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Particle shape measurement is an important analysis for a number of research and industrial disciplines. Most of the presently available 2-Dimensional shape analysis methods commonly suffer from at least two shortcomings: (1) particles, which are visually distinct, are not easily distinguished and (2) the parameter value is difficult to relate to physical attributes. This contribution seeks to address these issues by introducing the Curvature (κ) plot which can be used to identify particle corners and measure their sharpness. Using the κ plot, four shape parameters are proposed: number of corners, cumulative angularity, sharpest corner and straight fraction. This methodology is applied to synthetic geometrical shapes and 4000 natural sand particle data. The results demonstrate that the four new shape parameters can be collectively used to quantitatively describe particle shape which correlates closely with visual perceptions of particle boundary.

The number of stone aggregate sources that provide a high polished stone value and therefore a high skid resistance on road surfaces is relatively small. The factors that influence polished stone value have traditionally been associated with mineralogical and mechanical properties of specific rock-types. This study examines a group of variable lithologies from a single source, that exhibit similar laboratory measured polished stone value readings. It can be shown that multiple properties, including microstructural fabrics, can influence the potential PSV of an aggregate. 3D areal surface measurements of the aggregate surface, obtained using z-stacked digital images, can be combined with traditional mineralogy and microstructural observations, to track the development of surface microtexture from the source through to in-service trafficked sites. These combined methodologies can provide a detailed picture of how specific rock-types or regional geological settings can influence the polished stone value of aggregates and how different and varied factors can control the maintenance of a textured surface throughout its use.

The Upper Devonian Munster Basin of southern Ireland has traditionally been viewed as a post-orogenic molasse deposit that was sourced from the Caledonides of central Ireland and subsequently deformed by the end Carboniferous Variscan orogenic event. The basin fill is composed of super-mature quartz arenite sandstone that clearly represents a second cycle of deposition. The source of this detritus is now recognized as Lower Devonian Dingle Basin red bed sequences to the north. This genetic link is based on the degree of similarity in the detrital mica chemistry in both of these units; micas plot in identical fields and define the same trends. In addition, the two sequences show increased textural and chemical maturity up-sequence and define indistinguishable Ar-40/Ar-39 age ranges for the detrital mica grains. Partial resetting of the Ar ages can be attributed to elevated heat flow in the region caused by Munster Basin extension and subsequent Variscan deformation. The combined evidence from southwest Ireland therefore points to a Caledonian or possibly Taconian primary source area that initially shed detritus into the Lower Devonian Dingle Basin which was subsequently recycled into the Upper Devonian Munster Basin following mid-Devonian Acadian basin inversion. (C) 2014 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

Shape fabrics of elliptical objects in rocks are usually assumed to develop by passive behavior of inclusions with respect to the surrounding material leading to shape-based strain analysis methods belonging to the Rf/ϕ family. A probability density function is derived for the orientational characteristics of populations of rigid ellipses deforming in a pure shear 2D deformation with both no-slip and slip boundary conditions. Using maximum likelihood a numerical method is developed for estimating finite strain in natural populations deforming for both mechanisms. Application to a natural example indicates the importance of the slip mechanism in explaining clast shape fabrics in deformed sediments.

Geological strain analysis of sedimentary rocks is commonly carried out using clast-based techniques, which rely on assumptions regarding clast shapes and orientations as well as distances between clast centres. To evaluate the accuracy of these methods in low strain regimes we have compared strain analysis estimates to the anisotropy of magnetic susceptibility (AMS) in sandstone samples from the Wyoming salient. The Cordilleran Mountain Belt of North America is one of the world's classic foreland fold and thrust belts. The Sevier Belt represents the thin-skinned front of this orogenic event characterized by thrust faults and folds that shortened and translated sequences of Devonian to Cretaceous strata eastward. Deformation increases westward providing an ideal laboratory and geological setting to explore the potential correlation of anisotropy of magnetic susceptibility (AMS) to clast-based strain analyses. Studies attempting to define the relationship between AMS and finite strain have been in vogue since the link between layer parallel shortening and AMS was first established. This relationship, despite proven strong correlations between the AMS tensors and tectonic directions, is generally complicated by competing sub-fabrics as well as the various magnetic anisotropy properties of the minerals contributing to the AMS fabric. Even with these complications, we conclude that AMS is much more sensitive to incipient tectonic fabrics than clast based methods.

Geological strain analysis is a common task for structural geologists. This contribution presents software written on top of the Mathematica platform which allows for rapid semi-automatic strain analysis. After an initial step of manual identification of strain markers, the software performs image analysis, parameter extraction and strain analysis using the shape and relative spatial positioning of markers. Bootstrap estimates of sampling errors are calculated and suitable graphical output is generated. Three representative samples of lithologies typically used in strain analysis are analysed to test the software. We present an automated method for geological strain analysis techniques using Mathematica.Grain boundary parameters are automatically extracted using image analysis.Strain estimates are made using object separation and object shape properties.Both methods compare well to previously published manual estimates.A statistical technique allows error estimates to be calculated using the bootstrap method.

The computer strain analysis methods SAPE, MRL and DTNNM have permitted the characterization of finite strain in two different regions with contrasting geodynamic scenarios; (1) the Talas Ala Tau (Tien Shan, Kyrgyzs Republic) and (2) the Somiedo Nappe and Narcea Antiform (Cantabrian to West Asturian-Leonese Zone boundary, Variscan Belt, NW of Iberia). The performed analyses have revealed low-strain values and the regional strain trend in both studied areas. This study also investigates the relationship between lithology (grain size and percentage of matrix) and strain estimates the two methodologies used. The results show that these methods are comparable and the absence of significant finite strain lithological control in rocks deformed under low metamorphic and low-strain conditions.

Digital images obtained from X-ray computed tomography scans are analysed for the estimation of inclusion shape and orientation. Three-dimensional computer imagery and segmentation algorithms are used to visualise and isolate the regions of interest. These regions are then approximated by best-fit ellipsoids and the mean best-fit ellipsoid is used as a measure of preferred inclusion orientation. A Windows program is developed to implement these procedures and results found from both manufactured and natural data are presented. These results show that the radiodensity contrast plays a major role in the ability of the software to isolate inclusions from their matrix and hence determine rock fabric.

Existing methodologies that use ellipsoidal objects for the analysis of geological strain typically assume that these objects acted passively during deformation. This assumption, when not valid, can lead to significant underestimates of strain in rocks deformed under low-grade conditions in orogenic forelands. This is especially true when clastic sedimentary rocks are utilized to measure strain; the competency contrast between clasts and matrix possibly leading to marked ‘non-passive’ behaviour. The systematic nature of this finite strain underestimation may allow for a correction of strain estimates when this type of behaviour is evident.

A solution for the deformation of viscous layers during a two-dimensional general deformation is derived. Layer deformation is found to consist of components of pure shear, simple shear and pure rotation when considered with respect to the layer orientation. It is shown that for external bulk pure shear the layer deformation is typically vastly different to that outside the layer, ranging from pure to simple to super shear depending on layer orientation and viscosity contrast. Treagus' method for determining viscosity ratio from cleavage refraction is re-evaluated and is found to applicable under bulk pure shear. However, the presence of earlier LPS (layer-parallel shortening) can lead to erroneous viscosity ratio estimates. A new approach is developed, relying on the variation of cleavage orientation with layer orientation that is found to be robust with respect to LPS. The method is applied to examples from the Irish Variscides and the new approach has implications for determining whether or not LPS occurred in this foreland setting.

Constrained automated seeded region growing (CASRG) is an algorithm for automated grain boundary detection. It uses as input a single digitised microphotograph, such as ones obtained from a polarising microscope with an attached digital camera. In addition to this, it requires the user to click on the clasts within the microphotograph that the user wishes to obtain boundaries for. The algorithm requires no subsequent human input. The algorithm is based on the seeded region growing (SRG) algorithm of Adams and Bischof [Adams, R., Bischof, L., 1994. Seeded region growing. IEEE Transactions on Pattern Analysis Machine Intelligence 16, 641–647]. We have modified this algorithm to be guided by constraints and to adapt to the heterogeneity of colour information in the image. Imposition of these pre-determined additional conditions enables automated grain boundary detection without human intervention. The accuracy of CASRG has been validated through two benchmarking comparisons; one lithology with low tectonic strain and a second with high strain are used. The CASRG measurements are compared with those from hand drawn boundaries, which are used as a gold standard. Comparison is made using (a) a non-overlap statistic, (b) shape features, (c) strain estimates. In each case, the CASRG method compares very favourably with the gold standard.

SAPE (Semi-Automatic Parameter Extraction) is a program for extracting information relevant to strain analysis from input digital images. Input images are manually produced by tracing the outlines of objects of interest. This is a less onerous, more objective and much faster task than manual measurement or digitisation of object data (i.e. aspect ratio R i and orientation ϕ i ). SAPE rapidly extracts the required data by using a simple region-growing algorithm to identify regions of interest. Subsequently, the second moments are calculated for each region enabling the common strain analysis parameters to be readily computed. The performance of SAPE was tested on three samples (microphotographs of deformed sandstone and oolite as well as a meter scale photograph of a deformed conglomerate) using a non-overlap statistic. This statistic is a normalised measure of overlap between two regions and can be used to compare different fits applied to the same region. In each case SAPE out-performed the manual method. SAPE offers significant advantages in terms of speed, objectivity and robustness and should facilitate the collection of large datasets for strain analysis.

The Upper Palaeozoic sedimentary rocks of the Munster and South Munster Basins, southern Ireland, lie within the Rhenohercynian Zone of the European Variscan orogeny. This foreland region accommodated shortening during the Asturian phase of Variscan deformation at the end of the Carboniferous by the development of kilometre-scale and lower order folding, high angle reverse faulting and regional fabric development. At the southwestern extremity of the belt lies the Black Ball/White Ball Heads area of the Beara Peninsula where high-level igneous intrusions locally exhibit a close relationship with both the ductile and late brittle phases of Variscan deformation. 40 Ar/ 39 Ar ultra-violet laser analysis of phlogopite crystals from these intrusions (principally trachytic dykes) has yielded ages that, when combined with structural field relationships, help to constrain the timing of Variscan deformation in southern Ireland. These ages include 314.44±1.00 Ma for a penetratively deformed lamprophric pipe on Black Ball Head, 301.98±1.47–298.08±0.61 Ma for dyke material associated with later stage brittle deformation and a date of 296.88±0.60 Ma for an undeformed post-orogenic dyke on White Ball Head.

A new method for calculating finite sectional strain from distributions of elliptical objects is presented. The only assumptions required are that before deformation (1) long axis orientations are uniformly distributed and (2) the distribution of axial ratios is independent of orientation. Importantly, an estimate of the orientation of the long axis of the strain ellipse is not required before the method can be applied. The method is based on the conceptually simple fact that the mean radial length of a set of uniformly oriented ellipses in the unstrained state equates to that of a circle, so that after strain, the mean radial length evaluates to the strain ellipse. Errors associated with the method are calculated from the bootstrap, and a simulation study verifies both the applicability of the new method for finite strain estimation and the accuracy of errors calculated with the bootstrap. The method is applied to a large set of sandstone quartz clast data from the Irish Variscides, whilst cross-checking of results with those from established methods also validates the approach taken.

Detailed mesostructural and strain analysis investigations across the Killarney Mallow Fault, i.e. the traditional Variscan “Front” in southwest Ireland, reveal that this structural line separates two distinct tectonic regimes. North of the Killarney Mallow Fault bulk shortening orthogonal to orogenic strike is estimated to be 12%, all of which is accounted for by late stage buckling. Microscopic strain analysis reveals that there is only local development of a tectonic fabric. South of the front, bulk shortening is ≈ 40% due to combined layer parallel shortening (LPS), buckling and faulting. Variscan deformation is presented as being essentially coaxial. The regional finite strain pattern outlined above is thought to be primarily controlled by the combined effect of a buried basement obstacle in eastern Iveragh and increased sedimentary pile thickness at the western end of the orogen.

Combined microstructural and fluid inclusion studies on a multiple quartz vein from the Irish Variscides reveal a kinematic history consisting of three separate, dynamically distinct, extensional opening events. The first of these is characterized by trapped fluid at densities of 0.87–0.98 g cm−3, indicative of pressures approximating the estimated lithostatic load, while fluid densities associated with the following two events (0.76–0.91 g cm−3) clearly indicate sub-lithostatic fluid pressures. The combined evidence suggests that these later two opening events were extensional fractures occurring well below the critical depth for tensional σ3 (least effective principal stress). It is argued here that the preservation of such low density fluids, in an environment that conventionally requires supra-lithostatic fluid pressures for hydraulic failure, is due to (i) the influence of a preexisting material anisotropy in the rock that would result in hybrid extensional failure, (ii) the effect of stress heterogeneities associated with the development of crude boudins in the vein.

The direct isotopic dating of deformation in orogenic forelands has always been restricted by the lack of suitable syn-kinematic low-grade mineral phases. The mix of detrital, metamorphic and alteration phases in low-grade sedimentary rocks makes the task of dating deformation of such rocks especially difficult. Dating minerals that formed or cooled during the peak thermal event may not necessarily represent the timing of deformation, as there can be significant temporal lags between these two events. Lowgrade regional studies usually yield ages for cleavage development in a given orogenic event and are consequently limited in constraining the full temporal range of upper crustal deformation, including the late stage brittle deformation that is characteristic of foreland settings. The indirect approach of constraining the timing of regional deformation events by dating igneous intrusions associated with the deformation is limited in orogenic forelands. To date, this has essentially involved dating un-deformed, orogenic collapse related, post-orogenic granites that provide an upper age boundary to a given event. Ideally, the use of high-level crustal intrusions requires that these intrusions span the full temporal range of deformation. This is the case in southern Ireland, where it is possible to indirectly constrain the age of Variscan deformation by dating high-level crustal intrusions that are known to be intimately associated with the complete sequence of Variscan deformation. These intrusions, exposed in the Lower Palaeozoic rocks of the Blackball/Whiteball Heads area of SW Ireland, were emplaced as pipes, dykes and sills in both ductile and brittle deformational regimes. By adopting a multifaceted approach and combining these ages with existing chronostratigraphical constraints and observed field structural relationships, the temporal bracketing of Variscan deformation in southern Ireland is possible (Quinn et al., 2005). Ar/Ar UV laser analyses of phlogopite crystals from these intrusions has yielded ages which, when combined with structural field relationships, help constrain the timing of Variscan deformation in southern Ireland. These ages include 314.4 ± 1.0 Ma for a penetratively deformed lamprophric pipe on Blackball Head, 302.0 ± 1.5 to 298.1 ± 0.6 Ma for dyke material associated with later stage brittle deformation and a date of 296.9 ± 0.6 Ma for an un-deformed post-orogenic dyke on Whiteball Head (Quinn et al., 2005).