PRELIMINARY EXPERIMENTAL CONSTRAINTS ON THE RHEOLOGY OF MAFIC BLUESCHISTS

The strength and deformation behavior of the subduction-interface plays a central role in generating geological hazards. Yet, the rheology of many subduction lithologies that deform along the plate interface remain poorly constrained. Between the base of the subduction seismogenic zone and the sub-arc (~35-100 km), high-P low-T metamorphism of subducting ocean crust produces mafic blueschist. Observations of naturally deformed blueschists exhumed from these depths suggest they accommodate significant strain—largely partitioned into the ubiquitous sodic amphibole glaucophane (gln). Thus, constraining the absolute strength of gln, and by extension blueschist, will improve our understanding of plate interface rheology at these depths. In order to provide these rheological constraints, we experimentally deformed gln in a high-P high-T deformation apparatus at relevant P-T conditions. Fine grained gln aggregates (10-20 µm) were hot-pressed for 16 hours at T=650℃ and P=1.5 GPa, then deformed at temperatures from 650-750℃ at strain rates (γ ̇) of 10-3-10-4 s-1. Glaucophane’s peak strength decreases substantially from τ~1000 to ~200 MPa with a 100℃ temperature increase. Preliminary calculations estimate an activation energy (Q) of ~200-400 kJ/mol. Stress-stepping experiments yield a stress-exponent of n ≈ 5, consistent with deformation by dislocation-assisted motion. Compared to the hot-pressed starting material, microstructures in deformed samples show increased evidence of intragranular deformation, subgrain boundary recrystallization, and strong crystallographic preferred orientations supporting the activation of dislocation-creep-accommodated deformation. Similar microstructures in our lab-deformed samples and exhumed, naturally deformed blueschists from various subduction zones imply that our experiments accessed the deformation regime experienced by these naturally-deformed blueschists. This suggests that deformation along the subduction interface at blueschist conditions is largely accommodated by dislocation creep in gln. Based on the high temperature-sensitivity of gln’s peak strength in the dislocation-creep regime, we propose that blueschist strength along the plate interface rapidly decreases with increasing temperature/depth in the subduction zone.