Sustained strain localisation and coeval brittle-ductile deformation in an exhuming low-grade shear zone: Insights from the Saih Hatat Window (NE Oman)

Synkinematic phyllosilicates play a key role in controlling the rheology and the style of shear zones. At the same time, aqueous fluids released by metamorphic dehydration reactions may transiently increase pore pressure, triggering cyclic switching between brittle and ductile deformation. The Hulw Shear Zone in Oman is an exhumation-related shear zone wherein these processes can be studied. Its footwall is mostly composed of metapelites, with a modal enrichment in phyllosilicates matched by a progressive increase in their physical interconnectivity along its internal strain gradient. Similarly, marbles in the hanging wall evolve from mylonitic to ultramylonitic toward the core of the shear zone. The Hulw Shear Zone started from peak conditions of 300–350 °C and 0.9–1.2 GPa, followed by the main shearing event at 350–420 °C and 0.6–0.9 GPa and ended at 350 °C, 0.3–0.4 GPa between c. 76-75 Ma. Decompression-driven fluid-gain reactions facilitated the growth of phyllosilicates, which created a pervasive and interconnected network that promoted strain localisation, causing mechanical weakening as well as the potential compartmentalized fluid cells within the mylonitic foliation. Brittle structures formed due to aqueous fluid release by metamorphic dehydration reactions, transiently increasing pore pressure and triggering brittle failure, resulting in coeval mylonitic foliation and veins. Our findings support sustained shearing promoted by synkinematic K-rich white mica and pyrophyllite growth and cyclic switching between brittle and ductile deformation conditions. The studied structures might represent a record of deep episodic tremors and slow slip events during exhumation-related tectonics in the accretionary wedge of the Oman continental lithosphere.