Seismic Anisotropy of Temperate Ice in Polar Ice Sheets

We present a series of simple shear numerical simulations of dynamic recrystallization of two‐phase nonlinear viscous materials that represent temperate ice. First, we investigate the effect of the presence of water on the resulting microstructures and, second, how water influences on P wave (Vp) and fast S wave (Vs) velocities. Regardless the water percentage, all simulations evolve from a random fabric to a vertical single maximum. For a purely solid aggregate, the highest Vp quickly aligns with the maximum c‐axis orientation. At the same time, the maximum c‐axis development reduces Vs in this orientation. When water is present, the developed maximum c‐axis orientation is less intense, which results in lower Vp and Vs. At high percentage of water, Vp does not align with the maximum c‐axis orientation. If the bulk modulus of ice is assumed for the water phase (i.e., implying that water is at high pressure), we find a remarkable decrease of Vs while Vp remains close to the value for purely solid ice. These results suggest that the decrease in Vs observed at the base of the ice sheets could be explained by the presence of water at elevated pressure, which would reside in isolated pockets at grain triple junctions. Under these conditions water would not favor sliding between ice grains. However, if we consider that deformation dominates over recrystallization, water pockets get continuously stretched, allowing water films to be located at grain boundaries. This configuration would modify and even overprint the maximum c‐axis‐dependent orientation and the magnitude of seismic anisotropy

The Relevance of Grain Dissection for Grain Size Reduction in Polar Ice: Insights from Numerical Models and Ice Core Microstructure Analysis

The flow of ice depends on the properties of the aggregate of individual ice crystals, such as grain size or lattice orientation distributions. Therefore, an understanding of the processes controlling ice micro-dynamics is needed to ultimately develop a physically based macroscopic ice flow law. We investigated the relevance of the process of grain dissection as a grain-size-modifying process in natural ice. For that purpose, we performed numerical multi-process microstructure modelling and analysed microstructure and crystallographic orientation maps from natural deep ice-core samples from the North Greenland Eemian Ice Drilling (NEEM) project. Full crystallographic orientations measured by electron backscatter diffraction (EBSD) have been used together with c-axis orientations using an optical technique (Fabric Analyser). Grain dissection is a feature of strain-induced grain boundary migration. During grain dissection, grain boundaries bulge into a neighbouring grain in an area of high dislocation energy and merge with the opposite grain boundary. This splits the high dislocation-energy grain into two parts, effectively decreasing the local grain size. Currently, grain size reduction in ice is thought to be achieved by either the progressive transformation from dislocation walls into new high-angle grain boundaries, called subgrain rotation or polygonisation, or bulging nucleation that is assisted by subgrain rotation. Both our time-resolved numerical modelling and NEEM ice core samples show that grain dissection is a common mechanism during ice deformation and can provide an efficient process to reduce grain sizes and counter-act dynamic grain-growth in addition to polygonisation or bulging nucleation. Thus, our results show that solely strain-induced boundary migration, in absence of subgrain rotation, can reduce grain sizes in polar ice, in particular if strain energy gradients are high. We describe the microstructural characteristics that can be used to identify grain dissection in natural microstructures

A dynamic atlas of interference patterns in superimposed, opposite sense ductile shear zones

Ductile shear zones that reactivate a coplanar shear zone with opposite shear sense are known from a variety of tectonic environments. Recognition of reactivation requires understanding the interference patterns that form when microstructures produced in the first event (D1) are modified by the second event (D2). We use numerical modelling to demonstrate the effect of D1 structures on the development of shear zone interference patterns during a coplanar, opposite sense D2 ductile shearing event. Seven models were generated from increasing D1 dextral simple shear strains (γdextral = 2–14) and we then superimposed D2 sinistral shearing (γsinistral = 10). The interference patterns produced are highly variable with geometric relationships between weak layers and strong lithons determining deformation style. Shear zones with high D1 strain can more easily accommodate D2 strain because more strain is localised into long, weak phase C planes, which are readily inverted and reused during sinistral shear. Interference structures in models and naturally deformed rocks include rotated σ-clasts, folded D1 S planes, disharmonic and hook folds, and cuspate layers of weak phase. We present a dynamic atlas of interference patterns produced due to overprinting shear zones to facilitate identification of these zones in nature.Peer reviewe

Estudio de la anisotropía sísmica del hielo parcialmente fundido en zonas de basales de los casquetes polares

X Congreso Geológico de España, 5-7 Julio 2021, Vitoria - GasteizLos datos sísmicos registrados en las zonas basales de la Antártida y Groenlandia muestran un fuerte descenso de las velocidades de ondas S (Vs) mientras que las ondas P (Vp) mantienen una velocidad similar en todo el perfil del casquete polar. Una explicación propuesta a este descenso es la presencia de agua. En esta contribución investigamos la evolución de las velocidades sísmicas en el hielo polar parcialmente fundido. Éste se simula como un agregado de policristales de hielo con agua. La deformación se realiza con un método full-field de deformación viscoplástica con recristalización. Las velocidades sísmicas son calculadas a partir de las orientaciones cristalinas. Independientemente del porcentaje de fundido, todas las simulaciones evolucionan a una orientación preferente de cristales (CPO), aproximadamente perpendicular al plano de cizalla. Cuando no hay agua, la Vp máxima se alinea rápidamente con la CPO, y aumenta a la vez que aumenta la intensidad de ésta CPO. Cuando una fase fundida está presente, la CPO desarrollada es menos intensa, siendo la Vp y la Vs menor. A alta proporción de agua la Vp no se alinea con la CPO. Sin embargo, si se considera el módulo de compresibilidad del hielo para el agua, encontramos un descenso fuerte de las Vs. Este resultado sugiere que el descenso de la Vs observado en la base podría ser explicado por la presencia de agua sobrepresionada, y por tanto aislada en conjunciones triples de granos, sin formar bandas de agua que favorecerían el deslizamiento.Observations of P-wave (Vp) and S-wa (Vs) velocities in ice-sheets show a strong decrease of Vs in basal parts, while Vp remains constant. The presence of water has been proposed as the cause of this decrease. We investigate the evolution of seismic velocities during deformation of temperate ice by means of numerical modelling. Temperate ice is simulated as a non-linear viscous aggregate constituted by ice polycrystals and a water phase. The viscoplastic full-field numerical approach is coupled with dynamic recrystallisation processes. The seismic velocities are calculated from measurements of crystal preferred orientations (CPO). Regardless the melt percentage, all simulations evolve to a CPO approximately perpendicular to the shear plane. For a purely solid aggregate, the highest Vp quickly aligns with the CPO, which evolves into a strong single maximum, while Vp increases. When water is present the developed CPO is less intense, being both the Vp and Vs lower than in the purely solid case. At high percentage of water, Vp is not aligned with the CPO. However, if the bulk modulus of ice, instead of water, is assumed for the water phase, we found a remarkable decrease in the Vs. These results suggest that the decrease in the Vs observed in the base of the ice-sheets could be explained by the presence of overpressured melt, that would be unconnected at triple grain junctions and would not favour sliding between ice grains

A review of numerical modelling of the dynamics of microstructural development in rocks and ice: Past, present and future

This review provides an overview of the emergence and current status of numerical modelling of microstructures, a powerful tool for predicting the dynamic behaviour of rocks and ice at the microscale with consequence for the evolution of these materials at a larger scale. We emphasize the general philosophy behind such numerical models and their application to important geological phenomena such as dynamic recrystallization and strain localization. We focus in particular on the dynamics that emerge when multiple processes, which may either be enhancing or competing with each other, are simultaneously active. Here, the ability to track the evolving microstructure is a particular advantage of numerical modelling. We highlight advances through time and provide glimpses into future opportunities and challenges

Ice-sheet flow transitions: for how long can crystallographic preferred orientations be preserved?

GeoMod 2021 conference , September 19-23, Utrecht, The NetherlandsCreep due to ice flow is generally thought to be the main cause for the formation of crystallographic preferred orientations (CPOs) in polycrystalline anisotropic ice. However, linking the development of CPOs to the ice flow history requires a proper understanding of the ice aggregate's microstructural response to flow transitions. In this contribution the influence of ice deformation history on the CPO development is investigated by means of full-field numerical simulations at the microscale. We simulate the CPO evolution of polycrystalline ice under combinations of two consecutive deformation events up to high strain. Ice polycrystalline viscoplastic deformation was simulated using the Fast Fourier Transform algorithm (VPFFT), within the numerical open-source platform ELLE (http://www.elle.ws). A volume of ice is first deformed under co-axial boundary conditions, which results in a CPO. The sample is then subjected to different boundary conditions (co-axial or non-coaxial) in order to observe how the deformation regime switch impacts on the CPO. The model results indicate that the second flow event tends to destroy the first, inherited fabric, with a range of transitional fabrics. However, the transition is slow when crystallographic axes are critically oriented with respect to the second imposed regime. Therefore, interpretations of past deformation events from observed CPOs must be carried out with caution, particularly, in areas with complex deformation histories