Megathrust Shear Modulated by Albite Metasomatism in Subduction Mélanges

• Published in: Geochemistry, geophysics, geosystems : G3 geophysics, geosystems : G3, 2022-08, Vol.23 (8), p.n/a
• Main Authors: Ujiie, Kohtaro, Noro, Kazuya, Shigematsu, Norio, Fagereng, Åke, Nishiyama, Naoki, Tulley, Christopher J., Masuyama, Haruna, Mori, Yasushi, Kagi, Hiroyuki
• Format: Article
• Language: English
• Subjects: Amphibolite facies; Amphibolites; Basalt; Chemical reactions; Depth; Earthquakes; Isotopes; Plate boundaries; Plate tectonics; Quartz; Rheology; Rock; Sedimentary facies; Shear; Solifluction; Subduction; Uplift
• ISSN: 1525-2027; 1525-2027
• DOI: 10.1029/2022GC010569

Aseismic megathrust slip downdip 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. We examined exhumed subduction mélanges on Kyushu, Japan, which deformed at ∼370–500°C under greenschist to epidote‐amphibolite facies conditions, comparable to warm‐slab environments. The mélanges recorded fluid release and viscous shear localization associated with metasomatic reactions between juxtaposed metapelitic and metabasaltic rocks. Metasomatic reactions caused albitization of metapelite, resulting in depth‐dependent changes to megathrust rheology. In a mélange deformed at ∼370°C, very fine grained reaction products (metasomatic albite) facilitated grain boundary diffusion creep at stresses of ∼45 MPa, less than those in the surrounding metabasalt. Mineralogical and chemical changes during metasomatic reactions, and their field content, imply an onset of albite metasomatism at ∼350°C. Albite metasomatism therefore potentially contributed to decreased megathrust strength around the inferred thermally controlled base of the seismogenic zone. In a mélange deformed near the mantle wedge corner at ∼500°C, metasomatic reactions promoted local quartz vein formation and localized viscous shear at slow slip strain rates, during which the coarse‐grained metasomatic albite behaved as relatively rigid blocks in a viscous matrix. We suggest that albite metasomatism can facilitate changes in a megathrust slip mode with depth and may explain why slip mode changes from creep to SSEs with tremor with increasing depth. Plain Language Summary Along tectonic plate boundaries, where one plate slips beneath another, plate movement occurs by processes including large and devastating earthquakes slipping at meters/second, very small earthquakes called tectonic tremor, slow slip events (SSEs) slipping at millimeters/day, and steady creep slipping at centimeters/year. However, the factors controlling where these different slip styles occur remain poorly understood. On Kyushu, Japan, ancient plate boundary rocks have been exposed by uplift and erosion. Our measurements of structures and rock chemistry in these rocks revealed that chemical reactions between subducted basalts and sediments may influence the plate boundary slip behavior. In rocks that deformed near ∼370°C, chemical reactions produced very fine grained rocks that caused local weakening wit