Abstract
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.
Highlights
Mariana-type subduction settings are believed to represent the “aseismic”, non-accretionary end-member where moderatemagnitude seismicity is widespread but no large megathrust earthquakes (Mw>9.0) are so far reported (e.g. Emry et al, 2011 and references therein)
Emry et al (2011) and Eimer et al (2020) challenged this hypothesis demonstrating that the seismogenic region of the Mariana subduction zone spans the depth range 10 to 60 km along a highly serpentinized region of the plate interface (Fryer et al, 2020), demonstrating that brittle deformation is actively occurring there
We focus here on the Seghin complex (Fig. 1B), a km-thick tectonic sliver structurally sandwiched between the Ashin and Siah-Kuh complexes, composed of blueschist blocks embedded in a serpentinite matrix
Summary
Mariana-type subduction settings are believed to represent the “aseismic”, non-accretionary end-member where moderatemagnitude seismicity is widespread but no large megathrust earthquakes (Mw>9.0) are so far reported (e.g. Emry et al, 2011 and references therein). Recent discoveries demonstrate that some of the blueschist fragments may have been buried to greater depths of ∼50 km (590 ◦C at 1.6 GPa) before being exhumed (Tamblyn et al, 2019) These rock clasts were brought back to the surface through channel flow and serpentinite mud volcanism which is thought to be triggered by slab devolatilization and subsequent fluid flow along normal faults in the forearc (Fryer et al, 2020). These observations suggest that the Mariana forearc is extensively serpentinized and the ascent of fluidized serpentinite material (protrusion) and rare blueschist fragments is due to diapiric buoyancy forces associated with deep return flow in an extensional regime (Maekawa et al, 2004). This work addresses water-saturated frictional sliding processes combining petro-structural observations with mechanical modelling and compares the observed structural features with what is currently being imaged and dredged in the active Mariana subduction margin
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