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• 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. p. 105806–105806. doi: 10.1016/j.jsg.2026.105806. Full text in Research Archive
• Michalchuk, Stephen Paul; Lueder, Mona; Gies, Nils; Ohl, Markus; Hermann, Jörg & Plümper, Oliver [Show all 7 contributors for this article] (2025). Mechanisms of Aqueous Fluid Infiltration and Redistribution in a Lower-Crustal Pseudotachylyte-Bearing Fault. Geochemistry Geophysics Geosystems. 26(2). doi: 10.1029/2024GC011968. Full text in Research Archive Show summary

  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.

• Michalchuk, Stephen Paul; Zertani, Sascha; Renard, Francois; Fusseis, Florian; Chogani, Alireza & Plümper, Oliver [Show all 7 contributors for this article] (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. Full text in Research Archive Show summary

  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.

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• 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. Full text in Research Archive Show summary

  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 (

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