Join us to explore the impact of crystallographic preferred orientation (CPO), grain size, and dynamic recrystallization on the rheological behavior of Earth materials, alongside innovative techniques for stress analysis at various scales, from intragranular heterogeneity to plate boundary dynamics. We welcome contributions that employ numerical modelling, laboratory experiments, or observational studies that highlight the intersection of microstructural evolution and stress, emphasizing time-dependent processes, such as creep transients, and their role in viscous deformation.
This session aims to foster an inclusive, interdisciplinary dialogue, inviting researchers from all backgrounds to bridge scales and methodologies. We encourage participation from early career researchers to collectively advance our understanding of the stress-microstructure relationship and its implications for viscous deformations in the cryosphere, crust, and mantle.
Dynamic recrystallization plays a critical role in the texture evolution of polycrystalline materials undergoing high-temperature deformation, particularly in anisotropic materials such as ice. This study presents a novel, physically-based formulation to model texture evolution during dynamic recrystallization, leveraging detailed observations of ice microstructure under dislocation creep and recrystallization [1]. The formulation incorporates an orientation attractor that maximizes resolved shear stress on basal slip systems, coupled with an anisotropic viscoplastic law to capture mechanical responses. Implemented via finite-element methods in the R3iCe model [2], the approach successfully replicates experimental observations across diverse loading conditions, demonstrating its effectiveness in modeling texture-induced mechanical softening. While the model is validated for ice, it shows potential for application to other anisotropic materials such as olivine. Ongoing work is investigating the scalability and applicability of this formulation to large-scale models, such as glacial ice flow simulations, with a focus on addressing challenges related to computational efficiency and parameterization.
[1] Chauve, T., Montagnat, M., Dansereau, V., Saramito, P., Fourteau, K., & Tommasi, A. (2024). A physically-based formulation for texture evolution during dynamic recrystallization. A case study of ice. Comptes Rendus. Mécanique, 352(G1), 99-134. https://doi.org/10.5802/crmeca.243
[2] R3iCe repository : https://gricad-gitlab.univ-grenoble-alpes.fr/mecaiceige/tools/ice-polycrystal-models/rheolef_cti
How to cite: Chauve, T., Hilzheber, A., Montagnat, M., Dansereau, V., Saramito, P., Fourteau, K., and Tommasi, A.: A physically-based model for texture evolution during dynamic recrystallization: applicationsto ice and prospects for large-scale modeling, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12671, https://doi.org/10.5194/egusphere-egu25-12671, 2025.
Glacial ice is formed as snow is compressed under its own weight, forming ice crystals with initially random orientations i.e. isotropic ice. Over time, under sustained accumulation and overburden stress, the ice crystals transition from a random arrangement to a more aligned structure, forming anisotropic ice. With continued stress, the ice starts flowing, further modifying the anisotropy. Unlike isotropic ice, which responds equally to stress in all directions, anisotropic ice can deform up to 10 times faster due to its aligned crystal structure. Widely used glacier flow laws, such as Glens flow law, assume the ice to be isotropic. Anisotropy significantly impacts flow dynamics and should therefore be included in ice sheet and glacier models. While enhancement factors are sometimes used to mimic anisotropy, they often do not accurately represent these effects.
In order to correctly represent anisotropy in ice flow, in-situ measurements of ice fabric are needed. However, obtaining such measurements is challenging, particularly in dynamic regions such as ice streams and outlet glaciers. Due to the evolving stress patterns they are subjected to over time, ice streams and outlet glaciers develop distinct anisotropic characteristics. This anisotropic signal contrasts with areas dominated by vertical compression, such as accumulation zones, where anisotropic measurements are typically conducted through ice cores. By applying the concept of seismic anisotropy, specifically shear wave splitting (SWS), we can effectively determine the ice fabric in these fast-flowing areas. This approach provides insights into ice anisotropy of ice streams and glaciers that is difficult to achieve with other methods.
Here, we present ice fabric measurements at Sermeq Kujalleq in Kangia (Jakobshavn Isbræ), Greenland's fastest flowing outlet glacier, with flow velocities reaching 30–40 m/d. By utilizing shear wave splitting observed using basal icequakes, measured directly within the main ice stream, we are able to make a first estimate of the ice anisotropy in such a fast-flowing ice stream.
How to cite: Nap, A., Hudson, T. S., Walter, F., Wehrlé, A., Kneib-Walter, A., Rousseau, H., and Lüthi, M. P.: Assessing ice anisotropy using basal icequakes at Sermeq Kujalleq in Kangia, Greenland, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13323, https://doi.org/10.5194/egusphere-egu25-13323, 2025.
Ice streams are major contributors to ice sheet mass loss and critical regulators of sea level change. Despite their importance, standard viscous flow simulations of ice stream deformation and evolution have limited predictive power, mostly because our understanding of the involved processes is limited. This leads, for instance, to widely varying predictions of sea level rise during the next decades.
Here we report on a Distributed Acoustic Sensing experiment conducted in the borehole of the East Greenland Ice Core Project (EastGRIP) on the Northeast Greenland Ice Stream (NEGIS). For the first time, our observations reveal a brittle deformation mode that is incompatible with viscous flow over length scales similar to the resolution of modern ice sheet models: englacial ice quake cascades that are not being recorded at the surface. A comparison with ice core analyses shows that ice quakes preferentially nucleate near volcanism-related impurities, such as thin layers of tephra or sulfate anomalies. These are likely to promote grain boundary cracking, and appear as a macroscopic form of crystal-scale wild plasticity. A conservative estimate indicates that seismic cascades are likely to produce strain rates that are comparable in amplitude to those measured geodetically, thereby bridging the well-documented gap between current ice sheet models and observations.
How to cite: Hofstede, C., Fichtner, A., Kennett, B., Svensson, A., Westhoff, J., Walter, F., Ampuero, J.-P., Cook, E., Zigone, D., Jansen, D., and Eisen, O.: Brittle creep deformation observed in an ice stream from borehole distributed acoustic sensing , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-20791, https://doi.org/10.5194/egusphere-egu25-20791, 2025.
A subgrain-size piezometer is intended to be free from subsidiary effects of recrystallisation, such as phase mixing and pinning, unlike the classical grain-size piezometers, which are best limited to monomineralic samples to avoid these effects. Previously calibrated subgrain-size piezometers have a wide range of uncertainty in stress for a given intercept length. The log-log linear regression fits contribute to the large and impractical error ranges in linear space. The reason behind this behaviour could be the method applied to measure the representative intercept length of the experimental samples. We reanalysed the same calibration datasets used in the existing subgrain-size piezometer and observed that the distributions of intercept lengths are not log-normal. Instead of taking the arithmetic mean of such datasets, we propose that the median may be a better statistic to represent the central tendency of the datasets. Additionally, we have considered subgrains having misorientation angles from 2–10°. Removing 1–2° subgrain boundaries strikes a balance between data loss and noise reduction. Moreover, we propose a method whereby the measurement of subgrain intercepts is free from grain-boundary intercepts, which usually contribute to the largest values in the datasets. Care is taken to minimise the noise in the electron backscatter diffraction datasets whilst preserving the subgrain boundaries by conservatively choosing the halfQuadratic filter parameters. In this updated subgrain-size piezometer, the error ranges in the linear space are reduced from hundreds of megapascals to a few tens of megapascals. We compared the new calibration with the classical grain-size piezometers in two recrystallised monomineralic quartz-bearing natural rock samples. One sample is from a deformed quartzite in a shear zone and the other is from a sheared silicic vein inside a craton. Misorientation axes of subgrain boundaries indicate that basal and prism slip occurred in the respective samples, implying that the deformation temperatures are different. Recrystallisation regimes are confined to certain temperature ranges, and we tested the subgrain-size piezometer in two separate regimes. The range of the differential stress estimated from our recalibrated piezometer is narrowest amongst the available piezometers, for both samples, even when postdeformation grain growth is observed in one of them.
How to cite: Sikdar, A., Misra, S., and Wallis, D.: Subgrain-Size Piezometer: A Recalibration and its Application in Natural Samples, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-907, https://doi.org/10.5194/egusphere-egu25-907, 2025.
The exhumation of high-pressure metamorphic rocks from subduction zones involves dramatic pressure-temperature changes, triggering complex micromechanical responses at the grain-to-sub-grain scale. However, the mechanical aspects of these processes, particularly the origins and persistence of residual stresses within rock microstructures, remain poorly understood. To address this problem, we employed synchrotron-based three-dimensional X-ray diffraction to investigate residual strain, stress, and intra-grain misorientation in a garnet-quartz metamorphic rock from the Lago di Cignana ultra-high-pressure unit in the Western Alps. Our analysis reveals long-range residual stress heterogeneities spanning tens to hundreds of micrometers, with magnitudes reaching several hundred MPa. Significant intra-grain misorientations in both quartz and garnet provide insights into the interplay between plastic and elastic deformation processes. These stress signatures are preserved in a sample lacking apparent macroscopic deformation, suggesting that subtle mechanisms—such as decompression-induced anisotropic expansion, grain interactions, and garnet compositional gradients—play a key role in stress retention. These findings highlight the potential of synchrotron X-ray diffraction for capturing the stress field within polycrystalline rocks. The ability to resolve three-dimensional strain and stress distributions across scales offers new opportunities to advance our understanding of micromechanical processes associated with rock deformation and metamorphism.
How to cite: Jacob, J., van Schrojenstein Lantman, H., Cordonnier, B., Menegon, L., Wright, J., and Renard, F.: Exhumation-induced residual stress in undeformed, ultra-high-pressure metamorphic rock, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3540, https://doi.org/10.5194/egusphere-egu25-3540, 2025.
Transient creep of calcite controls the strength evolution of carbonate shear zones during postseismic deformation. However, a lack of information on the dominant microphysical mechanisms of transient creep of calcite hinders the development of constitutive equations. Specifically, for dislocation-mediated deformation, it is unclear whether strain hardening occurs primarily by short-range dislocation interactions and is therefore isotropic or by long-range elastic interactions and is therefore anisotropic. Here, I test whether mylonitic calcite marbles from the mid-crustal shear zone of the Karakoram Fault Zone, NW India, preserve residual stresses indicative of these long-range elastic interactions among dislocations. Previous work demonstrated that the mylonitic fault rocks experienced bulk stresses in the range 40–250 MPa as they were exhumed and cooled from approximately 480°C to 300°C. I analysed the microstructure and micromechanical state of three samples, including undeformed wall rock, protomylonite, and ultramylonite, using electron backscatter diffraction and high-angular resolution electron backscatter diffraction. The undeformed wall rock has a grain size of 130 µm, whereas the protomylonite and ultramylonite have grain sizes of 22 µm and 12 µm, respectively. Densities of geometrically necessary dislocations (GNDs) increase from the wall rock into protomylonite and ultramylonite. In the deformed lithologies, GND densities generally increase with proximity to grain boundaries over distances of 10–15 µm. Residual stresses in the wall rock are below the noise level of the HR-EBSD measurements, with a 99th percentile of 54 MPa. However, significant heterogeneity in residual stress is present in the protomylonite and ultramylonite, with 99th percentiles of 325 MPa and 742 MPa respectively. Both the spatial and probability distributions of the residual stresses reveal that they are imparted primarily by dislocations. Autocorrelation of the stress fields indicates that the typical length scale of stress heterogeneity increases from approximately 2 µm in the wall rock to 4 µm in the protomylonite and 7 µm in the ultramylonite. Collectively, these observations demonstrate that dislocations in calcite generate long-range internal stresses that cause elastic interactions. These elastic interactions are typically inferred to manifest as a backstress that counteracts the applied stress and generates a component of anisotropic kinematic hardening. The contribution of this mechanism of transient creep is missing from existing constitutive equations for calcite and should be represented by a backstress that is subtracted from the applied stress and can evolve with strain and time.
How to cite: Wallis, D.: The role of intragranular stress heterogeneity in transient dislocation-mediated deformation of calcite mylonites, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7403, https://doi.org/10.5194/egusphere-egu25-7403, 2025.
Seismic failure of dry lower-crustal rocks requires very high differential stress on the gigapascal-level. Among the mechanisms proposed to generate such high stresses is the so-called jostling block model, in which stress is amplified in rigid blocks within lower-crustal shear zone networks, leading to seismic failure. The model is based on field observations from the Musgrave ranges, Australia and the Nusfjord ridge, Lofoten, northern Norway, where pseudotachylytes (quenched frictional melts produced by coseismic slip) occur within the aforementioned structural setting.
Here we present numerical models to test if stress can be amplified in jostling blocks to the levels necessary to fracture dry, intact, lower-crustal rocks, and on which timescales such stress amplification can be achieved. Our models are based on the geometries and material properties determined in the Nusfjord locality. We systematically test the influence of strain rate, viscosity, loading conditions (pure vs. simple shear), and geometry (shear zone thickness, spacing, angle) and find that the bulk strain rate has the most significant impact on both the magnitude and rate of stress amplification. At high to moderate strain rates of 10-10-10-12 s-1 stress amplification to the required level is achieved in years to hundreds of years, while lower strain rates are insufficient to reach the required stress levels. Average long-term strain rates in the in the crust are on the order of 10-13-10-15 s-1, and transiently high strain rates are reported from both field localities mentioned above. Our numerical results are thus well-supported by the rock record. Furthermore, we find that a high viscosity contrast in our models is necessary to reproduce the geometries observed in the field. A third notable contributor to the magnitude of stress amplification that can be reached in the jostling-block geometry is the loading conditions. Specifically, we find that the impact of pure shear on stress amplification is greater compared to simple shear. Shear zone angle and spacing typically have a minor effect. In contrast, increased shear zone width leads to a reduction of stress in the blocks as strain is accommodated fully by the viscous shear zones, and elastic loading of the rigid blocks is no longer necessary to accommodate bulk strain.
Our results clearly demonstrate that, geometric and material properties contribute to stress amplification in different ways, but that strain rate is the controlling factor. In fact, our results indicate that at moderate to high strain rates, stress amplification to levels necessary for failure of intact lower-crustal rocks in shear zone networks is not only plausible, but inevitable.
How to cite: Zertani, S., Thielmann, M., and Menegon, L.: Stress amplification in rigid blocks of lower-crustal shear zones is controlled by bulk strain rate, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9407, https://doi.org/10.5194/egusphere-egu25-9407, 2025.
Due to the abundance of quartz in the continental crust, quartz rheology is fundamental to our understanding of many geodynamic processes. Quartz rheology is commonly characterized using a dislocation creep flow law with a stress exponent equal to 4; however, several recent studies indicate that the stress exponent for quartz aggregates can be as low as 2 at conditions where it has been proposed to deform by a combination of dislocation creep and grain boundary sliding (GBS), known as dislocation accommodated grain boundary sliding (disGBS). To address these differing hypotheses, we conducted axial compression load-stepping experiments in a Griggs apparatus at temperatures ranging from 800-950°C, 1.5 GPa, and differential stresses ranging from ~40 MPa to ~1430 MPa with water added. Quartz samples were prepared with different grain sizes of ~3, 5, 10, and 20 μm. For each experiment ~25 load steps were conducted during which the strain rate achieved a mechanical steady state. At the finest grain size, the mechanical data show a stress exponent of n = 1, which then transitions to n ~ 1.8 with increasing stress; for a given stress, strain rate increases with decreasing grain size in both regimes. For larger grain sizes over the same stress range, the stress exponent transitions from n ~ 4 to n ~ 1.8 to n ~ 3 with increasing stress, where only the intermediate stress regime (n ~1.8) shows a grain size sensitivity. We interpret the lowest stress and finest grain size mechanical data to represent grain boundary diffusion creep and assume a grain size exponent of 3. With increasing stress, the samples are interpreted to represent disGBS, where dislocation creep and GBS act in series, where GBS is determined to have a grain size sensitivity of 1. The highest stress data represents dislocation creep. Microstructurally, we observe minimal variation in the starting and final grain sizes, suggesting that the grain size was nominally constant throughout the experiments. Experiments quenched in the GBS regime show microstructures with straight grain boundaries consistent with observations from previous studies. Flow laws have been constrained for all four deformation mechanisms. Plotting a deformation mechanism map using our new flow laws extrapolated to geologic conditions, we show consistent relationships between our flow law estimates and c-axis fabric relationships with naturally deformed quartzites. These new mechanical relationships improve our understanding and constraints on grain-size sensitive rheologies in quartz as well as our ability to model quartz rheology over a wide range of geologic conditions.
How to cite: Tokle, L., Hirth, G., and Behr, W.: Characterizing quartz rheology through load-stepping experiments, from diffusion to dislocation creep, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12111, https://doi.org/10.5194/egusphere-egu25-12111, 2025.
Dynamic models of Earth's lithosphere and convecting mantle often simplify the rheological behavior of mantle rocks, for example by assuming constant grain size or considering limited changes in material properties with mineral assemblage. While these simplifications reduce computational requirements, they neglect key processes such as shear localization and transient rheological behaviour associated with phase transitions, which can profoundly impact mantle flow patterns. As incorporating the effect of an evolving grain size in dynamic models has garnered more interest in the geodynamics community, there is a growing need for accurate, scalable, and computationally efficient approaches to address this complexity.
Here, we present recent advancements in the finite-element code ASPECT that address this challenge. These include a higher-order particle method for tracking grain size evolution and the integration of the ARKode solver library, which offers adaptive time-stepping for solving the ordinary differential equation governing grain size evolution. Our implementation captures the simultaneous and competing effects of different mechanisms affecting grain size, such as dynamic recrystallization driven by dislocation creep, grain growth in multiphase assemblages, Zener pinning, and recrystallisation at phase transitions.
We showcase three applications that highlight the importance of grain size evolution—and its interaction with stress and strain rate—for mantle dynamics: (i) global-scale mantle flow, (ii) small-scale convection beneath lithospheric plates, and (iii) the collapse of passive margins. Our models reveal that grain size evolution induces viscosity variations spanning several orders of magnitude, promoting strain localization in all three settings. It therefore controls the shape of upwellings and downwellings as well as the onset time of instabilities. For instance, beneath oceanic plates, the development of large grain sizes before the onset of convection, when strain rates are low, can delay the initiation of cold downwellings. These initial downwellings, in turn, reduce both grain size and viscosity at the base of the lithosphere, allowing subsequent cold drips to form at younger plate ages. Grain damage can also facilitate the collapse of a passive margin through grain size reduction in the lower parts of the lithosphere—but only within a specific range of grain size evolution parameters. Furthermore, additional weakening mechanisms are required for breaking the upper ≥25 km of the plate for subduction initiation to occur. These applications illustrate the applicability of our method to large-scale 2D and 3D models of the convecting mantle and lithosphere and emphasize the critical role of grain-scale processes in shaping the dynamics of Earth’s interior.
How to cite: Dannberg, J., Gassmöller, R., Myhill, R., Saxena, A., Fraters, M., and Li, R.: Modelling Grain Size Evolution and its Role in Mantle Dynamics: From Small-scale Convection to Passive Margin Collapse, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13052, https://doi.org/10.5194/egusphere-egu25-13052, 2025.
Posters on site: Wed, 30 Apr, 10:45–12:30 | Hall X5
The long-term fluid-like movements in the Earth’s mantle largely depend on the rheological behaviour of olivine, the main rock-forming mineral in the upper mantle. Although the average viscosity of the mantle can be estimated from post-glacial rebound or geoid anomalies, the micromechanical mechanisms that facilitate the deformation of the solid mantle have been identified from rock mechanics experiments. Dislocation creep emerges as the predominant deformation mechanism in the uppermost mantle, aligning olivine crystals into a crystallographic preferred orientation (CPO) parallel to the flow, while this alignment of crystals also results in anisotropic viscous behaviour. Thus, anisotropic viscosity and CPO evolve hand in hand, and this interaction may impact many geodynamic processes. For example, beneath tectonic plates CPO evolves parallel to the plate motion direction, weakening the asthenosphere in that direction. However, if the plate motion direction changes, the asthenosphere will resist this change, leading to smaller velocities, less deformation and therefore a slow evolution of the CPO towards the new plate motion direction. In the ANIMA project, we aimed to find an efficient way of modelling CPO evolution and the related anisotropic viscosity in a fully coupled way within a geodynamic simulation. We developed a method that tracks CPO evolution on advected particles based on the D-REX method and utilizes the eigenvalues of the mean CPO orientation matrices to predict the anisotropic viscous parameters. These parameters allow us to calculate a tensor form of the viscosity, which we then feed back into our model solution. This method can be applied in combination with other rheologies, although with a cost of having to represent the viscosity as a tensor in the entire model domain, regardless of the dominant deformation mechanism. Despite an estimated increase in computational cost by up to an order of magnitude, incorporating anisotropic viscosity coupled to CPO evolution stands feasible for regional geodynamic models. This development will facilitate the study of a broad new range of geodynamics problems that involve olivine texture and anisotropic viscosity.
How to cite: Király, Á., Wang, Y., Conrad, C. P., Dannberg, J., Fraters, M., Gassmöller, R., and Hansen, L.: ANIMA the journey: how we model olivine CPO-related anisotropic viscosity, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16902, https://doi.org/10.5194/egusphere-egu25-16902, 2025.
During the gravity-driven flow of glaciers and ice sheets, polycrystalline ice tends to develop a strain-induced alignment of individual grains. This fabric development can act as a strain marker for understanding the recent-most deformation history, in addition to exerting significant rheological control on ice sheets compared to isotropic ice. We develop a new way to directly solve for depth-average fabric fields using satellite-derived velocities, assuming that velocities are approximately steady and that fabric evolution is dominated by lattice rotation, in a depth-averaged sense. We apply the method to the North East Greenland Ice Stream (NEGIS) and compare results to radar-derived observations of ice fabrics, suggesting the memory of past flow, stored in ice-stream fabrics, might be useful way to independently set bounds on the age of ice streams (assuming recrystallization is negligible in a depth-average sense). Source/sink flux terms for crystal orientations at the surface and basal boundary naturally appear in the problem as fabric-state-space attractors, and we discuss how the effect of ice—bed interactions on fabric evolution may be parameterized using such terms.
How to cite: Häußler, T., Rathman, N., and Grinsted, A.: What can modeling Steady-State Crystal Fabrics of Ice Streams tell Us about their Age?, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11108, https://doi.org/10.5194/egusphere-egu25-11108, 2025.
The crystallographic preferred orientation (CPO) of polycrystalline olivine affects both the viscous and seismic anisotropy of Earth's upper mantle with wide geodynamical implications. In this methods contribution, we present a continuous field formulation of the popular directors method for modeling the strain‐induced evolution of olivine CPOs, assuming the activation of a single preferred crystal slip system. The formulation reduces the problem of CPO evolution to a linear matrix problem that can easily be integrated alongside large‐scale geodynamical flow models, and conveniently minimizes the degrees of freedom necessary to represent CPO fields. We validate the CPO model against existing deformation experiments and naturally deformed samples, as well as the popular discrete grain model D‐Rex. A numerical model of viscoplastic thermal convection is built to illustrate how flow and CPO evolution may be two‐way coupled, suggesting that CPO‐induced viscous anisotropy does not necessarily strongly affect convection time scales, boundary (lid) stresses, and seismic anisotropy, compared to isotropic viscoplastic rheologies. As a consequence, geodynamical modeling that relies on an isotropic rheology (one‐way coupling) might suffice for predicting seismic anisotropy under some circumstances. Finally, we discuss limitations and shortcomings of our method, such as representing D‐ and E‐type fabrics or modeling flows with mixed fabric types, and potential improvements such as accounting for the effect of dynamic recrystallization.
How to cite: Rathmann, N., Prior, D., Mosegaard, K., Bekkevold, I., and Lilien, D.: A Spectral Directors Method for Modeling the Coupled Evolution of Flow and CPO in Polycrystalline Olivine, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15461, https://doi.org/10.5194/egusphere-egu25-15461, 2025.
The viscous deformation of glacier ice is governed by its temperature and the bulk ice crystal orientation fabric. Due to the mechanical anisotropy of ice crystals, the fabric’s influence on viscosity is directional: depending on the deformation direction, the ice becomes softer or harder. Representing the mechanical anisotropy in numerical ice sheet models is crucial for accurately predicting the future contributions of the Greenland and Antarctic ice sheets to global sea-level rise. However, the fabric strength, orientation, and its impact on viscosity are largely unexplored in fast-flowing ice streams and glaciers. Consequently, the fabric’s influence on ice dynamics is currently inadequately accounted for in ice sheet models. Advances in ground-based radar technologies and improved analysis methods enable the determination of depth profiles of the crystal orientation fabric. In this study, we investigate the fabric and its influence on the viscosity of the Rutford Ice Stream, Antarctica. We analyzed polarimetric measurements performed with an Autonomous phase-sensitive Radio Echo Sounder (ApRES) using a novel approach that allows the determination of fabric-depth profiles to significantly greater depths than previously possible. The results demonstrate a rapid increase in fabric strength within the upper 200 to 300 m depth, followed by a relatively stable fabric strength over depth. In the center of Rutford Ice Stream, our analysis revealed an average fabric strength ranging between 0.4 and 0.5 within the upper 1200 m and fabric rotation by 45° to the ice flow direction. Closer to the shear margin, the fabric strength increased up to 0.8, where the orientation is aligned with the ice flow direction. The findings indicate a substantial influence of the fabric on the effective viscosity, particularly near the shear margin where the ice is softened by a factor of three for horizontal-shear deformation. These findings contribute to a more comprehensive understanding of the distribution of fabric and its influence on the viscosity within ice streams and serve as validation for fabric evolution models.
How to cite: Zeising, O., Arenas-Pingarrón, Á., Brisbourne, A. M., and Martín, C.: Impact of fabric on viscosity of Rutford Ice Stream, Antarctica, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3075, https://doi.org/10.5194/egusphere-egu25-3075, 2025.
During the gravity-driven flow and spreading of ice shelves, polycrystalline ice tends to develop a strain-induced alignment of individual grains. This fabric development can exert significant rheological control on ice shelves, potentially softening or hardening anisotropic ice by several orders of magnitude compared to isotropic ice. We develop a new way to directly solve for depth-average fabric fields using satellite-derived velocities over ice shelves, assuming that velocities are approximately steady and that fabric evolution is dominated by lattice rotation, in a depth-averaged sense. We apply the method to Amery ice shelf, Antarctica, and compare results to previous observations of ice fabrics. Further, we calculate the equivalent isotropic enhancement-factor field using the “CAFFE” method, supposed to represent the first-order effect of fabric on ice viscosity. Because a significant fraction of the ice-shelf thickness on Amery is accreted marine ice, we explore how this may alter the depth-averaged estimate of fabric, and thus viscosity, by including an idealized source term to account for the sub-shelf flux of new grain orientations as ice accretes.
How to cite: Demuth, A., Rathmann, N., and Grinsted, A.: Fabric-induced flow enhancement of the Amery ice shelf inferred from satellite-derived surface velocities, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11382, https://doi.org/10.5194/egusphere-egu25-11382, 2025.
Olivine, the most abundant mineral in the Earth's upper mantle, affects seismic wave propagation through its crystallographic preferred orientation (CPO) developed during deformation. As a result, the seismic anisotropy of the crystals serves as a crucial tool for constraining large-scale geodynamic models, linking seismic observations to mantle flow processes via the orientation of olivine crystals.
Building on this link, we propose an optimization problem for inferring the crystal orientation fabrics of upper mantle olivine using oblique seismic data by adapting a method from ultrasound tomography, previously used to infer orientation fabrics of polycrystalline ice. The method relies on (i) a harmonic expansion of the grain orientation distribution function (unknown to be inferred), (ii) a fourth-order closure approximation of the distribution function (reducing the dimensionality of the problem), and (iii) a simple strain homogenization scheme (Voigt homogenization) over elastically orthotropic grains. We construct a one- and two-layer homogeneous slab model of olivine to demonstrate the feasibility of our method in idealized settings and discuss potential applications to regions where sufficient seismic data might exist for real-world application. We also discuss the limitations of our method and the caveats of the assumptions made, in particular the assumed orientation fabric symmetries assumed (hence the assumed mantle flow regime) and the well-posedness of our cost function approach.
How to cite: Hirche, L., Mosegaard, K., and Rathmann, N.: Inferring the Crystal Orientation Fabrics of Olivine from Oblique Seismic Data using a Spectral Fabric Representation, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11179, https://doi.org/10.5194/egusphere-egu25-11179, 2025.
Predictions of postseismic creep, glacial isostatic adjustment (GIA), and seismic-wave attenuation rely on a sound understanding of the microphysics of transient rheological behavior of olivine-rich rocks, the main constituent of the upper mantle. Recent work proposes that changes in dislocation density and dislocation interactions in olivine may explain the time-dependent evolution of the viscosity of the upper mantle as inferred from geodetic studies. We designed load-relaxation experiments to test whether this model (also known as the backstress model) can accurately predict the transient rheological behavior of polycrystalline olivine during load relaxations similar to those experienced by the upper mantle during postseismic creep and GIA.
We performed our experiments in a gas-medium apparatus at a confining pressure of 300 MPa and temperatures from 1100–1200℃ on dried and annealed Aheim dunite with a grain size of ~ 400 μm. In each experiment, we performed two load relaxations. The first relaxation was initiated after rapidly loading our annealed samples to a differential stress of ~ 200 MPa within 60 s, and the second relaxation was initiated after steady-state creep was reached at a similar, constant stress.
During the first relaxation, we find that viscosities are initially 1–2 orders of magnitude lower than steady-state viscosities before converging to the steady-state creep flow law over the course of minutes to hours. Meanwhile, such an interval of transient rheological behavior is absent during load relaxations from steady state creep. Microstructural analysis of our starting materials and deformed samples indicates that the observed transient behavior cannot be attributed to changes in grain size or crystallographic preferred orientation. Instead, the transient behavior likely corresponds to changes in dislocation density, which systematically increased during deformation following a piezometric relationship.
We compare these observations to numerical predictions of the backstress model, taking into account the stress history preceding the relaxations, the grain size and the initial dislocation density of our samples. We find that the backstress model accurately predicts the viscosity reduction during the interval of transient rheological behavior, although it slightly underestimates the duration of the transient. In addition, the absence of transient behavior during relaxation subsequent to steady-state creep indicates that the magnitude of backstress during steady-state creep is similar to the applied stress, in agreement with the model. However, the backstress model tends to overestimate strain rates during steady-state creep and subsequent relaxation. Analysis of decorated dislocations in our deformed samples indicates that this discrepancy may be due to the overestimation of dislocation density during steady-state creep by the backstress model. We discuss potential modifications to improve the model involving the effects of temperature and internal stress heterogeneity on the transient behavior of olivine.
How to cite: Hein, D., Hansen, L., and Dillman, A.: Mimicking postseismic creep in the laboratory: Testing models for transient creep in the upper mantle, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8817, https://doi.org/10.5194/egusphere-egu25-8817, 2025.
Olivine is the most abundant mineral in Earth’s mantle, and its rheological behaviour is likely to control upper-mantle deformation. While the rheological behaviour of olivine is widely studied, relatively little is known about the behaviour of individual olivine grain-boundaries. There is a pressing need to advance our understanding of their physical and chemical properties. Forsterite bicrystals, synthesized by direct bonding of highly polished single crystals at high temperature, were tested in a creep apparatus to investigate sliding along a single planar grain-boundary at high temperature (1300°C and 1400°C). Prior to deformation, the lateral surfaces of the bicrystals parallel to the shear direction were polished, and fiducial markers were scribed perpendicular to the grain-boundary trace to track grain-boundary sliding. Bicrystals were deformed in shear between two polycrystalline alumina pistons or two single crystal forsterite pistons, at 1 atm, with applied resolved shear stresses ranging from 1 to 30 MPa. Post-deformation microstructural analysis using a scanning electron microscope (SEM) shows discrete offsets of fiducial markers, which is the first direct evidence of grain-boundary sliding in olivine bicrystals. These results establish that the studied grain-boundaries are significantly weaker than crystal interiors, and that, crucially, grain-boundary sliding is controlled by the crystallography of crystal interiors and is favoured in a direction nearly parallel to the weakest slip direction in both crystals of the bicrystal. The measured effective grain-boundary viscosities fit well theoretical models of a dislocation grain-boundary sliding mechanism and are higher than measurements inferred from attenuation. This evidence may highlight the important role of boundary dislocations in accommodating grain-boundary sliding in large grain sizes. These new results indicate that grain-boundary sliding in olivine could play a crucial role in the development of crystallographic preferred orientation and the resulting seismic anisotropy in the upper mantle and should therefore be accounted for in geodynamic models of Earth’s interior.
How to cite: Mecklenburgh, J., Singh, S., Mariani, E., Thom, C., Marquardt, K., Wheeler, J., and Hansen, L.: Direct Measurement of Grain-Boundary Sliding in Forsterite Bicrystals, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-19836, https://doi.org/10.5194/egusphere-egu25-19836, 2025.
Quantifying rock rheology is fundamental to understanding and modelling the lithosphere’s dynamics. However, although most rocks of the lithosphere deform at high (> 0.5 GPa) – to very high (> 3 GPa) – pressure over geodynamic events, available mechanical laws have been produced at low pressure (0.3 GPa) using gas-medium deformation apparatuses. To explore rock rheology at higher pressure – typically above 1 GPa – a solid-medium apparatus is required, which involves substantial friction-related stress overestimations while the sample is deforming within the confining medium. Here we provide a series of deformation experiments that aim to quantify such a stress overestimation in the new generation Griggs-type apparatus. The main goal is to better estimate how the friction “baseline” evolves with pressure, alongside defining the starting point of the strain-stress curve more accurately. To do so, we performed general shear experiments of Carrara marble at a confining pressure ranging from 0.3 to 1.5 GPa, while systematically applying a temperature of 650 °C and a displacement rate of 10-4 s-1. Using relaxation steps to highlight the friction baseline in a ‘force-displacement’ plot, we document a slope that increases linearly with pressure, from 0.1° to 1.5°. Moreover, none of the highlighted baselines crosses the conventional hit-point, which is the commonly used reference to define the “zero” point of strain-stress curves in the Griggs-type apparatus. Such a mismatch involves additional stress overestimations that we propose to correct by using a new “hit-point” at the intersection between the baseline and mechanical curve. Thanks to the latter and applying a “baseline” correction, we document stress measurements equivalent to the ones documented for Carrara marble using the gas-medium Paterson press.
How to cite: Précigout, J., McGill, G., and Arbaret, L.: Rheological perspective using the new generation Griggs-type apparatus: New constraints from general shear experiments of Carrara marble, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10412, https://doi.org/10.5194/egusphere-egu25-10412, 2025.
The rheological properties of Earth's lower mantle have a strong impact on global mantle dynamics. Previous studies have shown that the deformation of the ferropericlase-bridgmanite mixture may be strongly controlled by the morphology of the weaker ferropericlase. Due to elongation of weak ferropericlase clusters, the bulk viscosity of the two-phase mixture is significantly lowered and become anisotropic. As a result, this transient microstructural evolution may have a strong impact on the overall rheology of the lower mantle.
Existing numerical models of this process often do not consider that the elongation of ferropericlase during deformation may be counteracted by interfacial diffusion. This diffusion reduces the interfacial energy and may result in an increased rounding rate that reduces the deformation-induced elongation. However, it is unclear under which conditions this process has an impact on the overall dynamics and bulk rheology of a two-phase mixture. A scaling analysis of the governing equations reveals that the dynamics of the given system are mainly influenced by the ferropericlase-bridgmanite viscosity ratio and by the ratio of viscous to interfacial forces.
To explore the impact of these two properties on the dynamics and bulk rheology of the ferropericlase-bridgmanite mixture, we employ numerical models. In these models, interfacial diffusion is approximated by adding a surface tension term to the governing equations and by directly resolving the ferropericlase-bridgmanite interface using body fitted meshes. The results show that for a range of model parameters, rounding due to surface tension may have a significant impact on the morphological evolution of the ferropericlase inclusions and may thus also exert some control over the rheology of the lower mantle.
How to cite: Thielmann, M. and Dabrowski, M.: Elongation inhibition in two-phase media due to surface tension effects, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16598, https://doi.org/10.5194/egusphere-egu25-16598, 2025.
Ludox colloidal dispersions exhibit viscous, elastic, plastic and brittle rheological properties depending on their water content. This makes these dispersions a relevant model system to study a wide variety of phenomena, from drying paint to columnar joints. As for now, they are the only system that enables to generate one-sided subduction from convection in the laboratory. Rayleigh numbers, constraining the intensity of convection, have a similar order of magnitude in the laboratory experiments and in the mantle. Prandtl numbers are much greater than 100, insuring negligible inertial effects. Ludox is thus a relevant analog system to study convection in planetary mantles, the water content playing the role of temperature in determining its rheological properties.
We investigate here convective patterns in a Ludox suspension (TM50) heated from below and dried and cooled from above, coupling neutron imaging (NeXT, ILL) and thermochromic liquid crystals (TLCs). Both imaging methods are complementary. Neutron imagery is used to estimate the local volume fraction of silica in the solution, which can be linked to the local rheological properties. TLCs give us access to the temperature field. We therefore can follow in situ the development of hot thermal plumes, and of a skin at the surface, that will eventually subduct spontaneously.
In addition to the imagery, the evaporation rate, the surface, ambient and heating temperatures, and the ambient humidity rate are recorded. They are used to estimate the heat and mass transfer at the surface and how the formation of a skin affects them compared to a case with an homogeneous newtonian solution.
How to cite: Remise Charlot, H., Aubertin, A., Helfen, L., Pépin, M., Alba-Simionesco, C., and Davaille, A.: Coupling neutron imaging and thermochromic liquid crystals to investigate the properties of a laboratory-made subducting slab. , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11010, https://doi.org/10.5194/egusphere-egu25-11010, 2025.
Although both ice and methane hydrates are hydrogen-bonded structures of water molecules, methane hydrates are orders of magnitude more creep resistant than ice. The power law scaling properties of this creep resistance was shown experimentally two decades ago, but a molecular-scale explanation for these exponents has still been lacking. Using molecular dynamics simulations over almost two orders of magnitude of stresses and three orders of magnitude of strain rates, we show that power law creep consistent with the creep experiments by Durham and coauthors in 2003 can emerge from a monatomic water model. A monatomic water model with an angular term resulting in tetrahedral ordering, a spherically symmetric methane model and the concept of a hydrate polycrystal are sufficient conditions for this behavior to emerge. We attribute a low-stress low-power relationship to shear of the amorphous layer on grain boundaries between hydrate grains, and show this by a separate set of simulations only containing amorphous hydrate. Higher power creep of polycrystalline hydrate at higher stresses scales with an exponent about twice that of the low-stress regime, but is slower than expected from the amorphous hydrate simulation results. We therefore attribute this creep to the degradation of hydrate corners that are carrying the compressional loading of the hydrate at stresses that cannot be carried by the grain boundaries.
How to cite: Sveinsson, H. A. and Cao, P.: Distinct creep regimes of methane hydrates can be predicted by a monatomic water model , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8556, https://doi.org/10.5194/egusphere-egu25-8556, 2025.
