[S-CG50] Rheology, fracture and friction in Earth and planetary sciences

[SCG50-P07] Ductile fractures evolved from creep cavitation in the down-dip extension of seismogenic fault domains

*Thomas Jing Yi Yeo^1,2, Norio Shigematsu², Simon Richard Wallis¹ (1.Department of Earth and Planetary Science, Graduate School of Science, Tokyo University , 2.Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology, AIST )

Keywords:Electron backscattered diffraction, creep cavitation, fractures, microstructures

Ductile fracture is a phenomenon where severe ductile deformation results in the failure of metals, alloys and rocks at elevated temperatures and has been proposed as an important mechanism to account for tectonic tremors or slow slip in the down-dip extension of the seismogenic faults. We are now studying the possibility that ductile fracture on the micro scale can evolve into large-scale geological structures and to the potential to account for the geophysically observed earthquake-related phenomena using samples of Ryoke myloinites along the Median Tectonic Line in SW Japan. It has been shown that ductile fracture in metals begins with the development of grain-scale cavities which then grow and coalesce (Ashby et al., 1979). To examine how this process affects mylonites, we examined the evolution of micro-voids with increasing ductile strain and in particular their relationship to fractures.

Microstructures of the mylonite samples were analysed using a combination of scanning electron microscope imaging and electron backscattered diffraction (EBSD). Previous work has shown that the samples were deformed at around 350 °C with varying degrees of mylonitisation (or strain) and that the mylonites are locally associated with a crushed zone. Ductile strain was quantified based on the quartz recrystallised fraction, revealed by applying the Gaussian mixture model to electron backscattered diffraction data (Yeo et al, 2023). Creep cavitation can be recognised through the presence of micro-voids, many of which are now filled with secondary precipitated minerals, and documenting these voids can reveal the relationship between cavity density and ductile strain. Current observations show that the number of micro-voids increase as the ductile strain increases towards the crushed zone. These spatial relationships suggest that growth and linking of micro-voids developed during mylonitic deformation may lead to the development of large fractures and rock failure.