Insights into deformation mechanisms of exhuming brittle-ductile shear zones (Oman Mountains)

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. Microstructural analysis suggests that interconnected C planes were formed due to an interplay of fracturing allowing fluid to preferentially flow along the newly formed fractures and precipitating phyllosilicates, and preferential grain boundary sliding and glide of the quartz-phyllosilicate grain boundaries, with additional precipitation of new phyllosilicates in dilatant sites.

Hyperspectral cathodoluminescence highlights different luminescence for the larger (several hundreds of µm) detrital quartz grains, producing a bright signal and containing yielded cracks, and smaller equant quartz grains (less than 70 µm), darker in cathodoluminescence and devoid of cracks. Electron backscatter diffraction analyses suggest that large quartz grains experienced grain size reduction by subgrain rotation recrystallization to form smaller equant grains. Interconnected chains of small quartz grains are located in contact with the phyllosilicates, suggesting preferential recrystallization along these planes.

Transmission Electron Microscope analyses highlight pyrophyllite-muscovite intergrowths at the submicron scale as small as 300-500 nm, with truncated boundaries likely reflecting dissolution and precipitation mechanisms. Muscovite and pyrophyllite appear to deform differently, suggesting that strain partitioning occurred down to the submicron scale.

Summarising, these results suggest that strain localization and weakening of this rock volume was achieved by an interplay of the following mechanisms: I) diffuse microcracking and subgrain rotation recrystallization leading to a finer grain size of quartz, II) synkinematic nucleation of retrograde mineral phases along discrete C and C’ planes, III) preferential recrystallization along the shear planes and IV) dissolution and precipitation processes of phyllosilicates. 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.

 

References

Agard, et al., (2010). Tectonics, 29(5). https://doi.org/10.1029/2010TC002669

Petroccia, et al., (2025). Journal of Structural Geology, 191. https://doi.org/10.1016/j.jsg.2024.105328

Ring, et al., (2024). Earth-Science Reviews, 250, 104711. https://doi.org/https://doi.org/10.1016/j.earscirev.2024.104711