Apatite is a key accessory phase in metamorphic rocks, capable of recording mineral growth, deformation, and fluid-rock interaction across a wide range of metamorphic conditions. To evaluate the mechanisms of apatite growth and deformation, we present an integrated microstructural and geochemical study of apatite in high-pressure granulite- to lower amphibolite-facies rocks from the Eastern Himalayan Syntaxis, Tibetan Plateau. Cathodoluminescence imaging, electron backscatter diffraction, electron probe microanalysis, and in situ laser ablation inductively coupled plasma mass spectrometry trace element data were combined to assess apatite growth histories and post-crystallization modifications. In high-pressure granulite-facies samples, apatite grains display strong shape-preferred orientation (SPO) and significant intragranular deformation but lack a crystallographic preferred orientation (CPO), consistent with pre-peak metamorphism growth and experienced peak metamorphic conditions. Medium-pressure granulite-facies apatite shows oscillatory zoning, weak SPO and CPO, and limited deformation, indicating post-tectonic growth from compositionally homogeneous fluids. Apatite from upper amphibolite-facies samples preserves both strong SPO and CPO, with only minor deformation features, suggesting syn-tectonic crystallization during foliation development. In lower amphibolite-facies rocks, apatite is characterized by core-mantle structures, weak SPO, but CPO subparallel to stretching lineation, and variable trace element patterns, consistent with episodic growth during fluid-mediated dissolution–precipitation under retrograde conditions. Systematic variations in apatite chemistry—including decreasing Sr/Y, ΣLREE, and (La/Yb)N, and increasing Th/U and F/Cl with decreasing metamorphic grade—reflect the influence of metamorphic fluid composition, mineral reaction pathways, and elemental partition among mineral phases at corresponding metamorphic conditions. Despite differences in deformation and growth history, consistent correlations between texture and composition suggest localized, short-range fluid-mediated element redistribution. Our results demonstrate that multiple generations of apatite can be distinguished through integrated microstructural and geochemical datasets, providing a robust archive of pressure–temperature (P–T) conditions and fluid evolution in metamorphic terranes.

The age of the mylonite belts in the basement rocks of the Pyrenees is a subject of debate in the structural geology and petrology communities because of its potential implication on the regional tectonothermal history and on the tectonic evolution of SW Europe. Here we address when and how mylonitisation took place in two key areas of the Eastern Pyrenees, where shear zones are associated with Giant Quartz Veins (GQVs). We conducted zircon U-Pb and muscovite Ar-40/Ar-39 dating coupled with structural, textural, and crystallographic preferred orientation (CPO) analyses of mylonites from the Cap de Creus and Canig & oacute; Massifs. U-Pb zircon dating of a dacite porphyry dyke crosscut by GQVs and mylonitic bands yields a maximum shear zone and GQV formation age of ca. 292 +/- 3 Ma. Ar-40/Ar-39 analyses of muscovite within mylonitised GQVs yield initial crystallisation ages between ca. 164 and 188 Ma, as well as younger recrystallisation ages of ca. 110-118 Ma. A qualitative assessment of the GQV history is inferred from step-heating spectra of muscovite and quartz CPOs. The results indicate that GQV formation and mylonitisation were coupled, coeval, and long-lasting processes that took place from early Jurassic to early Cretaceous times. A comparative evaluation of quartz CPOs reveals inconsistencies regarding the strain distribution, quartz slip systems activity, and deformation temperatures depending on the deformed rock type. Quartz mylonites have stronger CPOs dominated by basal , prism , or prism slip systems, whilst phyllonites and granite mylonites show weaker fabrics mostly dominated by mixed slip. This apparently suggests higher deformation temperatures in quartz mylonites than those inferred from more reliable proxies, such as mineral assemblages, brittle behaviour of K-feldspar, and fluid inclusion data. We suggest that the water-weakening effect caused by coeval formation and deformation of GQVs enabled easier dislocation glide and creep, allowing strain localisation and transitions between slip systems at lower temperatures than commonly inferred due to enhanced ductility. U-Pb zircon dating further suggests the existence of an early Carboniferous (ca. 332 +/- 4 Ma; Visean) magmatic episode in the Pyrenees, in agreement with a cyclic, rather than a progressive, geodynamic history of the region during Variscan times. The present work challenges classical interpretations stating that Pyrenean mylonite belts developed during the retrograde stages of the Variscan Orogeny, highlighting that the structural evolution of this region during Mesozoic times deserves further investigation. Results have implications for interpreting deformation localisation mechanisms and conditions in crustal rocks, for the formation mechanisms of GQVs in worldwide orogenic belts, and for the tectonothermal history of the Pyrenees since late-Variscan times.

Fast-flowing ice streams drain most of the inland ice from the Antarctic and Greenland ice sheets (GrIS). The Northeast Greenland Ice Stream (NEGIS), for example, extends for more than 500 km from the central GrIS ice divide to its outlets, with flow velocities up to ten times higher than in the surrounding ice. Despite extensive research, the mechanisms responsible for ice stream formation remain poorly understood. NEGIS, as a representative case of ice streams that are not topographically confined, has recently also been found to lack an area of elevated geothermal heat flux below. However, no model has so far been able to test whether ice stream initiation can solely result from the evolving internal properties of the ice itself, without relying on external forcing, given that slow-moving ice may be frozen to the bed before ice-stream formation.Ice is strongly anisotropic because it deforms more easily parallel to its crystallographic basal plane than perpendicular to it along the crystal's c-axis. During deformation, this difference leads to a preferred alignment of the crystal lattice orientations. This anisotropy has significant implications for ice flow. We present a three-dimensional full-Stokes model of an analogue to NEGIS. In our modelling, ice first shows convergent flow towards the outlet gate. During flow, c-axis rotations calculated by our model cause the directional alignment of the easy-glide crystallographic basal planes parallel to the vertical shear plane, which make the ice effectively softer. Shear zones usually form in pairs due to the localized shearing, known as shear margins that bound the ice stream that can now flow much faster and extend further inland. Our results show that a fully developed, fast-flowing ice stream can form in only 1000–2000 years solely due to the evolving ice anisotropy. We perform several model runs up to 4000 years to explore the effect of varying boundary conditions, which result in different geometries of an ice-stream system. Ice streams in the system can potentially initiate and evolve by the formation and movement of shear margins in relation to the location of outlet gates within the drainage basin. This work stresses the importance of evolving ice anisotropy on ice-sheet mass balance and sea-level rise during global climate change.

Satellite and airborne sensors have provided detailed data on ice surface flow velocities, englacial structures of ice sheets and bedrock elevations. These data give insight into the flow behaviour of ice sheets and glaciers. One significant phenomenon observed is large-scale folds (over 100 m in amplitude) in the englacial stratigraphy in the Greenland ice sheet. A large population of folds is located at ice streams, where the flow is distinctly faster than in the surroundings, such as the North-East Greenland Ice Stream (NEGIS). While there is no consensus regarding the formation of large-scale folds, unraveling the underlying mechanisms presents significant potential for enhancing our understanding of the formation and dynamics of ice streams.Ice in ice sheets is a ductile material, i.e., it can flow as a thick viscous fluid with a power-law rheology. Furthermore, ice is significantly anisotropic in its flow properties due to its crystallographic preferred orientation (CPO). Here, we use the Full-Stokes code Underworld2 (Mansour et al.,2022) for 3D modelling of the power-law and transversely isotropic ice flow, also in comparison with the isotropic ice models.Our simulated folds with anisotropic ice show complex patterns on a bumpy bedrock, and are classified into three types: large-scale folds (fold amplitudes >100 m), small-scale folds (fold amplitudes

The Northeast Greenland Ice Stream (NEGIS) is a fascinating, over 500 km long structure in the Greenland Ice Sheet. The ice stream shows many features, such as folds and shear zones, that are also common in other ductile rocks. Geological methods and expertise may contribute to a better understanding of NEGIS and similar deformation structures in ice sheets. It is standard practice in oil and gas exploration to create 3D-structural models from parallel seismic lines. This approach, applied to radar profiles, is relatively new in glaciology (Bons et al., Nat. Comm. 2016, DOI: 10.1038/ncomms11427) but provides far more insight into the structural architecture and evolution of ice sheets than single radar sections. A 3D-structural model of upstream NEGIS reveals how pre-existing folds are offset within the ice stream. With that, classical strain analysis methods can be applied to quantify the deformation of these folds in the shear margins. This reveals that the total offset at the level of the EGRIP drilling project is in the order of up to 75 km and that the finite shear strain in the shear margins is around 18. With present-day shear-strain rates in the shear margins, such a finite offset and shear strain are achieved in ≤2000 yrs. This strain analysis also proves that ice does not flow through shear margins, but that the shear margins instead advect with the ice. This means that 'flow lines' (which should better be called 'streamlines') are not the same as 'path lines', as is now often assumed. The two are only the same in a time-invariant velocity field, which does not apply to NEGIS. Shear zones in other ductile rocks show that rocks never flow through shear zones, but shear zones can shift or 'jump' to new locations, as is actually observed in NEGIS. Geological principles to analyse and date the formation and activity of salt diapirs and syn-sedimentary faults can also be applied to folds observed in and around NEGIS. This reveals that fold amplification inside the shear margins ceased about 2000 yrs ago, which can be explained by the formation of the shear margins and concomitant reorientation of the CPO. A combination of several structural geological methods thus enables constraining the age of NEGIS as we now know it to about 2000 yrs, which is much less than previously assumed. The surprisingly late appearance of NEGIS, as well as the demise of ice streams in the Holocene (based on 3D-analyses of folded stratigraphy; Franke et al., Nature Geosci. 2022, Doi: 10.1038/s41561-022-01082-2) indicates that ice sheets are very dynamic, mostly due to the highly non-linear (n=4) and anisotropic rheology of ice.

Only a few localised ice streams drain most ice from the Greenland Ice Sheet. Thus, understanding ice stream behaviour and their temporal variability is crucially important to predict future sea-level change. The interior trunk of the 700 km-long North-East Greenland Ice Stream (NEGIS) is remarkable for the lack of any clear bedrock channel to explain its presence. Here we use isochronous radar reflections from an airborne radar survey as passive tracers of ice deformation. We present the first-ever 3-dimensional analysis of folding and advection of stratigraphic horizons within an ice stream, which shows that the localised flow and shear margins in the upstream part were fully developed only ca. 2000 years ago. This indicates that this type of streaming in the interior of an ice sheet can be triggered on short time scales.

The deformation of the Himalaya-Tibetan plateau remains one of the hottest examples of Earth's tectonics and dynamic evolution. What drives deformation and growth of the plateau, and how this is partitioned into diverse structural styles and mechanisms remain heated discussions. Mesozoic orogens also contributed to crustal thickening of the plateau prior to the Cenozoic India-Asia collision and notably, they were reactivated due to its structural inheritance and low viscosity since the collision, for example, the first uplift in Qiangtang and the Paleo-Tethyan suture zone in its interior. However, few attention was paid to pre-collision reactivation and structural superimposition of Mesozoic orogens. The newly discovered NWW-trending Ejiumai shear zone with biotite 40Ar/39Ar and zircon and monazite U-Pb ages of ca. 80-70 Ma flanks the northern border of the Paleo-Tethyan suture zone and mainly includes reactivated Triassic basement gneisses and syntectonic pegmatite. Combined with the oblique moving kinematics including both sinistral and normal-sense shear, a transtension regime with lower crustal anatexis can be concluded for the genesis of Ejiumai shear zone. Concurrent granitic plutons were also found in the suture hundreds of kilometers to the east. Based on these observations, we present a schematic model of extensional collapse of the proto-Tibetan plateau induced by far-field northward indentation of Neo-Tethyan suture zone to the south in Late Cretaceous.

The rheological properties of mantle rocks are strongly dependent on their crystallographic preferred orientation (CPO). Olivine CPO, defined by the orientation of seismically fast [100] axes parallel to flow direction, is also thought to be a dominant contributor to seismic anisotropy in the Earth's upper mantle. However, the amount of deformation needed to overprint a new CPO on a pre-existing fabric and the impact of the inherited CPOs on the transient microstructure evolution, remain unknown. This study employs a full-field numerical approach (VPFFT-ELLE) to explore the dynamic recrystallization and microstructural evolution of olivine polycrystalline aggregates under complex deformation conditions. We test four combinations of successive pure shear and simple shear boundary conditions. Findings indicate that inherited CPOs influence subsequent deformation in a manner dependent on the kinematic relationship between successive stages. In all cases, a minor strain increment (epsilon similar to 0.3-0.6) is sufficient to erase the previous microstructure and CPO. However, when deformation conditions change dramatically (e.g., stretching direction changes orthogonally), the intensity of the new CPO developed is significantly lower and strain distribution are specially altered. During a transient strain stage, pre-existing microstructures undergo extensive reworking, especially when deformation conditions are changed dramatically, such as switching from simple shear to a pure shear condition with a parallel shortening direction relative to the stretching direction. We estimate the significance of these results in interpreting observations of seismic velocity anisotropy, concluding that P-wave seismic anisotropy is significantly and positively correlated with the evolution of olivine CPO with deformation history. This research underscores the transient nature of microstructural rearrangement in olivine aggregates and the necessity for caution in interpreting seismic anisotropy in regions with complex deformation histories, as inherited CPOs can influence current fabric development and induce deviation to the present deformation conditions.

The deformation of the upper mantle is predominantly governed by the mechanical behavior of olivine (Karato et al., 1989). During mantle flow, olivine undergoes crystal-plastic deformation, leading to the development of crystallographic preferred orientations (CPOs). In this process, the a-axes of olivine polycrystalline aggregates align with the flow direction (Hansen et al., 2012). Consequently, the observed CPOs in olivine-rich rocks serves as an indicator of the mantle flow direction. While the influence of plastic deformation is well understood, the role of dynamic recrystallization during deformation remains not fully comprehended, hindering our ability to interpret the deformation history of naturally-deformed rocks.This contribution employs microdynamic numerical simulations of olivine polycrystalline aggregates with varying iron content (fayalite content) to explore the CPO and grain size response to dynamic recrystallization. Utilizing a full-field approach with explicit simulation of viscoplastic deformation (http://www.elle.ws; Bons et al., 2008; Piazolo et al., 2019) and dynamic recrystallization processes under simple shear boundary conditions up to high strain, this study indicates that simulations with only dislocation glide and also those including recrystallization successfully reproduce such steady state conditions, without requiring other potential mechanisms. The model establishes a framework for understanding the development of olivine CPOs in mantle rocks, highlighting the interplay between plastic deformation and dynamic recrystallization processes, including grain boundary migration, intracrystalline recovery, and new grain nucleation.Acknowledgements: Yuanchao Yu acknowledges funding by the China Scholarship Council for a PhD scholarship (CSC-202008130104). This work has been developed using the facilities of the Laboratory of Geodynamic Modelling of GEO3BCN-CSIC.

Natural fractures controlled by faults in ultradeep carbonate strata play substantial roles as both fluid migration channels and storage spaces. However, characterizing the heterogeneous distribution of underground fractures within the complex three-dimensional geometry of strike-slip fault zones remains challenging. This study investigates the characteristics of natural fractures controlled by strike-slip faults in the fractured Middle and Lower Ordovician reservoirs of the central and northern Tarim Basin, China. Seismics, cores, and image logs were integrated to quantitatively analyze the intensity and dip angle of natural fractures and findings were verified using published sandbox simulations. The carbonate reservoir contains three main types of natural fractures: tectonic fractures, abnormal high-pressure-related fractures, and stylolites. Strike-slip faults control the distribution and characteristics of tectonic fractures across various scales. Generally, both fracture intensity and porosity exhibit a decreasing trend as the distance from the main fault surface increases. Compared with those in non-stepover zones along a strike-slip fault, natural fractures and faults in stepover zones are more developed along the fault strike, with significantly greater development intensity in central stepover regions than that at its two ends. Furthermore, strike-slip faults influence the dip angles of both natural fractures and secondary faults. The proportion of medium-to-low-dip angle fractures and faults in the stepover zone is greater than that in the non-stepover zone. Additionally, the proportion of medium-to low-dip angle fractures and faults in the middle of the stepover is greater than that at both ends. Therefore, strike-slip fault structures control the dip angle of natural fracture and the heterogeneity of secondary fault and fracture intensity. The linking damage zone in the stepover contains a larger volume of fractured rocks, making it a promising petroleum exploration target. The development of stepovers and the orientation of present-day in-situ stress substantially influence the productivity of fractured reservoirs controlled by strike-slip faults. The analysis in this study reveals that reservoir productivity increases as the angle between the strike-slip fault segment and the maximum horizontal principal stress decreases. This study provides valuable insights for quantitatively evaluating fracture heterogeneity in fractured reservoirs and establishing optimized selection criteria for favorable targets in fault-related fractured reservoirs.

Understanding the material properties and physical conditions of basal ice is crucial for a comprehensive understanding of Antarctic ice-sheet dynamics. Yet, direct data are sparse and difficult to acquire, necessitating geophysical data for analysis. We employed high-resolution ultra-wideband radar to map high-backscatter zones near the glacier bed within East Antarctica's Jutulstraumen drainage basin. In addition, we used radar forward modelling to constrain their material composition. Our results reveal along-flow oriented sediment-laden basal ice units connected to the basal substrate, extending to several hundred meters thick. Three-dimensional thermomechanical modelling suggests these units initially form via basal freeze-on of subglacial water originating upstream. We suggest that basal freeze-on and the entrainment and transport of subglacial material play a significant role in an accurate representation of the material, physical, and rheological properties of the Antarctic ice sheet's basal ice, ultimately enhancing the accuracy and reliability of ice-sheet modelling.

Rock salt, composed mainly of halite, flows viscoplastically over a wide range of geological conditions, strongly impacting the dynamic evolution of sedimentary basins and orogens. Understanding how dislocation creep, which involves dislocation glide, intracrystalline recovery and dynamic recrystallization, influences the microstructure and rheology of halite under various deformation kinematics and temperatures is crucial for enhancing knowledge of salt flow dynamics. This study employs a full-field numerical simulation method to compare the viscoplastic deformation of polycrystalline halite under simple shear and pure shear conditions up to a natural strain of epsilon = 1.5 at temperatures ranging from 100 degrees C to 300 degrees C. The results are presented in terms of crystallographic preferred orientation (CPO), grain shape preferred orientation (SPO), subgrain boundary direction, grain size and strain rate distribution. The results indicate that the crystallographic anisotropy of individual halite crystals is transferred to the polycrystalline scale, resulting in strain localization, particularly in simple shear simulations. The kinematics of deformation affect the evolution and distribution of high strain-rate bands, determining the direction of intragranular substructures and the morphology of strain-induced grain boundaries, with minimal impact on grain size. The intensity of grain boundary migration increases with temperature, significantly influencing grain morphology and size, thereby obscuring strain localization, while having little effect on CPOs. At low strain (epsilon < 1.0), CPOs relative to both the maximum shortening direction and the grain SPO are similar regardless of the deformation kinematics. At high strain (epsilon > 1.0), simple shear CPOs exhibit three stronger {100} maxima with a monoclinic symmetry relative to the grain SPO compared to the six {100} maxima with an orthotropic symmetry relative to the grain SPO generated under pure shear. Therefore, microstructures and CPOs can serve as indicators of the strain path in polycrystalline halite under various conditions, aiding in determining the shear sense and elucidating the deformation kinematics of salt structures.

The EastGRIP ice core is drilled through the Northeast Greenland Ice Stream, which has a surface velocity of 55 m/yr towards NNE at the drill site. Deriving a better understanding of internal deformation and the rheology within an ice stream is crucial for ice flow models and projections of future solid ice discharge. We use the line scanner to make the stratigraphy visible and document disturbances in the layering in the depth region from 1375 to 2120 m covering a large part of the Glacial Period. Disturbances are visible in cuts perpendicular to the ice flow direction, and not in cuts parallel to flow. Between these two extremes, we have a gradual change in type and amount of disturbances. As with all other ice cores, the ice in the EastGRIP ice core is thinned vertically. Due to the advanced thinning of layers, it is clear that the visible structures are not the remnants of surface features, such as sastrugi. However, the disturbances, or deformation structures, are the result of strain caused by the stress field at the EastGRIP site, which is described by a compressional component perpendicular to and an extensional component parallel to the ice flow direction. In most samples cut perpendicular to ice flow, i.e. with the compressional setting visible, we find structures, very similar to geological duplex structures. We identify duplex structures extending the width of the core by the sudden change of layer tilt within one bag at a time. Duplex structures are confined by layer parallel shear zones, with tilted layers in between them. The small-scale shear zones only become evident due to the deformation they cause and can extend well beyond these visible structures. We furthermore suggest, that shear zones are present parallel to layering, but do not show up, as a lateral displacement of layers, does not disrupt the vertical profiles. We discuss one example, from a depth of 1651 m (26 ky b2k), in detail. We further investigate approx. 30 m of chemical CFA data, mainly NH4+ and Ca++, from the same depth. We find peaks that double, in both the visual stratigraphy as well as the CFA data. These may be a result of the duplex structures that stack the stratigraphy and have the potential to disturb the climate record. Our results display the importance of understanding internal deformation when interpreting the climate record.

Stylolites are ubiquitous structures in carbonates that present a variety of morphologies. Besides being important structures in terms of rock compaction and deformation, stylolites control limestone permeability, which is influenced by their morphology and arrangement in connected networks. This study characterizes stylolite morphological parameters, such as length, thickness, filling by insoluble material, spacing and connectivity, to provide quantitative data on the relationships between stylolite morphology and carbonate host rock textures. A collection of thin sections from a variety of lithologies and geological settings was analyzed, showing that the stylolite length, thickness and morphology are interrelated and are controlled by the host rock textures. The results show that stylolite linearity, calculated as the ratio between the straight length and the length measured along the stylolite trace, varies according to the stylolite type. Higher stylolite thickness is observed in mud-dominated textures compared to grain-dominated ones but is independent of the stylolite type. There is no clear trend in terms of the amount of insoluble material collected by the stylolites and the rock texture or stylolite type. Analysis of stylolite spacing reveals that heterogeneity in grain size, grain sorting and composition controls stylolite formation.

Upright folds in ice sheets are observed on the cm scale in cloudy bands in drill cores and on the km scale in radargrams. We address the question of the folding mechanism for these folds by analysing the power spectra of fold trains to obtain the amplitude as a function of wavelength signal. Classical Biot-type buckle folds due to a rheological contrast between layers develop a characteristic wavelength, visible as a peak in the power spectrum. Power spectra of ice folds, however, follow a power law, with a steady increase in amplitude with wavelength. Such a power spectrum is also observed in a folded, highly anisotropic biotite schist and in a numerical simulation of the deformation of ice Ih with a strong alignment of the basal planes parallel to the shortening direction. This suggests that the folds observed in ice are primarily due to the strong mechanical anisotropy of ice, which tends to have a strong lattice preferred orientation due to ice-sheet flow.

This contribution presents (i) an overview of how the characteristics of sedimentary facies (composition, grain size distribution, and texture) control the morphology and spatial distribution of stylolite networks, revealing that muddy versus grainy facies and the skeletal component size are the primary controls on the degree of stylolite network connectivity; and (ii) an analysis of how bedding-parallel stylolites may act as barriers and/or conduits for fluids depending on the characteristics of their host rocks, the stress state, fluid pressure, and the forces that drive fluids. This analysis is based on outcrops from the Benicàssim area (Maestrat Basin, Eastern Spain).

Solid ice discharge from land-based ice masses into the ocean raises the global sea level and accelerates due to anthropogenic climate change. Modelling ice flow dynamics aims to provide better projections of future sea level rise. The Antarctic and Greenland ice sheets are predominantly drained through ice streams, which are regions of higher ice flow velocity than their surroundings, and thus play an important role in ice sheet dynamics. However, little is known about their rheology. Therefore, they may introduce large uncertainties in ice sheet models.In order to study the main deformation and recrystallization mechanisms dominant in an ice stream, we conducted microstructural analyses on samples from the EastGRIP ice core that was drilled in the largest Greenlandic ice stream, the Northeast Greenland Ice Stream (NEGIS).The data set contains 1064 samples, oriented vertically and horizontally to the ice core axis, from depths between 111 and 2121 m. Analyses of the deepest 550 m of the ice core are pending. All samples were scanned with 5 µm resolution under bright-field illumination with a Large Area Scanning Macroscope (LASM). The obtained microstructure, i.e. grain shape, size, and elongation, was extracted using digitalised grain boundary networks by means of a machine-learning based image analysis software. We determined six different rheological regimes through the ice column. Most microstructural changes were interpreted as changes in recrystallization mechanisms, whereas the dominant deformation mode, horizontal extension, appears to remain fairly constant below 500 m of depth. Previous numerical high-strain ice deformation simulations showed strain localisation with the development of visible shear bands. A similar setting was expected inside ice streams, but at the investigated depths of the EastGRIP ice core, no clear shear bands could be discerned so far for the applied sampling resolution.These results indicate that NEGIS has no strong high-strain localisation down to 2121 m depth but probably deforms as a block with extension along flow. The high ice flow velocities, therefore, might have to be compensated either in the lowest 500 m or below the ice.

Metasomatism is a ubiquitous process in the Earth's crust, exerting major controls on fluid, heat and mass transfer and rock deformation, and is commonly constituted by mineral replacement reactions. Different types of metasomatism may coexist and/or successively conceal each other in a given area. Deciphering the geochemical behaviour, regional extent and mineralogical changes of multi-stage metasomatism can be difficult because of the overprinting of successive events and their frequent relationship with deformation. Here, we investigate granitoid metasomatism, namely silicification, feldspathisation and sericitisation, in the Variscan basement rocks of the Canig & oacute; Massif (Eastern Pyrenees, SW Europe), which is spatially related to Giant Quartz Veins (GQVs) tens of metres wide and several kilometres long. Unaltered and altered granitic orthogneisses derived from Ordovician intrusives and late-Variscan granitoids, as well as GQV occurrences, are studied across scales through structural and textural characterisation, whole-rock geochemistry and Electron Backscatter Diffraction (EBSD). Geochemical analyses are further compared with a new database including more than 600 unaltered granite and orthogneiss samples from the Pyrenees and the Catalan Coastal Ranges (SW European Variscan Belt). Results show that silicification, the dominant metasomatic process, was related to regional-scale shear zones and contributed to form GQVs through mineral replacement. This is confirmed at the macro- (km), meso- (m-cm) and micro-scale (mu m) by relict fabrics, mineral phases and structural features of the precursor rocks within veins, by a progressive depletion of all major and trace elements, except silica, in rocks sampled along decreasing distances from GQV outcrops, and by the localisation of mylonitic deformation along GQVs. Feldspathisation and sericitisation are, in contrast, restricted to specific sectors and exposed as albitite, trondhjemite and pale green mica-rich outcrops. It is suggested that most of the exposed areas of the studied GQVs are, accordingly, not veins sensu stricto but metasomatic products where the original fabrics and features of precursor rocks were overprinted during coupled deformation and Si-metasomatism. Results presented here have major implications for the scale and geochemical behaviour of multi-metasomatic events, as well as on the kinetics of mineral replacement processes leading to changes in the physicochemical properties of crustal rocks.

Abstract. A better understanding of glacial ice flow and how it is influenced by internal deformation is required to improve the projections of future sea-level rise in a warming climate. Especially large ice streams, the main contributors to solid ice discharge to the ocean, still require more observational data to be represented sufficiently in numerical ice-sheet models. The East Greenland Ice-core Project (EastGRIP) successfully drilled the first continuous deep ice core through an active ice stream, the Northeast Greenland Ice Stream (NEGIS), focusing on investigating the dynamical processes that lead to its exceptionally high velocity. Here, we show Crystallographic Preferred Orientations (CPO) data in 5–15 m depth resolution throughout 2663 m, down to bedrock, to determine the deformation regimes in this ice stream setting complemented by grain-size and borehole temperature profiles for context. A broad single-maximum CPO pattern is present in the upper 200 m caused by overlying snow and ice layers. Below, a crossed girdle CPO is observed for the first time in a deep ice core and we discuss possible formation mechanisms. Between 500 and 1230 m of depth, we observe a vertical girdle CPO indicative of along-flow extensional deformation. A complementary simple-shear component and polygonization explain the CPO between 1230 and 2500 m, a vertical girdle with horizontal maxima of varying strength. Close to bedrock, a multi-maxima CPO originates from migration recrystallisation due to high temperatures close to the pressure melting point. Ice at this depth is characterised by centimetre-large, amoeboid-shaped grains, which, together with the conductivity data from the deepest 260 m, indicates that the core contains ice from the last Eemian. A comparison with other deep ice cores from Greenland and Antarctica shows the uniquely fast development of CPO at shallow depths in the EastGRIP ice core due to its location in an area of high strain rates while the grain-size evolution with depth remains similar to less dynamic sites confirming that it is mainly governed by the varying purity of ice deposited during varying climatic conditions. We further show that the overall plug flow of NEGIS is characterised by many small-scale variations, which remain to be considered in ice-flow models.