Blueschist facies deformation mechanisms, rheology, and fault slip styles: preliminary observations on an exhumed subduction complex (Ishigaki-jima, SW Japan)

Abstract:

Episodic tremor and slow slip (ETS) have been recorded in several subduction zones up-dip and down-dip of the subduction megathrust. These geophysical phenomena may reflect rheological behaviours transitional between seismic and aseismic slip, and can provide important clues on how tectonic plate boundaries accommodate deformation.

We present here preliminary observations on an exhumed subduction complex in the SW of Japan, in Ishigaki-jima, which represents an environment comparable to modern ETS downdip of the seismogenic zone. In particular, we focus on the Triassic high pressure-low temperature Tomuru metamorphic rocks, which comprise blueschist and greenschist metabasites and metapelite that underwent subduction-related deformation with peak metamorphic conditions of 460 ± 50°C and 0.6-1.3 GPa. Field observations suggest that the coexistence of greenschist and blueschist could set up an important competence contrast, creating a range of rheological behaviours. In fact, blueschist rocks contain an intensely foliated matrix, which appears to be deformed by distributed viscous shear. This matrix envelops two types of more rigid blocks: (1) blueschist lenses that are less intensely viscously deformed than the matrix; and (2) greenschist lenses characterized by brittle shear fractures and veins.

Through multiscale and multidisciplinary, field- and laboratory-based studies, including microstructural and petrographic observations, as well as image analysis, our research focuses on two key aspects: (i) understanding mineral-scale deformation mechanisms active in blueschist rocks and (ii) quantitative comparison of grainsize evolution and foliation development between foliated matrix and apparently more rigid lenses. These analyses are aimed at discriminating between varied rheological behaviours that occurred during subduction-related deformation, and ultimately the role of blueschist rheology and the greenschist-blueschist metamorphic facies transition in episodic tremor and slow slip events.