Brittle–ductile transients during continental subduction and exhumation: inheritance, fluids, and implications for seismicity

Temperature-dependent rheological changes control the long-term brittle-ductile transition, but several other factors may control the transient and localized switching between brittle and ductile rheology in the continental crust during orogen build-up. Rheological transients are inferred from, for example, mutual overprinting relationship between localized ductile and brittle deformation features (faults, veins, foliations, and shear zones) in the field. These occurrences are commonly used as a starting point for developing models of the mechanisms controlling seismicity outside the upper-crustal schizosphere, including lower-crustal earthquakes, intermediate-depth seismicity, and slow seismic phenomena such as tremor and slow slip. Current geophysical/seismological investigations show indeed the occurrence of different types of seismicity potentially related to continental subduction; however, most recorded seismicity appears to be linked to collision and exhumation processes. Based on field observations from subducted and exhumed Alpine continental units (Corsica and the Central Alps), this contribution addresses key challenges in interpreting brittle–ductile transient rheology from the geological record, discussing how structural inheritance, metamorphic overprinting, and fluid composition complicate interpretations of seismic versus aseismic deformation.

During prograde subduction, increasing temperature and pressure should promote a progressive transition from brittle to ductile rheology. The blueschist-ecogitic facies continental units of Alpine Corsica, prime example of continental subduction, show indeed a general brittle-to-ductile (and potentially seismic-to-aseismic) evolution, with distinct deformation features developed across increasing metamorphic grades. However, the post-kinematic increment in metamorphic conditions may overprint brittle structures with higher-grade assemblages, precluding us to understand if these field occurrences are really representative of (seismic) rheological transients during deep subduction, or if they simply result from structural inheritance from the pre-orogenic stages. New field observations from the Crystalline Massifs of the Central Alps (Aar massif, Gotthard nappe) further demonstrate the role of inherited structures in steering the retrograde rheological evolution of the continental crust during Alpine collision and exhumation, challenging models for mid-crustal seismicity and strain localization. Rheological transients are commonly associated with fluid flow and fluid pressure fluctuations, manifested in the field as mineralised veins precipitating from metamorphic fluids. Yet, the polyphase nature of metamorphic fluids (e.g., CO₂-, CH₄-bearing fluids), and the resulting variability in chemo-physical properties are rarely considered in rheological models. CO₂-rich fluids and the resulting carbonate-bearing mineral veins might lead to transient rheological switches in both the brittle and ductile fields, as documented by sheared carbonate-bearing breccias in several Alpine crystalline units and plutons.

Together, these observations highlight that brittle–ductile transients inferred from the geological record require careful evaluation of inheritance, metamorphic overprinting, and fluid composition before being extrapolated to crustal rheology and seismicity models.