Phyllosilicates do their job: insights into their role in exhuming subducted continental units

Exhuming shear zones are key structures in the dynamic evolution of orogens. Such shear zones accommodate most of the shear-related exhumation within relatively small rock-volumes. This is possible due to major strain partitioning occurring along weak rocks, frequently represented by phyllosilicate-rich rocks. Thus, the study of phyllosilicate-rich mylonites can provide fundamental insights into exhumation mechanisms responsible for the architecture of orogens.The Hulw Shear Zone in the Saih Hatat Window of Oman (Agard et al., 2010) is one of these exhuming shear zones juxtaposing two subducted continental tectonic units. This tectonic contact experienced sustained shearing, accommodating a delta pressure of circa 0.8 GPa between 1.2 and 0.4 GPa at a relatively constant temperature of circa 400 °C (Petroccia et al., 2025) between 77 and 74 Ma (Ring et al., 2024).In the field, micaschist belonging to the footwall displays a strain gradient moving toward the contact with the hanging wall, corresponding to a development of a S-C-C’ fabric and a modal enrichment in K-rich white mica and pyrophyllite matched by a progressive increase in the physical interconnectivity of these phyllosilicates. Electron backscatter diffraction analyses suggest that large (several hundreds of µm) detrital quartz grains experienced grain size reduction by subgrain rotation recrystallization to form equant grains of less than 100 µm in size.Hyperspectral cathodoluminescence highlights different luminescence for the larger detrital grains, producing a bright signal and containing yielded cracks, and smaller equant grains, darker in cathodoluminescence and devoid of cracks. Interconnected chains of small quartz grains are located in contact with the phyllosilicates, suggesting an interplay between pinning and grain growth from a fluid phase.In pyrophyllite-muscovite intergrowths, Transmission Electron Microscope analyses highlight more defects and kinking in pyrophyllite than in muscovite, intergrowths at the submicron scale and crystallites as small as 2 µm with truncated boundaries likely reflecting dissolution and precipitation mechanisms.Summarising, these results suggest that strain localization and weakening of this rock volume was achieved by an interplay of the following mechanisms: I) synkinematic nucleation of retrograde mineral phases along discrete C and C’ planes, forming an interconnected network of phyllosilicates, II) microcracking in larger quartz grains followed by subgrain rotation recrystallization leading to a finer grain size of quartz, III) pinning of the grain size and IV) dissolution and precipitation processes of phyllosilicates. Different types of phyllosilicates appear to differently accommodate strain by both plastic deformation and recovery by dissolution-reprecipitation.Concluding, this intimate and polyphase interplay between deformation and metamorphism is responsible for the formation and evolution of exhuming shear zones and the related structure of orogens. Giuntoli acknowledges financial support of grant N° MUR 2022X88W2Y _002. ReferencesAgard, P., Searle, M. P., Alsop, G. I., & Dubacq, B. (2010). Tectonics, 29(5). https://doi.org/10.1029/2010TC002669Petroccia, A., Giuntoli, F., Pilia, S., Viola, G., Sternai, P., & Callegari, I. (2025). Journal of Structural Geology, 191. https://doi.org/10.1016/j.jsg.2024.105328Ring, U., Glodny, J., Hansman, R., Scharf, A., Mattern, F., Callegari, I., van Hinsbergen, D. J. J., Willner, A., & Hong, Y. (2024). Earth-Science Reviews, 250, 104711. https://doi.org/https://doi.org/10.1016/j.earscirev.2024.104711