Patrick A. Meere

Finite strain estimation using the mean radial length of elliptical objects with bootstrap confidence intervals

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.

The structural evolution of the western Irish Variscides: an example of obstacle tectonics?

Abstract 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.

Upper crustal fluid migration: an example from the Variscides of SW Ireland

The density and composition of single generation fluids in quartz veins from two similar host lithologies located either side of the Irish Variscan front were investigated using fluid inclusion microthermometry and crush leach analysis. The front marks the northern boundary of a distinctive cleavage developed under sub-greenschist facies conditions and coincides with the margin of a fault-controlled inverted half graben, the Late Devonian Munster Basin. Trapped fluids in Variscan veins north of this basin have medium salinities (4–14 wt% NaCl eq.) and densities generally compatible with the expected P—T conditions of deformation in the area. Fluids in veins south of the half graben margin can be grouped into those in (a) syn-compressional veins with moderate salinities (8–16 wt%) and (b) late stage veins associated with post-orogenic extension which have trapped high salinity fluids (22–27 wt%). Fluid densities for both vein types are broadly compatible with estimated P–T conditions of both trapping events. Crush-leach analyses reveal that all the fluids analysed have Br/Cl ratios close to SMOW. Variations in I/Cl ratios and cation chemistry indicate significant water/rock interaction for medium salinity fluids on either side of the front. The marked internal consistency in Br/Cl ratios from northern and southern fluids coupled with the broad ranges of cation to chloride ratios suggests that an early homogenous marine brine was subsequently modified by differing migration histories during the Late Palaeozoic tectono-sedimentary evolution of the area. It is contended that on a megascopic scale, gravity driven fluid flow controlled by megascopic fold geometries was the dominant mechanism of fluid migration associated with the buckling phase of the orogen, while expulsion of supra-hydrostatically pressurised fluids is associated with late stage orogenic extension.

The effect of sample size on geological strain estimation from passively deformed clastic sedimentary rocks

Abstract The role of sample size in the estimation of geological strain, both finite strain ( R s ) and that of the orientation of the finite strain ellipse ( φ s ), is investigated for clastic sedimentary rocks. This study looks at four strain methods, the Robin method, the linearization method, the Mulchrone and Meere method and the mean radial length method that are initially tested using simulated strained data sets and subsequently by applying the methods to real data. It is found that the optimum strain analysis sample size for a clastic sedimentary rock is primarily dependant on the intensity of strain suffered by that rock because of the error behavior associated with R s estimates. An iterative process is therefore recommended starting with a minimum sample size of 150, which can be maintained or reduced based on the initial R s estimates.

Sub-greenschist facies metamorphism from the Variscides of SW Ireland an early syn-extensional peak thermal event

Illite crystallinity studies on metaclastics from the Variscides of SW Ireland range in value from 0.18 to 0.25°Δ2θ indicating a metamorphic grade in the upper anchizone-lower epizone (c. 275–325°C). Chlorite geothermometry yields a similar metamorphic temperature range (280–315°C). Combining these data with known overburden estimates for the area (c.5km) implies late Palaeozoic geothermal gradients in excess of 60°Ckm−1. Fluid inclusion studies on quartz veins in these metaclastics reveal two vein types characterized by fluid densities of 0.7–0.93 g cm−3 (Group 1) and 0.9–1.0gcm−3 (Group 2) respectively. This variation in fluid density is thought to be dominantly controlled by temperature, with Group 1 veins linked to late Palaeozoic extension and high geothermal gradients and Group 2 veins associated with subsequent Variscan deformation and lower temperature conditions.

Timing of deformation within Old Red Sandstone lithologies from the Dingle Peninsula, SW Ireland

Finite strain analysis of deformed clastic sedimentary rocks below and above the Acadian unconformity on the Dingle Peninsula, SW Ireland, reveals that this boundary represents a significant bulk strain discontinuity. The Late Emsian Acadian event is primarily responsible for penetrative cleavage development and high strain in rocks below the unconformity and not, as previously held, the later Variscan event, which overprinted the peninsula with a weak and localized disjunctive cleavage. The presence of apparent anticlockwise transecting cleavage and implied dextral closure of the Dingle Basin during the Acadian event is compatible with northward convergence of Armorica with respect to Eastern Avalonia.

High and low density fluids in a quartz vein from the Irish Variscides

Abstract 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.

Mathematica code for image analysis, semi-automatic parameter extraction and strain analysis

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.

WinDICOM: A program for determining inclusion shape and orientation

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.

Shear folding in low-grade metasedimentary rocks: Reverse shear along cleavage at a high angle to the maximum compressive stress

Shear folding, which is also referred to as slip folding, involves shear along planes that are oriented approximately parallel to the axial plane of the fold structure. These planes, which are typically axial-planar cleavage planes, facilitate high-angle reverse slip leading to fold limb rotation and amplification. This study builds on recent advances in our understanding of the role of weak fault zones in facilitating slip on misoriented faults; i.e., faults at a high angle to the maximum principal tectonic stress (σ 1 ). Analysis of folded marine sedimentary rocks from the Variscan of southern Ireland provides unambiguous microstructural evidence for reverse shear on chemically weakened cleavage domains. Significant silica loss in these cleavage domains, and as a consequence marked mechanical weakening, is seen as the primary cause for the reverse slip associated with the shear folding of these sedimentary rocks.