The Late Paleozoic Munster Basin of SW Ireland is predominantly composed of the non-marine siliciclastic-dominated fine-grained alluvial sediments of the Upper Old Red Sandstone magnafacies. Copper mineralisation in this sedimentary basin is important, either as sediment-hosted stratiform or locally abundant polymetallic vein-hosted copper. In the polymetallic extensional veins, the ore phases include chalcopyrite, tetrahedrite-tennantite, galena, and molybdenite, with gangue minerals commonly quartz, carbonates, chlorite, barite, and Fe-oxides. Recent Re-Os geochronology on molybdenite proved the latter veins opened ca. 367-366 Ma, during Upper Devonian basinal extension, and were deformed before ca. 316-312 Ma by the Variscan orogeny. However, the role of these two major geodynamic events on copper mineralisation was never studied in detail, such that the vein-hosted copper mineralisation and remobilisation processes are still poorly understood. A collection of mineralised vein samples from the western Munster Basin are characterised using reflected light microscopy, Raman spectrometry, and LA-ICPMS trace element analysis to better define the mineralised vein paragenesis. We have identified a pre-mineralisation chlorite veinlet generation. This generation appears to have been reopened by the quartz-rich polymetallic veins in a syntaxial manner, such that the chlorite rims the polymetallic veins. Both vein types show evidence of Variscan deformation (i.e., buckling, displacement). These new observations are critical as 1) the presence of chlorite may allow for precise geothermometry on the veins and 2) the veins appear to have used the same conduits, which may indicate important physicochemical variations (e.g., T, P, pH, fO2, etc.) and/or a pivotal switch in the fluid source(s).

How to cite: Bonin, F.-X., Meere, P., and Unitt, R.: Paragenesis of the Munster Basin Upper Devonian polymetallic veins, SW Ireland, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-1188, https://doi.org/10.5194/egusphere-egu26-1188, 2026.

This study investigates the coupled roles of solid-state deformation, hydrothermal fluid flow, and critical metal enrichment mineralization. Integrating structural analysis, mineral microtextures, EBSD, fluid inclusions, and H-O isotopes, we show that the deposit experienced Neoproterozoic Nb pre-enrichment in alkaline volcanic rocks, later overprinted by Early Paleozoic tectonic-hydrothermal events. Ductile shear zones that characterized by foliation and lineatio enhanced permeability and channeled F⁻-Cl⁻-CO₂-rich fluids from deep sources, with fluid inclusion planes confirming foliation-parallel migration. Syn-tectonic breakdown of amphibole, pyroxene, and titanite released Nb, Y, and REEs into solution as soluble complexes.

Fluid evolution—driven by alkali consumption and CO₂ influx, lowered pH and increased Nb solubility. Nb enrichment occurred via water-rock interaction within shear zones, though complex stability initially inhibited precipitation. Localized Nb deposition took place at altered mineral margins, where Fe²⁺ spikes destabilized complexes. Ore bodies formed in brittle–ductile to brittle domains, governed by the interplay of deformation-controlled permeability and chemical feedbacks.

Strain regime shifts further enhanced permeability, enabling mixing between deep NbF₆⁻- YF₆³⁻-CO₂-rich fluids and external alkaline magmatic or Ca-rich fluids. This mixing triggered pH and redox changes that destabilized metal complexes, precipitating Nb minerals as magnetite/ilmenite-hosted inclusions or microveins, which direct evidence of strain and fluid pulsation. Fault-valve cycling induced fluid immiscibility and boiling, further disrupting complex stability. Our findings underscore that tectonically driven fluid migration is fundamental to Nb enrichment, providing a structural framework for exploring orogenic rare-metal deposits.

How to cite: Cao, S., Li, X., Zhan, L., Zhou, D., Liu, J., Cheng, X., Tao, L., Guo, J., and Hu, Z.: Tectonics and Fluid Coupling in Critical Metal Enrichment, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17856, https://doi.org/10.5194/egusphere-egu26-17856, 2026.

The circulation of magmatic-hydrothermal fluids along crustal-scale fault systems plays a fundamental role in the formation of porphyry-type ore deposits, as these structures control magma emplacement, fluid pathways, and associated rock alteration. In the Rhodope magmatic-metallogenic belt of northern Greece, numerous Oligocene-Miocene porphyry-type ore deposits formed in an extensional back-arc environment. One example is the Maronia Cu-Mo±Re±Au porphyry deposit in the Mesozoic Circum-Rhodope metamorphic belt, where plutonic intrusion occurred during detachment fault activation. Despite this, the detailed sequence and timing of magmatic-hydrothermal fluid circulation related to ore formation remain poorly constrained.

In this study, we aim to unravel the relationship between magmatic and tectonic events to decipher the mechanisms of magmatic-hydrothermal fluid circulation along detachment faults associated with ore formation processes. A total of 20 rock samples were collected across the exposed detachment fault zone at Maronia, ranging from unaltered monzonite to the porphyry microgranite intrusion. We combined highly sensitive Anisotropy of Magnetic Susceptibility and Remanence (AMS and ARM) petrofabric tools with geochemical analyses (e.g., EPMA, LA-ICP-MS/MS). Petrofabric analyses identified multiple magnetic fabrics within individual samples, providing insights into magmatic intrusion emplacement, deformation, and fluid flow, as well as into whether magmatic and tectonic processes occurred concurrently or successively.

Preliminary geochemical and magnetic analyses of minerals and whole rocks constrain the genetic relationship between microgranite intruding the mylonitic rocks within the detachment fault. Petrofabric data are coaxial with observed field fabrics, whereas preliminary ARM results indicate that higher coercivity mineral phases deviate from both field observations and AMS results. Petrographic observations reveal the nature of mineralization and allow evaluation of textural changes related to fluid-rock interaction. We suggest that the combined dataset reflects a strain archive of multi-stage tectonomagnetic processes that drove fluid flow and possibly mineralisation in this sector of the Circum-Rhodope belt.

This study further demonstrates the potential of integrating petrofabric data with geochemical methods to better resolve fluid flow processes and tectono-magmatic evolution in detachment-controlled porphyry systems, providing new insights into the structural controls on mineralization at Maronia.

How to cite: Toivanen, E., McCarthy, W., Koehn, D., and Kleine-Marshall, B.: Tectono-magmatic controls on fluid flow in a detachment-related porphyry system: Insights from magnetic petrofabric analyses at the Maronia deposit, NE Greece, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14523, https://doi.org/10.5194/egusphere-egu26-14523, 2026.

Owing to the Dense Oceanfloor Network System for Earthquakes and Tsunamis (DONET) and borehole observatories, slow slip events (SSEs) have been detected in the shallow extension of the source region of the 1944 Tonankai earthquake (DONET-1). However, a localized seafloor pressure anomaly—characterized by uplift and subsidence at two DONET-1 stations in 2013—has yet to be reasonably explained.

In this study, we explore possible source models for this pressure anomaly by assuming pore-water migration from compacted reservoirs, either arranged in layered formations or represented as swarms of small spheres, toward a dilated zone beneath the décollement. We also compile observations of seafloor crustal deformation driven by SSEs and oceanographic phenomena under baroclinic conditions to refine the spatio-temporal scaling relationship of seafloor pressure variations.

Our main findings are as follows. (i) The potential compacted pore-water reservoirs spatially overlap with the hypocenters of very low-frequency earthquakes (VLFEs), whereas the dilated zone lies in a region with normal-fault-type VLFE activity. (ii) A Kuroshio meander associated with an abrupt fluctuation in sea surface height (SSH) occurred around DONET-1 during the pressure event. (iii) Taken together, (i) and (ii) suggest that the local seafloor pressure change may be explained by pore-water migration destabilized by the Kuroshio current meander. (iv) As this is the first reported case in which a local seafloor pressure anomaly has been identified from only two observation points, the suggested causal link—namely, that the Kuroshio meander may have promoted pore-water migration—provides a strong scientific motivation for future geological surveys, particularly those monitoring seismic activity and seafloor crustal deformation before and after similar pore-water migration events.

How to cite: Ariyoshi, K., Nagano, A., Hasegawa, T., Nakano, M., Matsumoto, H., Aiken, C., Araki, E., Takahashi, N., and Hori, T.: A Local Seafloor Pressure Anomaly Potentially Triggered by Pore Water Migration during Ocean Current Meander, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-271, https://doi.org/10.5194/egusphere-egu26-271, 2026.

Absolute dating of remagnetization events remains the holy grail in paleomagnetism, with the potential to unlock thousands of rock units for new tectonic, metamorphic, and paleointensity studies. Constraining the timing of remagnetizations is especially crucial for understanding fluid flow and mineralogical transformations in orogenic systems.

As a first step toward dating fluid-flow–related remagnetizations, we investigate three Cambrian carbonate units from the northwestern Iberian Peninsula—Tamames, Láncara, and Vegadeo—remagnetized during the Carboniferous. Our goal is to identify, characterize, and ultimately constrain the age of these fluid-induced remagnetization events.

In this presentation we will show an integration of rock magnetism, paleomagnetism, mineralogical, and geochronology results. Magnetic characterization includes room-temperature and low-temperature hysteresis cycles, IRM acquisition curves, First‑Order Reversal Curve diagrams (FORC), thermomagnetic curves, thermal and AF demagnetization, and anisotropy of magnetic susceptibility (AMS). In addition, targeted mineral separation procedures were performed to obtain magnetic sulfide fractions for Re–Os geochronology. The identification and spatial distribution of magnetic phases were examined using scanning electron microscopy (SEM) and quantum diamond microscopy (QDM), allowing us to distinguish primary from secondary magnetic minerals and to evaluate textural evidence of fluid-rock interaction. Complementary U–Pb carbonate geochemistry provides independent age constraints to compare with paleomagnetic and Re–Os datasets.

Together, these results initiate the development of a robust framework for identifying, characterizing, and dating fluid-induced remagnetizations, offering new insights into the tectonic and mineralogical evolution of Iberia’s orogenic systems.

How to cite: Galan, C., Pastor-Galán, D., and Martín Hernández, F.: Towards Absolute dating of Fluid-Flow Remagnetizations: Initial results from Variscan Carbonates, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-492, https://doi.org/10.5194/egusphere-egu26-492, 2026.

How to cite: Vogel, H., Unitt, R., and Meere, P.: Fluid migration, albitization, and metal concentration in the Munster Basin, SW Ireland, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-1362, https://doi.org/10.5194/egusphere-egu26-1362, 2026.

Lithium (Li) isotopes have been widely used as powerful tracers of chemical weathering processes, providing insights into the coupling between climate and silicate weathering. Although Li isotope fractionation does not occur under equilibrium conditions but rather during kinetically controlled mineral dissolution, the relationship between incipient mineral weathering and Li isotope fractionation remains poorly constrained in natural weathering systems, particularly with respect to the direction and magnitude of fractionation. Here, we investigate elemental and Li isotope geochemistry in two types of biotite—oxidized biotite and hydrobiotite (a 1:1 regularly interstratified biotite–vermiculite)—collected from in situ granitoid weathering profiles. Both biotite types exhibit negative correlations between elemental concentrations and depth; however, Li shows the most pronounced depletion. Elemental loss reaches up to ~70% for Li, with more extensive depletion observed in hydrobiotite compared to oxidized biotite, despite the progressive transformation of biotite into secondary phases such as vermiculite and kaolinite. Lithium isotope analyses are currently underway. By integrating elemental geochemistry with Li isotope compositions, we aim to constrain Li isotope behavior during the initial stages of silicate weathering and to quantify potential Li isotope fractionation associated with distinct biotite alteration pathways. These results will provide new constraints on kinetic controls of Li isotope fractionation during incipient weathering and improve the interpretation of Li isotope signatures in natural weathering systems, including glacial and weathering-limited environments.

How to cite: Ryu, J.-S., Park, H., Yang, M., and Jeong, G. Y.: Variation in elemental and Li isotope geochemistry during the weathering of two types of biotite, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-1973, https://doi.org/10.5194/egusphere-egu26-1973, 2026.

The Allihies region on the Beara Peninsula, SW Ireland possesses a mining history for vein-hosted Cu sulphide mineralisation. Structural and chronological control of the deposit has been studied extensively (Fletcher, 1969; Lang et al., 2020; Reilly, 1986; Sheridan, 1964). However, the spatial distribution of fluid alteration in the host rock and associated mineralogy remain unstudied. Several alteration minerals linked with the sulphide mineralisation have been recorded, such as chlorite, muscovite, siderite, calcite, dolomite, kaolinite, montmorillonite, and goethite (Fletcher, 1969).

Reflectance spectroscopy can be used for identifying alteration minerals. Hunt (1977) showed, due to the different electron and molecular structure of the compounds, most minerals absorb unique amounts of energy upon the incident of electromagnetic radiation, thus the reflected energy show characteristics absorption features in the spectra. Certain mineral groups exhibit unique features in the visible-near (400 – 900 nm) and short-wave infrared (900 – 2500 nm) wavelength ranges (Clark et al., 1990; Hunt, 1977). High-spectral resolution (hyperspectral) imaging (HSI) techniques provide a large amount of spectral information where each pixel contains hundreds of narrow, contiguous wavelength bands (Goetz et al., 1985; Lodhi et al., 2019). This gives the ability to identify wavelength positions of mineral absorptions and their subtle deviations that reveal the compositional variations.

Consequently, HSI can be used for analysing the host-rock alterations around the Mountain Mine, Allihies, which will reveal the spatial patterns. The target sulphide mineralisation/lodes are oriented in E-W and N-S (Reilly, 1986), and systematic sampling from the mineralized vein across the alteration zone  will help determine if the fluid alteration has a recognisable detectable spectral signature. Mineral groups such as chlorites, carbonates, and clays (Clark et al., 1990) possibly be differentiated of the existing propylitic and sericitic alteration phases (Fletcher, 1969) as  one moves away from the veins into the country rock.

The current study will use laboratory HS data from a rock scanner for initial analysis, followed by a HS drone survey for extending the spatial scale. Principal Component Analysis will be used for extracting the relevant spectral information (Burger & Gowen, 2011). Subsequently, Minimum wavelength mapper can be incorporated for further analysis of dominating mineral occurrences (Hecker et al., 2019), by studying unique absorption features and their feature depths, for mapping variations across the samples. Specifically, the wavelength range of 2100 - 2400 nm contains the diagnostic absorption features for phyllosilicates and carbonates that highlight the different alteration stages the region has undergone.

The research model has the potential to be further developed for identifying regions with similar spectral responses with mineral exploration potential.

How to cite: Kulugammana, M., Meere, P. A., and Unitt, R. P.: Characterizing the rock alteration associated with vein-hosted Cu sulphide mineralization using hyperspectral reflectance spectroscopy; A case study from the Allihies region, SW Ireland., EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-1223, https://doi.org/10.5194/egusphere-egu26-1223, 2026.

The presence of melt and associated fluids profoundly weakens the continental crust, promoting strain localization and establishing a close link between migmatites and ductile shear zones. Here we compare four migmatite case studies developed within major crustal-scale shear zones formed in contrasting tectonic settings, from collisional to extensional regimes: the Kinawa migmatite (Brazil), Opatica migmatite (Canada), Saint-Malo migmatite (France), and the Øksfjord Shear Zone (Norway). Our goal is to evaluate the connection between migmatites and shear zones, their impact on shear zone evolution, and the main macro- and microstructural features of migmatites in shear zones. We also examine the extent to which shear zones may serve as conduits for magma transport within the crust.

All migmatites formed at mid- to lower-crustal conditions (4–9 kbar; 650–820 °C) under both fluid-present and fluid-absent regimes. Macro- and microstructural observations reveal that the evolution of melt connectivity and permeability was strongly controlled by shear zone kinematics. In the Kinawa and Opatica examples, preservation of magmatic microstructures indicates that deformation ceased shortly after melt crystallization, suggesting limited post-melting deformation. In contrast, the Saint-Malo and Øksfjord shear zones record pervasive solid-state deformation overprinting magmatic fabrics, implying sustained deformation and continued microstructural reorganization after partial melting.

Across all examples, the spatial association between migmatites and shear zones highlights the role of deformation in enhancing melt segregation, extraction, and transient permeability. However, only some shear zones evolved into efficient pathways for melt migration. These and other case studies from the literature illustrate how ductile shear zones function as dynamic crustal domains in which deformation, partial melting, and fluid transport are tightly coupled, and where porosity and permeability evolve through time in response to changing rheology and strain.

How to cite: Carvalho, B. B. and Sawyer, E. W.: Interplay Between Migmatites and Deep Crustal Shear Zones, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3448, https://doi.org/10.5194/egusphere-egu26-3448, 2026.

This study quantifies spatial variations in acoustic seepage intensity offshore southwestern Taiwan and assesses whether margin setting or conduit continuity better explains the observed differences. Seepage variability was characterized using volume backscattering strength (Sv), plume geometry, and subsurface structural features. A total of 21 plumes from 14 seep sites were characterized based on Sv and geometry derived from Simrad EK60/EK80 echosounder data. After applying transmission-loss correction, seepage sites on the passive margin (e.g., Horseshoe Ridge, Pointer Ridge, and Formosa Ridge) exhibit higher Sv and taller plumes than those on the active margin. Integration with multichannel seismic profiles and sediment-core records reveals extensive free gas beneath bottom-simulating reflectors (BSRs) and gas chimneys, indicating sustained fluid migration through persistent conduits. In contrast, relatively weak Sv at the mixed-origin G96 site suggests partial conduit infilling. These observations indicate that although tectonic deformation establishes the first-order structural framework for fluid migration, the continuity and evolutionary state of seep conduits exert the dominant control on seepage intensity. Potential tidal modulation was further evaluated by comparing Sv with the rate of tidal pressure change, but only weak correlations were observed, suggesting that tidal forcing plays a secondary role in controlling seepage variability.

Keywords: seepage intensity, margin setting, conduit continuity, volume backscattering strength (Sv), offshore southwestern Taiwan

How to cite: Ou, C.-Y., Chen, T.-T., Hsu, H.-H., and Wu, Y.-C.: Acoustic Characterization of Fluid Seepage Controlled by Tectonic Structures Offshore Southwestern Taiwan, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6268, https://doi.org/10.5194/egusphere-egu26-6268, 2026.

Numerical study of phenomena in granular media is typically classified into two categories according to the scale of the domain: macro-scale modeling which relies primarily on continuum theories, such as the Finite Element Method (FEM), and micro-scale modeling, which is based on interparticle forces, commonly performed using the Discrete Element Method (DEM).

However, granular/porous media are inherently discontinuous due to their micro-structure, and traditional continuum-based approaches cannot accurately capture microstructure-induced anisotropy in these media. Particle-based approaches like DEM have therefore been widely used for the modeling of such discontinuous domains.

On the other hand, it is not computationally feasible to resolve the entire intricate microstructure of large domains using DEM. Thus, this work implements a multi-scale approach that combines the accuracy of DEM at the grain scale with the computational efficiency of FEM at the macro-scale.

The approach is called “Hierarchical FEM-DEM”, originally developed to study the mechanical response and strain localization (shear bands) in granular media [1]. It then has been extended to hydro-mechanical problems in saturated media [2]. In this framework DEM assemblies are treated as Representative Volume Elements (RVE) attached to Gauss (integration) points of a macroscopic FEM mesh. The DEM is used for the calculation of the homogenized effective stress corresponding to the interpolated strain field on each Gauss point, thereby eliminating the need for phenomenological constitutive assumptions for the solid skeleton, that are common in conventional nonlinear FEM analyses.

In this study we apply this method by implementing it in MATLAB to investigate the hydro-chemo-mechanics of granular media. The model is going to be used to study the effect of fluid flow and pore pressure on the solid skeleton deformation, and generation of shear bands, how micro-scale solute-related heterogeneities influence the macro-scale mechanical behavior, based on thin sections made from field sample collected by the authors.

We use periodic boundary conditions for DEM assemblies to satisfy the compatibility between the microscopic deformation and the macroscopic strain field imposed at the Gauss points, ensuring the condition for the satisfaction of Hill-Mandel micro-macro energy equivalence during homogenization. At the macroscopic level, the boundary conditions are prescribed to simulate the in-situ loading and hydraulic conditions, corresponding to the field sites from which the samples were extracted.

References

[1] Guo, N. and Zhao, J., 2014. A coupled FEM/DEM approach for hierarchical multiscale modelling of granular media. International Journal for Numerical Methods in Engineering, 99(11), pp.789-818.

[2] Guo, N. and Zhao, J., 2016. Parallel hierarchical multiscale modelling of hydro-mechanical problems for saturated granular soils. Computer Methods in Applied Mechanics and Engineering, 305, pp.37-61.

How to cite: Ahmadi Olyaei, E. and Koehn, D.: A multi-scale hierarchical FEM-DEM approach for hydro-chemo-mechanical modeling of granular media, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-12850, https://doi.org/10.5194/egusphere-egu26-12850, 2026.

Dolomitization is among the most widespread processes affecting carbonate rocks and may significantly overprint carbonate successions during post-diagenetic and deformation-related fluid infilling and circulation. The process is generally hindered in low-porosity/low-permeability carbonates (e.g., micritic limestones). However, primary (e.g., bedding interfaces) and secondary (e.g., fractures) rock planar anisotropies might compensate for this low porosity/permeability, acting as potential pathways for fluid ingress and as loci for the initiation of fluid-rock interaction. Among these anisotropies, burial stylolites are particularly widespread in carbonate successions, forming through progressive chemo-mechanical dissolution-precipitation over time. Their large lateral continuity (>1 km) and potentially high vertical frequency make stylolites key features in governing the syn-to-post diagenetic evolution of sedimentary successions.

Although stylolites have traditionally been considered fluid barriers, recent studies challenge this paradigm, a view that we further stress here. We present petrophysical data from micritic limestones of the Lessini Mountains (Italian Southern Alps), where a large portion of the exposed carbonate Jurassic-Cretaceous succession (>700 m thick) is almost entirely overprinted by a regional dolomitization event, which produced large volumes of massive, sandy, crystalline dolostones. We studied preserved patches of micritic limestone where the progression of dolomitization from initiation to complete overprint is clearly visible. Hg-porosimetry, SEM imaging, μ-CT and cathodoluminescence were combined to constrain petrophysical variations associated with dolomitization. Results show that burial stylolites (Hg injection capillary threshold pressure – HgP c. 4 Psi) affecting the micritic limestones (HgP c. 5140 Psi) were systematically exploited by the dolomitizing Mg-rich fluids, transiently aided by fluid overpressure surges, which locally induced brecciation and further enhanced fluid-rock interaction.

Progressive dolomitization increased rock porosity and density from ~1% to ~20% and from 2.65 g/cm³ to 2.9 g/cm³, respectively (from micritic limestone to massive dolostone). Pore characteristics (pore-size, sphericity, 3D-φ-angle and 3D-Eulerian characteristic - all constrained by μ-CT data and Hg-μporosimetry) indicate a complex evolution characterised by (i) early diffuse dolomitization followed by (ii) localised dedolomitization triggered by the later ingress along the porous stylolites of a Mg-poor fluid, which selectively infiltrated the dolomitized succession and created significant rock porosity. Dedolomitization appears to have been more efficient (and is better preserved) along the dolomitized stylolites than within the massive dolostones, where fluid-rock interaction was inhibited by the larger rock volume.

Spatio-temporal porosity variations related to dolomitization and dedolomitization, guided by- and preserved within stylolites, have significant implications for (i) reservoir quality evaluation, and (ii) the mechanical behaviour of carbonate rock masses during post-fluid-infiltration deformation phases. In these settings, dolomitization and dedolomitization promote long-term fluid ingress and circulation, thus even modulating further deformation localisation.

How to cite: Zuccari, C., Vignaroli, G., Balsamo, F., Berio, L., Buono, G., Pappalardo, L., and Viola, G.: Stylolite-controlled dolomitization and dedolomitization in low-porosity carbonates (Lessini Mountains, Southern Alps, Italy), EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14061, https://doi.org/10.5194/egusphere-egu26-14061, 2026.

The Upper Rhine Graben (URG) is the central part of the European Cenozoic Rift System and holds a huge geothermal potential due to a reduced Moho depth and active hot brine convection cells. In addition to that appealing potential, hydrothermal brines of the URG show high Lithium concentrations. Yet, investors-relying deep geothermal energy companies face difficulties to predict fracture network permeability before drilling operations. This problem induces techno-economic risks, which frighten investments and in turn hinder the wide development of deep geothermal energy use. The goal of this work is to provide input data to develop modelling tools to help predict fracture network permeability before drilling operations. We will integrate data from both the French and German side of the URG, which is seldomly done in studies. Here we present a timeline that we’ll use as a data compilation base to reconstruct the URG setup and hydrothermal fracture clogging history. Once identified, the fracture clogging events will be deeper characterized to be implemented in a reconstruction model. A characterization of mineralized fractures all around the URG shoulders will help to complete the list of hydrothermal clogging events identified in the Black Forest (Permian, Jurassic-Cretaceous and Paleogene) and will allow to simulate the rate of precipitation in mineralized fractures and thus their clogging potential.

How to cite: Mazzinghi, L.: A timeline for the reconstruction of Upper Rhine Graben hydrothermal fracture mineralization events  , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7761, https://doi.org/10.5194/egusphere-egu26-7761, 2026.

The aims of this study is to understand how concurrent fluids and deformation influence the behaviour of the Rb/Sr geochronometer in micas. This study is conducted within a classical deformation framework, shear zones, key structures in the geodynamic evolution of rifts and orogens which can accommodate kilometer-scale displacements and facilitate significant mass transfer. While the last two decades have led to a better understanding of the processes of nucleation and widening of these structures, as illustrated by the world-famous case of Cap de Creus, the extreme localization of deformation they exhibit raises fundamental questions about the rheological evolution of the lithosphere.

By conducting a multiscale petro-tecto-geochronological study of the Cap de Creus shear zones, whose age is debated, we have underlined that these shear zones record a major event of fluid-induced strain softening during the formation of the Pyrenean prism. This event is characterized by concomitant muscovite blast neocrystallization and intense quartz dynamic recovery, whose contemporaneity can be uniquely demonstrated by evidence of Fe-rich surface-derived fluids that penetrated to depth. This event occurred between 60 and 50 Ma, as shown by in situ and in-context Rb/Sr dating of two shear zones. This study highlights the critical role of fluid-induced rheological softening in ductile reactivation of polycyclic basements and provides a context-dependent framework for interpreting the behaviour of the Rb/Sr geochronometer in muscovite during deformation.

The implications of these results for the three-dimensional formation and evolution of the Pyrenean orogenic prism, will be discussed, including the role of structural inheritance, fluid circulation, and their contribution to shear-zone reactivation processes.

How to cite: Denele, Y., Nicolas, C., Bosse, V., Merle, O., Gardés, E., and Lotout, C.: Fluid-induced strain softening during the formation of the Pyrenean orogenic prism: In situ Rb/Sr dating from the Cap de Creus shear zones (Eastern Pyrenees, Spain), EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17657, https://doi.org/10.5194/egusphere-egu26-17657, 2026.

Fluid flow in the Earth’s crust governs heat and mass transfer, critical metal mineralisation, rock rheology, and the development of deep, non-photosynthetic biospheres, yet its direction and mechanical drivers remain poorly constrained in natural systems. Conventional approaches infer fluid pathways from fractures, models, or geochemical tracers but rarely capture flow direction or mechanism directly. This PhD project develops a novel combination of fabric-based methods—integrating anisotropy of magnetic susceptibility and remanence (AMS/ARM), crystal preferred orientation analysis, and hyperspectral mineral mapping—to directly identify and quantify fluid-induced petrofabrics within the thermal aureoles of igneous intrusions, independent of fault kinematics. Similar integrated approaches have demonstrated their ability to track volatile-rich liquid migration through texturally layered intrusions, where permeability contrasts control fluid focusing and the development of REE-enriched horizons. Together, these methods provide new constraints on how fluids modify host-rock properties, localise permeability, and generate chemical enrichment, representing a step-change in our ability to observe and model crustal fluid flow.We present new petrofabric data from the Sherwood Sandstone Group, Northern Ireland, to assess fluid flow in permeable sandstones surrounding basaltic dykes. The study examines: (i) the geometry and extent of hydrothermal flow pathways, (ii) the interaction between thermally driven fluid circulation and pre-existing sedimentary anisotropy, and (iii) the impact of alteration on host-rock porosity and permeability. The Sherwood Sandstone Group forms the lowermost unit of the Triassic New Red Sandstone succession and is cross-cut by Palaeogene basaltic dykes related to North Atlantic rifting. Preliminary field observations and hyperspectral data identify laterally zoned alteration halos defined by systematic variations in clay, mica, and Fe-oxide mineralogy. AMS and ARM data reveal that primary sedimentary fabrics are preserved more than ~10 m from the dyke but are progressively overprinted toward the intrusion. Ongoing analyses test whether these overprinting fabrics record convective hydrothermal flow, with fluids ascending along dyke margins before dispersing laterally along bedding planes. We further evaluate the controls on stratigraphic fluid focusing and blockage, constraining the reciprocal relationship between fluid flow and evolving rock properties.

How to cite: Pelletier, M. and McCarthy, W.: Go with the Flow : Investigating Petrofabric Evidence for Hydrothermal Flow in Thermal Aureoles, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-22152, https://doi.org/10.5194/egusphere-egu26-22152, 2026.

Mid-crustal shear zones localize strain and control the migration of heat and fluids, making them central to understanding metallic ore-forming processes. Mica are widely used to date deformation in shear zones, however their radiometric ages can be difficult to interpret because hydrothermal alteration and strain-induced microstructural defects can promote recrystallization and disrupt isotopic retention. In Archean gold camps, mica ages commonly postdate mineralization events by >100 Myr, raising questions about whether these ages reflect primary mineralization, metamorphic or hydrothermal growth, or post-orogenic remobilization. Robust interpretation of these ages requires direct integration of geochemical and micro- to nanoscale structural analysis. We examined mica from the Sunday Lake Deformation Zone, a regional scale deformation zone controlling gold mineralization at Agnico Eagle Mines Ltd. giant Detour Lake Mine (DLM), in the northwestern Abitibi greenstone belt, Canada. The DLM orogenic gold deposit is characterized by c. 2734-2724 Ma volcanic rocks, comprising ultramafic-dominated lower units and mafic volcanic and volcaniclastic upper units, metamorphosed under greenschist to lower amphibolite facies conditions. Mafic host rocks are intruded by felsic to mafic sills and dikes. The main regional foliation is subvertical and axial-planar to west-trending, shallowly-plunging tight to isoclinal folds, which transposes the intrusive relationships. Gold mineralization occurred at c. 2670-2640 Ma in a syn-orogenic setting. Microstructural analyses were conducted on muscovite from felsic meta-intrusive rocks, collected from drill core, that are comprised of quartz, muscovite ± biotite, plagioclase, K-feldspar, chlorite, garnet, amphibole, carbonates and sulfides. Quartz microstructures record bulging recrystallization and nascent subgrain rotation, indicating deformation temperatures of ~300-400°C. Plagioclase display tapered deformation twins and brittle fracturing, consistent with low to moderate temperature deformation. Mica constitute between <5 and 30 vol% of the rock and occur as euhedral porphyroblasts/neoblasts to subhedral poikilitic, skeletal grains ranging in size from 15 x 50 μm to 250 x 650 μm. High-resolution electron channeling contrast imaging of the mica reveals weak undulatory orientation contrast patterns perpendicular to cleavage planes in ~20-50% of the grains. Such contrast patterns suggest deformation in the mica is accommodated primarily by dislocation glide. Backscatter electron imaging of the mica also revealed concentric chemical zoning, typically expressed as irregular and discontinuous rims along grain margins, which are weakly enriched in Fe and Al and depleted in Mg and Si relative to mica cores. Muscovite Ar-Ar analyses from unmineralized rocks at DLM yield single-crystal dates of 2600-2100 Ma, and complementary K-Ca and Rb-Sr dating will be conducted to assess the roles of Ar loss and element mobility in producing these younger and dispersed ages. The timing of metamorphism and deformation has important implications for understanding the nature and controls on mineralization at DLM, and whether the original geometry and mineralogy of the deposit has been modified through later stages of syn‐metamorphic deformation.

How to cite: Komendat, I., Schneider, D., and Dubosq, R.: Wrinkled clocks in the crust: dating deformation in Archean gold-bearing shear zones, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8033, https://doi.org/10.5194/egusphere-egu26-8033, 2026.

Volcanogenic massive sulfide (VMS) deposits are among the world’s most important sources of copper (Cu), zinc (Zn), lead (Pb), gold (Au), and silver (Ag), metals that are critical for modern infrastructure and energy technologies. These deposits are characterized by systematic hydrothermal alteration halos that preserve mineralogical and chemical gradients generated by spatial and temporal variations in temperature, redox conditions, and hydrothermal fluid composition. Such alteration zones provide important vectors for mineralization; however, their traditional characterization is commonly qualitative, reliant on subjective geological interpretation, and difficult to collolate at scale across exploration projects. This study investigates the Rävliden deposits of the Paleoproterozoic (1.89 Ga) volcanic–sedimentary sequence of the Skellefte district, Sweden, to evaluate whether integrated rock magnetic and VNIR–SWIR hyperspectral data can be used to objectively characterize hydrothermal alteration in VMS deposits. In this study, rock magnetic measurements are cross-referenced with hyperspectral data and supported by mineral chemistry and sulfur isotope analyses to develop a quantitative and reproducible framework for fingerprinting hydrothermal alteration in both metalliferous and barren VMS systems. The approach comprises three objectives: (1) defining diagnostic magnetic and hyperspectral signatures of alteration mineral assemblages to construct a reference dataset, which is then validated using Raman spectroscopy; (2) evaluating trace-element variations in magnetite and associated sulfide minerals to assess their influence on magnetic properties across alteration zones; and (3) using sulfur isotope compositions (δ³⁴S) of sulfide minerals across alteration zones and structural domains to constrain fluid sources and reconstruct the hydrothermal fluid evolution of the system. This workflow systematically links observable physical alteration patterns to their underlying mineral-chemical controls and fluid origins, providing a robust and scalable tool for hydrothermal alteration characterization in VMS exploration.

How to cite: Aryani, L. and McCarthy, W.: Integrated Rock Magnetic and VNIR–SWIR Hyperspectral Characterization: A Quantitative Classification Tool for VMS Alteration Systems, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-22148, https://doi.org/10.5194/egusphere-egu26-22148, 2026.

Graphite formation in deep crust during granulite facies metamorphism is documented in the Proterozoic gneisses of the Lofoten-Vesterålen Complex, northern Norway. Regionally distributed graphite zones are hosted in banded gneisses dominated by orthopyroxene-bearing quartzofeldspathic gneiss, including marble, calcsilicate rocks and amphibolite. The schist has major graphite, quartz, plagioclase, pyroxenes, biotite (Mg# = 0.67-0.91; Ti < 0.66 a.p.f.u.) and K-feldspar/perthite. Pyroxene is orthopyroxene (En_69-74) and/or clinopyroxene (En_33-53Fs_1-14Wo_44-53). Although graphite is usually described in pelitic rocks or as vein deposits in the granulite facies rocks, we document graphite in assemblage with metamorphic orthopyroxene.

Phase diagram modelling (plagioclase + orthopyroxene (Mg#-ratio = 0.74) + biotite + quartz + rutile + ilmenite + graphite-assemblage) constrains pressure-temperature conditions of 810-835 °C and 0.73-0.77 GPa; Zr-in-rutile thermometry 726-854°C. COH-fluids stabilise graphite and orthopyroxene; high Mg#-ratio of biotite and pyroxenes, and apatite Cl < 2 a.p.f.u. indicate importance of fluids during metamorphism.

Stable isotopic δ¹³C_graphite in the graphite schist is -38 to -17‰; δ¹³C_calcite of marbles +3‰ to +10‰. Samples with both graphite and calcite present give lighter values for δ¹³C_calcite = -8.7‰ to -9.5‰ and heavier values for δ¹³C_graphite = -11.5‰ to -8.9‰. δ¹⁸O_calcite for marble shows lighter values ranging -15.4‰ to -7.5‰ (Engvik et al. 2023).  We interpret the graphite origin as organic carbon accumulated in sediments contemporaneous with the Early Proterozoic global Lomagundi-Jatuli isotopic excursion, while an isotopic exchange between graphite and calcite reflects metamorphic and hydrothermal re-equilibration.

The high-ordered graphite (< modality 39%) and biotite with a strong-preferred orientation defines the well-developed foliation. Increased graphite content resulted in high-conductivity zones with a contrast to the host low-conductive crust (Rodinov et al. 2013; Engvik et al. 2021). Enrichment of graphite resulted in zones with strong schistosity and a sharp strain gradient towards host massive granulite gneiss. The presence of graphite causes strain localisation in the granulite facies crust, reducing crustal strength and may thereby influence continental architecture and evolution of collision zones.

References:

Engvik AK et al. (2023) Proterozoic Deep Carbon—Characterisation, Origin and the Role of Fluids during High-Grade Metamorphism of Graphite (Lofoten–Vesterålen Complex, Norway). Minerals 13(10), 1279

Engvik AK et al. (2021) The control of shear-zone development and electric conductivity by graphite in granulite: An example from the Proterozoic Lofoten-Vesterålen Complex of northern Norway. Terra Nova, https://doi.org/10.111/ter.12545

Rodinov A et al. (2013) Helicopter-borne magnetic, electromagnetic and radiometric geophysical survey at Langøya in Vesterålen, Nordland. NGU Report 2013.044

How to cite: Engvik, A. K., Gautneb, H., and Knežević Solberg, J.: Characterisation, origin, petrophysical properties and the role of fluids during high-grade metamorphism of graphite (Lofoten-Vesterålen Complex, Norway), EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21791, https://doi.org/10.5194/egusphere-egu26-21791, 2026.

Abstract: The Beishan Orogenic Belt, located along the southern margin of the Central Asian Orogenic Belt, is one of the key mineral resource regions in northwestern China. The Hongjianbing fluorite deposit, located in the northern part of this belt, is a well-known quartz-vein-type fluorite deposit that has attracted considerable attention from researchers. Through field geological mapping, UAV and remote sensing measurements, and borehole structural recording, two east–west-trending strike-slip fault systems were identified in the study area, separated by a distance of 5 km, with nearly vertical dips. These faults exhibit multi-stage activity, with early deformation characterized by dextral strike-slip motion. The intervening blocks experienced ductile deformation, with S-C fabric development in the shear zones.39Ar-40Ar dating of biotite from the mylonite in the ductile shear zone yielded a plateau age of approximately 330 Ma, marking the timing of ductile deformation. Later, these two faults evolved into brittle left-lateral strike-slip faults, forming a Riedel shear system (R, R', T shears), and displaying fault breccia and fault gouge. K–Ar dating of authigenic illite from the fault gouge yielded an age of approximately 220 Ma, indicating a transition from ductile to brittle deformation over time.The host rocks of the fluorite deposit are mainly intermediate to acidic volcanic rocks from the Carboniferous Baishan Formation. Zircon U–Pb dating of these rocks yielded ages of approximately 330 Ma, suggesting that the host rocks may have formed during contemporaneous magmatic activity. All fluorite orebodies are located within the main damage zone of the southern brittle fault system, which exhibits left-lateral, right-stepping characteristics. The development of this brittle fault system provided the necessary space and conduits for ore-forming fluid migration, facilitating fluorite mineralization.The cataclastic texture of the ores further suggests that mineralization occurred after significant faulting, reflecting high fluid mobility within the fault damage zones. This high fluid mobility is also reflected in the characteristics of the fluorite ore bodies. After migrating along the brittle fault zones, mineralizing fluids precipitated fluorite as fine veinlets, which is the current form of mineralization observed in the deposit. A three-dimensional geological model of the ore body was constructed using GOCAD software, revealing the close temporal and spatial relationship between fluorite mineralization and fault activity.
Keywords: Structural control of mineralization; Fluorite deposit; Geochronology; Three-dimensional mineralization model;

How to cite: Bo, C., Wang, G., Zhang, P., and Zhang, J.: Strike-slip fault system and fluorite mineralization in the Hongjianbing area, Mazong Mountain, Gansu China, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9560, https://doi.org/10.5194/egusphere-egu26-9560, 2026.

Foreland basins often host important ore deposits (like MVT deposits; Bradley and Leach, 2003) which are associated with deep and shallow fluid circulation. Those fluids, expelled from the orogen, can have different origins like meteoric water, diagenetic fluid, metamorphic fluid or even deeper crustal origins (Oliver J., 1986). However, whether these fluids are expelled to the foreland as continuous flow or as series of rapid pulses remains largely unexplored. Here, we combine numerical modelling with geochemical data and petrographic observation of a sandstone and its associated veins and reaction halos to identify spatial and temporal fluid flow and its transport mechanism(s) in foreland basins.

Thin sections from a Rotliegend red arkose-sandstone formation (German Permian Variscan foreland) were investigated using microprobe analysis and BSE-EDS-SEM imaging. The arkose-sandstone exhibited tapering lighter reaction halos around veinlets, most likely produced through redox reactions upon fluid infiltration into the sandstone. The model Elle from Koehn et al., (2022) was subsequently applied to link fluid transport mechanism to the patterns and geometry observed in the samples. Pore pressure applied from a crack toward the host rock and a concentration gradient were used to create fluid flow in the sandstone from which a range of values for advection, diffusion and a reaction rate were deduced. In this way, the model allowed to mimic the same pattern/geometry as the sample on several scales and enabled a systematic assessment whether fluid flow may have been constant or pulsating.

Combining petrographic, geochemical and modelling investigations revealed that the reaction halos in the sandstone were in fact formed upon a single rapid fluid flow event, that presumably was fast and channelised in the vein, and pervasive and comparatively slow in the surrounding host rock. These preliminary results imply that fluid flow and transport in foreland basins may be of a more pulsating nature rather than continuous steady state fluid flow and transport mechanisms may thus be similar to what has been previously reported for subduction zone settings (e.g., Kleine et al., 2016).

Bradley, D. C., & Leach, D. L. (2003). Tectonic controls of Mississippi Valley-type lead–zinc mineralization in orogenic forelands. Mineralium deposita, 38(6), 652-667.

Kleine, B. I., Zhao, Z., & Skelton, A. D. (2016). Rapid fluid flow along fractures at greenschist facies conditions on Syros, Greece. American Journal of Science, 316(2), 169-201.

Koehn, D., Kelka, U., Toussaint, R., Siegel, C., Mullen, G., Boyce, A., & Piazolo, S. (2022). Outcrop scale mixing enhanced by permeability variations: the role of stationary and travelling waves of high saturation indices. Geological Magazine, 159(11-12), 2279-2292.

Oliver, J. (1986). Fluids expelled tectonically from orogenic belts: their role in hydrocarbon migration and other geologic phenomena. Geology, 14(2), 99-102.

How to cite: Lebrun, L., Kleine-Marshall, B., and Koehn, D.: Fluid flow in foreland basins: spatial and temporal scaling of their transport mechanisms, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11377, https://doi.org/10.5194/egusphere-egu26-11377, 2026.

Mobile shales strongly influence deformation, uplift, and fluid migration in compressional sedimentary basins, yet the mechanical pathway from "normal" shale to mobile shale is still debated. This study tests the idea that shear-induced dilation under high pore-fluid overpressure can trigger a positive feedback among shear localization, porosity–permeability increase, and fluid flow, thereby promoting long-lived, ductile-like shale mobility. We focus on the Zhongliao Tunnel area in southwestern Taiwan, where rapid uplift and sharp spatial gradients in vertical motion have been reported near major faults.

We develop a two-phase hydro-mechanical numerical model that couples a poro–visco–elasto–plastic solid with Darcy fluid flow. Porosity evolves through competing compaction and a strain-rate–dependent dilation term that is activated under elevated overpressure, allowing fault-related shear zones to dynamically transform into high-permeability conduits. In the reference experiment, a high-pressure layer sealed beneath a low-permeability cap sustains overpressure within mudstone. Once shear localizes, dilation increases porosity and permeability along damage zones, enhancing focused fluid discharge. The resulting seepage forces and reduced effective strength further intensify shear localization, producing sustained fault creep and pronounced uplift of the block bounded by the principal fault systems. The modeled uplift pattern reproduces key first-order observations: a sharp vertical-velocity contrast across the main fault and a more gradual decay of uplift away from it, with peak uplift rates reaching the order of centimeters per year.

Sensitivity tests demonstrate that overpressure alone generates only modest uplift without dilation-enabled conduit formation, while shear compaction suppresses localization and distributes deformation. Permeability exerts a non-monotonic control: very low permeability limits fluid flux and seepage forcing, whereas very high permeability drains overpressure too efficiently and weakens sustained creep. Overall, the results provide a mechanistic framework for how overpressured mudstone can evolve into mobile shale through coupled dilation and fluid flow, and offer testable criteria for identifying similar processes in other shale-dominated orogenic settings.

How to cite: Tan, E., Lin, C.-H., Wang, W.-H., Le Beon, M., and Gerya, T.: Hydro-Mechanical Modeling of Over-Pressured Mobile Shale: Insights into Shear Dilation Effects on the Uplift at Zhong Liao Tunnel, Taiwan, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11563, https://doi.org/10.5194/egusphere-egu26-11563, 2026.

In the past decades, the extent of fluid-induced reaction halos in metabasaltic sills within the Argyll Group of the Dalradian Supergroup in the SW Scottish Highlands has been intensively used to constrain metamorphic fluid flow velocities (Skelton, 2011). However, recent findings revealed that reaction front propagation within numerous sills was primarily controlled by preferred fabric alignment at the margins during deformation events, as well as by mineralogical and chemical heterogeneities across the sills. Here, we revisit hydration and carbonation fronts in metabasaltic sills in the vicinity of major fluid pathways, i.e., the Loch Awe Syncline and Ardrishaig Anticline, to reevaluate fluid-induced reaction front propagation and constrain metamorphic fluid flow velocities.
This study integrates field observations, detailed petrological-textural analyses, and whole-rock geochemistry, including carbon and water contents as well as trace element data, along a transect across compositionally homogeneous metabasaltic sills. The aim is to constrain the mechanisms controlling fluid-induced reaction progress at the contact between metasedimentary rocks and metabasaltic sills.
The selected basaltic sills were metamorphosed under greenschist- and epidote-amphibolite-facies conditions and record at least four deformation events. In the sill margins, the rocks show increased calcite and chlorite contents and replacement of garnet, amphibole, and dark mica, reflecting localized retrogression. This retrograde overprint is also characterized by mobilization of large-ion lithophile elements (LILE; e.g., K, Na, Sr). In contrast, the sill interior preserves textural and mineralogical equilibrium among amphibole, dark mica, epidote, garnet, and titanite. Textural variations indicate a progressive decrease in hydration and carbonation toward the sill interior.
Carbon contents decrease systematically from 1.22-1.16 wt.% in the sill margins to 0.07-0.02 wt.% toward the sill interior. Similarly, water contents are highest in the sill margins (up to 1.95 wt.%) and lowest in the sill interior (0.52 wt.%). Petrographic observations further suggest that fluid infiltration and reaction are controlled by structural anisotropy inherited from earlier deformation during retrogression, rather than mineralogical heterogeneity. Fluid flow is preferentially channelized along lithological contacts and deformation-related weaknesses, such as foliation and mineral lineation, which are most developed near the sill margins.
The new dataset enables a recalculation of the true spatial extent of metamorphic fluid infiltration and allows time-integrated estimates of fluid fluxes based on carbonation and hydration reaction front geometries, as well as their relationships with trace element redistribution. Understanding the rates of CO₂ release and sequestration during orogenic processes provides new insights into the role of structural anisotropies and brittle-ductile processes in controlling the volume, pathways, and metal-enriching potential of metamorphic fluid flow.

How to cite: González-Ixta, Y., Kleine-Marshall, B., Skelton, A., and Koehn, D.: Carbon gradients as tracers of structurally controlled fluid flow in homogeneous metabasaltic sills (Loch Stornoway, Scottish Highlands)., EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8377, https://doi.org/10.5194/egusphere-egu26-8377, 2026.

Rocks in the upper crust are generally subjected to true triaxial stress conditions, in which all three principal stresses are unequal (σ₁ > σ₂ > σ₃). Pore and fracture networks respond to anisotropic loading by opening in certain directions while closing in others, potentially inducing strong permeability anisotropy. However, most experimental constraints on stress-dependent permeability are derived from conventional triaxial tests, where two principal stresses are equal and permeability is measured in only one direction.

Here, we use a true triaxial apparatus equipped with a pore-fluid system to measure permeability parallel to all three principal stress axes in cubic samples of Etna basalt (EB) and Crab Orchard sandstone (COS) subjected to anisotropic loading.

For an initially isotropic EB sample, increasing stress along a single axis results in a sharp permeability decrease in the corresponding maximum stress direction, reaching up to two orders of magnitude (from ~10⁻¹⁶ to ~10⁻¹⁸ m²) at a differential stress (σ₁ − σ₃) of 215 MPa. In contrast, permeability parallel to σ₂ decreases mildly when stresses are increased up to ~75 MPa while permeability parallel to σ₃ remains largely unchanged. During unloading, permeability parallel to σ₁ recovers by approximately 1.5 orders of magnitude once σ₁ is reduced to 75 MPa.

Similarly, samples of the initially anisotropic COS also experience a decrease in permeability of two orders of magnitude (from ~10⁻¹⁷ to ~10⁻¹⁹ m²) along the maximum compressive stress at (σ₁ − σ₃) of just 100 MPa. Permeability along σ₁ recovers only partially after unloading, up to 10⁻¹⁸ m², indicating that some permanent compaction occurred along the maximum compression.

These results demonstrate that true triaxial stress conditions can induce pronounced permeability anisotropy through directional closure of pores and microcracks, with important implications for fluid transport in the upper crust, including fault zones, geothermal systems, and stressed reservoirs.

How to cite: Meredith, P., Stanton-Yonge, A., Mitchell, T., Browning, J., and Healy, D.: Permeability anisotropy under true triaxial stress states: strong flow reduction parallel to the maximum principal stress., EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-20074, https://doi.org/10.5194/egusphere-egu26-20074, 2026.