From oceanic to continental subduction and collision in the Western Alps: P-T-time evolution of the Briançonnais/Liguro-Piemont plate contact

Understanding the evolution of convergent plate boundaries and the mechanisms of strain accommodation through time and space is made possible by studying exhumed subduction complexes within orogenic belts. This study uses the internal zones of the Western Alps, one of the largest and best-preserved fossil subduction complexes in the world, to track the transition from subduction to collision. We herein combine in-situ Ar-Ar and Rb-Sr data on white micas with pressure-temperature estimates derived from pseudosection modeling and Raman thermometry on carbonaceous material, along eleven transects crossing the mountain belt.

Results (i) confirm the preservation of similar peak pressure-temperature conditions on both sides of the Briançonnais/Liguro-Piemont contact (as proposed by Mendes et al., 2023), (ii) indicate that the Briançonnais cover units reached their metamorphic peak around 50 ± 5 Ma and likely correspond to the former cover of the Dora-Maira massif, (iii) document the progressive slicing of large basement units at the end of the subduction process and the evolution of deformation (from localized at interface-scale to  distributed at crustal-scale), and (iv) allow refining the initial structure of the continental margin and its role during convergence.

This study also highlights the merits and limitations of Ar-Ar and Rb-Sr radiochronological systems, and in particular the complexity of the record associated with multiple metamorphic recrystallizations. Although the variable and in places marginal extent of excess argon complicates the interpretation of Ar-Ar ages, this study shows that the Ar-Ar system is likely more robust than the Rb-Sr system for tracking recrystallization history. The latter system appears sensitive to late re-equilibration episodes, potentially linked to fluid circulation.

Mendes, K., Agard, P., Plunder, A., Herviou, C., 2023. Lithospheric-scale dynamics during continental subduction: Evidence from a frozen-in plate interface. Geology 51, 1153–1157. https://doi.org/10.1130/G51480.1