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 99^th percentile of 54 MPa. However, significant heterogeneity in residual stress is present in the protomylonite and ultramylonite, with 99^th 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.