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

Protracted Subduction of the European Hyperextended Margin Revealed by Rutile U‐Pb Geochronology Across the Dora‐Maira Massif (Western Alps)

“…Above Monviso, the Schistes Lustrés are interpreted as an oceanic sedimentary sequence, with small ophiolitic bodies that are interpreted as oceanic core complexes (e.g., Lagabrielle et al., 2015). The Schistes Lustrés are divided in three main bodies (e.g., Herviou et al., 2022; Plunder et al., 2012) with increasing P ‐ T conditions from west to east, from 1.2 GPa to 300°C to 2.3 GPa and 480°C (Agard et al., 2001; Plunder et al., 2012), with the western exposures lying structurally above the eastern ones. The peak metamorphism of these units is thought to have occurred around 62–55 Ma, followed by progressive retrogression from 51 to 35 Ma (Agard et al., 2002; Villa et al., 2014).…”

Paleocene‐Eocene High‐Pressure Carbonation of Western Alps Serpentinites: Positive Feedback Between Deformation and CO2‐CH4 Fluid Ingression Responsible for Slab Slicing?

“…Most of the Liguro‐Piemont slices expose an almost complete lithostratigraphic succession, with a preserved serpentinite basement often dismembered during exhumation (Herviou et al., 2022). For the LPU and LPM units, which contain this complete succession, the décollement layer was therefore necessarily in the serpentinites.…”

“…Several independent subduction slices were identified in transects across the complex (Agard, 2021;Fudral, 1996;Lagabrielle, 1987;see Herviou et al, 2022 for a compilation). Recently, through the compilation of metamorphic, structural, lithostratigraphic, and radiochronogical data, Herviou et al (2022) reappraised the overall structure of the complex, highlighting a trimodal distribution of units with an almost continuous eastward increase in peak burial conditions from the Liguro-Piemont Upper (LPU, 320°C-400°C; 1.2-1.9 GPa) to the Middle (LPM, 415°C-475°C; 1.7-2.2 GPa) and Lower units (LPL, 500°C-580°C; 2.2-2.8 GPa; Figure 1). The LPU and LPM units, metamorphosed under blueschist-facies conditions at peak P-T conditions, are dominated by sediments (>90%), whereas the LPL units, metamorphosed under eclogite-facies conditions at peak burial, are much 3 of 21 richer in mafic-ultramafic rocks (>40%, Herviou et al, 2022).…”

“…Recently, through the compilation of metamorphic, structural, lithostratigraphic, and radiochronogical data, Herviou et al (2022) reappraised the overall structure of the complex, highlighting a trimodal distribution of units with an almost continuous eastward increase in peak burial conditions from the Liguro-Piemont Upper (LPU, 320°C-400°C; 1.2-1.9 GPa) to the Middle (LPM, 415°C-475°C; 1.7-2.2 GPa) and Lower units (LPL, 500°C-580°C; 2.2-2.8 GPa; Figure 1). The LPU and LPM units, metamorphosed under blueschist-facies conditions at peak P-T conditions, are dominated by sediments (>90%), whereas the LPL units, metamorphosed under eclogite-facies conditions at peak burial, are much 3 of 21 richer in mafic-ultramafic rocks (>40%, Herviou et al, 2022). Petrological and lithostratigraphic data suggest that offscraping of the LPU and LPM units was mostly controlled by lithological contrasts within pelagic shales or along contacts within the uppermost serpentinized mantle and possibly enhanced by high fluid pressure (Herviou et al, 2022).…”

“…The LPU and LPM units, metamorphosed under blueschist-facies conditions at peak P-T conditions, are dominated by sediments (>90%), whereas the LPL units, metamorphosed under eclogite-facies conditions at peak burial, are much 3 of 21 richer in mafic-ultramafic rocks (>40%, Herviou et al, 2022). Petrological and lithostratigraphic data suggest that offscraping of the LPU and LPM units was mostly controlled by lithological contrasts within pelagic shales or along contacts within the uppermost serpentinized mantle and possibly enhanced by high fluid pressure (Herviou et al, 2022). In contrast, major dehydration reactions (such as lawsonite breakdown in sediments) likely controlled the slicing of the LPL units at eclogite-facies conditions, possibly through high fluid pressure conditions and rock embrittlement (Herviou et al, 2022).…”