The Olympic Subduction Complex (OSC) in the central Olympic Mountains is a deeply exhumed continuation of the offshore modern accretionary wedge of the Cascadia subduction zone. Metamorphic grade and thermochronology reveal that the OSC’s central core likely accreted by underplating at seismogenic depths. Duplexes of underplated sediments bounded above and below by abandoned paleomegathrust interfaces could therefore be preserved during exhumation. We characterize a previously unknown ~500 m wide belt of block-in-matrix mélange containing an anastomosing system of 9 major fault strands, which in turn include mm to cm wide discrete principal slip surfaces consistent with brittle-frictional, likely coseismic, slip. The lithologies are turbiditic sandstones and mudstones; other elements of ocean plate stratigraphy are absent. Raman spectroscopy of carbonaceous material refines peak paleotemperatures to 260-280°C, consistent with the seismogenic zone. We interpret these intermingled structures as a composite fault zone that records both slow and fast slip of the seismic cycle through coeval coseismic brittle-frictional and interseismic viscousdeformation. We show that the mélange forms by cataclasis, pressure solution, and development and abandonment of localized shear surfaces, while the fault strands are dominated by concentrated cataclasis and brecciation. We calculate the degree of pressure solution experienced by the mélange at the thin section scale using scanning electron microscopy and compare the accumulated strain across the fault zone through anisotropy of magnetic susceptibility. We interpret this fault zone as an exhumed paleomegathrust interface, the first direct analog for the modern Cascadia subduction zone. The absence of basalt indicates that the megathrust fault was localized within the incoming plate stratigraphic sequence in the past, facilitating sediment underthrusting, similar to offshore structures observed via seismic reflection imaging in Cascadia and elsewhere today.
How to cite: Ledeczi, A., Tobin, H., Chen, T.-W., and Mulcahy, S.: Structure and properties of the Cascadia plate interface: evidence from a newly-described exhumed paleomegathrust in the Olympic subduction complex, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-301, https://doi.org/10.5194/egusphere-egu25-301, 2025.
Seismic activity often triggers liquefaction, a process where water-saturated sediments lose their strength due to an increase in pore water pressure. This process leads to the development of soft-sediment deformation structures (SSDS), such as load casts, clastic dykes, flame structures etc. However, distinguishing seismogenic SSDS from those triggered by other mechanisms (e.g., storms, overloading) can be challenging.
This study investigates the micromorphological changes in quartz grains derived from SSDS caused by liquefaction triggered by seismic shocks., Laboratory simulations mimicking seismic conditions revealed characteristic quartz alterations, including microcracks, edge corrosion, and grain fragmentation. These features were found to be closely linked to the duration of seismic exposure.
Particularly notable was the discovery of gold within quartz cracks, which serve as direct evidence of seismic events and underscore the role of seismicity in mineral redistribution. This novel finding highlights the potential of quartz grains as micro-scale markers for reconstructing past seismic activities. Geochemical factors, such as pH and redox potential, further influenced the behavior of liquefied sediments and the extent of quartz grain deformation, demonstrating the complex interplay between seismic forces and geochemical conditions in shaping sedimentary records.
How to cite: Świątek, S., Lewińska, K., Pisarska-Jamroży, M., and Günter, C.: Seismically-induced quartz grain alterations as indicators of past earthquake events, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12829, https://doi.org/10.5194/egusphere-egu25-12829, 2025.
Micro-scale porosity is a feature commonly found in viscously deformed quartz-rich mylonites. However, the processes which may form such porosity are actively debated, and whether or not pores are formed syn-kinematically to shear zone activity remains uncertain. Yet, the production of micro-pores during rock deformation may have several critical implications, such as affecting the rock strength, possibly through the brittle-ductile transition, and/or providing fluid pathways through active shear zones.
In this study we focus on quartz-rich shear bands produced during extensional deformation of a granitic pluton below the detachment of Ikaria (Cyclades, Greece). Nearby to the detachment, quartz aggregates are often decorated by micrometric and sub-micrometric pores, of which a large proportion adopt angular, crystallographically controlled shapes. Quartz in such decorated shear bands primarily deformed by crystal plasticity and underwent dynamic recrystallisation by subgrain rotation. Using a combination of standard and High-angular Resolution (HR) Electron Back-Scatter Diffraction (EBSD) analyses alongside Scanning and Transmission Electron Microscopy (SEM/TEM), we highlight that micro-pores decorate primarily grain boundaries, as well as some intragrain substructures including subgrain boundaries. EBSD analyses show that pore-decorated substructures are characterised by high (~4°) Kernel Average Misorientation (KAM), which describes the mean lattice misorientation of one EBSD pixel with respect to its closest neighbours. (HR)EBSD maps indicate a high lattice curvature gradient across these pore-decorated substructures, which can be seen by Geometrically Necessary Dislocation (GND) densities as high as 1015 per m2.
TEM analyses of Focused Ion Beam (FIB) sections across grain and intragrain boundaries reveal that quartz contains free dislocation densities around 1013 per m2, which matches our (HR)EBSD estimates for the interior of grains and subgrains. GND estimates of some porosity-decorated subgrain boundaries are between 1014 to 1015 dislocations per m2, which are not visible in TEM. Instead, nm-scale layers of amorphous SiO2 are seen, into which porosity is often partially or fully embedded.
Our results suggest that amorphous SiO2 and porosity are formed from the same process, since pores are embedded into amorphous SiO2. Furthermore, in the case of pore-decorated substructures where amorphous SiO2 is present, a factor other than dislocation climb likely accounts for their quartz lattice distortion, possibly related to a stress concentration. Although the origin of quartz amorphization remains a matter of discussion, we hypothesise a stress concentration which caused quartz to amorphize, followed by subsequent pore formation through fluid exsolution while stress was released. If this is the case, it would strongly suggest that pore nucleation occurred syn-kinematically in Ikaria.
How to cite: McGill, G., Précigout, J., Prigent, C., Arbaret, L., Airaghi, L., and Cordier, P.: The origin of micro-porosity in quartz mylonites: insights from quartz-rich shear bands in the Ikaria granite, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17663, https://doi.org/10.5194/egusphere-egu25-17663, 2025.
Fault zones are complex systems where mineralogy, fabric, and frictional properties interplay on fault strength and slip behavior. While prior investigations have focused on post-experimental microstructures to relate fault friction to deformation processes, the evolution of fault fabric coupled with Acoustic Emissions (AEs) during deformation remains elusive. In this study, we present experimental data integrating systematically microstructural, mineralogical, frictional, and AEs analysis coming from a suite of frictional experiments in a double direct shear configuration. These experiments aim to elucidate deformation micro-mechanisms and associated acoustic activity in heterogeneous fault systems. We performed a set of experiments in quartz-phyllosilicate mixtures and another set of tests where a quartz layer is sandwiched between two muscovite horizons in contact with the forcing blocks. The first set represents typical cataclastic rocks with random fabric while layered mixtures were designed to replicate the deformation behavior of block-and-matrix shear zones.
Pure quartz gouges exhibit cataclastic deformation (grain fragmentation and shear localization) which generates high AE rates and amplitudes. In contrast, muscovite-rich gouges deform through distributed sliding along anastomosed foliation and are characterized by low AE activity. In quartz-phyllosilicate mixtures, increasing muscovite content reduces friction, AE rate, and AE average amplitude inhibiting quartz grain interactions. Layered mixtures introduce additional complexity. While the two muscovite layers govern frictional strength and accommodate distributed deformation, cataclastic processes in the central quartz layer dominate AE activity. These layered systems combine the AE characteristics of quartz and muscovite, with high AE rates similar to pure quartz despite the overall weakening from muscovite. Microstructural observations support these findings, showing deformation concentrated at muscovite interfaces but also revealing localized shear bands in the quartz layer that significantly contribute to AE activity. Experiments performed at varying strain rates reveal that higher strain rates amplify AE rate and amplitude. In the layered mixtures, at high strain rate, AE rates and amplitudes are in the range of pure quartz gouge whereas at low strain rate, they are in the range of pure muscovite gouge.
The evolution of the frequency-magnitude distribution of AEs (b-values) with strain provides additional insights on micromechanical processes: quartz-dominated gouges show increasing b-values from yield to steady-state, suggesting a transition from distributed to localized deformation. In contrast, muscovite-dominated gouges maintain low b-values, reflecting consistent distributed deformation. Layered systems exhibit b-value evolution with strain similar to pure quartz, indicating the dominant role of quartz in the AE properties. Our findings emphasize that fault friction and acoustic behavior are controlled by mineralogical and structural heterogeneities and modulated by strain rate. Weak muscovite layers primarily control frictional strength, while strong quartz layers generate significant AEs, highlighting the potential for aseismic slip coupled with seismic activity in heterogeneous faults.
How to cite: Casas, N., Volpe, G., Collettini, C., and Scuderi, M. M.: How mineralogy and fault structure influence frictional and acoustic properties of quartz-phyllosilicate mixtures, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5757, https://doi.org/10.5194/egusphere-egu25-5757, 2025.
The strength and deformation behaviour of the Earth are necessary parameters to model and fundamentally understand crustal scale deformation. In the case of the Earth’s continental middle crust, so far, most models use extrapolated physical parameters from monomineralic deformation experiments, assuming pure quartz to represent the weakest rheological phase at mid-crustal conditions. This is an oversimplification as the Earth’s continental middle crust mostly consists of polymineralic granitoid rocks, the rheology of which is unknown so far.
We present the first experimental study investigating the viscous rheology of a natural, fine-grained, granitoid rock. To unravel the complex deformational behaviour, it is crucial to combine an in-depth microstructural analysis with thorough estimations of rheological parameters. Cylindrical natural granitoid ultramylonite samples, composed of qtz + ab + K-fsp + bt + ep, with grain sizes of 125-15 μm are deformed in a Griggs type apparatus at T=650°C, confining pressure=1.2 GPa, strain rates=10-3 to 10-5s-1, and 0.2 wt% H2O added. Mechanical data are combined with light microscope, SEM, TEM, and quantitative image analysis to connect microstructures with stress and strain evolution.
Only through this combination we can show that grain size sensitive deformation processes, namely dissolution precipitation creep (DPC) lead to extreme grain size reduction, and pinning prevents grain growth and produces a stable microstructure. These processes lead strain localization and overall very weak viscous behaviour – weaker than can be extrapolated from monomineralic quartz. We further can show through two different experimental setups how strain is localizing with and without preexisting fracture in an initially foliated rock. Verified with microstructural observations, we fit parameters into a constitutive equation for this DPC, based on an exponential diffusion creep flow law, to model our experiments and tackle the extrapolation to various natural conditions.
Our findings imply that the Earth’s granitic middle crust deforms faster in localized shear zones than previously modelled. This would result in faster stress buildup at the viscous to brittle transition, promoting seismic ruptures in the overlaying brittle crust than predicted so far. Thereby, our new insights can improve models investigating mechanics of extensions of earthquakes to the middle crust, where they supposedly nucleate and help understand seismic cycles or improve models of stress buildups and thermal flow below and influences on hydro/geothermal systems. We further highlight the importance to improve modelling of polymineralic systems.
How to cite: Nevskaya, N., Berger, A., Stünitz, H., Plümper, O., Zhan, W., Ohl, M., and Herwegh, M.: Weaker than quartz? – Strain localization mechanisms and rheology of fine-grained polymineralic rocks, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4855, https://doi.org/10.5194/egusphere-egu25-4855, 2025.
Pseudotachylytes (quenched frictional melts produced during coseismic slip) represent unambiguous evidence of fossil earthquakes. They are routinely studied to investigate the processes operating before, during, and immediately after seismicity. To place this knowledge into the relevant tectonic context, requires the capability of determining the age of seismicity. However, pseudotachylytes are thin (typically below 1 cm thick) and extremely fine grained, rendering the application of geochronological tools challenging. In the upper crust, the 40Ar/39Ar method has been successfully used to date pseudotachylytes, however, no successful attempts of dating lower-crustal pseudotachylytes have been reported until now.
We present results from in-situ apatite U-Pb geochronology applied to lower-crustal pseudotachylytes exposed in Lofoten, northern Norway. The sample suite includes a pristine pseudotachylyte, a mylonitized pseudotachylyte, and a mylonite with pseudotachylyte veins transposed into the foliation. The results are scrutinized by detailed microstructural investigations using electron backscatter diffraction and cathodoluminescence (CL) imaging. By doing so, we are able to identify apatite that deformed by crystal plasticity in response to the seconds-to-minutes-long thermal pulse generated by the earthquake. The corresponding apatite U-Pb data define a tight regression with a lower intercept at ~426 Ma; the age of lower-crustal seismicity in the Lofoten exposures.
Our analyses also yield an age population significantly younger than the age of seismicity. The single spot dates of this population are microstructurally controlled, i.e., spots within the same microstructural framework yield similar ages, and characterized by significant variations in CL intensity within single grains. The U-Pb data of this age population do not define a clear regression, but rather indicate several Pb-loss events. Patchy CL zoning in some of these apatites indicates modification by dissolution-precipitation, and thus the involvement of a fluid in partial resetting of the apatite U-Pb system. We interpret this age group to reflect protracted fluid percolation along the pathways provided by the pseudotachylytes and shear zones. In-situ apatite U-Pb dating coupled with microstructural investigations thus provides (1) the first robust age of a fossil lower-crustal earthquake, which indicates that seismicity occurred during the early stages of continental collision, as well as (2) evidence that such structures serve as long-lived fluid pathways long after seismicity occurred.
How to cite: Zertani, S., Menegon, L., Whitehouse, M. J., Jeon, H., and Jamtveit, B.: In-situ apatite U-Pb geochronology coupled with microstructural analysis reveals the age of lower-crustal seismicity, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10325, https://doi.org/10.5194/egusphere-egu25-10325, 2025.
Furtschaglschiefer is a thick layer of graphitic schists belonging to the Greiner unit in the western Tauern Window. The precise tectonostratigraphic assignment of these rocks is debated, but they are considered the post-Variscan cover of the Zillertal-Riffl nappe, which was metamorphosed only during the Alpine orogeny.
These schists display a peculiar structure, where biotite adds to garnet to form porphyroblasts up to 1 cm long, defining a marked lineation. The texturally well-equilibrated mineral assemblage of the schists comprises quartz, plagioclase, garnet, biotite, ilmenite, muscovite, graphite ± chlorite ± epidote ± staurolite.
Garnet occurs as euhedral rhombic dodecahedral porphyroblasts 2-3 mm in diameter. Biotite occurs in three textural generations, of which the most abundant and coarser is represented by microboudinaged porphyroblasts with cleavage at high angle to the foliation, showing evidence of repeated crystals opening at cleavage planes and sealing by new biotite and in places by quartz. The evidence of this process is given by the variable distribution of graphite inclusions within biotite. The abundant graphite is disseminated in layers defining the main foliation of the rock, probably an isoclinally folded sedimentary bedding. The resulting "accordion-like" biotite porphyroblasts display an average total strain of 175%. Like garnet and ilmenite, these porphyroblasts show symmetric synkinematic features (rotation of cleavage and internal foliations) and strain shadows. When observed, sinistral and dextral shear sense indicators are equally recorded. The 2D analysis performed on thin sections was complemented by a 3D µCT study on two samples. Microboudinaged biotites show a prolate strain, with the long axis parallel to the rock lineation. Many coin-shaped ilmenite porphyroblasts are oriented parallel to the main foliation. The microboudinaged biotite of the Furtschaglschiefer probably formed in a two-stage process including 1) random static growth followed by 2) pure shear, intense microboudinage and precipitation of new biotite (and quartz) with very little rotation.
Continuous biotite and garnet growth occurred during and after deformation and foliation development, under almost constant PT conditions as evidenced by the constant chemical composition of synkinematic phases like biotite.
Phase equilibrium modelling and thermometry based on Raman spectroscopy of carbonaceous matter and titanium in biotite converge to indicate metamorphic temperatures of c. 550 °C, in agreement with previous studies. Constraints on metamorphic pressure are still poor, and at present there is no evidence for significant decompression from higher-P conditions predating the development of the main Grt-Bt-Ilm±Chl assemblage.
Preliminary results of in situ U-Pb dating by LA-MC-ICPMS provide an age 24.0 ± 2.6 Ma for garnet and of 26.1 ± 3.3 Ma for ilmenite, confirming the Oligocene age of amphibolite-facies metamorphism in this part of the Tauern Window.
How to cite: Cesare, B., Bedon, S., Salvadori, L., Bartoli, O., Macente, A., Behr, W., and Beranoaguirre, A.: Microstructural, petrological and petrochronological study of the graphitic schists ("Furtschaglschiefer") of Passo di Vizze (SW Tauern Window, Eastern Alps, Italy), EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9019, https://doi.org/10.5194/egusphere-egu25-9019, 2025.
In situ garnet U-Pb geochronology by laser ablation-inductively coupled mass spectrometry (LA-ICPMS) is a powerful tool for rapid and high-spatial resolution dating of metamorphic pressure-temperature–time histories. However, ultra-low U and Pb contents in many metamorphic garnets pose a significant analytical challenge, including the risk of contamination by inclusions. Here we use split-stream analysis to simultaneously measure U, Th, and Pb isotopes and trace-element contents of garnet from eclogite-facies metamafic rocks (As Sifah, Oman). The data show strong linear correlations in U vs Zr contents, the slopes of which can only be explained by zircon contamination. Time-resolved U, Th, and Pb signals show some irregularities, but often lack sharp spikes typically diagnostic of inclusions. Abundant micro-zircon (<2 µm diameter) inclusions were observed by SEM in all five samples. Interestingly, garnet–zircon mixing lines in Tera-Wasserburg space project to well-defined concordia intercept dates of 94–89 Ma, whereas a sample with sufficient zircon-free analyses gave an intercept date of 71 ±7 Ma. The latter date overlaps within uncertainty of published Sm-Nd garnet–whole rock isochron ages (81–77 Ma) and U-Pb zircon ages (82–78 Ma), whereas ‘garnet’ dates dominated by the zircon-inclusion are 8 to 17 Myr older. The discrepancy between zircon inclusion and published zircon ages from the same samples likely arises from the fact that such small zircon size fractions are rarely dated and may represent a different age population from that of lager grains. We suggest that the <2 µm zircon fraction formed by diffusion-limited local nucleation from the expulsion of Zr from igneous precursor minerals during (sub-)greenschist facies metamorphism. Recrystallization and Ostwald ripening of matrix grains produced zircons that record peak metamorphic ages, whereas the micro-zircon grains remained isolated and were shielded by garnet. We therefore suggest that (1.) the ablation of inclusions may produce linear trends in Tera-Wasserburg space that may be easily misinterpreted as U-Pb garnet ages, and (2.) bulk inclusion dating of zircons by LA-ICPMS is a promising technique that requires further investigation. Finally, we establish more rigorous criteria for the screening of samples for inclusions prior to and during LA-ICPMS U-Pb garnet geochronology analysis.
How to cite: Walters, J., Garber, J., Beranoaguirre, A., Millonig, L., Gerdes, A., and Marschall, H.: What are we dating? The role of micro-inclusions for in situ garnet geochronology, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6118, https://doi.org/10.5194/egusphere-egu25-6118, 2025.
To assess the metamorphic and deformational history preserved in apatite, we selected high-pressure granulite to lower-amphibolite facies rocks from the East Himalaya Syntaxis (EHS) in the Tibetan orogen for in situ apatite analysis. Through the cross-correlation of cathodoluminescence (CL) images, Electron Backscatter Diffraction (EBSD) analysis, and in situ trace element and U-Pb geochronology, we elucidate the evolution of apatite growth and deformation. In a high-pressure granulite facies sample, apatite has no crystallographic preferred orientation (CPO) but instead a strong shape preferred orientation (SPO) and intragranular deformation, indicating that the apatite experienced peak metamorphism/deformation. Apatite from medium-pressure granulite facies rocks exhibits obvious oscillatory zoning, grow over the main foliation, and have weak intragranular deformation, as well as SPO and CPO, suggesting crystallization after the main phase of metamorphism/deformation. Euhedral apatite grains from upper-amphibolite facies samples display strong SPO and CPO, but only weak intragranular plastic deformation, indicating apatite growth coeval with the main deformation phase. Apatite from lower-amphibolite facies samples have core-and-mantle microstructures and a CPO subparallel to the stretching lineation, with a weak SPO and intragranular deformation, suggesting multiple phases of growth induced by fluid activity during and after peak metamorphism. Almost all apatites in high-pressure granulite facies, medium-pressure granulite facies, and upper-amphibolite facies samples exhibit similar rare earth element (REE) characteristics within individual samples. Despite undergoing different growth and deformation processes, this similarity likely reflects either chemical reequilibration under high-temperature conditions or the retention of comparable initial chemical compositions during their formation. In contrast, apatites within lower-amphibolite facies samples display inconsistent REE characteristics, suggesting that chemical reequilibration did not occur after their formation. Based on microstructural analysis, in situ U-Pb ages reveal peak and retrograde ages in the EHS at ~21 Ma and <16 Ma, respectively, overlapping with published chronology results. Therefore, multiple generations of apatite can be identified through careful cross-correlation of CL, EBSD, and geochemical data, providing a robust record of a rock's P-T-t evolution.
How to cite: Zhao, Z.: Growth and deformation of apatite in different metamorphic scenarios, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2190, https://doi.org/10.5194/egusphere-egu25-2190, 2025.
Deformation of the oceanic and continental lithosphere induce by plates motion on Earth’s surface implies the existence of an oblique deformation component at plate boundaries. Oblique tectonic systems, including transpressional and transtensional regimes, represent complex oblique boundaries and in-between movements combining transcurrent with either convergent or divergent deformation. These oblique strain regimes could generate a wide variety of structures and strain patterns in the ductile domain of the middle to lower crust, that makes interpretation of fabrics challenging. Studied example of transtensional regimes in orogenic domains are still rare and deformation process in this context remains poorly understood. However, transtensional systems are expected to play a key role by accommodating tectonic forces within a rheologically and mechanically heterogeneous lithosphere, in particular during orogenic collapse and exhumation of high-grade metamorphic rocks by thinning the orogens.
Here we present a new multidisciplinary and multiscale study by combining (micro)structural, thermobarometric and geochronological analyses in the Variscan Tanneron massif to reconstruct its late P-T-t-D evolution. The Tanneron massif represent the most internal part of the Maures-Tanneron Variscan belt (SE France), which was mainly structured during the late stage Variscan orogeny. The aim of this study was to precise the nature, organisation and evolution of ductile deformation and associated structures inside a transtensional tectonic regime. Structural and microstructural analysis (AMS and 3D finite strain ellipsoids calculation) of the Tanneron massif indicates that the late tectonic event that structured the massif is a general transtensional regime characterised by a strong subhorizontal stretching that originated through two phases. The first phase is a pure shear- dominated transtension with the development of a subhorizontal constrictional flow associated with L>S tectonites and minority gently-dipping foliations. The second phase is a simple shear-dominated transtension characterised by a plane strain flow (S-L tectonites) and the development of vertical foliations and dextral shear zones. Thermobarometric modelling (X-Ray compositional maps and mineral composition using EPMA) and in-situ U-Th-Pb dating (monazite and xenotime, LA-ICPMS) of the migmatitic units allow us to precise the tectonic evolution of this transtensional deformation regime. This regime represents a tectonic continuum between ~ 325 and 300 Ma and defines a progressive deformation event synchronous with the exhumation of high-grade metamorphic units. Deformation was initiated at high-temperature conditions associated with partial melting of the crust between 7.2 - 10.0 Kbar and 730 - 770 °C, then progressed mainly under subsolidus conditions during the retrograde path of the migmatitic units until low greenschist metamorphic facies conditions reaching 4.1 Kbar and 370 °C. Linking structural, microtextural observations and thermobarometric data, we propose a rheologically driven strain path partitioning during the progressive exhumation of this deep crust. During the second phase of the transtensional regime, deformation was localised preferentially in the hydrated meta-sedimentary units, rather than in meta-igneous rocks.
How to cite: Gremmel, J., Duclaux, G., Corsini, M., Bosse, V., and Bascou, J.: Tectonic evolution in transtensional regimes: the example of the Variscan Tanneron massif (SE France), EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-20090, https://doi.org/10.5194/egusphere-egu25-20090, 2025.
The rheology of granite under geological conditions cannot be determined directly by experiment, because of the different flow parameters for the constituent minerals, and changes in grain-size and microstructure during deformation. I use a recent geologically calibrated dislocation creep flow law for quartz, experimentally determined flow laws for feldspars, and a grain-size sensitive pressure-solution creep flow law for quartz-feldspar-mica ultramylonite utilizing a new stress/grain-size relationship for feldspar derived from subgrain piezometry. These flow laws are combined using various rheological mixing laws depending on the evolving grain-size and fabric anisotropy to give bulk rheological parameters. This allows prediction of the effective viscosity for granite as a function of stress, temperature, and strain as the rock evolves from a load-bearing framework to an interconnected weak layer microstructure. The results have implications for tectonic processes such as rates of crustal thickening during continental collision, crustal thinning during rifting, channel flow, and diapirism.
How to cite: Platt, J.: Rheology of granite under middle and lower crustal conditions: tectonic implications, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2947, https://doi.org/10.5194/egusphere-egu25-2947, 2025.
High-angle misorientations, including grain boundary domains within quartz aggregates, may exert significant control on how strain is accommodated in quartz-bearing domains of the continental lithosphere. Among the high-angle boundaries in quartz, Dauphiné twin boundaries (DTBs) are the most prominent type that displays coincident site lattice (CSL) relationships, a special grain boundary geometry commonly found in metals. In metals, it is known that CSL boundaries stabilize the microstructure by reducing the overall grain boundary surface energy, and thereby impart certain special properties to the material. The present study explores CSL relationships of DTBs and how they control strain accommodation and deformation mechanisms in quartz-rich rocks by combining optical microscopy, Scanning Electron Microscope-Electron Backscatter Diffraction (SEM-EBSD), and Atomic Force Microscopy (AFM). This investigation was carried out on thin sections of granulite facies quartzo-feldspathic gneisses prepared following a standard protocol, culminating with chemical mechanical polishing (CMP) using colloidal silica. The CMP procedure causes preferential material removal along less-compact random high-angle grain boundaries (RHAGBs) and forms indented channels that are prominent in high-resolution, nanoscale AFM images. The absence of corresponding channels along DTBs and the presence of 'bridge-like' structures at RHAGB-DTB intersections suggest greater compactness of DTBs in quartz, which is compatible with CSL relations. Importantly, there is a demonstrable change in misorientation across adjacent quartz grains between consecutive RHAGB-DTB intersections. Grains adjacent to these RHAGB segments have angles between their c-axes varying from 61-66° and 81–84° with parallel rhombohedral faces. These symmetries represent the Japan and Sardinian twin laws of quartz, indicating that the RHAGB segments are modified by DTBs into low-energy twin boundaries, thereby reducing the overall surface energy of the aggregate. Contextually, these quartzofeldspathic gneisses record multiple deformations of the previous ultra-high-temperature (UHT) fabric under granulite facies conditions, with grain boundary migration recrystallization (GBM) as the dominant dynamic recrystallization process. Owing to the GBM recrystallization, the interaction of RHAGBs with DTBs increases, thereby producing more intersection-induced twin boundaries. The relict vs. recrystallized quartz grain maps with DTBs indicate a higher frequency of DTBs in the relict grains. Crystallographic preferred orientations (CPOs) of these relict grains do not define any slip system pattern. In contrast, the recrystallized grains show the operation of prominent slip system patterns, suggesting differences in strain accommodation. Therefore, the formation of DTBs and DTB-RHAGB intersections in quartz can cause strain partitioning due to the higher compactness of DTBs, which will eventually control the response to the deformation of high-grade quartz aggregates. As a result, interpreting the timing of DTB formation becomes important while deciphering the deformation history of rocks based on quartz CPOs from a poly-deformed high-grade terrane.
How to cite: Dey, S., Chatterjee, S., Dobe, R., Mandal, S., and Gupta, S.: Grain boundary microstructures and their control on deformation mechanisms in high-grade quartz-rich rocks, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5264, https://doi.org/10.5194/egusphere-egu25-5264, 2025.
Deformed quartz veins next (1-1.5m) to an exhumed pseudotachylyte-bearing (i.e. anciently seismic) fault within the Schobergruppe (Austroalpine Crystalline Complex, Eastern Alps) contain intensely kinked quartz grains. In general, kinking requires the presence of a planar mechanical anisotropy, e.g. a multi-layered structure with a regular periodic alternation of thin weakly bounded layers (or of high viscosity layers interleaved with thin low viscosity ones) such as in minerals with a strong cleavage, e.g. micas or in industrial metallic nano-laminates. Since quartz does not commonly have a strong mechanical anisotropy, we address the question of why and how kinking of quartz may develop during the seismic cycle.
The monoclinic symmetry of kink bands is consistent with the slip sense of the fault. Cathodoluminescence images show a very high density of intragranular, sub-planar, lamellae accompanied by nanometre-scale fluid-related porosity visible in electron backscatter orientation contrast. Based on the oscillating orientation variation across subgrain boundaries (misorientation angle 1-9°) these lamellae (oriented (sub)parallel to a rhomb plane and spaced 4-10 µm apart) are identified as short-wavelength undulatory extinction microstructures (SWUE). Transmission electron microscopy reveals a high degree of recovery (low dislocation density) across the SWUE. Only grains with SWUE oriented parallel to the vein boundary are kinked. We infer following history for the kink evolution related to the seismic cycle: (I) Deformation lamellae formed during high differential stresses preceding the earthquake rupturing or associated with seismic rupture propagation. The initial high dislocation density within the deformation lamellae provided the mechanical anisotropy in quartz required for (II) the subsequent coseismic initiation of kinking. The lamellae acted as a geometric filter that only allowed r<a> slip of dislocations parallel to the lamellae. These athermal dislocations were able to glide fast over a relatively large distance before piling up and initiating kinking during the coseismic event. Progressive build-up of dislocations resulted in deformation bands which accumulated the final misorientation angle between host domain and kink domain. (III) During post-seismic deformation dislocations were dynamically re-arranged under residual stress into sub-parallel subgrain boundaries which now characterize the kink band boundary region. We suggest that kinking in quartz potentially indicates coseismic deformation and is an important mechanism for incipient strain accommodation during high strain rates.
How to cite: Bestmann, M., Grasemann, B., Kilian, R., Wheeler, J., Morales, L. F. G., Bezold, A., and Pennacchioni, G.: Seismically induced kinking in quartz, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3597, https://doi.org/10.5194/egusphere-egu25-3597, 2025.
In natural shear zones, micro-porosity is found to decorate the grain boundaries of rocks which have been deformed in viscous conditions. Whether porosity is formed during or after deformation is widely debated, and requires further investigation to test how micro-pores may be produced, particularly in monomineralic aggregates.
Using a new-generation Griggs-type apparatus, we performed two general shear experiments using a fine grained (∼ 3 μm) quartzite (white novaculite) with low to no primary porosity (< 1%). The experiments were performed at a temperature of 900 °C and pressures of 1.2 and 1.5 GPa, with bulk strain rates of ≅ 1.2×10-4 and 2.3×10-5 s-1, respectively. In both, 1 wt% of water was added to the starting sample.
During deformation, both samples record a significant stress drop following a high peak of differential stress, after which a progressive strain weakening occurs over several gamma of shear strain. In the experiment at 1.2 GPa, the sample deformed above the Goetze criterion at peak stress, where σ1-σ3 > 1.2 GPa, which gave rise to a highly fractured sample. Within the sample, shear planes < 1 µm thick contain a material which is brighter than quartz in SEM-BSE, despite being composed of SiO2. Microstructural observations of the same sample show the production of a penetrative secondary porosity, where most pores are < 1 µm in diameter. Pores decorate most grain boundaries, which are open and easily identifiable in the SEM.
In contrast, the experiment at 1.5 GPa did not experience any fracturing, and the maximum differential stress remained below the Goetze criterion at σ1-σ3 ≅ 0.8 GPa. In this experiment, a porosity of microns to tens of microns in size developed along apparent conjugate bands. Outside of these bands, there is no porosity nor open grain boundaries. Electron backscatter diffraction (EBSD) analyses reveal quartz which deformed viscously, both inside and outside of the porosity-decorated bands. However, quartz grains within the pore-decorated bands have a stronger intragrain misorientation and higher lattice curvature gradients, as well as slightly weaker lattice-preferred orientation than in the surrounding, non-decorated quartz. Finally, an interesting feature of EBSD maps is the lower indexation rate of quartz within the porosity-decorated bands, at 54% within compared to 83% outside. While the reason for this is unknown, the non-indexed area is considerably larger than the area of pores.
How to cite: Arbaret, L., McGill, G., Précigout, J., Prigent, C., and Airaghi, L.: Experimental deformation of natural monomineralic quartz to produce micro-porosity, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13040, https://doi.org/10.5194/egusphere-egu25-13040, 2025.
Metamorphic textures are often complex because they reflect a long and protracted history across a range of conditions. When these textures develop in settings where deformation also occurs it becomes increasingly difficult to accurately pull apart their spatiotemporal history.
While experimental petrology and deformation studies have been instrumental in understanding metamorphic textures, traditional approaches only capture ‘before’ and ‘after’ states of metamorphic texture formation. This limitation has prevented direct observation of the dynamic evolution of internal reactions and processes within rocks as they occur throughout time. Fundamental questions remain unanswered, such as the spatial distribution of nucleation sites, the temporal evolution of growth patterns, and critically, how deformation influences these processes. Understanding these dynamics is particularly crucial along pressure-temperature-time (P-T-t) paths, where natural rocks preserve evidence of competing processes between textural inheritance and overprinting.
To address these questions, we conducted time-resolved microtomography (4D µCT) experiments on alabaster gypsum (Volterra, Italy) that cycled between dehydration, rehydration and dehydration to understand how complex metamorphic textures formed. We ran one experiment at hydrostatic confining conditions and another for comparison in a non-hydrostatic stress state. Initial results reveal differences between how the microstructures evolve in the two stress states. For the non-hydrostatic environment we have observed a sequence where any initial gypsum fabric is uniformly over-printed to form a foliated texture determined by the stress field, but the rehydration reaction occurs in both a localized and patchy fashion. This in-turn determines where any further dehydration occurs. These experiments give insight into how a rock texture can evolve through a prograde and retrograde metamorphism on a clockwise P-T-t path.
How to cite: Vass Payne, E., Butler, I., Gilgannon, J., Freitas, D., King, A., Rizzo, R. E., Eberhard, L., and Fusseis, F.: Imaging the textural evolution of gypsum through a metamorphic cycle, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18290, https://doi.org/10.5194/egusphere-egu25-18290, 2025.
The Swiss deep geological repository for radioactive waste is to be sited in the easternmost part of the Jura fold-and-thrust belt that forms the external allochthonous units in the Western Alps. Although it is commonly accepted that the Jura is a thin-skinned fold- and-thrust belt, which is detached from the underlying autochthonous units along Middle Triassic evaporites, this is not so clear for Nagra’s siting region. There, the amount of shortening accommodated in the so-called Siglistorf Anticline is only some 150-220m (Jordan et al., 2015, Nagra Arbeitsbericht NAB 14‐105). The location of the anticline above the northern boundary fault of a crustal-scale Paleozoic graben gave rise to a dedicated discussion whether the calculated shortening is in fact related to a thin-skinned detachment or related to thick-skinned deformation involving the underlying basement (e.g., Schöpfer et al., 2023, Terra Nova, and references therein).
To test the competing thin- and thick-skinned models, we analyzed cores from four wells drilled by Nagra through the evaporitic detachment for structures which can be used for quantifying shear strain and, hence, the thrust displacement. Cores of the drilled anhydrite and halite succession are oriented allowing to determine the true orientation of structures. Variably oriented stretching lineations along with sigma clasts, winged inclusions, shear bands and asymmetrical boudins prove polyphase kinematics with different transport directions arguing against a continuous high-strain detachment. Recorded shear directions are top-NNW, top-NNE and top-W. The preservation of primary sedimentary and early diagenetic fabrics, shapes of winged inclusions, angles between S and C planes in shear bands, elongations calculated from boudinaged layers and the abundance and sizes of survivor grains are used to estimate the finite shear strain for the different lithotypes, which are categorized in undeformed (tangent of the shear angle γ=0), low shear strain (γ<2), medium shear strain (γ<7), and high shear strain (γ<15). The total maximum displacement, calculated from the sum of the thickness-shear strain products for each strain category, are 41m, 66m, 79m and 123m, for the four investigated boreholes.
Balanced cross-sections from the area of interest based on the thin-skinned deformation model state a shortening of approximately 150 to 220m which is accommodated in the Siglistorf Anticline north of the investigated boreholes. In this thin-skinned model the whole respective displacement must be accommodated by the regional evaporitic detachment, which, however, is considering the estimated total displacements from the respective core intervals not fully supported. We propose that shortening accomodated in the Siglistorf Anticline is also related to shortening involving the strata below the regional décollement level. It is concluded that deformation of the easternmost part of the Jura fold-and-thrust belt is in fact a combination of thin-skinned and thick-skinned tectonics.
How to cite: Mohideen, A., Decker, K., and Grasemann, B.: Fabric and shear strain of a potential halite detachment below the Swiss Eastern Tabular Jura, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13163, https://doi.org/10.5194/egusphere-egu25-13163, 2025.
The various extensional structures developed at the shallow surface are the coupled embodiment of the deep dynamics, which have played significant roles in the Himalayan tectonic evolution. The two sets of intersecting extensional structures in the Cona area of eastern Himalayan orogen are studied, this paper presents new detailed field investigations, microstructures, quartz [c] axis CPO patterns, kinematic vorticity, deformation temperatures, zircon U-Pb, and mica 40Ar/39Ar geochronology. The results suggest that the Cona Detachment (CD) is mainly in pure shear deformation and the ductile deformation temperature ranges from 280 to 517℃. It was active between 19 and 16 Ma, and ceased at 15 Ma. However, the Cona Rift (CR) is mainly in simple shear deformation and its top-down-to-the-E ductile deformation is recorded at temperatures from 500 to 608℃. It initiated at ~16 Ma, and moved until 10 Ma. Statistically, it was found that the cessation of STDS and the initiation of NSTR seem to follow a youthful trend from west to east along the Himalayan orogen, and there is an overlap in the period of activity between them. Combined with previous studies, we speculate that this process resulted from the Indian plate tearing and the asthenosphere upwelling. Ultimately, this tectonic event leads to the mid-Miocene regime transition. The shallow surface reflects the change from N-S to E-W extensional movement in the mid-Miocene, while the fluid content, heat source, and stress field conditions are also changed in response.
How to cite: Dong, H., Tang, Z., Fei, R., Song, Y., Zhao, L., Gao, L., Wang, Y., Yan, L., and Zeng, L.: Structural and geochronological studies of the Cona Detachment and Cona Rift: Implications for the Miocene evolution of the eastern Himalayan extensional structures, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4148, https://doi.org/10.5194/egusphere-egu25-4148, 2025.
In the last decades, titanite has gained popularity in the petro- and micro-structural community due to its advantageous compositional and microstructural properties that make it an important tracer of fluids, chemical reactions and deformation mechanisms in the Earth’s crust. Thanks to its crystal structure, titanite incorporates a wide range of minor and trace elements, including OH, the equivalent chemical component of water. Indeed, although titanite is considered nominally anhydrous, it can incorporate significant amounts of OH (up to 0.1 wt.%). Understandings of hydrogen concentration, which constitutes the water molecule, in titanite is important since it can affect the physical and chemical properties of minerals and rocks of the upper mantle/lower crust, such as deformation mechanisms, rheology and fluid flow. Moreover, these studies could be useful to understand the reservoirs and the water cycle in the Earth system. Nevertheless, advanced studies regarding water content in titanite are still lacking. However, thanks to recent improvements in both petrochronological and microstructural techniques, the investigation of the link between hydrogen mobility and deformation processes at the nanoscale is now possible.
In this study, we combined the Atom Probe Tomography (APT) to obtain 3D maps showing the distribution of OH with Photo-induced Force Microscopy (PiFM) to quantify the amounts of OH in selected deformed and undeformed domains of mylonitic titanite. In particular, we investigate OH variations between titanite domains showing different dislocation densities in rock layers with different composition (amphibolite vs calcsilicate). Preliminary results show interesting OH variations though the titanite grains from the different layers and a positive correlation between OH and dislocation densities, suggesting a heterogeneous distribution of water during deformation strongly dependent by the bulk rock composition. This study highlights the importance of a multi-disciplinary, multi-technique approach to advance our understanding the deformation/chemical processes occurring at the nanoscale ultimately governing the large scale rheological and chemical evolution of rocks during major tectonic events.
How to cite: Corvò, S., Chen, Y.-S., Holmes, N., Förster, M. W., Yaxley, G., Maino, M., Langone, A., Cairney, J., and Piazolo, S.: Deformation control on OH distribution at the nanoscale: a case of mylonitic titanite, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4781, https://doi.org/10.5194/egusphere-egu25-4781, 2025.
The Sierra de Juárez Complex (SJC), located in Oaxaca, Mexico, comprises a N-S trending belt of deformed igneous and metamorphic rocks with a complex history of multistage deformation and metamorphism. The SJC exhibits penetrative mylonitic deformation attributed to Mesozoic tectonics; however, the timing and kinematics of this event remain poorly constrained.
New Rb-Sr geochronology on muscovite reveals ages ranging from ~164 to ~151 Ma, corresponding to a Middle to Late Jurassic event, and a younger age of ~67 Ma, associated with the Laramide Orogeny. Ongoing microstructural analyses, including traditional optical methods and Electron Backscatter Diffraction (EBSD), aim to characterize the kinematics and deformation style of the mylonitic belt.
The integration of geochronological and microstructural data will enhance our understanding of the role of the SJC in the tectonic evolution of southern Mexico during the Mesozoic, providing new insights into the broader tectonic framework of western equatorial Pangea.
How to cite: Monreal Roque, E., Weber, B., and Tajčmanová, L.: Combined microstructural and geochronological analysis of ductile deformation in the Sierra de Juarez Complex (Southern Mexico)., EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18704, https://doi.org/10.5194/egusphere-egu25-18704, 2025.
Pristine microstructures preserved in a pseudotachylyte (coseismic-derived quenched frictional melt) are recordkeepers of the time-lapse processes associated with the dynamic rupture propagation and the earthquake slip. Unlocking these near-instantaneous processes from the microstructures in a pseudotachylyte provides insights into the chemo-mechanical processes operating during, and immediately following an earthquake.
Inclusion-rich, mutually intergrown garnet aggregates with a morphology akin to a cauliflower are a common product phase in lower-crustal pseudotachylytes. Pseudotachylyte veins in gabbroic rocks from Lofoten (Norway) formed at ambient temperatures of 650–700 °C and display pristine quenching microstructures in the matrix such as plagioclase microlites, dendritic clinopyroxene, cauliflower garnet aggregates, survivor lithoclasts of plagioclase, olivine, and orthopyroxene, and coronas of cauliflower garnet at the interphase boundary between orthopyroxene and the pseudotachylyte matrix. There is a lack of hydrous phases such as amphibole or biotite. Quantitative compositional maps across an entire vein show an irregular or patchy intercrystalline major element distribution in cauliflower garnet. Pyrope and spessartine contents are higher in garnet coronas around survivor lithoclasts comprised of orthopyroxene and olivine, while grossular is highest near plagioclase survivor lithoclasts and near the contact with the plagioclase-rich wall-rock. In some instances, trace elements maps show sharp compositional zoning within single garnet grains. In addition, electron backscattered diffraction data indicate that the garnet corona grains in contact with orthopyroxene along the pseudotachylyte vein boundary show evidence of crystal-lattice distortion through dislocation glide. Collectively, these microstructures indicate that the anhydrous pseudotachylyte melt quenched extremely quickly. The quenching of garnet proceeded faster than the chemical homogenisation in the frictional melt, freezing in and preserving local compositional variations without any later recrystallization at amphibolite-facies ambient conditions.
Using garnet-clinopyroxene geothermometry on cauliflower garnet cores and rims in contact with clinopyroxene inclusions and microlites, respectively, and using a cooling model for the pseudotachylyte vein, we estimate that garnet quenched from the frictional melt starting at ~1100–900 ºC, as recorded in garnet cores, and ceased growing upon reaching the ambient temperature of ~650–700 ºC in <1 hour. In the short duration of mineral growth, garnet captured the incipient distribution of major and minor elements from the frictional melt and was able to record the post-seismic stress relaxation that localized in the form of solid-state creep along the pseudotachylyte vein margin during quenching. High-T microstructures are preserved because dislocations in garnet are immobile at ambient lower-crustal temperatures, and anhydrous conditions inhibit recrystallisation, diffusion, and viscous deformation.
How to cite: Michalchuk, S. P., Zertani, S., Markmann, T., Hermann, J., Lanari, P., Rubatto, D., and Menegon, L.: High-T quenching microstructures in cauliflower garnet capture instantaneous post-seismic processes in a lower-crustal seismogenic fault, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12837, https://doi.org/10.5194/egusphere-egu25-12837, 2025.
Dora Maira is one of the internal crystalline massifs of the Western European Alps, formed by basement nappes of the Penninic Domain. The massif is characterized by high-pressure (HP) and ultra-high-pressure (UHP) rocks and is famous for the presence of coesite-bearing whiteschists. These rocks can be employed as source of information for Earth’s subduction-exhumation cycle, as well as window into (U)HP mechanical processes. For these reasons, Dora Maira whiteschists have attracted the attention of petrologists for the last four decades. However, most of the previous studies focused on petrological aspects and little attention has been given to the peculiar microstructures of these intriguing rocks. This contribution shows the preliminary results of a microstructural and compositional study performed on the whiteschists, with a focus on garnet crystals.
These rocks are characterized by a spatially variable foliation, defined by the alignment of phengite and garnet crystals. Where the foliation is spaced, palisade quartz develops between phengite crystals with an orientation mostly at high angle to the main schistosity. Palisade quartz also formed within garnet crystals, surrounding or completely substituting pre-existing coesite inclusions.
Garnet grains are both elongated parallel to the rock’s foliation or rounded in shape. They show two sets of fractures: a parallel set developed at high angle to the rock schistosity, and radial fractures around coesite/palisade quartz inclusions. The formation of this second set of radial fractures is due to the large volumetric change involved in the coesite-quartz transition.
We adopted SEM-EDS, (HR-)EBSD techniques and performed microprobe analyses to study both microstructures and composition of garnet crystals. Their composition is ca. 88 up to 98% pyrope. However, they also show a distinctive chemical zoning around inclusions, which results in a higher grossular content.
These observations raise questions on the mechanical behaviour of garnets at UHP. In particular, the coesite-quartz transition provokes large volumetric changes which likely result in a mechanical modification of the host garnet. The question is whether the volumetric change of the phase transition and related fractures can trigger also a chemical redistribution. More investigations are still needed, however a strong influence of mechanics on garnet crystals’ behaviour in these rocks is undeniable. Only a meticulous microstructural and compositional analysis can shed light on the history written in the pyrope crystals.
How to cite: Tagliaferri, A., Tajčmanova, L., and Duretz, T.: A Garnet tale: chemical and mechanical responses in Dora Maira Whiteschists, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13122, https://doi.org/10.5194/egusphere-egu25-13122, 2025.
The petrographic study and structural analysis of high-pressure rocks can bring to light some of the mechanisms involved in the subduction and subsequent exhumation of the crustal materials affected by these processes. In the Cabo Ortegal Complex (NW Spain) the emplacement, amalgamation and progressive deformation of still hot peridotites between the Chimparra gneisses and the Bacariza high-pressure granulites imposed a thermal gradient in the shear zones developed at the contact with the high-pressure gneissic formation. This gradient lead to a change of deformation mechanisms and operative intracrystalline slip systems in all the constituent minerals during initial stages of the exhumation of the complex in the subduction channel.
In external areas of the shear zone affecting the gneissic materials, large garnets accommodated strain by rigid rotation. However, close to the shear zones elongated garnets behaved more plastically assisted by 1/2<111>{110} and <100>{010} intracrystalline slip systems. Kyanite, in turn, shows kink bands, sigmoidal geometries and subgrain boundaries disposed subperpendicular to the X structural direction. This feature suggests the activation of [001](100) and [001](010) intracrystalline slip systems in outer sectors and glide on (100) planes along the [001] direction in areas closer to the contact. Although oligoclase does not show systematic orientation distribution patterns in distant sectors the lattice preferred orientation (LPO) obtained at the contact indicates clearly the activation of [100](001) intracrystalline slip systems which have been recognized in rocks deformed under medium- to high-grade conditions. This is in accord with the operation of prism-<a> slip systems in quartz for those samples closer to the ultramafic massif, indicative of temperature conditions up to 700º C. Away from the contact, the temperature decreases, leading to activation of basal- and rhomb-<a> systems in this phase suggesting lower deformation temperature or, more likely, higher strain rates along narrower sectors of the subduction channel accommodating deformation.
How to cite: Puelles, P., Abalos, B., and Esteban, J. J.: Constraining the thermal gradient in shear zones bounding the Chimparra high-pressure gneisses of the Cabo Ortegal Complex (NW Spain): an EBSD approach , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9048, https://doi.org/10.5194/egusphere-egu25-9048, 2025.
We investigated the microstructures in the pelitic schists of the high-P/low-T Sanbagawa metamorphic belt in the Shibukawa area, northwestern Shizuoka Prefecture, Japan, which are mostly classified as low-grade, non-spotted schists, and delineated the thermal structure and microstructure of the Sanbagawa pelitic schists in the vicinity of an ultramafic block using a Raman carbonaceous material (CM) geothermometer and SEM-EBSD crystallographic analysis. The pelitic schists consist of alternating mica-rich and quartzofeldspathic layers. The mica-rich layers consist mainly of muscovite, chlorite, and CM, whereas the quartzofeldspathic layers consist mainly of quartz and albite. The maximum experienced temperatures estimated from the carbonaceous material (CM) were 277–354°C, corresponding to the metamorphic temperature of the chlorite zone. In the quartzofeldspathic layers, both quartz and albite showed subgrain boundaries with undulatory extinctions, indicating plastic deformation. The crystallographic preferred orientations (CPOs) of the quartz grains within the pelitic schists show weak but distinct patterns somehow resembling a type-I cross girdle, whereas those of the albite locally show (100)[001] patterns. The average sizes of both quartz and albite grains were at around 10 µm for all samples and increased slightly with increasing modal composition, independent of the Raman CM temperature. This suggests that the duration time of the peak metamorphic temperature may not be long enough to mature metamorphic textures in the pelitic schists. As a consequence, the microstructures in the pelitic schists would result from the interaction between the grain growth due to heating and the grain size reduction due to deformation in the subduction zone.
How to cite: Katagiri, S., Kouketsu, Y., and Michibayashi, K.: Structural and thermal characteristics around an ultramafic body in the low-grade Sanbagawa pelitic schists, central Honshu Island, Japan, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8742, https://doi.org/10.5194/egusphere-egu25-8742, 2025.
We present detailed 1:10’000-scale geological maps of the eastern flank of the Lepontine Dome, focusing on the Tambo nappe. These maps, produced for the Grono, Mesocco, and Hinterrhein sheets of the Geological Atlas of Switzerland 1:25’000 (GA25), reveal the nappe’s complex internal deformation history, influenced by inherited rheological anomalies, localized fluid percolation, and varying metamorphic conditions. The study of basement nappes internal deformation provides crucial insights into contrasting exhumation mechanisms, a topic central to understanding orogenic dynamics. While nappes exhumed through channel flow mechanisms typically exhibit incoherent internal deformation, those exhumed via Stokes flow or within an accretionary wedge are more structurally coherent.
The Tambo nappe is composed of heterogeneous paragneisses, micaschists, amphibolites, and metagranitoids. This 20 km-long body was emplaced northward as part of the Alpine nappe stack derived from the Briançonnais paleogeographic domain since the Paleocene. The dominant structural elements (foliation, fold axes, and lineation) are predominantly eastward-oriented.
In the southern part of the nappe, the Truzzo meta-granitoid, a folded Permian batholith, acted as a competent, elongated body embedded within micaschists and paragneisses. Below the batholith, pervasive greenschist facies mineral assemblage (chlorite and phengite bearing), associated with top-to-the-east shearing, overprinted earlier amphibolite-facies metamorphism. Deformation subsequently localized along the Forcola Fault. Quartz veins supplied the fluids responsible for the greenschist-facies metamorphism during orogen-parallel shearing that postdated nappe emplacement.
In the northern part, approximately 2 km from the nappe’s front, the lithologies steepen into a cusp-like geometry oblique to the upper nappe boundary. This sector is characterized by sporadic metacarbonate lenses within metasomatized gneisses and schists. Over several hundred meters, syn-foliation veins were folded and boudinated at local peak metamorphic (lower amphibolite) conditions. Locally, some veins cut across the foliation, indicating fluid percolation during deformation. Unlike the southern part, deformation and fluid activity here occurred at peak metamorphic conditions, obliterating earlier textures. We postulate that metacarbonates at the cusp’s core may represent remnants of squeezed Permo-Mesozoic grabens or half-grabens.
These findings suggest that the Tambo nappe behaved as an incoherent body incorporating Mesozoic fragments during nappe emplacement. Subsequent orogen-parallel shearing further thinned the sequence. Field evidence highlights the crucial role of fluids in both deformation stages, influencing rheology and metamorphism.
How to cite: Schenker, F. L., Czerski, D., Scapozza, C., De Pedrini, A., Ambrosi, C., De Paoli, R., Bonazzi, O., Troger, M., and Gouffon, Y.: The Tambo nappe: insights into its internal deformation from geological mapping (Swiss-Italian Alps), EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8302, https://doi.org/10.5194/egusphere-egu25-8302, 2025.
Strain localization and softening in metastable crustal rocks involve complex feedbacks between deformation mechanisms, metamorphic reactions and fluid circulation, as long pointed out for shear zones by previous authors. These feedbacks, however, have rarely been scrutinized and documented precisely at grain-scale. Furthermore, while recent studies have shown that high-grade metamorphic rocks (T°C>550°C) deform through a combination of dislocation creep (DC), diffusion creep and dissolution-precipitation creep (DPC), available creep laws only account for dislocation creep and/or solid-state diffusion processes. Deciphering the role and contribution of DPC to strain accommodation at grain-scale is therefore important to better understand the rheological behavior of rocks, as well as of plate boundaries (for example, deep mechanical coupling in subduction zones likely occurs when/where DC takes over fluid-assisted DPC).
This study investigates the evolution of deformation mechanisms and metamorphic reequilibration of the metamorphic sole of the Bay of Islands ophiolitic complex (BOIC, Newfoundland) and its associated overlying mantle, which jointly preserve evidence for deformation-reaction-fluid feedbacks leading to gradual strain localization at plate boundary scale. Detailed patterns and chronology of deformation-reaction-fluid interactions are constrained by structural, textural, chemical and microstructural data acquired in both the metamorphic sole and the basal mantle. A new method based on EPMA and EBSD maps overlay was also used to track and quantify grain-scale deformation mechanisms, as well as the interplay between grain size reduction, mineral reactions and material transfer. Results show progressive cooling of the mantle associated with increasing deformation (from protomylonitic to ultramylonitic stages) and fluid-related metasomatization of peridotites towards the contact. Fluids are interpreted as coming from the metamorphic sole in which activation of dissolution-precipitation processes are dominant.
How to cite: Mérit, L., Agard, P., Labrousse, L., Dubacq, B., Prigent, C., and Guilmette, C.: Metamorphic re-equilibration, deformation and rheology along a nascent plate boundary: the case study of the Bay of Islands Ophiolitic Complex, Newfoundland, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18133, https://doi.org/10.5194/egusphere-egu25-18133, 2025.
Subduction zone dynamics depend on the rheological behavior of shear zones defining the plate interface, which can accommodate a spectrum of slip from earthquakes to creep. In cold subduction zones, high-pressure, low-temperature metamorphism of the oceanic crust forms blueschist-facies rocks within the interface. Geologic observations demonstrate that deformation at the plate interface is often localized into blueschists with fabrics dominated by glaucophane, a sodic amphibole. Microstructural observations of exhumed blueschists suggest that glaucophane typically deforms by dislocation-accommodated mechanisms. Recent experiments have made progress in developing new flow laws for diffusion creep and dislocation creep. However, the rheological behavior of blueschists remains under-constrained due to challenges arising from phase stability that limits experimental temperatures. Therefore, we conducted experiments at very high pressures to suppress brittle deformation and favor dislocation-accommodated mechanisms to investigate low-temperature plasticity in glaucophane. We conducted cyclical loading experiments in the deformation-DIA on blueschist cores, both parallel and perpendicular to foliation, and cold-pressed glaucophane powders. Experimental conditions covered confining pressures of 6–8 GPa and temperatures of 20–800 °C at strain rates of ~10–4 s–1. We observe temperature-dependent yield and flow stresses and nearly temperature-independent backstresses. These mechanical results as well as the microstructures are characteristic of deformation dominated by dislocation glide. These experiments provide the foundation for a constitutive law describing low-temperature plasticity in glaucophane, which will serve as an input for geodynamic models that aim to understand the physics of strain localization, quantify steady-state interface strength, and explain the mechanics of slip transients.
How to cite: Seyler, C., Kotowski, A., James, K., Thom, C., Hansen, L., and Hein, D.: Dislocation-accommodated deformation in blueschists from high-pressure experiments, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7583, https://doi.org/10.5194/egusphere-egu25-7583, 2025.
Experimental and field-based research has shown that the presence of mica minerals drastically reduces the mechanical resistance of rocks during both brittle and viscous deformation, which allows the localization of deformation into narrow regions such as shear zones. Furthermore, experimental work has proved that the resistance of phlogopite-quartz assemblages with a mica abundance of 30% is as weak as a sample composed entirely of mica. This phenomenon occurs as a combination of several processes, including developing interconnected networks of mica domains and grain size decrease in quartz. Nevertheless, it remains unclear which other processes enhance weakening in these rocks, to what extent each process is significant, in which stage of deformation they take place, and how the presence of mica assists the deformation of quartz. In this work, we address these questions by analyzing the microstructure of six experimental shear zones carried on samples composed of 30% muscovite and 70% quartz with different amounts of strain. By combining microstructural and mechanical information, we aim to infer how and when different weakening processes occur.
Simple shear experiments were conducted using a Griggs-type apparatus to deform 0.1 g of a powder composed of 30% muscovite with an initial grain size of 62-125 µm and 70% quartz with an initial grain size of 10-20 µm. The samples reached different amounts of gamma strain ranging from 0 to 6. The experiments were carried out at T=800°C, P=1 GPa, added H2O=0.1%, and ė≈1x10-5s-1. Afterward, in the post-mortem samples, a detailed microstructural analysis was carried out comparing SEM-BSE images, cathodoluminescence in SEM (CL), and EBSD maps. Some microstructural parameters were acquired such as the interconnectivity of each phase, grain size distribution, and grain lattice misorientation, which were compared to the CL-signal.
As strain increases, the interconnectivity of mica grains does not increase or decrease significantly, but rather, mica grains decrease in size through breaking. Quartz grains located in mica-rich domains preserve their original size and shape while micas take most of the deformation. On the other hand, in quartz-rich domains, the grain size is intensely reduced as strain increases. Additionally, a blue material in the CL maps appears along grain boundaries and microcracks. This blue material becomes more and more abundant and interconnected as deformation increases, which is the main feature appearing as a consequence of progressive deformation. The correlation of CL and EBSD maps indicates that some of the newly formed grains (blue material) present low misorientation to the parental grain, some other grains preserve exactly the orientation of the parental grain, and others present a random misorientation to the parental grain. This suggests that the coupling between continuous dissolution-precipitation and crystal-plastic deformation of quartz is the most suitable mechanism behind the formation of the new material. However, the source of luminescence of the precipitated material in the CL spectra remains unclear.
How to cite: Serrano López, G., Airaghi, L., Raimbourg, H., Precigout, J., and Alaoui, K.: Weakening mechanisms on mica-bearing rocks: an experimental approach, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17810, https://doi.org/10.5194/egusphere-egu25-17810, 2025.
The polymineralic nature of most rocks induce changes in deformation mechanisms with respect to those observed in monomineralic aggregates, and challenges our understanding of the feedbacks between microstructure and rheology. In-situ information from high pressure experiments, such as X-ray tomography and X-ray diffraction, offer the opportunity of quantifying the fabric and stresses, and their evolutions, in polymineralic rocks, under high pressures and high temperatures relevant for the deep earth. Here we present results relevant to the deformation of serpentinized peridotites. Interconnected weak layers (IWL) of serpentine can cause morphological anisotropy and strain localization in serpentinized peridotite, with important implications for the mechanical properties of the lithosphere. We quantify the morphological anisotropy, topology, and interconnectivity of serpentine, in serpentine + olivine aggregates, under torsion. We use in-situ X-ray absorption-contrast tomography at pressures of ca. 4 GPa and temperatures 300-400°C. At shear strains γ >= 4 and ~10 vol. % serpentine fraction, the topology of the serpentine clusters becomes simple, with few interconnections between long isolated serpentine clusters. Conversely, for ~20 vol. % serpentine, the clusters increase in length and topological complexity, resulting in large interconnected serpentine network for γ > 4. This study reveals how serpentinized peridotite with ~20 vol.% serpentine can develop a deformation-induced IWL of serpentine, where strain can preferentially localize. IWL of serpentine may not happen when the serpentine content is ~10 vol. % because of the formation of a serpentine disjoint network. The results will be put in perspective with published in-situ stresses distribution within various serpentine+olivine aggregates, deforming under similar pressures and temperatures. These experiments participate to set the ground for exploring the deformation distribution, stresses and fabric evolution in polymineralic rocks, using in-situ information.
How to cite: Hilairet, N., Mandolini, T., Chantel, J., Merkel, S., Le Godec, Y., Thielmann, M., Guignot, N., and King, A.: Layered or interconnected ? In-situ connectivity and topology from X-ray tomography on deformed serpentine+olivine aggregates, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13729, https://doi.org/10.5194/egusphere-egu25-13729, 2025.
