Rheological Heterogeneity Along the Deep Subduction Interface: Insights from Exhumed HP Metamorphic Rocks Exposed on Syros Island, Greece

Abstract:

Rheological properties of subduction interface shear zones control several aspects of subduction zone dynamics, including shear tractions along the plate interface, rates and amounts of exhumation, and depths and styles of seismicity. We document the rheological properties of a deep subduction interface using exhumed eclogite and blueschist-facies rocks from Syros Island, Greece. These rocks were subducted to ~60 km depth during the Eocene, were exhumed part way along the top of the subducting slab, and were then exhumed to upper-crustal levels beneath Miocene detachment faults. Localization of strain during exhumation allowed prograde fabrics to be preserved. The PT conditions (400-550^°C, 12-16kb) of these fabrics are comparable to conditions of episodic tremor and slow slip (ETS) observed in some modern subduction zones, including Cascadia. Two types of prograde fabrics were distinguished after analyzing macro-scale distributions of strain and microphysical mechanisms of creep in metamafic rocks. Type 1 fabrics contain eclogite pods boudinaged within a blueschist matrix. The eclogites show brittle deformation with cross-cutting veins containing high-pressure minerals. Deformation in matrix blueschists is accommodated by rigid rotation of amphibole and diffusion creep in plagioclase. Type 2 fabrics contain blueschists and eclogites that are isoclinally folded at similar wavelengths, thus are approximately isoviscous. Deformation is again accommodated by diffusion creep in blueschists, but by dislocation creep of omphacite in eclogites. These deformation types characterizing boudin-matrix and isoviscous rheologies of blueschist-eclogite assemblages appear to reflect varying amounts of finite strain, but work is in progress to determine whether they also record different PT conditions. The transition from Type 1 to 2 fabrics represents a significant change in bulk viscosity and seismic anisotropy, and may correspond to a transition from ETS-type behavior—a coupled seismic (brittle)-aseismic (ductile creep) phenomena—to fully ductile aseismic creep. Rheological heterogeneity appears to be an inherent feature of subduction terranes, involving changes in deformation mechanisms from subduction to exhumation and strain partitioning between lithologies in the subduction shear zone.