Evidence for strain localisation and episodic tremors and slow slip events in exhuming continental shear zones (Saih Hatat Window, Oman)

Phyllosilicates play a key role in controlling the rheology of shear zones, the style of deformation and the syndeformational fluid budget. The latter, including aqueous fluids released by metamorphic reactions, can transiently increase pore pressure and trigger cyclic switching between brittle and ductile deformation conditions. Unfortunately, it is still unclear how these processes act together in exhuming low-grade shear zones in a continental collisional framework.
To tackle this scientific question, we studied the top-to-the N/NE Hulw Shear Zone in the Saih Hatat Window of Oman. This shear zone is responsible for part of the exhumation of the subducted continental crust, but its pressure-temperature (P–T) and deformation behaviour remain largely unconstrained. Its footwall is mostly composed of metapelites, with a modal enrichment in K-rich white mica and pyrophyllite, matched by a progressive increase in the physical interconnectivity of phyllosilicates along its internal strain gradient. Similarly, marbles in the hanging wall evolve from mylonitic to ultramylonitic towards the core of the shear zone. 
In the Hulw Shear Zone coexist two opposite deformation behaviours, with ductile deformation accommodated preferentially along laterally continuous phyllosilicate-rich bands and brittle deformation in the form of hybrid/dilational hydroshear veins found regularly at the outcrop. To constrain the metamorphic conditions of dehydration reactions during the exhumation path, we integrated forward thermodynamic modelling with Raman Spectroscopy on Carbonaceous Material, and K-rich white mica multiequilibrium barometry on a representative mylonite from the shear zone footwall. The resulting metamorphic evolution of 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 with sustained shearing at low-P conditions (350 °C, 0.3-0.4 GPa). Therefore, the Hulw Shear Zone accommodated progressive shearing while exhuming its footwall from epidote blueschist to low-pressure greenschist facies conditions. 
Decompression-driven fluid-gain reactions facilitated the growth of synkinematic phyllosilicates, which created a pervasive and interconnected K-rich white mica and pyrophyllite network that promoted strain localisation, causing significant mechanical weakening as well as the potential for discrete and compartmentalised fluid cells within the mylonitic foliation. Brittle structures formed due to aqueous fluid release by metamorphic dehydration reactions close to peak-P conditions (e.g., kaolinite-out reaction) or along the exhumation trajectory, transiently increasing pore pressure and triggering brittle failure, resulting in coeval mylonitic foliation and crack-seal hybrid veins. 
Our findings support the idea that sustained shearing was promoted by synkinematic growth of K-rich white mica and pyrophyllite and by cyclic switching between brittle and ductile deformation conditions. Therefore, the studied structures might also represent a record of deep episodic tremors and slow slip events during exhumation-related tectonics in the accretionary wedge above the subduction interface of the Oman continental lithosphere.