shzertan

• 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
• Dunkel, Kristina G.; Zertani, Sascha; Roddatis, Vladimir & Jamtveit, Bjørn (2026). Zoned feldspar records hydration-dehydration cycles in lower-crustal shear zones. Contributions to Mineralogy and Petrology. ISSN 0010-7999. 181(5). doi: 10.1007/s00410-026-02317-w. Fulltekst i vitenarkiv Vis sammendrag

  Metamorphic reactions provide crucial insights into the conditions that rocks experienced and significantly influence their rheological behaviour. However, some reactions are challenging to identify in the rock record. This study focuses on zonation and microstructures in plagioclase as indicators of metamorphic history, employing a case study from Lofoten, northern Norway, where various types of plagioclase microstructures are preserved together. We use a combination of electron microscopy techniques (electron backscatter diffraction, transmission electron microscopy, and electron microprobe analyses) complemented with local equilibrium thermodynamic modelling to examine the relationships between composition, microstructures, and underlying metamorphic processes, and put that in context with previous observations of metamorphically zoned plagioclase, often referred to as “complex feldspar”. Needle-shaped Ca-richer zones in plagioclase represent pseudomorphs after zoisite/clinozoisite, indicative of plagioclase breakdown following fluid infiltration at high pressure and subsequent reversal of the reaction. Polygonal plagioclase with complex zonations also originates from the hydration/dehydration cycle but has additionally undergone deformation during hydration. Minor changes in pressure and temperature at upper amphibolite-facies conditions can significantly affect the stability and microstructures of plagioclase and thereby the rheological behaviour of the lower crust.

• 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.

• 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.

• 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.

• 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 >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.

• Zertani, Sascha; John, Timm; Brachmann, Caroline; Vrijmoed, Johannes Christiaan & Plümper, Oliver (2022). Reactive fluid flow guided by grain-scale equilibrium reactions during eclogitization of dry crustal rocks. Contributions to Mineralogy and Petrology. ISSN 0010-7999. 177. doi: 10.1007/s00410-022-01928-3. Fulltekst i vitenarkiv Vis sammendrag

  Abstract Fluid flow in crystalline rocks in the absence of fractures or ductile shear zones dominantly occurs by grain boundary diffusion, as it is faster than volume diffusion. It is, however, unclear how reactive fluid flow is guided through such pathways. We present a microstructural, mineral chemical, and thermodynamic analysis of a static fluid-driven reaction from dry granulite to ‘wet’ eclogite. Fluid infiltration resulted in re-equilibration at eclogite-facies conditions, indicating that the granulitic protolith was out of equilibrium, but unable to adjust to changing P – T conditions. The transformation occurred in three steps: (1) initial hydration along plagioclase grain boundaries, (2) complete breakdown of plagioclase and hydration along phase boundaries between plagioclase and garnet/clinopyroxene, and (3) re-equilibration of the rock to an eclogite-facies mineral assemblage. Thermodynamic modelling of local compositions reveals that this reaction sequence is proportional to the local decrease of the Gibbs free energy calculated for ‘dry’ and ‘wet’ cases. These energy differences result in increased net reaction rates and the reactions that result in the largest decrease of the Gibbs free energy occur first. In addition, these reactions result in a local volume decrease leading to porosity formation; i.e., pathways for new fluid to enter the reaction site thus controlling net fluid flow. Element transport to and from the reaction sites only occurs if it is energetically beneficial, and enough transport agent is available. Reactive fluid flow during static re-equilibration of nominally impermeable rocks is thus guided by differences in the energy budget of the local equilibrium domains.

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• 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.  The Nusfjord locality (Lofoten, Norway) represents a natural laboratory to study the interplay between seismic and aseismic deformation in the Earth’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  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 (

• 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

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