[S-CG40] Science of slow-to-fast earthquakes

[SCG40-P32] The role of ductile fractures in continental crustal faults from micro to macro scale

Thomas Yeo^1,2, *Norio Shigematsu¹, Simon Richard Wallis², Tatsuya Sumita³, Ayumu Miyakawa³, Takuma Katori⁶, Kohtaro Ujiie⁴, Keita Kobayashi⁵, Chunjie Zhang⁷ (1.Research Institute of Earthquake and Volcano Geology, Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology, 2.School of Science, University of Tokyo, 3.Research Institute of Geology and Geoinformation, Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology, 4.Earth Evolution Sciences, University of Tsukuba, 5.Research Institute for Material and Chemical Measurement, National Metrology Institute of Japan, National Institute of Advanced Industrial Science and Technology, 6.Fossamagna Museum, 7.Earthquake Research Institute, University of Tokyo)

Keywords:Median Tectonic Line, Ductile fracture, Ductile deformation, Creep cavitation

The base of the seismogenic zone of large continental crustal faults is clearly defined by a rapid decrease in seismicity generally at depths of around 10 kilometres. This boundary which occurs around 300℃, marks the fundamental change in the deformation styles of rocks from brittle at shallow levels to ductile deformation (mylonites) at deeper levels. Beneath the seismogenic zone characterised mainly by ductile-dominated rock deformation, seismic signals are still recognised, and they are potentially important as triggers to larger events at shallow levels. However, the mechanism responsible for the nucleation of seismic signals (earthquakes and tectonic tremors) occurring in the ductile regime is poorly understood and there is still no consensus on how these phenomena relate to rock deformation and geological structures.
Fracturing in rocks is typically associated with brittle deformation and is usually interpreted as a mechanism for releasing accumulated tectonic elastic energy resulting in earthquakes. In metallurgy, fractures are also known to develop through ductile processes. These fractures, termed ductile fractures, manifest in materials subjected to large ductile strain, alongside the occurrence of creep cavitation. The occurrence of creep cavitation and ductile fractures are reported to exhibit transient deformation behaviour which may offer a viable model for explaining seismic activities in the ductile regime. The goal of this study is to show evidence of ductile fractures in naturally deformed mylonites, and their continuity within a kilometre-scale fault.
In this study, we examined the exhumed Ryoke mylonites along the Median Tectonic Line (MTL) of SW Japan. These mylonites are deformed at the temperature of ~350 ℃, heterogeneously sheared with a sinistral sense of shear and constitute a differential stress of ~130 MPa. They represent the deformation at the brittle-ductile transition zone that potentially hosts seismic activities.
To identify the presence and evolution of ductile fractures in rocks, microstructural analyses by SEM and TEM imaging are conducted while SEM-EBSD is applied to determine the quartz recrystallised fraction. The quartz recrystallised fraction was used as a proxy for strain and a comparative analysis between cavity density and the quartz recrystallised fraction was performed. The results reveal that the cavity density increases in proportion to the quartz recrystalised fraction, with ductile fractures emerging when cavity density approaches 7.5%.
Kilometre-scale structure due to ductile fracture is crucial for identifying the role of ductile fractures in controlling fault behaviour. Owing to limitations in outcrop exposure, ductile fractures can only be observed with dimensions of up to 30 metres. Thus, by leveraging high-precision digital outcrop model (DOM), the continuity of the geological structure can be analysed. The DOMs enable the identification of zones of high ductile fracture density with a minimum length of ~1200 m along the strike and a thickness of ~100 m in the MTL fault zone.
These results show the significant role of ductile fracturing in controlling fault zone behaviour within continental crust. The widespread presence of ductile fractures in strongly deformed mylonites implies a non-steady-state fault deformation behaviour. These findings imply ductile fracturing is a plausible mechanism for generating tectonic tremors or earthquakes within continental crustal fault settings.