Luca Menegon

• Clivet, Flavio; Piazolo, Sandra; Michalchuk, Stephen P.; Zertani, Sascha & Menegon, Luca (2026). Shear zone growth by repeated generation of pseudotachylytes: Implications for the seismogenic potential of the lower crust. Journal of Structural Geology. ISSN 0191-8141. s. 105806–105806. doi: 10.1016/j.jsg.2026.105806. Fulltekst i vitenarkiv
• Zertani, Sascha; Lantman, Hugo Willem van Schrojenstein; Kaatz, Lisa; Chogani, Alireza; Plümper, Oliver & Menegon, Luca [Vis alle 7 forfattere av denne artikkelen] (2026). Grain-scale residual stress distribution and dislocation reorganization during plagioclase-breakdown reactions. Tectonophysics. ISSN 0040-1951. 936, s. 231313–231313. doi: 10.1016/j.tecto.2026.231313. Fulltekst i vitenarkiv
• Rosenqvist, Marija Plahter; Dunkel, Kristina; Nogueira, Liebert Parreiras; Planke, Sverre; Polteau, Stephane & Ardit, M. [Vis alle 8 forfattere av denne artikkelen] (2026). Natural CO<inf>2</inf>sequestration: mechanisms and conditions of carbonate precipitation in basalt. Geochimica et Cosmochimica Acta. ISSN 0016-7037. 427, s. 381–402. doi: 10.1016/j.gca.2026.07.013. Fulltekst i vitenarkiv
• Zertani, Sascha; Thielmann, Marcel & Menegon, Luca (2025). Lower‐Crustal Earthquakes: Strain Rate Controls the Magnitude and Rate of Stress Amplification in Rigid Blocks. Geophysical Research Letters. ISSN 0094-8276. 52(7). doi: 10.1029/2024gl114350. Fulltekst i vitenarkiv Vis sammendrag

  Earthquakes in the dry lower continental crust are enigmatic, as they require very high deviatoric stresses. Field observations suggest that stress amplification in rigid blocks surrounded by ductile shear zones leads to seismic failure: the jostling block model. Here we quantify this model by systematically testing numerically how variations in geometry, material properties, and loading conditions impact magnitude and rate of stress amplification. We demonstrate that bulk strain rate is the dominant factor controlling stress amplification. High strain rates of 10 −10 –10 −12  s −1 cause stresses on the 10 2 –10 3 –MPa level within 10 0 –10 2  years, while lower strain rates are insufficient to generate the stresses required for lower‐crustal earthquakes. While geometries and material properties play a subordinate role in causing stress amplification, tests with varying loading conditions show that pure shear is more effective in generating high stress amplifications compared to simple shear in the case of the given geometry.

• Dobe, Ritabrata; Giuntoli, Francesco; Olivieri, Orlando Sèbastien; Siron, Guillaume; Viola, Giulio & Menegon, Luca [Vis alle 7 forfattere av denne artikkelen] (2025). Tracking molecular hydrogen migration along a subduction shear zone. Geological Society of America Bulletin. ISSN 0016-7606. 137(11-12), s. 5241–5264. doi: 10.1130/b38007.1. Fulltekst i vitenarkiv Vis sammendrag

  Abstract This manuscript presents integrated field, petrographic, microstructural, and thermodynamic modeling results documenting the high-pressure channelization of H2 and the rheological impact of its reactivity with carbonate (dolomite and calcite)-rich rocks along a serpentinite-hosted shear zone from Alpine Corsica (France). Microstructures within the carbonates attest to the occurrence of a deformation continuum, evolving from initial brittle fracturing to strain localization by viscous deformation. Raman spectra of fluid inclusions within the carbonates reveal the circulation of H2 and CH4 at all stages of the microstructural evolution, the latter interpreted to be the result of H2-carbonate interactions. Thermodynamic models suggest that carbonate phase stability at pressure-temperature conditions representative of a subduction setting is modified by the presence of H2, with dolomite being progressively replaced by calcite + graphite + magnetite with increasing H2 in the system. An initial phase of overpressure created by H2-rich fluids led to the brecciation of dolomite, creating a fine-grained aggregate, which facilitated a switch to a semi-brittle mode of deformation and created high-permeability pathways for subsequent phases of H2 infiltration. Subsequent phases of infiltration of H2 were accompanied by transformation of dolomite to calcite, the degree of transformation dependent upon the efficiency of H2 percolation. Calcite, being rheologically weaker than dolomite at these temperatures, underwent viscous flow in domains of extensive dolomite reduction, whereas adjacent dolomite-rich domains contain minimal imprints of extensive plastic deformation. Our results demonstrate extensive fossilized H2-carbonate reactivity and show that the infiltration of H2-rich fluids strongly affects the rheology of carbonates by inducing reactivity and phase transitions.

• Jacob, Jean-Baptiste; Lantman, Hugo Willem van Schrojenstein; Cordonnier, Benoît; Menegon, Luca; Wright, Jonathan & Renard, Francois (2025). Exhumation-induced residual stress in undeformed ultra-high-pressure metamorphic rock. Earth and Planetary Science Letters. ISSN 0012-821X. 671. doi: 10.1016/j.epsl.2025.119615. Fulltekst i vitenarkiv Vis sammendrag

  High-pressure metamorphic rocks in subduction zones undergo extreme pressure-temperature changes during exhumation, triggering complex micromechanical responses at the grain-to-sub-grain scale. While mineralogical and thermodynamic aspects of these processes are well studied, the development and preservation of residual stress in exhumed rocks remain poorly understood, besides very localized effects in host-inclusion systems. Here, we apply synchrotron scanning three-dimensional X-ray diffraction (s3DXRD) to an ultra-high-pressure garnet–quartz rock from the Lago di Cignana unit (Western Alps), revealing intra-crystalline misorientation, residual strain, and stress distributions at the ten-micrometer scale. Results show long-range residual stresses reaching several hundred Megapascals (MPa) over 10–100 μm, despite negligible strain at the macroscopic scale and absence of an oriented rock fabric. Quartz displays intra-grain misorientation up to 5∘, reflecting very local crystal plastic deformation resulting from dislocation activity. Garnet shows lower misorientation below 0.8∘, likely reflecting elastic strain gradients. These residual stresses are interpreted to result from anisotropic decompression during exhumation, locally amplified by grain interactions such as indenting tips and multi-anvil geometries. Composition gradients in garnet may also contribute to lattice distortion and stress retention. These long-range stress heterogeneities may influence rock rheology, metamorphic reactions, and affect elastic barometry results, underscoring the importance of rock microstructures in macroscale processes. Finally, our study demonstrates the value of synchrotron-based diffraction microscopy techniques to investigate metamorphic rocks. Besides in situ characterization of residual strain and stress, these methods open new avenues for multi-scale, three-dimensional investigation of intra-crystalline deformation and metamorphic reactions.

• Lantman, Hugo Willem van Schrojenstein; Carvalho, Patricia Almeida; Wallis, David & Menegon, Luca (2025). Residual stress in diopside: insight into localized transient high stress in seismogenic faults in the lower crust, Lofoten, Norway. Journal of Structural Geology. ISSN 0191-8141. 200. doi: 10.1016/j.jsg.2025.105517. Fulltekst i vitenarkiv Vis sammendrag

  Pseudotachylytes in the metamorphosed anorthosites of the Lofoten archipelago, Norway, preserve a record of seismic rupture in dry lower crust at temperatures and pressures of 650–750 °C and 0.8 GPa. Transient gigapascal-level stresses are suggested from microstructural evidence, however such high stresses have not been quantified. In this contribution, we combine microstructural analysis with the mapping of heterogeneity in residual stress in diopside from a lower-crustal pseudotachylyte from Nusfjord (Lofoten) using high-angular resolution electron backscatter diffraction (HR-EBSD). We aim to elucidate the deformation processes that led to this residual stress and to its spatial heterogeneity in the diopside grains. The diopside contains micro-to nanoscale deformation twins within 3 mm of the fault and in clasts in the pseudotachylyte. Within clasts, the diopside lattice strongly undulates, indicating crystal plasticity at high driving stress. Residual stress heterogeneity ranges between ∼200 MPa and ∼800 MPa for in-plane normal stress, with greater values occurring closer to and in the pseudotachylyte. This trend is not apparent for the in-plane shear stress, which has residual stress heterogeneity between ∼150 and ∼250 MPa, not correlating with distance to the fault. The greatest residual stresses are present in a clast that exhibits lattice distortion resulting from dislocation glide. Mechanical twins, lattice undulations, and the distribution patterns of residual stress are truncated by coseismic fractures, suggesting that the microstructures and residual stress are the result of stress build-up prior to slip. Given the extreme spatial localization of the residual stress heterogeneity, we conclude that it results from deformation occurring during earthquake rupture propagation. Despite high temperatures during frictional heating, thermal pressure did not contribute significantly to the residual stress. The behaviour of diopside as a stress recorder is influenced by mechanical twins: stress build-up in diopside may have partially dissipated by the formation of twins, and twins also appear to affect the residual stress, in particular shear stress.

• Michalchuk, Stephen Paul; Lueder, Mona; Gies, Nils; Ohl, Markus; Hermann, Jörg & Plümper, Oliver [Vis alle 7 forfattere av denne artikkelen] (2025). Mechanisms of Aqueous Fluid Infiltration and Redistribution in a Lower-Crustal Pseudotachylyte-Bearing Fault. Geochemistry Geophysics Geosystems. 26(2). doi: 10.1029/2024GC011968. Fulltekst i vitenarkiv Vis sammendrag

  Coseismic fracturing in the strong, dry, and metastable plagioclase-rich lower-crust is an effective mechanism for creating pathways for fluids to infiltrate the host rock, kick-start metamorphism, and potentially lead to rheological weakening. In this study, we have characterized the damage zone flanking a lower-crustal pseudotachylyte (solidified frictional melt produced during seismic slip) within an anorthosite to determine the mechanisms of incipient aqueous fluid infiltration and redistribution in a lower-crustal seismogenic fault. Pulverization-style fracturing of the host anorthosite resulted in the comminution of the host plagioclase (plagioclase1) grains and the growth of very fine (<20 μm) grained secondary plagioclase neoblasts (plagioclase2) filling the fractures. Fluid-assisted grain growth accompanied surface- and strain-energy minimization grain growth in the healing and sealing of the fractures. This process was not associated with the densification nor the creation of new reaction-induced porosity. Fourier transform infrared maps transecting the damage zones show the presence of H2O species along the plagioclase1 and plagioclase2 grain boundary regions, as well as incorporated into plagioclase2 grain interiors. Grain-size sensitive creep of fine-grained plagioclase localized along the pseudotachylyte margin where fracturing was most pervasive. In the absence of reaction-induced porosity, strain localization is determined by repeated occurrences of extreme grain-size reduction in addition to the mobilization of aqueous fluid to the grain boundary regions, to the extent in which these fine-grained wet plagioclase2 layers are volumetrically dominant over dry, coarse plagioclase1 fragments. This forms a layer capable of deforming by grain-size sensitive creep and sustaining the mobility of fluids.

• Rogowitz, Anna; Schorn, Simon; Huet, Benjamin; Grasemann, Bernhard & Menegon, Luca (2025). Melt-Supported Strain Localization at Eclogite-Facies Conditions Triggered by Syn-Metamorphic Strength Inversion. Geochemistry Geophysics Geosystems. 26(5). doi: 10.1029/2024GC012110. Fulltekst i vitenarkiv Vis sammendrag

  In convergent high-pressure settings dominated by dry mafic rocks, strain has been proposed to be predominantly localized in eclogite. However, the processes initiating strain localization, accommodating strain and rheological consequences remain to be identified. We combine field, microstructural, petrographic and geochemical data to investigate the origin of strain localization under eclogite-facies conditions. The studied shear zone bears rocks with two eclogite-facies mineral assemblages: The host-rock eclogite contains clinopyroxene, garnet, zoisite, amphibole, quartz, kyanite and rutile, in which sigmoidal enclaves dominated by zoisite, hornblende and garnet occur. Protolith assemblage calculations suggest that enclaves have originally been plagioclase-rich cumulates within a gabbro. Strength estimates indicate that the enclaves were initially less competent than the gabbro. However, the sigmoidal shape of the enclaves surrounded by ultramylonitic eclogite suggests that the enclaves were stronger during shear zone development. Microstructural investigations of the ultramylonitic eclogite reveal a fabric dominated by euhedral clinopyroxene. Triple- and quadruple-junctions and melt traces at grain boundaries suggest that eclogite is dominantly deformed by melt-supported grain boundary sliding. On the other hand, the microstructure of the lenses is dominated by elongated zoisite and sigmoidal amphibole aggregates deforming by combined dislocation creep and diffusion-mediated grain growth. Our findings demonstrate that high-pressure metamorphism led to a strength inversion, with lenses initially weaker than their host becoming stronger during eclogite-facies metamorphism. Strength-inversion initiated stress concentration at the lithological contact and subsequent strain localization in the weaker eclogitic mineral assemblage. Our results highlight the critical role of compositional heterogeneities on strain localization at high pressures.

• Giuntoli, Francesco; Menegon, Luca; Siron, Guillaume; Cognigni, Flavio; Leroux, Hugues & Compagnoni, Roberto [Vis alle 8 forfattere av denne artikkelen] (2024). Methane-hydrogen-rich fluid migration may trigger seismic failure in subduction zones at forearc depths. Nature Communications. 15. doi: 10.1038/s41467-023-44641-w. Fulltekst i vitenarkiv Vis sammendrag

  Metamorphic fluids, faults, and shear zones are carriers of carbon from the deep Earth to shallower reservoirs. Some of these fluids are reduced and transport energy sources, like H2 and light hydrocarbons. Mechanisms and pathways capable of transporting these deep energy sources towards shallower reservoirs remain unidentified. Here we present geological evidence of failure of mechanically strong rocks due to the accumulation of CH4-H2-rich fluids at deep forearc depths, which ultimately reached supralithostatic pore fluid pressure. These fluids originated from adjacent reduction of carbonates by H2-rich fluids during serpentinization at eclogite-to-blueschist-facies conditions. Thermodynamic modeling predicts that the production and accumulation of CH4-H2-rich aqueous fluids can produce fluid overpressure more easily than carbon-poor and CO2-rich aqueous fluids. This study provides evidence for the migration of deep Earth energy sources along tectonic discontinuities, and suggests causal relationships with brittle failure of hard rock types that may trigger seismic activity at forearc depths.

• Lehnert, Oliver; Almqvist, Bjarne; Anderson, Mark; Andersson, Jenny; Cuthbert, Simon & Calner, Mikael [Vis alle 19 forfattere av denne artikkelen] (2024). The COSC-2 drill core and its well-preserved lower Palaeozoic sedimentary succession – an unexpected treasure beneath the Caledonian nappes. Estonian journal of earth sciences. ISSN 1736-4728. 73(2), s. 134–140. doi: 10.3176/earth.2024.13. Fulltekst i vitenarkiv Vis sammendrag

  The Collisional Orogeny in the Scandinavian Caledonides (COSC) project focuses on processes related to the closure of the Iapetus Ocean, causing the Ordovician–Silurian continent–continent collision between Baltica and Laurentia. The rock succession in the second drill core (COSC-2) from the Jämtland County, central Sweden, provides the base for detailed sedimentological, stratigraphic, geophysical, geochemical, geothermal and structural studies. The basement, comprising 1.66–1.65 Ga Transscandinavian Igneous Belt porphyries intruded by 1.47 Ga and 1.27–1.26 Ga mafic dykes and sills, is heavily weathered towards the top. Here it grades into typical saprock and saprolite (including immature soil reflecting the sub-Cambrian peneplain). The overlying sedimentary sequence starts with basal conglomerates and heterogeneous sediments with shell fragments, indicating an early Cambrian rather than a Neoproterozoic age for the marine transgression in the area. The developing early Cambrian basin was rapidly filled, initially by mostly coarse-grained sediment gravity flows. These strata are covered by sandstone turbidites that show an upward transition into the Alum Shale Formation, representing a tectonically quieter period (mid-Cambrian/Maolingian to Early Ordovician/Tremadocian). The upper part of the Alum Shale Formation is overlain by a late Early Ordovician turbidite succession. Local sources of sediments below the Alum Shale Formation and the extended deposition period may indicate continuous sedimentation in a pull-apart basin preserved in a window beneath the Caledonian thrust sheets.

• Zertani, Sascha; Menegon, Luca; Whitehouse, Martin J.; Jeon, Heejin & Jamtveit, Bjørn (2024). Dating fossil lower-crustal earthquakes by in-situ apatite U-Pb geochronology. Earth and Planetary Science Letters. ISSN 0012-821X. 630. doi: 10.1016/j.epsl.2024.118621. Fulltekst i vitenarkiv Vis sammendrag

  Determining the precise age of fossil earthquakes is essential for understanding the tectonic setting in which they occurred. Pseudotachylytes are quenched frictional melts that occur along coseismic fractures and represent evidence of fossil earthquakes in the rock record. Here, we present the first in-situ apatite U-Pb ages for three samples from Lofoten in the northern Norwegian Caledonides, that occur within and adjacent to lower-crustal pseudotachylytes: a pristine pseudotachylyte, a mylonitized pseudotachylyte, and a mylonite that contains pseudotachylyte transposed into the shear plane. The apatite U-Pb ages are combined with cathodoluminescence and electron backscatter diffraction analysis to provide microstructural constraints on the interpretation of ages. Apatite shows evidence of either fragmentation and annealing, or dislocation creep in response to coseismic rupture. The results suggest that the short-lived thermal pulse generated by the earthquake is sufficient to reset the apatite U-Pb system. The age of 426 ± 19 Ma is interpreted to date lower-crustal seismicity during the early Caledonian orogeny. Later fluid-rock interaction along the same structures led to partial resetting of the apatite U-Pb systematics, indicating that seismic brittle failure of the lower continental crust occurs at the incipient stages of continental collision, and that the resulting structures are preferential pathways for later fluid flow in the otherwise impermeable anhydrous lower crust.

• Toffol, Giovanni; Pennacchioni, Giorgio; Menegon, Luca; Wallis, David; Faccenda, Manuele & Camacho, Alfredo [Vis alle 7 forfattere av denne artikkelen] (2024). On-fault earthquake energy density partitioning from shocked garnet in an exhumed seismic midcrustal fault. Science Advances. 10(9). doi: 10.1126/sciadv.adi8533. Fulltekst i vitenarkiv Vis sammendrag

  The energy released during an earthquake is mostly dissipated in the fault zone and subordinately as radiated seismic waves. The on-fault energy budget is partitioned into frictional heat, generation of new grain surface by microfracturing, and crystal-lattice distortion associated with dislocation defects. The relative contribution of these components is debated and difficult to assess, but this energy partitioning strongly influences earthquake mechanics. We use high-resolution scanning-electron-microscopy techniques, especially to analyze shocked garnet in a fault wall-rock, to provide the first estimate of all three energy components for a seismic fault patch exhumed from midcrustal conditions. Fault single-jerk seismicity is recorded by the presence of pristine quenched frictional melt. The estimated value of energy per unit fault surface is ~13 megajoules per square meter for heat, which is predominant with respect to both surface energy (up to 0.29 megajoules per square meter) and energy associated with crystal lattice distortion (0.02 megajoules per square meter).

• Zertani, Sascha; Morales, Luiz F. G. & Menegon, Luca (2024). Omphacite breakdown: nucleation and deformation of clinopyroxene-plagioclase symplectites. Contributions to Mineralogy and Petrology. ISSN 0010-7999. 179. doi: 10.1007/s00410-024-02125-0. Fulltekst i vitenarkiv Vis sammendrag

  The breakdown of omphacite plays an important role in the exhumation and retrogression of eclogites. Additionally, metamorphic reactions associated with grain size reduction have the potential to significantly impact deformation mechanisms and the rheology of crustal rocks. We analyze the breakdown reaction omphacite → diopsidic clinopyroxene + plagioclase ± amphibole and associated microstructures by electron backscatter diffraction. The reaction results in the formation of (diopsidic) clinopyroxene-plagioclase symplectites. Samples were chosen from localities on Holsnøy (western Norway) and Lofoten (northern Norway), that are representative of vermicular symplectites, partly recrystallized symplectites, and deformed symplectites. Interphase misorientation analysis based on the electron backscatter diffraction results reveals that the nucleation of (diopsidic) clinopyroxene-plagioclase symplectites was crystallographically controlled, with the diopside copying the lattice orientation of the omphacite, and the plagioclase growing along diopside planes with favorable, i.e., similar, interplanar spacing. Deformation of the (diopsidic) clinopyroxene-plagioclase symplectites occurred by fracturing, transitioning into grain boundary sliding accommodated by diffusion creep. The results indicate that the formation of vermicular symplectites is not associated with enhanced permeability and fluid flow. Subsequent recrystallisation and grain-size sensitive deformation of the symplectites facilitates fluid redistribution and weakening of the retrogressed eclogites.

• Zertani, Sascha; Menegon, Luca; Pennacchioni, Giorgio; Buisman, Iris; Corfu, Fernando & Jamtveit, Bjørn (2023). Protracted localization of metamorphism and deformation in a heterogeneous lower-crustal shear zone. Journal of Structural Geology. ISSN 0191-8141. 176. doi: 10.1016/j.jsg.2023.104960. Fulltekst i vitenarkiv Vis sammendrag

  Deformation of the continental crust is influenced by the pre-existing structural framework, fluid availability, strain rate, and pressure-temperature conditions. We investigate the evolution of a large (hundreds-of-meters wide) heterogeneous shear zone and associated brittle and ductile deformation structures (Lofoten, Norway) using structural analysis, mineral-chemical and microstructural observations, and U-Pb dating. The shear zone developed through a sequence of metamorphic stages: (1) migmatization and granulite-facies metamorphism, (2) eclogite-facies metamorphism, and (3) amphibolite-facies metamorphism. Stage (1) was related to magmatic activity at lower-crustal conditions prior to 1.7 Ga. Stage (2) likely occurred during the early phases of the collision between Baltica and Laurentia (Caledonian collision). Stage (3) was related to shortening during the main phase of the Caledonian collision (∼430–400 Ma) and was accompanied by hydration of the shear zone. We demonstrate how mechanical heterogeneities influence the deformation style from the centimeter to meter scale. Zones with a pre-existing fabric deformed by ductile shearing and folding, whereas homogeneous, dry rocks fractured. Fractures provided precursors for small-scale shear zones. These contrasting deformation styles occurred repeatedly. Consequently, pre-existing structures define the deformation style and serve as conduits for the channelization of fluids over extended periods of time.

• Cavdar, B; Morris, Antony; Anderson, Mark; Menegon, Luca & Parlak, O (2023). Magnetic anisotropy of a sub-ophiolitic metamorphic sole (Mersin ophiolite, Türkiye). Bulletin of the Mineral Research and Exploration. ISSN 0026-4563. s. 1–14. doi: 10.19111/bulletinofmre.1081170. Fulltekst i vitenarkiv
• Wang, Jianfeng; Liu, Yuke; Yang, Chao; Zheng, Yangcheng; Jiang, Wenmin & Menegon, Luca [Vis alle 9 forfattere av denne artikkelen] (2023). Upscaling the creep behavior of clay-rich and quartz-rich shales from nanoindentation measurements: Application to the Wufeng-Longmaxi shale, China. International Journal of Rock Mechanics And Mining Sciences. ISSN 1365-1609. 171. doi: 10.1016/j.ijrmms.2023.105580. Fulltekst i vitenarkiv
• Gratier, Jean-Pierre; Menegon, Luca & Renard, Francois (2023). Pressure Solution Grain Boundary Sliding as a Large Strain Mechanism of Superplastic Flow in the Upper Crust. Journal of Geophysical Research (JGR): Solid Earth. ISSN 2169-9313. 128(4). doi: 10.1029/2022JB026019. Fulltekst i vitenarkiv Vis sammendrag

  Abstract A mechanism accommodating large strain in superplastic flow with elongation of 100%–300%, is described for fine‐grained calcareous shales deformed at temperature of 200–335°C and depths of 5–8 km up to slate schists in the Oisans massif, Western Alps, France. Using electron microscopy techniques on thin sections parallel to the principal finite strain axes, we show that the shape ratios of the slightly elongated grains (1.4–1.6), mainly calcite and quartz, do not match the finite strains recorded by the markers of the deformation (truncated belemnites, folded veins) in the maximum elongation and shortening plane (ratio 6.7) or in the maximum and minimum elongations plane (ratio 2.4). Consequently, a grain boundary sliding mechanism is required to explain the measured large finite strains. The most soluble minerals (quartz, calcite, dolomite, and albite), which represent about 95% of the rock, accommodate deformation by pressure solution grain boundary sliding whereas the least soluble minerals (muscovite, chlorite, Fe‐Ti oxides) act as indenters or are passively reoriented. Pressure solution is especially efficient in polymineralic rocks. Soluble minerals, which have been healed together in veins or fossils, are much more resistant to deformation and act as rigid objects. Models with idealized tessellation of hexagonal grains and creep laws derived from pressure solution indentation experiments provide deformation maps. We discuss the main parameters of this ductile deformation in the upper crust (thermodynamic conditions, strain rate, stress, distance of mass transfer) and show possible drastic decrease of mass transfer efficiency with decrease of stress and strain rate.

• Michalchuk, Stephen Paul; Zertani, Sascha; Renard, Francois; Fusseis, Florian; Chogani, Alireza & Plümper, Oliver [Vis alle 7 forfattere av denne artikkelen] (2023). Dynamic Evolution of Porosity in Lower-Crustal Faults During the Earthquake Cycle. Journal of Geophysical Research (JGR): Solid Earth. ISSN 2169-9313. 128(8). doi: 10.1029/2023JB026809. Fulltekst i vitenarkiv Vis sammendrag

  Abstract Earthquake‐induced fracturing of the dry and strong lower crust can transiently increase permeability for fluids to flow and trigger metamorphic and rheological transformations. However, little is known about the porosity that facilitates these transformations. We analyzed microstructures that have recorded the mechanisms generating porosity in the lower crust from a pristine pseudotachylyte (solidified earthquake‐derived frictional melt) and a mylonitized pseudotachylyte from Lofoten, Norway to understand the evolution of fluid pathways from the coseismic to the post‐ and interseismic stages of the earthquake cycle. Porosity is dispersed and poorly interconnected within the pseudotachylyte vein (0.14 vol%), with a noticeably increased amount along garnet grain boundaries (0.25–0.41 vol%). This porosity formed due to a net negative volume change at the grain boundary when garnet overgrows the pseudotachylyte matrix. Efficient healing of the damage zone by fluid‐assisted growth of feldspar neoblasts resulted in the preservation of only a few but relatively large interconnected pores along coseismic fractures (0.03 vol% porosity). In contrast, porosity in the mylonitized pseudotachylyte is dramatically reduced (0.02 vol% overall), because of the efficient precipitation of phases (amphibole, biotite and feldspars) into transient pores during grain‐size sensitive creep. Porosity reduction on the order of &gt;85% may be a contributing factor in shear zone hardening, potentially leading to the development of new pseudotachylytes overprinting the mylonites. Our results show that earthquake‐induced rheological weakening of the lower crust is intermittent and occurs when a fluid can infiltrate a transiently permeable shear zone, thereby facilitating diffusive mass transfer and creep.

• Ceccato, Alberto; Menegon, Luca & Hansen, Lars N. (2022). Strength of Dry and Wet Quartz in the Low-Temperature Plasticity Regime: Insights From Nanoindentation. Geophysical Research Letters. ISSN 0094-8276. 49(2). doi: 10.1029/2021GL094633. Fulltekst i vitenarkiv Vis sammendrag

  At low-temperature and high-stress conditions, quartz deformation is controlled by the kinetics of dislocation glide, that is, low-temperature plasticity (LTP). To investigate the relationship between intracrystalline H2O content and the yield strength of quartz LTP, we have integrated spherical and Berkovich nanoindentation tests at room temperature on natural quartz with electron backscatter diffraction and secondary-ion mass spectrometry measurements of intracrystalline H2O content. Dry (<20 wt ppm H2O) and wet (20–100 wt ppm H2O) crystals exhibit comparable indentation hardness. Quartz yield strength, which is proportional to indentation hardness, seems to be unaffected by the intracrystalline H2O content when deformed under room temperature, high-stress conditions. Pre-indentation intracrystalline microstructure may have provided a high density of dislocation sources, influencing the first increments of low-temperature plastic strains. Our results have implications for fault strength at the frictional-viscous transition and during transient deformation by LTP, such as seismogenic loading and post-seismic creep.

• Marchesini, Barbara; Menegon, Luca; Schwarz, G; Neff, C; Schmidt, P Keresztes & Garofalo, Paolo Stefano [Vis alle 10 forfattere av denne artikkelen] (2022). Strain-induced trace element mobility in a quartz-sulphide vein system: An example from the ONKALO™ spent nuclear fuel repository (Olkiluoto, SW Finland). Journal of Structural Geology. ISSN 0191-8141. 154. doi: 10.1016/j.jsg.2021.104473. Fulltekst i vitenarkiv Vis sammendrag

  This work investigates element mobility and deformation mechanisms in sulphide-bearing quartz veins associated with a strike-slip fault exposed in the ONKALO™ Finnish deep repository for spent nuclear fuel (Olkiluoto Island, southwest Finland). It combines petrography, trace element mapping by Laser Ablation Inductively Coupled Plasma Time-of-Flight Mass Spectrometry (LA-ICP-TOFMS) and Electron Backscattered Diffraction (EBSD) analysis of representative microstructures. The fault core was reactivated by brittle deformation episodes assisted by hydrothermal fluid batches with distinct trace element signatures. LA-ICP-TOFMS element distribution maps and EBSD on sulphides reveal local, syn-deformational intragrain enrichment of primary and secondary elements (i.e., As, Co, Cu, Ag, Sn, Sb, Pb, Se, In, Te). Trace element enrichments occur by a combination of microscale plastic distortions and microfracturing at the reaction fronts. Fluid ingress along microcracks enhanced chemical replacement of pyrite that combined with element diffusion along dislocations and tilt boundaries controlled trace element mobility in sulphides at the small scale. At the scale of the vein system, the competence contrast between inclusions of soft sulphides in the harder host quartz may have favored local fracture nucleation and fluid flow.

• Lorenz, Henning; Rosberg, Jan-Erik; Juhlin, Christopher; Klonowska, Iwona; Lescoutre, Rodolphe & Westmeijer, George [Vis alle 17 forfattere av denne artikkelen] (2022). COSC-2 - Drilling the basal décollement and underlying margin of palaeocontinent Baltica in the Paleozoic Caledonide Orogen of Scandinavia. Scientific Drilling. ISSN 1816-8957. 30, s. 43–57. doi: 10.5194/sd-30-43-2022. Fulltekst i vitenarkiv Vis sammendrag

  The Collisional Orogeny in the Scandinavian Caledonides (COSC) scientific drilling project aims to characterise the structure and orogenic processes involved in a major collisional mountain belt by multidisciplinary geoscientific research. Located in western central Sweden, the project has drilled two fully cored deep boreholes into the bedrock of the deeply eroded Early Paleozoic Caledonide Orogen. COSC-1 (2014) drilled a subduction-related allochthon and the associated thrust zone. COSC-2 (2020, this paper) extends this section deeper through the underlying nappes (Lower Allochthon), the main Caledonian décollement, and the upper kilometre of basement rocks. COSC-2 targets include the characterisation of orogen-scale detachments, the impact of orogenesis on the basement below the detachment, and the Early Paleozoic palaeoenvironment on the outer margin of palaeocontinent Baltica. This is complemented by research on heat flow, groundwater flow, and the characterisation of the microbial community in the present hard rock environment of the relict mountain belt. COSC-2 successfully, and within budget, recovered a continuous drill core to 2276 m depth. The retrieved geological section is partially different from the expected geological section with respect to the depth to the main décollement and the expected rock types. Although the intensity of synsedimentary deformation in the rocks in the upper part of the drill core might impede the analysis of the Early Paleozoic palaeoenvironment, the superb quality of the drill core and the borehole will facilitate research on the remaining targets and beyond. Protocols for sampling in the hard rock environment and subsequent sample preservation were established for geomicrobiological research and rock mechanical testing. For the former, a sparse sample series along the entire drill core was taken, while the target of the latter was the décollement. COSC-2 was surveyed by a comprehensive post-drilling downhole logging campaign and a combined borehole/land seismic survey in autumn 2021. This paper provides an overview of the COSC-2 (International Continental Scientific Drilling Project – ICDP 5054_2_A and 5054_2_B boreholes) operations and preliminary results. It will be complemented by a detailed operational report and data publication.

• Campbell, Lucy & Menegon, Luca (2022). High Stress Deformation and Short-Term Thermal Pulse Preserved in Pyroxene Microstructures From Exhumed Lower Crustal Seismogenic Faults (Lofoten, Norway). Journal of Geophysical Research (JGR): Solid Earth. ISSN 2169-9313. 127(7). doi: 10.1029/2021JB023616. Fulltekst i vitenarkiv Vis sammendrag

  Earthquake rupture in strong, anhydrous lower continental crust requires high brittle failure stresses unless high pore fluid pressures are present. Several mechanisms proposed to generate high stresses at depth imply transient loading driven by a spectrum of stress changes, ranging from highly localized stress amplifications to crustal-scale stress transfers. High transient stresses up to GPa magnitude are proposed by field and modeling studies, but the evidence for transient prerupture loading is often difficult to extract from the geological record due to overprinting by coseismic damage and slip. However, the local preservation of deformation microstructures indicative of crystal-plastic and brittle deformation associated with the seismic cycle in the lower crust offers the opportunity to constrain the progression of deformation before, during and after rupture, including stress and temperature evolution. Here, detailed study of pyroxene microstructures characterizes the short-term evolution of high-stress deformation and temperature changes experienced before and during lower crustal earthquake rupture. Pyroxenes are sampled from pseudotachylyte-bearing faults and damage zones of lower crustal earthquakes recorded in the exhumed granulite facies terrane of Lofoten, northern Norway. The progressive sequence of microstructures indicates localized high-stress (at the GPa level) prerupture loading accommodated by low-temperature plasticity, followed by coseismic pulverization-style fragmentation and subsequent grain growth triggered by the short-term heat pulse associated with frictional sliding. Thus, up to GPa-level transient high stress (both differential and shear) leading to earthquake nucleation in the dry lower crust can occur in nature, and be preserved in the fault rock microstructure.

• Ceccato, Alberto; Goncalves, Philippe & Menegon, Luca (2022). On the petrology and microstructures of small-scale ductile shear zones in granitoid rocks: An overview. Journal of Structural Geology. ISSN 0191-8141. 161. doi: 10.1016/j.jsg.2022.104667. Fulltekst i vitenarkiv Vis sammendrag

  Since the pioneering works of John Ramsay in the 1970's and 1980's, the analysis of exceptional exposures of small-scale shear zones (i.e. 10-3 – 10−1 m thick) in granitoid rocks provided invaluable insights into the processes controlling strain localisation in the middle and lower continental crust. Indeed, recent advancement in field, microstructural and petrological analyses of such small-scale shear zone have shed new light on the metamorphic, tectonic and fluid conditions promoting shear zone nucleation and development in granitoid rocks. In this paper we provide an overview of these new insights, comparing and integrating the results obtained from field, and microstructural and petrological analyses of small-scale shear zones in granitoid plutons and meta-granitoids from the Alps. A review of the deformation temperature shows that the granitoid shear zones development occurs between 350 and 600 °C, with most of them localising in a restricted temperature window between 450 and 500 °C. At these conditions, the magmatic assemblage is metastable and subjected to a series of metamorphic reactions. Furthermore, the development of shear zone does not occur under-closed system conditions. Introducing or expelling fluids and mass (i.e. metasomatism) during deformation has mineralogical consequences that control the rheology and the way shear zone evolves. Among the main mineralogical and microstructural changes, the breakdown of magmatic feldspar(s) into fine-grained aggregates steers both the rheology and fabric evolution of shear zones in granitoid rocks, triggering further mechano-chemical feedback mechanisms. Future research should consider the occurrence of feedback processes between deformation, metamorphic and metasomatic processes to understand and quantify the evolution with time and strain of shear zone geometry and rheology, as well as of the development of larger-scale shear zone networks.

• Cabrita, Dina I.G.; Faleiros, Frederico M.; Ribeiro, Bruno V.; Menegon, Luca; Cawood, Peter A. & Campanha, Ginaldo A.C. (2022). Deformation, thermochronology and tectonic significance of the crustal-scale Cubatão Shear Zone, Ribeira Belt, Brazil. Tectonophysics. ISSN 0040-1951. 828. doi: 10.1016/j.tecto.2022.229278. Fulltekst i vitenarkiv
• Menegon, Luca; Campbell, Lucy; Mancktelow, Neil S.; Camacho, Alfredo; Wex, Sebastian & Papa, Simone [Vis alle 8 forfattere av denne artikkelen] (2021). The earthquake cycle in the dry lower continental crust: insights from two deeply exhumed terranes (Musgrave Ranges, Australia and Lofoten, Norway). Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences. ISSN 1364-503X. 379(2193). doi: 10.1098/rsta.2019.0416. Fulltekst i vitenarkiv
• Menegon, Luca & Fagereng, Åke (2021). Tectonic pressure gradients during viscous creep drive fluid flow and brittle failure at the base of the seismogenic zone. Geology. ISSN 0091-7613. 49(10), s. 1255–1259. doi: 10.1130/G49012.1. Fulltekst i vitenarkiv Vis sammendrag

  Abstract Fluid-pressure cycles are commonly invoked to explain alternating frictional and viscous deformation at the base of the seismogenic crust. However, the stress conditions and geological environment of fluid-pressure cycling are unclear. We address this problem by detailed structural investigation of a vein-bearing shear zone at Sagelvvatn, northern Norwegian Caledonides. In this dominantly viscous shear zone, synkinematic quartz veins locally crosscut mylonitic fabric at a high angle and are rotated and folded with the same sense of shear as the mylonite. Chlorite thermometry indicates that both veining and mylonitization occurred at ∼315–400 °C. The vein-filled fractures are interpreted as episodically triggered by viscous creep in the mylonite, where quartz piezometry and brittle failure modes are consistent with low (18–44 MPa) differential stress. The Sagelvvatn shear zone is a stretching shear zone, where elevated pressure drives a hydraulic gradient that expels fluids from the shear zone to the host rocks. In low-permeability shear zones, this hydraulic gradient facilitates build-up of pore-fluid pressure until the hydrofracture criterion is reached and tensile fractures open. We propose that hydraulic gradients established by local and cyclic pressure variations during viscous creep can drive episodic fluid escape and result in brittle-viscous fault slip at the base of the seismogenic crust.

• Pellegrino, Luca; Menegon, Luca; Zanchetta, Stefano; Langenhorst, Falko; Pollok, Kilian & Tumiati, Simone [Vis alle 7 forfattere av denne artikkelen] (2021). Reaction-induced mantle weakening at high-pressure conditions: An example from garnet pyroxenites of Ulten Zone (Eastern Alps, N Italy). Journal of Geophysical Research (JGR): Solid Earth. ISSN 2169-9313. 126(12). doi: 10.1029/2021JB022584. Fulltekst i vitenarkiv Vis sammendrag

  Peridotites of Ulten Zone (Eastern Alps, N Italy) show a transition from coarse protogranular spinel lherzolites to fine-grained amphibole + garnet peridotites, recorded by the crystallization of garnet coronas around spinel. Pyroxenite veins, transposed along the peridotite foliation, show a similar metamorphic evolution from coarse-grained (garnet-free) websterites to fine-grained garnet websterites. In both peridotites and websterites, garnet previously exsolved from porphyroclastic high-temperature pyroxenes and later crystallized along the foliation. This evolution has been interpreted to reflect cooling and pressure increase of websterites and host peridotites from spinel- to garnet-facies conditions. Microstructures and crystallographic orientation data indicate that the re-equilibration of garnet websterites in the garnet stability field occurred during deformation. Porphyroclastic pyroxenes have been interpreted to deform by dislocation glide and creep. In particular, TEM observations indicate the activation of the (100)[010] slip system in orthopyroxene. Core-and-mantle microstructures also suggest that dislocation creep was aided by subgrain rotation recrystallization, leading to the formation of neoblastic pyroxenes. These recrystallized grains deformed by diffusion-accommodated grain boundary sliding, as indicated by the occurrence of quadruple junctions and straight, aligned grain boundaries. The transition from dislocation creep to diffusion creep in websterites was accompanied by the crystallization of garnet along foliation, which triggered the pinning of the recrystallized matrix and stabilized the fine-grained microtexture for diffusion creep, promoting rheological weakening. Garnet websterites of Ulten Zone thus offer a unique opportunity to investigate the effects of reaction softening during the corner flow in the supra-subduction lithospheric mantle induced by the descending slab.

• Papa, Simone; Pennacchioni, Giorgio; Menegon, Luca & Thielmann, Marcel (2020). High-stress creep preceding coseismic rupturing in amphibolite-facies ultramylonites. Earth and Planetary Science Letters. ISSN 0012-821X. 541. doi: 10.1016/j.epsl.2020.116260. Fulltekst i vitenarkiv Vis sammendrag

  Coeval pseudotachylytes (solidified melts produced during seismic slip) and mylonites are generally regarded as the geological record of transient seismic events during dominant ductile flow. Thermal runaway has been proposed as a model to explain the pseudotachylyte-mylonite association. In the Mont Mary unit (Western Alps), pseudotachylyte fault veins occur along the amphibolite-facies (ca. 550 °C; 0.35 GPa) ultramylonitic foliation of paragneisses. These veins formed at the same metamorphic conditions of the ultramylonites, thus potentially recording thermal runaway. We analysed the microstructure of quartz in ultramylonite and of ultramylonite clasts in pseudotachylyte to investigate the possible occurrence of thermal runaway. Quartz aggregates show an evolution under constant temperature to ultrafine-grained recrystallised grain size (2.5 μm), reflecting creep under high differential stresses (> 200 MPa) and high strain rates (10−9 s−1), along very narrow foliation-parallel layers. In the ultrafine aggregates, viscous grain boundary sliding became dominant and promoted cavitation leading to disintegration of quartz aggregates and precipitation, in the pore space, of biotite, oriented parallel to the main ultramylonitic foliation. The strain rate-limiting process was aseismic fluid-assisted precipitation of biotite. The potential occurrence, at the deformation conditions of the Mont Mary ultramylonites, of thermal runaway in pure quartz layers was investigated by numerical modelling. The models predict a switch from stable flow to thermal runaway at background strain rates faster than 10−9 s−1 for critical differential stresses that are comparable to the brittle strength of rocks. Deformation of ultramylonites occurred close to the conditions for thermal runaway to occur, but based on the microstructural record we conclude that the Mont Mary pseudotachylyte-mylonite association is best explained by brittle failure, triggered by transients of high differential stress and strain rate causing a downward deflection of the brittle-ductile transition.

• Chiara, Anita Di; Morris, Antony; Anderson, Mark W.; Menegon, Luca & Tremblay, A (2020). Magnetic anisotropy reveals Acadian transpressional fabrics in an Appalachian ophiolite (Thetford Mines, Canada). Geophysical Journal International. ISSN 0956-540X. 222, s. 1034–1045. doi: 10.1093/gji/ggaa173. Fulltekst i vitenarkiv
• Ceccato, Alberto; Menegon, Luca; Warren, Clare & Halton, Alison M. (2020). Structural and metamorphic inheritance controls strain partitioning during orogenic shortening (Kalak Nappe Complex, Norwegian Caledonides). Journal of Structural Geology. ISSN 0191-8141. 136. doi: 10.1016/j.jsg.2020.104057. Fulltekst i vitenarkiv Vis sammendrag

  The occurrence of pre-collisional structural and metamorphic fabrics may control the development of new structures during subsequent orogenic deformation. Structural, petrological and geochronological analyses have been performed on selected samples collected along a NW-SE cross section of the Kalak Nappe Complex (KNC) exposed on Kvaløya Island (Finnmark, Norway), in order to define pre-Caledonian or Caledonian affinity of deformation fabrics. Nappes within the KNC experienced different pre-collisional tectonometamorphic histories, resulting in contrasting pre-Caledonian fabrics, which in turn controlled orogen-scale strain partitioning and metamorphic re-equilibration during Caledonian shortening. Caledonian deformation during top-to-SE-directed thrusting occurred at 550–675 °C and 0.8–1.0 GPa in the presence of fluids. Suitably-oriented pre-collisional fabrics were firstly exploited as zones of localized shearing internal to the KNC and subjected to metamorphic re-equilibration during shortening. Fold geometry during Caledonian thrusting was also controlled by the orientation of pre-Caledonian fabrics. SE-verging asymmetric folds were developed after minor tilting of pre-Caledonian upright folds with orogen-parallel hinge in the hinterland consistently with top-to-SE shearing. Shear-parallel folds displaying orogen-perpendicular hinge lines resulted from top-to-SE general shearing of pre-collisional upright folds showing pre-collisional orogen-perpendicular hinge lines. Caledonian metamorphism appears to have been accompanied by infiltration of radiogenic 40Ar-rich fluids, which affected the Ar isotopic system in synkinematic micas.

• Giuntoli, Francesco; Menegon, Luca; Warren, Clare; Darling, James R. & Anderson, Mark W. (2020). Protracted shearing at mid crustal conditions during large-scale thrusting in the Scandinavian Caledonides. Tectonics. ISSN 0278-7407. 39(9). doi: 10.1029/2020TC006267. Fulltekst i vitenarkiv Vis sammendrag

  During continental collision, large tracts of crust are mobilized along major shear zones. The metamorphic conditions at which these zones operate, the duration of thrusting, and the deformation processes that facilitated hundreds of km of tectonic transport are still unclear. In the Scandinavian Caledonides, the Lower Seve Nappe displays a main mylonitic foliation with thickness of ~1 km. This foliation is overprinted by a brittle‐to‐ductile deformation pattern localized in C‐ and C′‐type shear bands proximal to the tectonic contact with the underlying Särv Nappe. Thermobarometry of amphibolites and micaschists suggests a first high‐pressure stage at 400–500°C and 1–1.3 GPa recorded in mineral relics. The main mylonitic foliation developed under epidote amphibolite facies conditions along the retrograde path from 600°C and 1 GPa to 500°C and 0.5 GPa. Age dating of synkinematic titanite grains in the amphibolites indicates that this mylonitic fabric formed at around 417 ± 9 Ma but older ages spanning 460–430 Ma could represent earlier stages of mylonitization. The shear bands developed at lower metamorphic conditions of 300–400°C and ~0.3 GPa. In the micaschists, the recrystallized grain size of quartz decreases toward the shear bands. Monomineralic quartz layers are eventually dismembered to form polyphase aggregates deforming by dominant grain size sensitive creep accompanied by slip in muscovite and chlorite. Plagioclase zoning truncations suggest that the shear bands originated by fracturing followed by ductile deformation. The results suggest protracted and long‐lasting shearing under amphibolite to greenschist facies conditions during the juxtaposition, stacking, and exhumation of the Lower Seve Nappe.

• Graziani, Riccardo; Montomoli, Chiara; Iaccarino, Salvatore; Menegon, Luca; Nania, Laura & Carosi, Rodolfo (2020). Structural setting of a transpressive shear zone: insights from geological mapping, quartz petrofabric and kinematic vorticity analysis in NE Sardinia (Italy). Geological Magazine. ISSN 0016-7568. 157(11), s. 1898–1916. doi: 10.1017/S0016756820000138. Fulltekst i vitenarkiv Vis sammendrag

  Abstract The Posada–Asinara Line is a crustal-scale transpressive shear zone affecting the Variscan basement in northern Sardinia during Late Carboniferous time. We investigated a structural transect of the Posada–Asinara Line (Baronie) with the aid of geological mapping and structural analysis. N-verging F2 isoclinal folds with associated mylonitic foliation (S2) are the main deformation features developed during the Posada–Asinara Line activity (D2). The mineral assemblages and microstructures suggest that the Posada–Asinara Line was affected by a retrograde metamorphic path. This is also confirmed by quartz microstructures, where subgrain rotation recrystallization superimposes on grain boundary migration recrystallization. Crystallographic preferred orientation data, obtained using electron backscatter diffraction, allowed analysis of quartz slip systems and estimation of the deformation temperature, vorticity of flow and rheological parameters (flow stress and strain rate) during the Posada–Asinara Line activity. Quartz deformation temperatures of 400 ± 50 °C have been estimated along a transect perpendicular to the Posada–Asinara Line, in agreement with the syn-kinematic post-metamorphic peak mineral assemblages and the late microstructures of quartz. The D2 phase can be subdivided in two events: an early D2 early phase, related to the metamorphic peak and low kinematic vorticity (pure shear dominated), and a late D2 late phase characterized by a lower metamorphic grade and an increased kinematic vorticity (simple shear dominated). Palaeopiezometry and strain rate estimates associated with the D2 late deformation event showed an intensity gradient increasing towards the core of the shear zone. The D2 early deformation developed under peak temperature conditions, while the D2 late event was active at shallower structural levels.

• Prando, Francesca; Menegon, Luca; Anderson, Mark W; Marchesini, Barbara; Mattila, Jussi & Viola, Giulio (2020). Fluid-mediated, brittle-ductile deformation at seismogenic depth – Part 2: Stress history and fluid pressure variations in a shear zone in a nuclear waste repository (Olkiluoto Island, Finland). Solid Earth (SE). ISSN 1869-9510. 11, s. 489–511. doi: 10.5194/se-11-489-2020. Fulltekst i vitenarkiv Vis sammendrag

  Abstract. The microstructural record of fault rocks active at the brittle–ductile transition zone (BDTZ) may retain information on the rheological parameters driving the switch in deformation mode and on the role of stress and fluid pressure in controlling different fault slip behaviours. In this study we analysed the deformation microstructures of the strike-slip fault zone BFZ045 in Olkiluoto (SW Finland), located in the site of a deep geological repository for nuclear waste. We combined microstructural analysis, electron backscatter diffraction (EBSD), and mineral chemistry data to reconstruct the variations in pressure, temperature, fluid pressure, and differential stress that mediated deformation and strain localization along BFZ045 across the BDTZ. BFZ045 exhibits a mixed ductile–brittle deformation, with a narrow (&lt;20 cm thick) brittle fault core with cataclasites and pseudotachylytes that overprint a wider (60–100 cm thick) quartz-rich mylonite. Mylonitic deformation took place at 400–500 ∘C and 3–4 kbar, typical of the greenschist facies metamorphism at the base of the seismogenic crust. We used the recrystallized grain size piezometry for quartz to document a progressive increase in differential stress, from ca. 50 to ca. 120 MPa, towards the shear zone centre during mylonitization and strain localization. Syn-kinematic quartz veins formed along the mylonitic foliation due to transiently high pore fluid pressure (up to lithostatic value). The overprint of the veins by dynamic recrystallization and mylonitic creep is further evidence of the occurrence of brittle events under overall ductile conditions. We propose a conceptual model in which the ductile–brittle deformation cycle was controlled by transient oscillations in fluid pressure and progressively higher differential stress, possibly occurring in a narrowing shear zone deforming towards the peak strength of the crust at the BDTZ.

• Giuntoli, Francesco; Brovarone, Alberto Vitale & Menegon, Luca (2020). Feedback between high-pressure genesis of abiotic methane and strain localization in subducted carbonate rocks. Scientific Reports. 10(1). doi: 10.1038/s41598-020-66640-3. Fulltekst i vitenarkiv Vis sammendrag

  Fluid-rock interactions exert key control over rock rheology and strain localization. Redox may significantly affect the reaction pathways and, thereby, the mechanical properties of the rock. This effect may become critical in volatile-rich, redox sensitive rocks such as carbonate-rich lithologies, the breakdown of which can significantly modify the net volume change of fluid-mediated reactions. Subduction focus the largest recycling of crustal carbonates and the most intense seismic activity on Earth. Nevertheless, the feedbacks between deep carbon mobilization and deformation remain poorly investigated. We present quantitative microstructural results from natural samples and thermodynamic modeling indicating that percolation of reducing fluids exerts strong control on the mobilization of carbon and on strain localization in subducted carbonate rocks. Fluid-mediated carbonate reduction progressed from discrete domains unaffected by ductile deformation into localized shear zones deforming via diffusion creep, dissolution-precipitation creep and grain boundary sliding. Grain-size reduction and creep cavitation along localized shear zones enhanced fluid-carbonate interactions and fluid channelization. These results indicate that reduction of carbonate rocks can exert an important positive feedback on strain localization and fluid channelization, with potential implications on seismic activity and transport of deep hydrocarbon-bearing fluids.

• Campbell, Lucy; Menegon, Luca; Fagereng, Åke & Pennacchioni, Giorgio (2020). Earthquake nucleation in the lower crust by local stress amplification. Nature Communications. 11. doi: 10.1038/s41467-020-15150-x. Fulltekst i vitenarkiv Vis sammendrag

  Deep intracontinental earthquakes are poorly understood, despite their potential to cause significant destruction. Although lower crustal strength is currently a topic of debate, dry lower continental crust may be strong under high-grade conditions. Such strength could enable earthquake slip at high differential stress within a predominantly viscous regime, but requires further documentation in nature. Here, we analyse geological observations of seismic structures in exhumed lower crustal rocks. A granulite facies shear zone network dissects an anorthosite intrusion in Lofoten, northern Norway, and separates relatively undeformed, microcracked blocks of anorthosite. In these blocks, pristine pseudotachylytes decorate fault sets that link adjacent or intersecting shear zones. These fossil seismogenic faults are rarely >15 m in length, yet record single-event displacements of tens of centimetres, a slip/length ratio that implies >1 GPa stress drops. These pseudotachylytes represent direct identification of earthquake nucleation as a transient consequence of ongoing, localised aseismic creep.

• Marchesini, Barbara; Garofalo, Paolo Stefano; Menegon, Luca; Mattila, Jussi & Viola, Giulio (2019). Fluid-mediated, brittle–ductile deformation at seismogenic depth – Part 1: Fluid record and deformation history of fault veins in a nuclear waste repository (Olkiluoto Island, Finland). Solid Earth (SE). ISSN 1869-9510. 10(3), s. 809–838. doi: 10.5194/se-10-809-2019. Fulltekst i vitenarkiv Vis sammendrag

  The dynamic evolution of fault zones at the seismogenic brittle–ductile transition zone (BDTZ) expresses the delicate interplay between numerous physical and chemical processes. Deformation and fluid flow at the BDTZ are closely related and mutually dependent during repeating and transient cycles of frictional and viscous deformation. Despite numerous studies documenting in detail seismogenic faults exhumed from the BDTZ, uncertainties remain as to the exact role of fluids in facilitating broadly coeval brittle and ductile deformation at that structural level. We combine structural analysis, fluid inclusion, and mineral chemistry data from synkinematic and authigenic minerals to reconstruct the temporal variations in fluid pressure (Pf), temperature (T), and bulk composition (X) of the fluids that mediated deformation and steered strain localization along BFZ300, a strike–slip fault originally active at the BDTZ. BFZ300 deforms the Paleoproterozoic migmatitic basement of southwestern Finland and hosts in its core two laterally continuous quartz veins formed by two texturally distinct types of quartz – Qtz I and Qtz II, with Qtz I older than Qtz II. Veins within the damage zone are formed exclusively by Qtz I. Mesostructural and microstructural analysis combined with fluid compositional data indicate recurrent cycles of mutually overprinting brittle and ductile deformation triggered by oscillations of fluid pressure peaking at 210 MPa. Fluid inclusion microthermometry and mineral pair geothermometry indicate that the two documented quartz types precipitated from different fluid batches, with bulk salinities in the 1 wt % NaCleq–5 wt % NaCleq range for Qtz I and in the 6 wt % NaCleq–11 wt % NaCleq range for Qtz II. The temperature of the fluids involved with initial strain localization and later fault reactivation evolved through time from &gt; 350 ∘C during Qtz I precipitation to &lt; 300 ∘C at the time of Qtz II crystallization. The peak fluid pressure estimates constrain pore pressure oscillations between 80 and 210 MPa during the recorded faulting episodes. Our results suggest variability of the physico-chemical conditions of the fluids steering deformation (Pf, T, X), reflecting the ingress and effects of multiple batches of fluid in the fault zone. Initial fluid-mediated embrittlement generated a diffuse network of joints and/or hybrid–shear fractures in the damage zone; subsequent strain localization led to more localized deformation within the fault core. Localization was guided by cyclically increasing fluid pressure and transient embrittlement of a system that was otherwise under overall ductile conditions. Our analysis suggests that fluid overpressure at the BDTZ can play a key role in the initial embrittlement of the deforming rock and steer subsequent strain localization.

• Jamtveit, Bjørn; Petley-Ragan, Arianne Juliette; Incel, Sarah; Dunkel, Kristina G; Aupart, Claire Olga Maryse & Austrheim, Håkon Olaf [Vis alle 9 forfattere av denne artikkelen] (2019). The effects of earthquakes and fluids on the metamorphism of the lower continental crust. Journal of Geophysical Research (JGR): Solid Earth. ISSN 2169-9313. 124(8), s. 7725–7755. doi: 10.1029/2018JB016461. Fulltekst i vitenarkiv Vis sammendrag

  Rock rheology and density have first‐order effects on the lithosphere's response to plate tectonic forces at plate boundaries. Changes in these rock properties are controlled by metamorphic transformation processes that are critically dependent on the presence of fluids. At the onset of a continental collision, the lower crust is in most cases dry and strong. However, if exposed to internally produced or externally supplied fluids, the thickened crust will react and be converted into a mechanically weaker lithology by fluid‐driven metamorphic reactions. Fluid introduction is often associated with deep crustal earthquakes. Microstructural evidence, suggest that in strong highly stressed rocks, seismic slip may be initiated by brittle deformation and that wall‐rock damage caused by dynamic ruptures plays a very important role in allowing fluids to enter into contact with dry and highly reactive lower crustal rocks. The resulting metamorphism produces weaker rocks which subsequently deform by viscous creep. Volumes of weak rocks contained in a highly stressed environment of strong rocks may experience significant excursions toward higher pressure without any associated burial. Slow and highly localized creep processes in a velocity strengthening regime may produce mylonitic shear zones along faults initially characterized by earthquake‐generated frictional melting and wall rock damage. However, stress pulses from earthquakes in the shallower brittle regime may kick start new episodes of seismic slip at velocity weakening conditions. These processes indicate that the evolution of the lower crust during continental collisions is controlled by the transient interplay between brittle deformation, fluid‐rock interactions, and creep flow.

• Campbell, Lucy & Menegon, Luca (2019). Transient High Strain Rate During Localized Viscous Creep in the Dry Lower Continental Crust (Lofoten, Norway). Journal of Geophysical Research (JGR): Solid Earth. ISSN 2169-9313. 124(10), s. 10240–10260. doi: 10.1029/2019JB018052. Fulltekst i vitenarkiv Vis sammendrag

  Understanding the ability of the lower crust to support transient changes in stresses and strain rates during the earthquake cycle requires a detailed investigation of the deformation mechanisms and rheology of deep crustal fault rocks. Here, we show that lower crustal pseudotachylyte‐bearing shear zones are able to accommodate short‐term episodes of high strain rate and high stress deformation by accelerated viscous creep, followed by a reduction in stresses to some ambient deformation condition. Quartz microstructure within pseudotachylyte‐bearing shear zones in otherwise undeformed granulites from Lofoten, Norway, indicates that dynamic recrystallization occurred during viscous creep under rapid strain rates and high stresses of ~10−9 s−1 and ~100 MPa, respectively. Lower stress microstructures (i.e., foam textures) are also recorded in the shear zones, indicating spatial and temporal variations of stress and strain rate during deformation cycles. Both the high and lower stress quartz recrystallization took place under granulite facies conditions of 650°C–750°C and 0.7–0.8 GPa and represented a record of highly localized viscous creep within the lower crust. This implies that lower crustal pseudotachylytes are potentially able to form extremely localized weak zones within strong lower crust, enabling a deep mechanical response to perturbations in stress and strain rate such as those experienced during the seismic cycle, for example, seismogenic loading followed by subsequent postseismic relaxation.

• Ceccato, Alberto; Menegon, Luca; Pennacchioni, Giorgio & Morales, Luiz F.G. (2018). Myrmekite and strain weakening in granitoid mylonites. Solid Earth (SE). ISSN 1869-9510. 9, s. 1399–1419. doi: 10.5194/se-9-1399-2018. Fulltekst i vitenarkiv
• Zibra, Ivan; White, Joseph clancy; Menegon, Luca; Dering, G & Gessner, Klaus (2018). The ultimate fate of a synmagmatic shear zone. Interplay between rupturing and ductile flow in a cooling granite pluton. Journal of Structural Geology. ISSN 0191-8141. 110, s. 1–23. doi: 10.1016/j.jsg.2018.02.001. Fulltekst i vitenarkiv
• Giuntoli, Francesco; Menegon, Luca & Warren, Clare (2018). Replacement reactions and deformation by dissolution and precipitation processes in amphibolites. Journal of Metamorphic Geology. ISSN 0263-4929. 36(9), s. 1263–1286. doi: 10.1111/jmg.12445. Fulltekst i vitenarkiv
• Menegon, Luca; Pennacchioni, Giorgio; Malaspina, Nadia; Harris, Kathrin & Wood, Elliot (2017). Earthquakes as Precursors of Ductile Shear Zones in the Dry and Strong Lower Crust. Geochemistry Geophysics Geosystems. 18(12), s. 4356–4374. doi: 10.1002/2017GC007189. Fulltekst i vitenarkiv
• Gilgannon, James; Fusseis, Florian; Menegon, Luca; Regenauer-Lieb, Klaus & Buckman, J (2017). Hierarchical creep cavity formation in an ultramylonite and implications for phase mixing. Solid Earth (SE). ISSN 1869-9510. 8, s. 1193–1209. doi: 10.5194/se-8-1193-2017. Fulltekst i vitenarkiv
• Macente, Alice; Fusseis, Florian; Menegon, Luca; Xiao, Xianghui & John, Timm (2017). The strain-dependent spatial evolution of garnet in a high-pressure ductile shear zone from the Western Gneiss Region (Norway): a synchrotron X-ray microtomography study. Journal of Metamorphic Geology. ISSN 0263-4929. 35(5), s. 565–583. doi: 10.1111/jmg.12245. Fulltekst i vitenarkiv
• Alessandrini, Giulia Degli; Menegon, Luca; Malaspina, Nadia; Dijkstra, Arjan & Anderson, Mark W (2017). Creep of mafic dykes infiltrated by melt in the lower continental crust (Seiland Igneous Province, Norway). Lithos. ISSN 0024-4937. 274-275, s. 169–187. doi: 10.1016/j.lithos.2016.12.030. Fulltekst i vitenarkiv
• Molli, Giancarlo; Menegon, Luca & Malasoma, Alessandro (2017). Switching deformation mode and mechanisms during subduction of continental crust: a case study from Alpine Corsica. Solid Earth (SE). ISSN 1869-9510. 8, s. 767–788. doi: 10.5194/se-8-767-2017. Fulltekst i vitenarkiv
• Ceccato, Alberto; Pennacchioni, Giorgio; Menegon, Luca & Bestmann, Michel (2017). Crystallographic control and texture inheritance during mylonitization of coarse grained quartz veins. Lithos. ISSN 0024-4937. 290-291, s. 210–227. doi: 10.1016/j.lithos.2017.08.005. Fulltekst i vitenarkiv
• Viegas, Gustavo; Menegon, Luca & Archanjo, Carlos J (2016). Brittle grain-size reduction of feldspar, phase mixing and strain localization in granitoids at mid-crustal conditions (Pernambuco shear zone, NE Brazil). Solid Earth (SE). ISSN 1869-9510. 7, s. 375–396. doi: 10.5194/se-7-375-2016. Fulltekst i vitenarkiv
• Kjøll, Hans Jørgen; Viola, Giulio; Menegon, Luca & Sørensen, Bjørn Eske (2015). Brittle-viscous deformation of vein quartz under fluid-rich lower greenschist facies conditions. Solid Earth (SE). ISSN 1869-9510. 6(2), s. 681–699. doi: 10.5194/se-6-681-2015. Fulltekst i vitenarkiv Vis sammendrag

  We studied by Electron BackScatter Diffraction (EBSD) and optical microscopy a coarse-grained (ca. 0.5–6 mm) quartz vein embedded in a phyllonitic matrix to gain insights into the recrystallization mechanisms and the processes of strain localization in quartz deformed under lower greenschist facies conditions, broadly coincident with the brittle–viscous transition. The vein deformed during faulting along a phyllonitic thrust of Caledonian age within the Porsa Imbricate Stack in the Paleoproterozoic Repparfjord Tectonic Window in northern Norway. The phyllonite hosting the vein formed at the expense of a metabasaltic protolith through feldspar breakdown to form interconnected layers of fine, synkinematic phyllosilicates. In the mechanically weak framework of the phyllonite, the quartz vein acted as a relatively rigid body. Viscous deformation in the vein was initially accommodated by quartz basal slip. Under the prevailing deformation conditions, however, dislocation glide- and possibly creep-accommodated deformation of quartz was inefficient, and this resulted in localized strain hardening. In response to the (1) hardening, (2) progressive and cyclic increase of the fluid pressure, and (3) increasing competence contrast between the vein and the weakly foliated host phyllonite, vein quartz crystals began to deform by brittle processes along specific, suitably oriented lattice planes, creating microgouges along microfractures. Nucleated new grains rapidly sealed these fractures as fluids penetrated the actively deforming system. The grains grew initially by solution precipitation and later by grain boundary migration. We suggest that the different initial orientation of the vein crystals led to strain accommodation by different mechanisms in the individual crystals, generating remarkably different microstructures. Crystals suitably oriented for basal slip, for example, accommodated strain mainly viscously and experienced only minor fracturing. Instead, crystals misoriented for basal slip hardened and deformed predominantly by domainal fracturing. This study indicates the importance of considering shear zones as dynamic systems wherein the activated deformation mechanisms may vary through time in response to the complex temporal and spatial evolution of the shear zone, often in a cyclic fashion.

• Gasser, Deta; Jerabek, Petr; Faber, Carly; Stunitz, Holger; Menegon, Luca & Corfu, Fernando [Vis alle 8 forfattere av denne artikkelen] (2015). Behaviour of geochronometers and timing of metamorphic reactions during deformation at lower crustal conditions: phase equilibrium modelling and U–Pb dating of zircon, monazite, rutile and titanite from the Kalak Nappe Complex, northern Norway. Journal of Metamorphic Geology. ISSN 0263-4929. doi: 10.1111/jmg.12131. Fulltekst i vitenarkiv
• Gasser, Deta; Jeřábek, Petr; Faber, Carly; Stunitz, Holger; Menegon, Luca & Corfu, Fernando [Vis alle 8 forfattere av denne artikkelen] (2015). Behaviour of geochronometers and timing of metamorphic reactions during deformation at lower crustal conditions: Phase equilibrium modelling and U-Pb dating of zircon, monazite, rutile and titanite from the Kalak Nappe Complex, northern Norway. Journal of Metamorphic Geology. ISSN 0263-4929. 33(5), s. 513–534. doi: 10.1111/jmg.12131. Fulltekst i vitenarkiv
• Menegon, Luca; Fusseis, Florian; Stunitz, Holger & Xiao, Xianghui (2015). Creep cavitation bands control porosity and fluid flow in lower crustal shear zones. Geology. ISSN 0091-7613. 43(3), s. 27–30. doi: 10.1130/G36307.1. Fulltekst i vitenarkiv
• Negrini, Marianne; Stunitz, Holger; Nasipuri, Pritam; Menegon, Luca & Morales, Luiz F.G. (2014). Semibrittle deformation and partial melting of perthitic K-feldspar: An experimental study. Journal of Geophysical Research (JGR): Biogeosciences. ISSN 2169-8953. 119(4), s. 3478–3502. doi: 10.1002/2013JB010573. Fulltekst i vitenarkiv
• Menegon, Luca; Stunitz, Holger; Nasipuri, Pritam; Heilbronner, Renée & Svahnberg, Henrik (2013). Transition from fracturing to viscous flow in granulite facies perthitic feldspar (Lofoten, Norway). Journal of Structural Geology. ISSN 0191-8141. 48, s. 95–112. doi: 10.1016/j.jsg.2012.12.004. Fulltekst i vitenarkiv
• Rosenbaum, Gideon; Menegon, Luca; Glodny, Johannes; Vasconcelos, Paulo; Ring, Uwe & Massironi, Matteo [Vis alle 8 forfattere av denne artikkelen] (2012). Dating deformation in the Gran Paradiso Massif (NW Italian Alps): Implications for the exhumation of high-pressure rocks in a collisional belt. Lithos. ISSN 0024-4937. 144-145, s. 130–144. doi: 10.1016/j.lithos.2012.04.016. Fulltekst i vitenarkiv
• Bistacchi, Andrea; Massironi, Matteo; Menegon, Luca; Bolognesi, Francesca & Donghi, Valeriano (2012). On the nucleation of non-Andersonian faults along phyllosilicate-rich mylonite belts. Geological Society Special Publication. ISSN 0305-8719. 367, s. 185–199. doi: 10.1144/SP367.13. Fulltekst i vitenarkiv
• Menegon, Luca; Piazolo, Sandra & Pennacchioni, Giorgio (2011). The effect of Dauphiné twinning on plastic strain in quartz. Contributions to Mineralogy and Petrology. ISSN 0010-7999. 161(4), s. 635–652. doi: 10.1007/s00410-010-0554-7. Fulltekst i vitenarkiv Vis sammendrag

  We present an electron backscatter diffraction analysis of five quartz porphyroclasts in a greenschist facies (T = 300–400°C) granitoid protomylonite from the Arolla unit of the NW Alps. Mechanical Dauphiné twinning developed pervasively during the incipient stage of deformation within two porphyroclasts oriented with a negative rhomb plane {z} almost orthogonal to the compression direction (z-twin orientation). Twinning was driven by the anisotropy in the elastic compliance of quartz and resulted in the alignment of the poles of the planes of the more compliant positive rhomb {r} nearly parallel to the compression direction (r-twin orientation). In contrast, we report the lack of twinning in two porphyroclasts already oriented with one of the {r} planes orthogonal to the compression direction. One twinned porphyroclast has been investigated with more detail. It shows the localization of much of the plastic strain into discrete r-twins as a consequence of the higher amount of elastic strain energy stored by r-twins in comparison to z-twins. The presence of Dauphiné twins induced a switch in the dominant active slip systems during plastic deformation, from basal <a> (regions without twinning) to {π} and {π′} <a> (pervasively twinned regions). Dynamic recrystallization is localized along an r-twin and occurred dominantly by progressive subgrain rotation, with a local component of bulging recrystallization. Part of the recrystallized grains underwent rigid-body rotation, approximately about the bulk vorticity axis, which accounts for the development of large misorientation angles. The recrystallized grain size piezometer for quartz yields differential stress of 100 MPa. The comparison of this palaeostress estimate with literature data suggests that mechanical Dauphiné twinning could have a potential use as palaeopiezometer in quartz-bearing rocks.

• Menegon, Luca; Nasipuri, Pritam; Stunitz, Holger; Behrens, Harald & Ravna, Erling J Krogh (2011). Dry and strong quartz during deformation of the lower crust in the presence of melt. Journal of Geophysical Research (JGR): Solid Earth. ISSN 2169-9313. 116. doi: 10.1029/2011JB008371. Fulltekst i vitenarkiv Vis sammendrag

  Granulite facies migmatitic gneisses from the Seiland Igneous Province (northern Norway) were deformed during deep crustal shearing in the presence of melt, which formed by dehydration melting of biotite. Partial melting and deformation occurred during the intrusion of large gabbroic plutons at the base of the lower crust at 570 to 520 Ma in an intracontinental rift setting. The migmatitic gneisses consist of high-aspect-ratio leucosome-rich domains and a leucosome-poor, restitic domain of quartzitic composition. According to thermodynamic modeling using synkinematic mineral assemblages, deformation occurred at T = 760°C–820°C, P = 0.75–0.95 GPa and in the presence of ≤5 vol % of residual melt. There is direct evidence from microstructural observations, Fourier transform infrared measurements, thermodynamic modeling, and titanium-in-quartz thermometry that dry quartz in the leucosome-poor domain deformed at high differential stress (50–100 MPa) by dislocation creep. High stresses are demonstrated by the small grain size (11–17 μm) of quartz in localized layers of recrystallized grains, where titanium-in-quartz thermometry yields 770°C–815°C. Dry and strong quartz forms a load-bearing framework in the migmatitic gneisses, where ∼5% melt is present, but does not control the mechanical behavior because it is located in isolated pockets. The high stress deformation of quartz overprints an earlier, lower stress deformation, which is preserved particularly in the vicinity of segregated melt pockets. The grain-scale melt distribution, water content and distribution, and the overprinting relationships of quartz microstructures indicate that biotite dehydration melting occurred during deformation by dislocation creep in quartz. The water partitioned into the segregated melt crystallizing in isolated pockets, in the vicinity of which quartz shows a higher intracrystalline water content and a large grain size. On the contrary, the leucosome-poor domain of the rock, from which melt was removed, became dry and thereby mechanically stronger. Melt removal at larger scale will result in a lower crust which is dry enough to be mechanically strong. The application of flow laws derived for wet quartz is not appropriate to estimate the behavior of such granulite facies parts of the lower crust.

• Massironi, Matteo; Bistacchi, Andrea & Menegon, Luca (2011). Misoriented faults in exhumed metamorphic complexes: Rule or exception? Earth and Planetary Science Letters. ISSN 0012-821X. 307(1-2), s. 233–239. doi: 10.1016/j.epsl.2011.04.041. Fulltekst i vitenarkiv
• Menegon, Luca & Pennacchioni, Giorgio (2010). Local shear zone pattern and bulk deformation in the Gran Paradiso metagranite (NW Italian Alps). International journal of earth sciences. ISSN 1437-3254. 99(8), s. 1805–1825. doi: 10.1007/s00531-009-0485-6. Fulltekst i vitenarkiv
• Pennacchioni, Giorgio; Menegon, Luca; Leiss, Bernd; Nestola, F. & Bromiley, G. (2010). Development of crystallographic preferred orientation and microstructure during plastic deformation of natural coarse-grained quartz veins. Journal of Geophysical Research (JGR): Solid Earth. ISSN 2169-9313. 115. doi: 10.1029/2010JB007674. Fulltekst i vitenarkiv
• Menegon, Luca; Piazolo, S. & Pennacchioni, G. (2010). The effect of Dauphiné twinning on plastic strain in quartz. Contributions to Mineralogy and Petrology. ISSN 0010-7999. doi: 10.1007/s00410-010-0554-7. Fulltekst i vitenarkiv
• Bistacchi, A.; Massironi, M. & Menegon, Luca (2010). Three-dimensional characterization of a crustal-scale fault zone: The Pusteria and Sprechenstein fault system (Eastern Alps). Journal of Structural Geology. ISSN 0191-8141. 32(12), s. 2022–2041. doi: 10.1016/j.jsg.2010.06.003. Fulltekst i vitenarkiv
• Menegon, Luca & Pennacchioni, Giorgio (2009). Local shear zone pattern and bulk deformation in the Gran Paradiso metagranite (NW Italian Alps). International journal of earth sciences. ISSN 1437-3254. doi: 10.1007/s00531-009-0485-6. Fulltekst i vitenarkiv
• Menegon, Luca; Pennacchioni, G.; Heilbronner, R & Pittarello, R. (2008). Evolution of quartz microstructure and c-axis crystallographic preferred orientation within ductilely deformed granitoids (Arolla Unit, Western Alps). Journal of Structural Geology. ISSN 0191-8141. 30, s. 1332–1347. Fulltekst i vitenarkiv
• Menegon, Luca; Pennacchioni, G. & Spiess, R. (2008). Dissolution-precipitation creep of K-feldspar in mid-crustal granite mylonites. Journal of Structural Geology. ISSN 0191-8141. 30, s. 565–579. Fulltekst i vitenarkiv
• Menegon, Luca; Pennacchioni, Giorgio & Stunitz, Holger (2006). Nucleation and growth of myrmekite during ductile shear deformation in metagranites. Journal of Metamorphic Geology. ISSN 0263-4929. 24(7), s. 553–568. doi: 10.1111/j.1525-1314.2006.00654.x. Fulltekst i vitenarkiv
• Pennacchioni, G.; Toro, G. Di; Brack, P.; Menegon, Luca & Villa, I. M. (2006). Brittle-ductile-brittle deformation during cooling of tonalite (Adamello, Southern Italian Alps). Tectonophysics. ISSN 0040-1951. 427, s. 171–197. Fulltekst i vitenarkiv

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• Menegon, Luca (2026). Earthquake-induced Damage in the Lower Crust.
• Menegon, Luca; Zertani, Sascha & Lantman, Hugo Willem van Schrojenstein (2026). Transient high stresses during the earthquake cycle in the lower crust and their preservation in the geological record. Fulltekst i vitenarkiv
• Clivet, Flavio; Piazolo, Sandra; Michalchuk, Stephen Paul; Zertani, Sascha & Menegon, Luca (2025). Shear zone growth by repeated generation of pseudotachylytes in the lower crust. doi: 10.5194/egusphere-egu25-10795. Fulltekst i vitenarkiv Vis sammendrag

  Understanding the deformation modes of the lower crust is crucial if we are to predict the rheological behaviour of the crust in space and time.&#160; The Nusfjord locality (Lofoten, Norway) represents a natural laboratory to study the interplay between seismic and aseismic deformation in the Earth&#8217;s lower crust. The area exposes pseudotachylytes (quenched frictional melt produced during coseismic slip) within a network of ductile shear zones bounding strong low-strain domains of granulitic anorthosites. Pseudotachylytes formed within the low-strain domains, during ongoing viscous creep in the ductile shear zones, at a depth of 25-35 km. The ductile shear zones themselves contain several generations of mylonitized pseudotachylytes suggesting repeated switches from frictional to viscous deformation within shear zones. The underlying reasons and rheological consequence of mutual overprinting relationships between ductile shear zones (generally considered to be weak) and several generations of pseudotachylytes remains enigmatic.Field investigations, photogrammetry, structural logs, and microstructural analysis reveal that (1) pseudotachylytes invariably nucleate within the low strain domains of the anorthosite host rock located between subparallel shear zones, and not along the shear zones themselves; and (2) that the rupture migrates along the material interface provided either by the shear zone/host rock boundary or by the shear zone foliation. The observed relationships suggest transient stress pulses that are supported by variations in the recrystallized grain size of quartz along individual shear zones.We propose that repeated episodes of pseudotachylyte generation and associated host-rock fracturing &#160;represent a mechanism of shear zone growth and thickening, because the pseudotachylyte veins are mylonitized and become part of the actively deforming shear zones, which in turn control the further development of pseudotachylytes in the adjacent rigid blocks (low-strain domains). Structural logs show that shear zone width depends on the initial spacing between subparallel shear zones: when shear zones are widely spaced (>1 m), the rigid block in between is essentially undeformed, it contains a low density of pseudotachylytes and the shear zones themselves are thin (

• Rosenqvist, Marija Plahter; Dunkel, Kristina; Planke, Sverre; Nogueira, Liebert Parreiras; Polteau, Stephane & Menegon, Luca (2025). Porosity structures and local compositional variations associated with natural CO2 sequestration in basalts offshore Norway. doi: 10.5194/egusphere-egu25-11073. Fulltekst i vitenarkiv Vis sammendrag

  Permanent storage of CO2 as carbonates in basalt deposits utilises natural weathering reactions between silicate minerals and carbonic water. By studying how CO2-enriched water has altered basalt in 43 samples from the V&#248;ring Margin, offshore Norway, we demonstrate the pathways and reactions of CO2 within a basalt reservoir.The pore spaces occupied by carbonates in the samples were studied through &#956;-CT (computer tomography) studies on three minicores and detailed microanalyses on 32 thin sections via scanning electron microscopy (SEM), electron microprobe analysis, and electron backscatter diffraction (EBSD). Single crystal X-ray diffraction on 15 samples determined the type of carbonate minerals and precipitation ages were determined for two samples through U/Pb-radiomeric dating. &#948;18O and &#948;13C-isotope analyses were used to determine the fluid origin in 11 samples.Three types of carbonate precipitation were characterised based on the pore structures they fill and the associated mineralogy. Type I carbonates, seen in 18 thin sections, fills vesicles, mainly along lava flow margins. SEM analyses showed that all vesicles are initially coated with a smectite layer which incorporates most of the Mg2+ and Fe2+ from dissolving silicate minerals. As pH and Ca concentrations increase, calcite precipitates in some vesicles. The secondary mineral assemblage and precipitation order fit with low-temperature alteration of basalt with pore water and atmospherically balanced CO2 levels (pH&#8805;8). Type II carbonates appear in seven thin sections as

• Zertani, Sascha; Morales, Luiz F. G. & Menegon, Luca (2024). Correction to: Omphacite breakdown: nucleation and deformation of clinopyroxene-plagioclase symplectites (Contributions to Mineralogy and Petrology, (2024), 179, 5, (44), 10.1007/s00410-024-02125-0). Contributions to Mineralogy and Petrology. ISSN 0010-7999. 179(8). doi: 10.1007/s00410-024-02140-1. Fulltekst i vitenarkiv
• Angiboust, Samuel; Cambeses, Aitor; Fagereng, Ake; Menegon, Luca; Klein, Frieder & Raimondo, Tom [Vis alle 7 forfattere av denne artikkelen] (2023). Fluids in the Earth's lithosphere: From petrology to geodynamics. Lithos. ISSN 0024-4937. 452-453. doi: 10.1016/j.lithos.2023.107226. Fulltekst i vitenarkiv
• Menegon, Luca & Stewart, Iain (2019). A safer future with clues from earthquakes past. Science Impact. 2019(9), s. 6–8. doi: 10.21820/23987073.2019.9.6. Fulltekst i vitenarkiv
• Gasser, Deta; Stunitz, Holger; Nasipuri, Pritam & Menegon, Luca (2013). Constraints on strain rates during large-scale mid-crustal shearing: An example from the basal Vaddas shear zone, Northern Caledonides. Fulltekst i vitenarkiv
• Gasser, Deta; Nasipuri, Pritam; Menegon, Luca; Stunitz, Holger & Corfu, Fernando (2013). Neoproterozoic partial melting and ca. 10 m.y. of Caledonian shearing in the south-western Kalak nappe complex, Northern Norway. Fulltekst i vitenarkiv
• Menegon, Luca; Stunitz, Holger; Nasipuri, Pritam; Svahnberg, Henrik & Heilbronner, Renee Stunitz (2011). Grain size reduction of feldspar and pyroxene, phase mixing, and strain localization in lower crustal shear zones (Lofoten, Norway). Abstracts AGU Fall meeting 2011. T43C-2347. Fulltekst i vitenarkiv
• Negrini, Marianne; Stunitz, Holger; Menegon, Luca & Nasipuri, Pritam (2011). Experimental Deformation, Partial Melting, and Compositional Changes in Perthitic K-feldspar at High Pressure and Temperature. Abstracts AGU Fall meeting 2011. T43C-2349. Fulltekst i vitenarkiv
• Stunitz, Holger; Menegon, Luca; Nasipuri, Pritam & Jerabek, Petr (2011). Localization of deformation and rheological evolution of the lower crust controlled by dehydration and hydration: examples from mm-scale shear zones to nappe thrusts in Norway. Abstracts AGU Fall meeting 2011. T53B-06. Fulltekst i vitenarkiv
• Nasipuri, Pritam; Stunitz, Holger; Menegon, Luca; Ravna, Erling J Krogh; Kullerud, Kåre & Berger, Alfons (2011). Metamorphic reactions, deformation localization and rheological evolution of gabbro to eclogite transformation: A case study from Lofoten Anorthosite Complex, Norway. Abstracts AGU Fall meeting 2011. T43C-2347. Fulltekst i vitenarkiv
• Menegon, Luca; Stunitz, Holger; Nasipuri, Pritam; Svahnberg, Henrik & Heilbronner, Renee Stunitz (2011). Grain size reduction of feldspar and pyroxene, phase mixing, and strain localization in lower crustal shear zones (Lofoten, Northern Norway). Fulltekst i vitenarkiv
• Stunitz, Holger; Menegon, Luca; Nasipuri, Pritam; Jerabek, Petr; Fusseis, Florian & Ravna, Erling J Krogh [Vis alle 8 forfattere av denne artikkelen] (2011). Strength evolution of the lower crust – examples from Northern Norway. Fulltekst i vitenarkiv
• Negrini, Marianne; Stunitz, Holger; Menegon, Luca & Nasipuri, Pritam (2011). Experimental deformation, partial melting, and compositional changes in perthitic K-feldspar at high pressure and temperature. Fulltekst i vitenarkiv
• Menegon, Luca; Stunitz, Holger; Nasipuri, Pritam; Nardini, Livia & Heilbronner, Renee Stunitz (2011). Strain localization in lower crustal shear zones due to granular flow of syndeformational reaction products. Fulltekst i vitenarkiv
• Negrini, Marianne; Stunitz, Holger; Menegon, Luca & Nasipuri, Pritam (2011). Crustal Processes:Experimental Studies. Fulltekst i vitenarkiv
• Nasipuri, Pritam; Stunitz, Holger; Ravna, Erling J Krogh; Menegon, Luca & Kullerud, Kåre (2011). Interaction between metamorphism and deformation in eclogite facies shear zones, Lofoten, Norway. Fulltekst i vitenarkiv
• Stunitz, Holger; Menegon, Luca; Nasipuri, Pritam; Behrens, Harald; Jerabek, Petr & Fusseis, Florian [Vis alle 9 forfattere av denne artikkelen] (2011). Evidence for a mechanically strong lower cust – examples from Northern Norway. Fulltekst i vitenarkiv
• Nasipuri, Pritam; Stunitz, Holger; Ravna, Erling J Krogh & Menegon, Luca (2011). Orthopyroxene-amphibole bi-phase corona from the Arnøya metagabbro, Norway: an example of open system disequilibrium microstructure. Fulltekst i vitenarkiv
• Negrini, Marianne; Stunitz, Holger & Menegon, Luca (2011). Experimental deformation, partial melting, and compositional changes in perthitic K-feldspar at high pressure and temperature. Fulltekst i vitenarkiv
• Stunitz, Holger; Menegon, Luca; Nasipuri, Pritam; Behrens, H.; Jerabek, P. & Fusseis, F. [Vis alle 9 forfattere av denne artikkelen] (2011). Evidence for a mechanically strong lower crust – examples from Northern Norway. Abstracts and Proceedings of the Geological Society of Norway. 1, s. 96–96. Fulltekst i vitenarkiv
• Menegon, Luca; Pennacchioni, Giorgio & Stunitz, Holger (2010). Fluid-assisted deformation processes of K-feldspar at mid-crustal conditions. Fulltekst i vitenarkiv
• Negrini, Marianne; Stunitz, Holger & Menegon, Luca (2010). Deformation processes in experimentally deformed perthitic K-fsp single crystals. Fulltekst i vitenarkiv
• Pennacchioni, Giorgio; Menegon, Luca; Leiss, Bernd & Heilbronner, Renee Stunitz (2009). Evolution of the crystallographic preferred orientation and microstructure in natural quartz veins during progressive simple shear at T>500°C. Fulltekst i vitenarkiv
• Menegon, Luca; Piazolo, Sandra & Pennacchioni, Giorgio (2009). The influence of Dauphiné twinning on activation of slip systems and incipient dynamic recrystallization in quartz. Fulltekst i vitenarkiv

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