Blueschist-facies paleo-earthquakes in a serpentinite channel (Zagros suture, Iran) enlighten seismogenesis in Mariana-type subduction margins

The architecture and pressure-temperature conditions reached by a Cretaceous block-in-matrix serpentinite mélange exposed in the Zagros suture resemble those imaged in the active Mariana subduction zone. There, large magnitude-earthquakes (M>w9) have never been recorded but smaller events – of poorly-constrained physical origin – in the range M∼w3-6 are widespread. Field and petro-structural constraints led to a first report of blueschist-facies seismic fault-related rocks in the Zagros serpentinite melange, including breccias, foliated cataclasites and ultracataclasites; all observed within a foliated mafic metatuffaceous block embedded in serpentinite schists. Fine-scale petrological characterization of ultrafine-grained, fluidized cataclastic material reveals the presence of newly-formed glaucophane, lawsonite, phengite, albite and pumpellyite, an assemblage inferred (based on thermodynamic modelling) to have crystallized in the lower lawsonite-blueschist facies at ∼0.6-1.0 GPa and 230-300 °C. Extensional veins containing similar mineral assemblages are observed crosscutting the aforementioned rocks but are also identified as comminuted fragments in all fault-related lithologies. Crosscutting relationships among the multiple generations of fluidized ultracataclasites and brecciated blueschists suggest that episodic faulting and hydrofracturing were contemporaneous processes at ∼20-35 km depth, i.e., at similar conditions as reported for metabasalts expelled by Mariana serpentinite mud volcanoes. Mechanical modelling confirms that the studied fault-related features can only have formed under nearly lithostatic pore fluid pressure conditions, maintaining the system in a critically unstable regime that promoted recurrent seismic faulting, as monitored in the Mariana seismogenic zone. These fluids are likely associated with externally and deeply-generated fluid pulses that may have reached the seismogenic window, imprinting a Ta-Th-Nb-HREEs-enriched trace element signature. This new faulted blueschist occurrence highlights the physical nature and the mechanical processes operating within fluid-saturated fault zones in the serpentinized subduction channel.Zeynab Gharamohammadi, Ali Kananian, Philippe Agard and Jafar Omrani are acknowledged for logistical assistance. Olga Cazalla, Laura Crespo and Miguel Angel Hidalgo Laguna are acknowledged for their technical support at Centre of Scientific Instrumentation, University of Granada . We are very grateful to Sophie Nowak for further support during XRD analytical sessions. Onno Oncken is warmly thanked for insightful discussions and support regarding mechanical earthquake-related processes. Hugues Raimbourg and Isabelle Martinez are also acknowledged for exciting discussions and suggestions regarding fluid-rock interaction processes. The editorial handling by Alexander Webb is very much appreciated. Whitney Behr and Volker Schenk are warmly acknowledged for detailed and constructive reviews, but also for their stimulating words that helped improving this work. This project has been funded by an Initiative D'EXcellence (IDEX) grant 16C538 and the TelluS Program of CNRS/INSU to S.A. Partial funding was also provided by the University of Granada at CIC