Lithospheric-scale dynamics during continental subduction: Evidence from a frozen-in plate interface

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 the pressure-temperature (P-T) and spatio-temporal evolution of rocks on each side of the tectonic contact (Briançonnais−Liguro-Piemont [Br-LP] contact) separating the subducted oceanic remnants from the subducted continental fragments along the Western Alps. 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 no significant metamorphic gap exists. The preservation of similar P-T conditions on both sides of the Br-LP contact is interpreted as resulting from offscraping of the Liguro-Piemont and later Briançonnais units at similar depths, as supported by the ∼10 m.y. gap between peak burial ages of both zones. The similar depth range reached by the various units reflects systematic variations of slicing and mechanical coupling along the plate interface suggesting that (1) similar slicing mechanisms and strain localization prevailed during both oceanic and continental subduction, and (2) the Br-LP contact represents a frozen-in subduction interface. The end of high-pressure and low-temperature metamorphism and continental subduction at ca. 33 Ma would mark the stalling of subduction interface dynamics and the onset of strain distribution across the plate interface and into the lower plate.