and petrofabrics, which can be recognized and accommodated more readily by this new generation of analytical techniques.

Abstract The Moine Thrust zone (MTZ) marks the Caledonian foreland-to-hinterland transition zone at the base of the Scandian ( c . 430 Ma) orogenic wedge. In the Loch Eriboll region, the upper ductile part of the MTZ is composed in ascending order of two regionally extensive thrust sheets (Upper Arnaboll-Creag na Faoilin and Creagan) and is overlain by the Moine Nappe. Quartz crystal fabrics, kinematic vorticity (W m ), and strain estimates from the ductile thrust sheets in this region are used to determine how pure and simple shear components of deformation are partitioned, and indicate that these processes may be thermally, structurally, and lithologically dependent. At the lowest structural levels, quartzite and gneiss in the Upper Arnaboll-Creag na Faoilin (UA-CNF) thrust sheet yield rigid grain-based arithmetic mean minimum (W m min) and mean maximum (W m max) vorticity estimates of 0.57 and 0.67, respectively (60–53% pure shear). Creagan thrust sheet mylonites yield W m min and W m max estimates of 0.59 and 0.72 (59–48% pure shear). At the highest structural levels, Moine Nappe mylonites yield W m min and W m max estimates of 0.59 and 0.71 (59–49% pure shear). Quartz c - and a -axis fabrics qualitatively indicate an increase in non-coaxial deformation (top-to-the-west) traced towards structurally higher levels, which is accompanied by increases in deformation temperature ( c . 370 °C to c . 550 °C). Integrated strain and vorticity estimates indicate that significant sub-vertical foliation normal shortening has occurred as nappe stacking progressed.

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.