Shattered veins elucidate brittle creep and slip processes in deep subduction interfaces

Abstract

In cold subduction margins, Deep Slow Slip and Tremors (SSTs) and regular seismicity occur under blueschist-facies conditions, but their physical source mechanisms remain poorly documented. On the one hand, SSTs occur at upper blueschist-facies conditions and consist of transient clusters of tectonic tremors and slow slip associated with extremely elevated fluid pressures, reflecting a transition from viscous to brittle subduction zone rheology. On the other hand, regular seismicity occurs under incipient blueschist-facies conditions and seismic events in the range Mw∼3-6 are widespread. Firstly, we document vein networks precipitated and brecciated within the deep SST region. These lawsonite-bearing vein sets exhibit evidence of brittle failure and are spatially related to localized, cataclastic shear bands. Petro-geochemical data reveal that the formation of these brittle fabrics was associated with the pulse-like injection of ultramafic-, mafic- and metasedimentary-derived fluids. Thus, imprinting characteristic Cr, high field strength elements, and light over heavy rare earth elements positive anomalies in the vein breccias, while leaching light rare earth elements from the cataclastic blueschist host. We suggest that metamorphic veins represent zones of mechanical anisotropy within the rock volume prone to localized shearing, brittle deformation and episodic injection of externally derived fluids. Secondly, we report blueschist-facies breccias, foliated cataclasites and ultracataclastic injection veins sharply crosscutting a foliated mafic metatuffaceous block embedded in serpentinite schists. Petrological characterization of ultrafine-grained, fluidized cataclastic material shows the presence of newly-formed glaucophane, lawsonite, phengite, albite and pumpellyite. This assemblage and fabrics are inferred to have crystallized in the incipient lawsonite-blueschist facies from a seismic event. Extensional veins containing similar mineralogy occur crosscutting these fault rocks, but also as comminuted fragments in all fault-related lithologies. Crosscutting relationships among the multiple generations of fluidized ultracataclasites and brecciated blueschists suggest episodic faulting and hydrofracturing at ∼20-35 km depth. Mechanical modelling confirms that the studied fault-related features likely formed under nearly lithostatic pore fluid pressure conditions, allowing recurrent seismic faulting. As in the previous case, petro-geochemical data demonstrate that faulting was accompanied by fluids imprinting a Ta-Th-Nb-HREEs-enriched trace element signature. For both scenarios, the infiltrating fluids during shearing are likely externally produced and derived from the blueschist-to-eclogite dehydration transition, located at greater depths than those reached by these blueschists. Pulses of fluids generated at greater depths may have reached the deep SST window, closer to the dehydration front. Ongoing deformation during SST could have enhanced fault zone permeability, facilitating upward migration of fluids along the subduction interface until reaching the seismogenic region at incipient blueschist-facies, potentially in a less periodic manner. Consequently, large-scale fluid circulation may play a more crucial role in generating a diverse range of seismic manifestations in subduction environments compared to in situ metamorphic dehydration reactions, as also supported by recent geophysical observations.