Plate interface frozen at the very end of continental subduction

Continental subduction and collision are not merely follow-ups of oceanic subduction but mark the transition from lithospheric-scale deformation localized along the subduction interface to crustal-scale deformation distributed across the orogen. In order to unravel the processes typifying the dynamic changes from oceanic subduction to collision, we have characterized pressure-temperature (P-T) and spatio-temporal evolution of rocks on either side of the tectonic contact (Briançonnais/Liguro-Piemont contact – Br/LP contact) separating the subducted oceanic remnants from the subducted continental fragments along the length of the Western Alps. West of the contact, the Briançonnais zone is considered as a micro-continent composed of pre-Alpine basement and Paleozoic to Meso-Cenozoic cover units. East of the contact, the Liguro-Piemont zone corresponds to a nappe-stack, with three groups of oceanic (upper, middle and lower) units. The Piemont zone is pinched in between the two in the southern part of the Western Alps and correspond to the distal part of the Briançonnais continental margin.

Results indicate that the maximum temperature and pressure difference on each side of the contact is generally < 30°C and < 0.3 GPa, evidencing that (i) no significant metamorphic gap exists between both sides and that (ii) offscraping of the continental fragments occurred at the same depth as the oceanic ones. The dataset also shows a northward increase of peak P-T conditions from ~300°C-1.2 GPa to ~500°C-2.0 GPa. The preservation of similar P-T conditions on both sides of the Br/LP contact can tentatively be assigned to either (1) offscraping of the Liguro-Piemont and later of the Briançonnais at similar depths or (2) entrainment and joint burial of the Liguro-Piemont (previously accreted or subducted) fragments together with the Briançonnais margin. The latter hypothesis, however, is not supported by the ~10 My gap between the peak burial of the Briançonnais and Liguro-Piemont zones. The recurrent depth range of the various units, which reflects systematic variations of slicing and mechanical coupling along the plate interface (Herviou et al., 2022), suggests that (1) similar slicing mechanisms and strain localization prevailed during both oceanic and continental subduction and (2) shows that the Br/LP contact represents a frozen-in subduction interface. The end of high-pressure and low-temperature metamorphism and continental subduction at ~33 Ma would thus mark the stalling of subduction interface dynamics and the onset of strain distribution across the plate interface and into the lower plate.

 

• Herviou, C., Agard, P., Plunder, A., Mendes, K., Verlaguet, A., Deldicque, D., Cubas, N., 2022. Subducted fragments of the Liguro-Piemont ocean, Western Alps: Spatial correlations and offscraping mechanisms during subduction. Tectonophysics 827, 229267. https://doi.org/10.1016/j.tecto.2022.229267