[S-CG39] Science of slow earthquakes: Toward unified understandings of whole earthquake process

[SCG39-03] Localized megathrust slip controlled by chemical reactions in subduction mélanges

*Kohtaro Ujiie¹, Kazuya Noro², Norio Shigematsu³, Ake Fagereng⁴, Naoki Nishiyama¹, Christopher Tulley⁴, Haruna Masuyama², Yasushi Mori⁵ (1.Faculty of Life and Environmental Sciences, University of Tsukuba, 2.Graduate School of Life and Environmental Sciences, University of Tsukuba, 3.Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology, 4.School of Earth and Environmental Sciences, Cardiff University, 5.Kitakyushu Museum of Natural History and Human Histor)

Megathrust slip down-dip of the seismogenic zone is accommodated by either steady creep or episodic slow slip events (SSEs). However, the geological conditions defining the rheology of megathrust slip remain elusive. Field observation and laboratory analyses show that subduction mélanges deformed at ~370–500 °C in warm-slab environments record fluid release and viscous shear localization associated with chemical reactions between juxtaposed metapelitic and metabasaltic rocks. In the mélange deformed near the base of the seismogenic zone, very fine-grained reaction products facilitated grain boundary diffusion creep at lower shear stresses than surroundings, whereas in the mélange deformed near the mantle wedge corner, chemical reactions led to viscous shear at two orders of magnitude faster strain rate than surroundings. We suggest that chemical reactions facilitate localized changes in megathrust slip mode with depth, potentially providing a mechanism for change from viscous creep to SSEs.