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Seamount subduction in the Zagros Suture Zone : structural and petrologic characterization and implications for seismogenesis

Abstract : Eamounts are for the most part subducted with the downgoing oceanic plate. They are expected to critically impact the mechanical and seismogenic behavior of subduction zones, but their exact role is strongly debated (i.e. as to whether they represent barriers to propagation or asperities promoting nucleation). Rare natural examples of metamorphosed seamounts, that got sliced off the slab along the plate interface are therefore precious witnesses to document processes operating at depth.This PhD thesis reports the existence of a former seamount in the Zagros suture zone (the Siah Kuh unit).This 20x12 kilometer-large, minimum 1.5 km-high unit composed of pillow basalts capped by reef limestone was formed in an arc environment during the Late Cretaceous.HP-LT minerals (lawsonite, aragonite veins, blue amphibole) found across the whole structure, particularly in zones of localized compressive deformation, indicate that this seamount was shallowly subducted at 30 km. Syn-subduction deformation is assisted by dŽcollement rooting in serpentinite and/or oceanic sediments and is related to the internal slicing of the seamount. The presence of soft layers may prevent most of the seismic activity, since only one pseudotachylite, recording a Mw2-3 earthquake has been found. The Siah Kuh unit is also a perfect target to investigate oceanic processes such as slope destabilization and subsidence, and subduction processes such as mechanical coupling, fluid/tectonic overpressure and the nature of subduction fluids. We finally build a model for the tectonic evolution of the Siah Kuh unit and its relationship with other ophiolites.
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Guillaume Bonnet. Seamount subduction in the Zagros Suture Zone : structural and petrologic characterization and implications for seismogenesis. Earth Sciences. Sorbonne Université, 2018. English. ⟨NNT : 2018SORUS526⟩. ⟨tel-02946749⟩

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