Blueschist-facies paleo-earthquakes in a serpentinite channel (Zagros suture, Iran) enlighten seismogenesis in Mariana-type subduction margins
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Mariana-type marginsBlueschistsEarthquakesPaleo-earthquakesUltracataclasitesZagros suture
J. Muñoz-Montecinos, S. Angiboust and A. Garcia-Casco. Blueschist-facies paleo-earthquakes in a serpentinite channel (Zagros suture, Iran) enlighten seismogenesis in Mariana-type subduction margins. Earth and Planetary Science Letters 573 (2021) 117135. [https://doi.org/10.1016/j.epsl.2021.117135]
SponsorshipInitiative D'EXcellence (IDEX) 16C538; TelluS Program of CNRS/INSU; University of Granada at CIC
The architecture and pressure-temperature conditions reached by a Cretaceous block-in-matrix serpentinite melange exposed in the Zagros suture resemble those imaged in the active Mariana subduction zone. There, large magnitude-earthquakes (M-w> 9) have never been recorded but smaller events - of poorly-constrained physical origin - in the range M-w similar to 3-6 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 similar to 0.6-1.0 GPa and 230-300 degrees 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 similar to 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.