Experimental Investigation of Geologic Controls on Blueschist Deformation in Slow Slip and Tremor Regions

Subduction zones produce large earthquakes and tectonic tremor, and the dynamics of these plate boundaries control the rates of plate tectonics. Blueschist forms at depths of >20 km in the subducting oceanic crust and can be used to study deformation styles at in situ temperature and stress conditions appropriate to the mineralogy. Very little is known about the properties of blueschists, in particular the role of rock fabrics in controlling deformation. We studied exhumed metabasic blueschist-facies rocks from the Franciscan Complex, which are commonly strongly foliated, producing a strength anisotropy. We performed a series of axial deformation experiments in a Griggs-type apparatus with acoustic emission (AE) sensors spanning a range of pressures and temperatures within the blueschist stability field (PC = 0.9-1.3 GPa, σd = 1.8-2.6 GPa, T = 350-450 °C), varying the angle between the natural foliation and the axial load (30° and 60°). These pressure and temperature conditions correspond to depths of episodic tremor and slip at about 25-35 km in a cold subduction zone. All experiments show localization, strain hardening and increased AE activity near the yield point without an associated stress drop. Due to natural fabrics in the rock, experimental deformation was distributed over multiple natural and newly-formed structures. Understanding the coevolution of these structures could be relevant to brittle-ductile behaviour in the tremor source region of subduction zones.