[S-CG58] 岩石―流体相互作用の新展開：表層から沈み込み帯深部まで

[SCG58-13] Albite metasomatism may control the downdip limit of the seismogenic megathrust

*氏家 恒太郎¹、重松 紀生² (1.筑波大学、2.産業技術総合研究所)

For warm-slab subduction beneath accretionary prism, the downdip limit of the seismogenic megathrust is inferred to occur where temperature exceeds ~350 ℃. The proposed thermal control on the downdip limit of the seismogenic zone was the onset of crystal plasticity in quartz or pressure solution creep in subducting sediments, but remains controversial. Here, we show a new factor controlling the downdip limit of the seismogenic megathrust based on observations and analyses of the subduction mélanges deformed under warm-slab environments. In the mélange deformed at ~370℃, subduction-related shear was accommodated by multiple viscous shear zones formed in association with metasomatic reactions between juxtaposed metapelite and metabasaltic rocks, but such metasomatism-related viscous shear zones were absent in the mélange deformed at ~330℃. Metasomatic reactions caused albitization of metapelite that was characterized by the consumption of muscovite and carbonaceous materials from the metapelite and the formation of very fine grained albite. The metasomatic reaction zones include quartz veins subparallel to boundaries of reaction zones and lack of boudinaged quartz veins suggest a small viscosity contrast between quartz veins and surrounding metasomatic albite. Deformation microstructures of quartz veins indicate the development of subgrains with some grain boundaries showing evidence of strain-induced grain bulging. Quartz c-axes show a Type-I crossed girdle, consistent with megathrust shear at peak temperature conditions of the mélange. Assuming that the deformation of metasomatic albite occurred at the same shear stress and strain rate as quartz dislocation creep, we made a deformation mechanism map for wet albite, using the wet albite flow law for diffusion or dislocation creep. The measured albite grain size is compatible with a preference for diffusion creep over dislocation creep. The calculated shear stress is ~45 MPa, less than those in the surrounding metabasalt. The observed albite metasomatism at temperatures > 330℃ but ≦ 370℃ and the hydrothermal experimental result showing the onset of albitization at 350 ℃ suggest that albite metasomatism occurred around the thermally inferred base of the seismogenic zone. We propose that when multiple albite metasomatism-related viscous shear zones accommodated megathrust shear, weaking and diffusion creep associated with metasomatic reactions and grain size reduction can potentially contribute to decreased megathrust strength near the seismic-aseismic transition.

Reference
Ujiie, K., Noro, K., Shigematsu, N., Fagereng, Å., Nishiyama, N., Tulley, C. J., et al. (2022). Megathrust shear modulated by albite metasomatism in subduction mélanges. Geochemistry, Geophysics, Geosystems, 23, e2022GC010569. https://doi.org/10.1029/2022GC010569