Embrittlement Within Viscous Shear Zones Across the Base of the Subduction Thrust Seismogenic Zone

Abstract

Brittle fracturing occurred locally within viscously deforming hydrated oceanic crust and subducted sediments.• Fracturing is localised at viscosity contrasts and P -T conditions of metamorphic 10 dehydration reactions. 11• Rocks that deformed at P -T conditions away from dehydration reactions record 12 dominantly viscous behaviour.

Rheology of Naturally Deformed Antigorite Serpentinite: Strain and Strain‐Rate Dependence at Mantle‐Wedge Conditions

“…Outcrops at Mie show a layer of serpentinite with a foliation-normal thickness ∼130 m within schistose, intercalated metabasalt and metasediment (Figures 1b and 1d). The peak temperature experienced by foliation-defining carbonaceous material in NMR metasediment near Mie is 474°C ± 30°C (Mori et al, 2019), consistent with thermodynamic modeling of a metabasalt mineral assemblage (Tulley et al, 2022). The foliation is continuous between metasediment, metabasalt and serpentinite units (Figures 1b and 1d) and generally dips gently to the east (Figure 1c), although it is folded near lithological contacts (Figure 1d).…”

“…S/C foliation geometry (Figure 2a) implies top‐south shear, consistent with north‐trending subduction along the eastern Eurasian margin in the Late Cretaceous (Whittaker et al., 2007). As foliation is continuous between metasediment, metabasalt and serpentinite (Figures 1b and 1d), we infer that the serpentinite foliation developed at 474°C ± 30°C, as inferred for metasediment (Mori et al., 2019) and metabasalt (Tulley et al., 2022) foliations in the Mie area.…”

Megathrust Shear Modulated by Albite Metasomatism in Subduction Mélanges

“…The Makimine mélange in the Late Cretaceous Shimanto accretionary complex of eastern and central Kyushu (Figure 1b) records progressive subduction-related deformation and prehnite-actinolite to greenschist facies metamorphism at ∼300-350°C, comparable to the conditions near the downdip limit of the seismogenic zone in warm subduction zones (Hara & Kimura, 2008;Mackenzie et al, 1987;Palazzin et al, 2016;Tulley et al, 2022;Ujiie et al, 2018). The stratigraphic sequence of the mélange shows a repetition, possibly resulting from duplex underplating.…”

“…The Nishikashiyama mélange in the Nishisonogi metamorphic rocks of western Kyushu (Figure 1c) also records subduction-related deformation under epidote-amphibolite facies metamorphic conditions (Mori et al, 2014;Nishiyama, 1989;Tulley et al, 2022). The thickness of the mélange is at least 50 m (Nishiyama, 1989).…”

“…Quartz vein formation in locations of metamorphic dehydration within the dominantly viscous regime could explain the tremor signal commonly observed alongside SSEs (Fagereng et al, 2018;Fisher & Brantley, 2014;Tulley et al, 2022;Ujiie et al, 2018). Quartz veins, related to H 2 O and silica release from the metapelite during metasomatic reactions that potentially generate pore space through volume loss (Figure 5d), may also be involved in tremor-forming processes as may metasomatic reactions involving interaction between the mantle wedge and subducting sediments (Tarling et al, 2019).…”