On thrusting, regional unconformities and exhumation of high-grade greenstones in Neoarchean orogens. The case of the Waroonga Shear Zone, Yilgarn Craton

Abstract During the Neoarchean, the dominant tectonic style progressively changed from an episodic-overturn/stagnant-lid regime to modern-style plate tectonics. The Neoarchean strengthening of continental lithosphere changed the style of deformation of orogenic belts. The case study presented here provides insights into how such transition in tectonic style occurred, a matter that is generally controversial. We present structural and metamorphic data from the c. 2660 Ma Waroonga Shear Zone (WSZ) in the Neoarchean Yilgarn orogen (Western Australia). The WSZ contains a syntectonic pluton and older, high-grade greenstones. The tectonic fabric in the pluton developed during melt-present thrusti…

Investigation of fabrics in quartz and ice: comparison and applications of different analytical methods

Microstructural analysis of shearband boudin - preliminary results

The internal structure of a shearband boudin resulting from an original igneous, hydrothermal or metamorphic segregation tabular rigid body is a subject of scientific interest. It allows understanding the deformation mechanisms acting on homogeneous quartz aggregate activated during simple shear progressive deformation. This communication is focused on the characterization of two main structural aspects: (i) the existence of different internal domains in shearband boudins; (ii) the development of internal structures related with its genesis and evolution, namely, secondary shear planes c’ type-I and c’ type-II, parameter B-t (mass accumulation sector on blunt tip of shearband boudin) and S-…

Microstructure and fabric development in ice: Lessons learned from in situ experiments and implications for understanding rock evolution

Abstract In this contribution we present a review of the evolution of microstructures and fabric in ice. Based on the review we show the potential use of ice as an analogue for rocks by considering selected examples that can be related to quartz-rich rocks. Advances in our understanding of the plasticity of ice have come from experimental investigations that clearly show that plastic deformation of polycrystalline ice is initially produced by basal slip. Interaction of dislocations play an essential role for dynamic recrystallization processes involving grain nucleation and grain-boundary migration during the steady-state flow of ice. To support this review we describe deformation in polycr…

P-T path development derived from shearband boudin microstructure

This work focuses on the development of a regional P-T-path from the Malpica-Lamego Ductile Shear Zone, NW Portugal, based on the microstructures of shearband boudins evolved during progressive simple shear. The combination of microstructural analysis, fluid inclusion studies, crystallographic pre- ffered orientation and fractal geometry analyses, allows to link several stages in the internal evolution of the boudin to regional P-T conditions. The boudinage process is initiated under differential stress after the original layer achieved sufficient viscosity contrast relative to the surrounding matrix. Two main transformations occur simultaneously: i) change in the external shape with contin…

Cretaceous tectonic evolution of the Sava-Klepa Massif, Republic of North Macedonia – Results from calcite twin based automated paleostress analysis

Abstract The Sava-Klepa Massif represents an approximately 5 × 2 km sized fault-bounded block of dominantly basaltic rocks located within the Sava-Zone, an important suture zone between the Eurasian (Europe) and Gondwana (Adria) continental plates in the Balkans. Its nature and tectonic evolution is controversial: It is either interpreted as a remnant of the youngest Tethyan oceanic realm left behind after the main closure in the Late Jurassic or as the delimiter of a diffuse tectonic boundary between Adria and Europe, which had already collided in the Late Jurassic and was dominantly controlled by transtensional tectonics during Cretaceous times. In order to strengthen one or the other mod…

Quantification of the microstructural evolution of polycrystalline fabrics using FAME: Application to in situ deformation of ice

Abstract In geology, glaciology and material science new technological advances result in an ever increasing amount of data and datasets, in particular when in situ experiments are conducted. Rapid, rigorous and reliable statistical treatment is needed to allow researchers to access these large datasets for further analysis. Here, we present FAME (Fabric Analyser based Microstructure Evaluation), a suite of Matlab® scripts that utilize the Matlab® open-source toolboxes MTEX and PolyLX (optional) for rapid quantification of thin section data. The data has been collected using an automated Fabric Analyser at a spatial resolution of 5 μm/pixel. From the dataset, grain maps are reconstructed an…

Reconstruction of 3D grain boundaries from rock thin sections, using an advanced polarised-light microscopy method

Grain boundaries play a significant role in materials by initiating reactions and collecting impurities. Here we present FAGO (Fabric Analyser Grain boundary recOnstruction), a first step towards the automatic determination of three-dimensional (3D) grain boundary geometry using polarised light. The trace of the grain boundaries from 2D rock thin sections are determined primarily from data acquired using an automatic fabric analyser microscope and the FAME software (fabric analyser-based microstructure evaluation; Peternell and colleagues and Hammes and Peternell). Based on the Fabric Analyser G50's unique arrangement of nine differently oriented light sources the retardation can be determi…

Effect of strain rate cycling on microstructures and crystallographic preferred orientation during high-temperature creep

Strain rate histories and strain magnitude are two crucial factors governing the evolution of dynamic recrystallized grain size and crystallographic preferred orientation (CPO) in rocks and ice masses. To understand the effect of cyclic variations in strain rate or non-steady-state deformation, we conducted two-dimensional, coaxial plane strain experiments with time-lapse observations from a fabric analyzer. There is a continuous reequilibration of microstructure and CPO development associated with constant and oscillating strain rate cycles. These can be correlated with c -axis small circle distributions, diagnostic of dynamic recrystallization involving new grain nucleation and grain boun…

Strain rate dependence for evolution of steady state grain sizes: Insights from high-strain experiments on ice

Abstract Understanding of the microstructural evolution and equilibrium grain size development during steady state tertiary flow is essential in order to improve our knowledge of ice and rock deformation. This contribution presents results from in situ transmitted light deformation experiments of natural glacier ice, with the development of the microstructure in a tertiary flow regime. We conducted one relative slower ( 1 × 10 − 6 1/s) and two relative faster-strain rate ( 2 × 10 − 6 1/s) pure shear experiments at −10 °C, up to a shortening of ∼57%. Microstructure development was followed by time-lapse observations, and two new microstructure-based indicators, the ‘seeding rate’ and the ‘mi…

On shearing, magmatism and regional deformation in Neoarchean granite-greenstone systems: Insights from the Yilgarn Craton

Abstract The structure of the Neoarchean Yilgarn Craton is dominated by craton-scale high-strain zones, mostly associated with highly-deformed elongate granitic bodies and transposed greenstone belts. These shear zones developed during widespread and prolonged magmatic activity that led to a nearly complete reworking of the felsic continental crust. The spatial, temporal and genetic relationships between such a voluminous and protracted event of crustal reworking and the development of the craton-scale shear zone network are unclear. Here, we combine new structural, geophysical and geochemical data to investigate the relationship between crustal-scale shear zones and large syntectonic pluto…

FAME: Software for analysing rock microstructures

Determination of rock microstructures leads to a better understanding of the formation and deformation of polycrystalline solids. Here, we present FAME (Fabric Analyser based Microstructure Evaluation), an easy-to-use MATLAB®-based software for processing datasets recorded by an automated fabric analyser microscope. FAME is provided as a MATLAB®-independent Windows® executable with an intuitive graphical user interface. Raw data from the fabric analyser microscope can be automatically loaded, filtered and cropped before analysis. Accurate and efficient rock microstructure analysis is based on an advanced user-controlled grain labelling algorithm. The preview and testing environments simplif…

Experimental deformation of deuterated ice in 3D and 2D: identification of grain-scale processes

Major polar ice sheets and ice caps experience cycles of variable flow during different glacial periods and as a response to past warming. The rate and localisation of deformation inside an ice body controls the evolution of ice microstructure and crystallographic fabric. This is critical for interpreting proxy signals for climate change, with deformation overprinting and disrupting stratigraphy deep under ice caps due to the nature of the flow. The final crystallographic fabric in polar ice sheets provides a record of deformation history, which in turn controls the flow properties of ice during further deformation and affects geophysical sensing of ice sheets. For example, identification o…

Influence of pre-existing microstructure on mechanical properties of marine ice during compression experiments

AbstractMarine ice is an important component of ice shelves in Antarctica. It accretes in substantial amounts at weak points and below ice shelves. It is likely to exhibit peculiar rheological properties, which are crucial to understanding its potential role in stabilizing ice-shelf flow. Due to its location and consolidation processes, marine ice can present a variety of textures which are likely to influence its rheological properties. We present a new dataset of unconfined uniaxial compression experiments on folded marine ice samples that have been cut at various angles to the folds. Texture and fabric analyses are described ‘before’ and ‘after’ the deformation experiment. It is shown th…

Interpretative model of shearband boudins internal evolution in HT ductile shear zones

The internal structure of a shearband boudin resulting from an original igneous, hydrothermal or metamorphic segregation tabular rigid body is a subject of scientific interest. It allows understanding the deformation mechanisms acting on homogeneous quartz aggregate activated during simple shear progressive deformation. This work is focused on the characterization of the internal evolution of shearband boudins, using microtextural analysis, fluid inclusions studies, fractal and OCP analysis. The proposed interpretative model shows the several structural stages that can be well established during the process of the internal evolution of shearband boudin.

Evaluating ice fabrics using fabric analyser techniques in Sørsdal Glacier, East Antarctica

AbstractIce cores (∼4 m long) obtained from areas of different surface velocities near the terminus of Sørsdal Glacier, East Antarctica, have been investigated using two versions of a fabric analyser (G50). In sections parallel to the flow plane, the microstructure is typically interlocking with elongate grains that parallel air-bubble elongation, X, reflecting their development in an earlier ductile regime. The c-axis fabric patterns vary with respect to X and vary from single–double maxima to asymmetric small-circle girdles oblique to the planar foliation, which can be attributed to a simple shear regime. The siteto-site variations in the c-axis patterns can be related to areas of differe…

Dynamics of ice mass deformation: Linking processes to rheology, texture, and microstructure

[1] Prediction of glacier and polar ice sheet dynamics is a major challenge, especially in view of changing climate. The flow behavior of an ice mass is fundamentally linked to processes at the grain and subgrain scale. However, our understanding of ice rheology and microstructure evolution based on conventional deformation experiments, where samples are analyzed before and after deformation, remains incomplete. To close this gap, we combine deformation experiments with in situ neutron diffraction textural and grain analysis that allows continuous monitoring of the evolution of rheology, texture, and microstructure. We prepared ice samples from deuterium water, as hydrogen in water ice has …

The influence of strain rate and presence of dispersed second phases on the deformation behaviour of polycrystalline D2O ice

ABSTRACTThis contribution discusses results obtained from 3-D neutron diffraction and 2-D fabric analyser in situ deformation experiments on laboratory-prepared polycrystalline deuterated ice and ice containing a second phase. The two-phase samples used in the experiments are composed of an ice matrix with (1) air bubbles, (2) rigid, rhombohedral-shaped calcite and (3) rheologically soft, platy graphite. Samples were tested at 10°C below the melting point of deuterated ice at ambient pressures, and two strain rates of 1 × 10−5 s−1 (fast) and 2.5 × 10−6 s−1 (medium). Nature and distribution of the second phase controlled the rheological behaviour of the ice by pinning grain boundary migratio…

The ~2730 Ma onset of the Neoarchean Yilgarn Orogeny

The timing of the onset of an orogeny is commonly constrained indirectly, because early orogenic structures are rarely exposed, or are overprinted. Establishing the onset of an Archean orogeny is considerably more challenging, because of the more fragmented geological record and the general lack of consensus about Archean geodynamics. We combine existing tectono-stratigraphic data with new structural and geophysical datasets to establish the onset of the Neoarchean Yilgarn Orogeny (Yilgarn Craton, Western Australia). We show that the surface of the c. 2960–2750 Ma deep-marine Yilgarn greenstone sequence was uplifted, eroded and unconformably overlain by a c. 2730 Ma, syntectonic clastic seq…

Ice deformed in compression and simple shear: control of temperature and initial fabric

AbstractLayered and polycrystalline ice was experimentally deformed in general shear involving axial compression (strain magnitude 0.5-17%) and simple shear (strain magnitude γ = 0.1-1.4). As the temperature is increased from -20°C to -2°C, there is at least a twofold enhancement in octahedral shear strain rate, which coincides with the onset of extensive dynamic recrystallization and a change in grain-size distribution at -15°C. Between -150C and -10°C the c-axis preferred orientation rapidly evolves with the initiation of two-maxima fabrics in shear zones. From -10°C to -2°C there is progressive evolution of a final c-axis pattern that is asymmetric with respect to the direction of shorte…

Quantification of synmagmatic flow structures: a tool for Rock Quality Evaluation

The result of the analysis using methods based on fractal geometry provides information about magmatic flux directions and mineral equilibrium processes in the crystallizing magma chamber. Additionally, the used methods may also provide important information for the rock industry, because they allow a fast and automatically evaluation of rock quality.