A NORTHERN CORDILLERAN BLUESCHIST-ECLOGITE TRANSITION ZONE AS A SEISMICITY SOURCE REGION: LAWSONITE ECLOGITES FROM THE YUKON-TANANA TERRANE, YUKON, CANADA (Invited Presentation)

Exhumed high-pressure metamorphic rocks offer a unique window into the processes that occur in subduction zones at depth. Metamorphism and deformation go hand-in-hand and mineral phase transitions and reactions can strongly influence deformation mechanisms, volume changes, and pore fluid budget, and thus rheological behaviour. The blueschist-eclogite transition zone has been linked to various types of seismicity, including intermediate depth earthquakes and episodic tremor and slow slip (ETS). Experiments indicate intermediate depth earthquakes may result from dehydration reactions (such as lawsonite dehydration) or stress changes resulting from mineral nucleation and growth (e.g. clinopyroxene). A range of mechanisms have been proposed for ETS, including combined brittle and viscous flow, changes in frictional properties from velocity-weakening to strengthening, fluid migration/pressurization, and metamorphic reactions.

Cordilleran lawsonite-eclogites from Faro in the Yukon-Tanana Terrane, Yukon Territory, Canada, preserve varying degrees of prograde and retrograde metamorphism. These high-pressure rocks formed during Permian subduction resulting in collision between the Yukon-Tanana continental island arc and the western edge of Palaeo-North America. The tectonostratigraphic position and shear fabrics of the rocks suggest they formed at or near the subduction interface. Their prograde path, through the blueschist-eclogite transition, is particularly well preserved. Mineral relationships, zoning and microstructures record subduction from blueschist facies conditions (~420 °C and ~1.5 GPa), into the eclogite facies (with lawsonite-dehydration in low eclogite facies), followed by peak eclogite facies metamorphism at 490 – 580 °C and 1.9 – 2.2 GPa. By using a combination of electron backscatter diffraction (EBSD), element mapping, and phase equilibrium modelling we describe the deformation mechanisms and mineral reactions during subduction, systematically examining how they facilitate deformation at the plate interface. Embrittlement, volume reduction, and fluid release at ~2 GPa lithostatic pressure is consistent with the deeper edge of the tremor source and low Vp/Vs region in the modern Cascadia megathrust.