Subducted fragments of the Liguro-Piemont ocean, Western Alps: Spatial correlations and offscraping mechanisms during subduction

Clément Herviou (1) , Philippe Agard (1) , Alexis Plunder (2) , Kevin Mendes (1) , Anne Verlaguet (1) , Damien Deldicque (3) , Nadaya Cubas (1)

1 iSTeP - Institut des Sciences de la Terre de Paris
2 BRGM - Bureau de Recherches Géologiques et Minières (BRGM)
3 LGENS - Laboratoire de géologie de l'ENS

Fragments of subducted slow-spreading oceanic lithosphere are exposed continuously in the Liguro-Piemont domain of the Western Alps. By combining new and literature petrological data, interpolated maps of maximum temperatures, maximum Si contents of phengite as a proxy for peak pressure and thermodynamic modelling, we provide a detailed framework of the peak metamorphic conditions experienced by the distinct subduction slices. High-resolution mapping confirms the marked eastward increase in metamorphic grade throughout the domain, as well as within some slices. The compilation of lithostratigraphic, structural and radiochronological data and the estimation of sediment/mafic-ultramafic ratio for each slice allow refining the origin of these tectonometamorphic units within the former oceanic domain. The refined structural sketchmap allows to restore the geometries of the Alpine subduction at peak burial conditions. Results point to a trimodal distribution of the units with an increase in metamorphic conditions from the Upper (LPU; 320-400°C- 1.2-1.9 GPa) to the Middle (LPM; 415-475°C- 1.7-2.2 GPa) and to the Lower units (LPL; 500-580°C- 2.2-2.8 GPa). The blueschist-facies LPU and LPM units are dominated by sediments (>90%), whereas the eclogitic LPL units are far richer in mafic-ultramafic rocks (>40%). These characteristics, along with lithostratigraphic differences, reflect major differences in their initial paleogeography and/or in the mechanisms responsible for material offscraping from the downgoing slab. The peak burial depths of the LPU, LPM and LPL units are similar to those inferred for slicing and underplating in both modern and fossil subduction zones. Petrological and lithostratigraphic data suggest that the offscraping of the LPU and LPM units was mostly controlled by lithological contrasts, within pelagic shales or along contacts within the uppermost serpentinized mantle. In contrast, major dehydration reactions (such as lawsonite breakdown in sediments) likely controlled the offscraping of the LPL units at eclogite-facies conditions, possibly through high fluid pressure conditions and rocks embrittlement.

Mots clés: Oceanic subduction; Liguro-Piemont; Schistes Lustrés; Western Alps; Tectonic slicing; HP-LT metamorphism

Domaines: Planète et Univers [physics]

Clément Herviou, Philippe Agard, Alexis Plunder, Kevin Mendes, Anne Verlaguet, et al.. Subducted fragments of the Liguro-Piemont ocean, Western Alps: Spatial correlations and offscraping mechanisms during subduction. Tectonophysics, 2022, 827, ⟨10.1016/j.tecto.2022.229267⟩. ⟨insu-03636647⟩