The rheology and mechanical anisotropy of a foliated blueschist

Abstract:

Blueschists are a major constituent rock type along the subduction zone interface and therefore critical to our understanding of subduction zone dynamics. Previous experimental work on natural blueschists focus on either seismic anisotropy or on the process of eclogization of a blueschist aggregate. We have begun an experimental investigation to constrain the rheology and mechanical anisotropy of a naturally foliated blueschist from the Condrey Mountain Window, CA. General shear experiments were performed in a Griggs apparatus using cores of the natural blueschist at 700°C, 1 GPa, and a shear strain rate of ~10^-5 s^-1. The starting material consists of ~55% glaucophane, ~40% epidote, <5% quartz and <5% mica where both glaucophane and epidote have strong crystallographic fabrics and shape-preferred orientations that define the foliation. Two types of experiments were performed: 1) with the foliation parallel to the shear plane and 2) with the foliation parallel to the sigma1 direction. Both experiments achieved a similar peak shear stress of ~250 MPa; however, the sample with the foliation parallel to the shear plane shows strain weakening while the sample with the foliation parallel to the sigma1 direction shows no strain weakening. We also observed several stress drops of ~20-30 MPa in the sample with the foliation parallel to the sigma1 direction prior to peak stress conditions. Additional experiments will be performed on these two deformation geometries at various stress and temperature conditions. A third type of deformation experiment will also be performed where the starting material has no foliation. A detailed microstructural analysis will accompany the mechanical results.