The island of Holsnøy in the Bergen Arcs, which belong to the Caledonides of western Norway, represents an excellent example of how fluid-induced eclogitization modifies material deeply buried by subduction and continental collision. We produced a new detailed map of the northwestern part of Holsnøy, differentiating not only the magnitude of eclogitization but also the strain intensity at different spatial scales: from the outcrop to the entire massif. Using structural data from eclogite-facies shear zones and eclogitized low-strain domains, we show that fluid-mediated eclogitization not only progresses via the development of shear zones (dynamic eclogitization) but occurs over large areas of the island without associated deformation, creating a characteristic static eclogite-facies overprint. Static eclogitization preserves the structural features of the granulitic protolith while the rock body is transformed from a granulite-to an eclogite-facies mineral assemblage. The extent of static eclogitization was underestimated strongly in the past, a finding also relevant for the interpretation of seismological images in currently active orogens, where presumably similar processes are currently occurring. In addition, we find that the general structure of the eclogite-facies shear zones is scale-independent over several orders of magnitude. Although crustal-scale eclogite-facies complexes are rarely preserved without significant modification during exhumation, this implies that similar geometrical configurations are likely produced at the scale of the whole lower crust during subduction or continental collision and therefore shape the crustal geophysical signature.

Seismic imaging of subduction and continental collision zones suggests that slabs often comprise strong and dry lower crustal rocks, which progressively transform into denser eclogites (

Subduction zone processes and the resulting geometries at depth are widely studied by large‐scale geophysical imaging techniques. The subsequent interpretations are dependent on information from surface exposures of fossil subduction and collision zones, which help to discern probable lithologies and their structural relationships at depth. For this purpose, we collected samples from Holsnøy in the Bergen Arcs of western Norway, which constitutes a well‐preserved slice of continental crust, deeply buried and partially eclogitized during Caledonian collision. We derived seismic properties of both the lower crustal granulite‐facies protolith and the eclogite‐facies shear zones by performing laboratory measurements on cube‐shaped samples. P and S wave velocities were measured in three perpendicular directions, along the principal fabric directions of the rock. Resulting velocities agree with seismic velocities calculated using thermodynamic modeling and confirm that eclogitization causes a significant increase of the seismic velocity. Further, eclogitization results in decreased VP/VS ratios and, when associated with deformation, an increase of the seismic anisotropy due to the crystallographic preferred orientation of omphacite that were obtained from neutron diffraction measurements. The structural framework of this exposed complex combined with the characteristic variations of seismic properties from the lower crustal protolith to the high‐pressure assemblage provides the possibility to detect comparable structures at depth in currently active settings using seismological methods such as the receiver function method.

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.

Seismological studies of large-scale processes at convergent plate boundaries typically probe lower crustal structures with wavelengths of several kilometers, whereas field-based studies typically sample the resulting structures at a much smaller scale. To bridge this gap between scales, we derive effective petrophysical properties on the 20-m, 100-m, and kilometer scales based on numerical modeling with the finite element method. Geometries representative of eclogitization of crustal material are extracted from the partially eclogitized exposures on Holsnøy (Norway). We find that the P wave velocity is controlled by the properties of the lithologies rather than their geometric arrangement. P wave anisotropy, however, is dependent on the fabric orientation of the associated rocks, as fabric variations cause changes in the orientation of the initial anisotropy. As a result, different structural associations can result in effective anisotropies ranging from~0-4% for eclogites not associated with ductile deformation to up to 8% for those formed during ductile deformation. For the kilometer-scale structures, a scale that in principle can be resolved by seismological studies, we obtained P wave velocities between 7.7 and 8.0 km s −1. The effective P wave anisotropy on the kilometer scale is~3-4% and thus may explain the backazimuthal dependence of seismological images of, for example, the Indian lower crust currently underthrusting beneath the Himalaya. These results imply that seismic anisotropy could be the key to visualize structures in active subduction and collision zones that are currently invisible to geophysical methods and thus can be used to unravel the underlying processes active at depth.

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<p>In the dry lower crust, earthquake-induced fracturing can increase permeability for fluids to infiltrate and flow, thus facilitating fluid-rock interactions, and potentially altering the strength and rheology of fault systems. Understanding the mechanisms that create and reduce porosity requires a detailed microstructural analysis. Here, we analyze microstructures that have recorded primary and secondary porosity generated by the dynamic rupture propagation of a lower crustal earthquake, and that were subsequently reworked during post- and interseismic viscous creep.</p> <p>An exhumed lower crustal section comprised largely of anhydrous anorthosites cross-cut by a coeval network of pseudotachylytes (solidified melts produced during seismic slip) and mylonitized pseudotachylytes (overprinted during the post- and interseismic viscous creep), is found at Nusfjord, Lofoten, Norway. We study the microstructures using synchrotron X-ray microtomography (SμCT), focused ion beam scanning electron microscopy (FIB-SEM) nanotomography, electron backscatter diffraction (EBSD) analysis, and SEM imaging.</p> <p>SμCT data reveals that porosity is dispersed and poorly interconnected within a pseudotachylyte vein (0.16 vol% porosity overall), and noticeably increased along the grain boundaries of garnet grains (1.07 – 1.87 vol%). The increased porosity around garnet is formed due to a net negative volume change (-DV) during garnet growth, as there is a localized increase in density of ~1.00 g/cm<sup>3</sup> when a recrystallizing garnet overgrows a pseudotachylyte matrix (plagioclase + amphibole). Efficient healing of the earthquake damage zone (0.03 vol% porosity) resulted in the preservation of only a few but relatively large interconnected primary pores along fractures in the anorthosite. Fractures were healed by the growth of plagioclase neoblasts nucleated from extremely comminuted fragments of the host anorthosite, and by the precipitation of barium-enriched K-feldspar filling intragranular pores. Fluid-rock interaction was so efficient at sealing the porosity that a FIB-SEM transect along one of these microfractures revealed a myrmekite intergrowth replacing K-feldspar.</p> <p>Porosity is dramatically decreased in the mylonitized pseudotachylyte (0.03 vol% overall), and focused mainly within monomineralic domains of plagioclase (0.07 – 0.11 vol%). These are interpreted as recrystallized and sheared survivor clasts of wall-rock fragments, while the polymineralic domains are primarily derived from the overprint of the original pseudotachylyte veins. The plagioclase grains in both domains are more-or-less equant, very fine grained (< 25 μm), lack a crystallographic preferred orientation, grain boundaries occasionally aligned to form quadruple junctions, and are well-mixed amongst the hydrous phases (polymineralic domain), suggesting that both domains deformed primarily by grain-size sensitive diffusion creep and viscous grain boundary sliding. The polymineralic domain has the least porosity (~0.01 vol%), which reflects the efficient precipitation of phases (amphibole, biotite, and feldspars) into transient pores during creep cavitation.</p> <p>A porosity reduction on the order of 90% from a pristine to a mylonitized pseudotachylyte may eventually result in shear zone hardening, and development of new pseudotachylytes overprinting the mylonites. Therefore, earthquake-induced rheological weakening of the lower crust is intermittent, occurs when a fluid can infiltrate a transiently permeable shear zone, and may stop when the porosity becomes clogged.</p> <div> <div> <div> </div> </div> </div>