Deformation Experiments on Blueschist and Greenschist: Implications for the Rheology of Subducted Oceanic Crust

Abstract:

To understand the spatial and temporal distribution of deformation (e.g., underplating and exhumation of metamorphic rocks) and earthquakes in subduction zones, it is important to constrain the rheological properties of metamorphic rocks (i.e., altered oceanic crust and sediments), and how they evolve during metamorphic reactions following hydration, carbonation and dehydration of the down-going slab.

We conducted triaxial deformation experiments on three mafic schists with various peak metamorphic conditions: a lawsonite-blueschist, a greenschist, and an epidote-amphibole schist, using Griggs-type solid pressure- medium apparatus. Constant strain rate experiments and strain rate stepping experiments were conducted at confining pressures (P_c) from 0.76-2GPa, temperatures (T) from 300-600C and strain rates from 10^-5-10^-71/s.

At a confining pressure of 1 GPa, temperature of 400C and strain rate of 10^-5 1/s, differential stresses σ_d for all mafic schists were higher than 1 GPa. The lawsonite-blueschist and greenschist samples were weaker than epidote-amphibolite samples under all experimental conditions. All types of samples exhibit high stress exponent (> 15) and strain rate strengthening; frictional behavior that inhibits earthquake nucleation. Differential stress increased with increasing confining pressure, while friction coefficient decreased with increasing confining pressure and temperature. The nominal friction coefficient for the lawsonite-blueschist and the greenschist samples was 0.3 to 0.35, values which predict stresses below the Goetze criterion (σ_d < P_c). Microstructures of recovered samples showed modest buckling and several localized shear zones. These features suggest that the deformation of mafic schist is accommodated by semi-brittle deformation resulting in strain localization on faults. Such weak and aseismic fault zones in subducting slab might promote detachment of oceanic crust from the subducting slab and allow underplating to forearc crust.