Widening of Hydrous Shear Zones During Incipient Eclogitization of Metastable Dry and Rigid Lower Crust—Holsnøy, Western Norway

Abstract

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 (

The key role of µH2O gradients in deciphering microstructures and mineral assemblages of mylonites: examples from the Calabria polymetamorphic terrane

“…This indicates that pumpellyite was more ductile than glaucophane during the shearing event that characterised the final stage of exhumation of the Diamante-Terranova blueschist from ~ 1.1 GPa and 380 °C to 0.7 GPa and 290 °C (Tursi et al 2020a), in particular at the P-T conditions of veins development. Hence, the formation of reaction products allowed sustaining ductile shearing, in line with the experimental results by Holyoke and Tullis (2006a), with the reaction products that formed in narrow zones where µH 2 O is locally higher than in the surroundings, which is compatible with narrowing of ductile shear zones when fluid in the system is limited (Menegon et al 2017;Kaatz et al 2021).…”

Transient weakening during the granulite to eclogite transformation within hydrous shear zones (Holsnøy, Norway)

“…The widely accepted scenario invoked to explain the formation of Holsnøy eclogite shear zones (Austrheim, 1987;Boundy et al, 1992;Raimbourg et al, 2005;Jamtveit et al, 2018;Zertani et al, 2019;Kaatz et al, 2021) Once the eclogitised halos were sufficiently weakened, they started to deform in a ductile manner, so that they were the precursor structures of small-scale eclogite shear zones that localised deformation. (5) The shear zones widened owing to diffusion of the fluid along a pressure gradient orthogonal to the shear direction.…”

“…These boudins are the same size as the blocks delimited by minor shear zones in the outer zone of the major shear zones, and can be considered as their later evolution at larger finite strain. (3) Using a scale-independent numerical model, Kaatz et al (2021) also showed that Holsnøy shear zones widen over time, owing to synchronous fluid infiltration and deformation. (4) Deformation experiments by Incel et al (2020) showed that fluid influx and subsequent metamorphic reactions in plagioclase-rich rocks result in the widening of shear zones at eclogite facies conditions.…”

“…In Holsnøy, the widening of the shear zones is inferred to occur by weakening of the granulite host-rock, due to its eclogitisation promoted by fluid infiltration (e.g. Austrheim, 1987;Kaatz et al, 2021). As eclogite facies deformation was localised within eclogite shear zones, eclogite is considered to be a mechanically weaker rock than granulite (e.g.…”

“…As eclogite facies deformation was localised within eclogite shear zones, eclogite is considered to be a mechanically weaker rock than granulite (e.g. Austrheim, 1991;Bjørnerud et al, 2002;Jolivet et al, 2005;Raimbourg et al, 2005), and the widening is considered to be only caused by weakening of the host rock (Kaatz et al, 2021). However this does not take into account the kinetics of the metamorphic reactions responsible for eclogitisation, and the potential complex change in strength during the granulite to eclogite transition.…”