Large-scale fluid circulation in deep subduction interfaces: implications on fast and slow earthquake-related processes

Devolatilization and fluid-rock interaction processes along subduction interfaces, in particular at depths where episodic tremor and slip events (ETS) are inferred, are evidenced by the occurrence of metamorphic veins in exhumed metamorphic terranes. We investigate the late Cretaceous lawsonite blueschist-facies Seghin complex, part of the Zagros suture zone (Iran), a well-preserved paleo-subduction mélange composed of an antigorite-rich matrix wrapping foliated metatuffs and minor carbonate-bearing metasediments. We first focus on characterizing the relative chronology, conditions of deformation and potential fluid source(s) of Lws+Cpx+Gln veins and aragonite-filled explosive hydraulic breccias. Petrological, geochemical as well as O-C and Sr-Nd isotopic systematics of silicate-rich veins suggest formation mostly from internal devolatilization. This stage is followed at near peak burial conditions by pervasive, externally-derived fluid influx events, with fluids characterized by REE enrichments, and geochemical signatures indicating mixing between metasedimentary-derived fluids and far-traveled mafic-ultramafic-derived fluids. Our geochemical and petrological observations suggest that a host rock-buffered isotopic homogenization occurred between the infiltrating fluids and the rock matrix.

The high pore fluid pressures that enabled the formation of these deep veins also enabled the formation of shallower fault-related rocks including breccias, foliated cataclasites and fluidized ultracataclasites, intimately associated with extensional Gln-bearing veins and Lws+Gln+Ph+Ab fluid-filled pockets. Mineral assemblages reveal that this faulting occurred upon exhumation throughout the lawsonite blueschist-facies (i.e. 35 to 20 km depth). Crosscutting relationships among multiple generations of fluidized ultracataclasites and extensional veins show that episodic seismic faulting and hydrofracturing were contemporaneous processes. Mechanical modelling confirms that the studied fault-related features can only form under nearly lithostatic pore fluid pressure conditions, maintaining the system in a critically unstable regime that promotes recurrent seismic faulting. We propose a large-scale tectonic model in which deeply produced H₂O-rich fluids are transported as highly pressurized “pulses” over tens of km parallel to the subduction interface, triggering episodic hydrofracturing and host rock-buffered isotopic homogenization within the ETS region. The mechanical consequence of these events is the triggering of unstable slip within the seismogenic window, as deduced in this unique record of blueschist-facies crustal paleo-earthquakes. These results shed a new light on the physical nature of the numerous moderate magnitude events (Mw=3-6) that are extensively recorded nowadays in Mariana-type plate boundary systems.