First finding of continental deep subduction in the Sesia Zone of the Western Alps and implications for subduction dynamics

Abstract

Continental deep subduction after the closure of large oceanic basins is commonly ascribed to gravitational pull of the subducting oceanic slab. However, it is not clear how continental lithosphere adjacent to small oceanic basins was subducted to mantle depths. The Sesia Zone in the Western Alps provides an excellent target for exploration of subduction dynamics in such a tectonic setting. Here we report the first finding of coesite in a jadeite-bearing orthogneiss from the Sesia Zone, providing the first evidence for deep subduction of the continental crust to mantle depths for ultrahigh-pressure (UHP) metamorphism in this zone. Three coesite inclusions were identified by laser Raman spectroscopy in two garnet grains. Based on zircon U-Pb dating and trace element analysis, the UHP metamorphic age was constrained to be 76.0 ± 1.0 Ma. The phase equilibrium modelling yields peak metamorphic pressures of 2.8-3.3 GPa, demonstrating the continental deep subduction to mantle depths of >80 km. The subducted continental crust was a rifted hyperextended continental margin, which was converted to the passive continental margin during seafloor spreading and then deeply subducted during the oblique convergence between Adria microplate and Eurasian plate in the Late Cretaceous. Because the slab pull could only play a limited role in closing small oceanic basins for continental collision, the distal push of either continental breakup or seafloor spreading is suggested as the major driving force for the deep subduction of continental crust in the Western Alps. Therefore, deep subduction of the continental crust bordering small oceanic basins would be induced by the far-field stress of compression, whereas that bordering large oceanic basins was spontaneous due to the oceanic slab pull. This provides a new insight into the geodynamic mechanism of continental deep subduction.

First finding of coesite in the eclogitic continental crust of the Sesia Zone, Italian Western Alps

“…This immense work was synthesized in the Carta Geologica delle Alpi Occidentali , an amazing map at 1 : 400.000 scale published in 1908, showing in particular the Eclogitic micaschist complex of the Sesia-Val di Lanzo (shortly Sesia) Zone which is presently studied by Chen et al. [ 2 ]. Previously envisaged as pre-Permian, the metamorphism of this eclogitic continental complex was re-classified by Dal Piaz et al.…”

“…The first finding of coesite in the Western Alps was due to Chopin [cited in Ref. 2 ], as evidence of UHP metamorphism in the continental crust of the Penninic (European) southern Dora Maira nappe. Subsequently, coesite and microdiamonds were found by Reinecke [cited in Ref.…”

“…Subsequently, coesite and microdiamonds were found by Reinecke [cited in Ref. 2 ] and Frezzotti et al. [cited in Ref.…”

“…[cited in Ref. 2 ] in eclogitic metabasalts of Cignana (Aosta valley), within the Zermatt-Saas ophiolitic unit below the Austroalpine Dent Blanche nappe. Two other coesite sites have been recently discovered by Manzotti et al.…”

“…[ 2 ] in the Eclogitic micaschist complex of the Sesia Zone, Argand's ‘root zone’ of Dent Blanche nappe [ 3 , 4 ]. Among them, Chen's paper [ 2 ] deserves to be highlighted for its original contribution to the geochronology, geochemistry and petrology on leucocratic metagranitoids of the Sesia eclogitic complex in the lower Aosta valley: this unit underwent UHP metamorphism at 76.0 ± 1.0 Ma (U-Pb zircon ages), 2.8–3.3 GPa and 450–520°C [ 2 ], corroborating previous regional dating and P-T estimates [ 5 , 6 and references therein]. In this view, all the continental and oceanic units of the Austroalpine-Penninic collisional wedge [ 7 ] experienced subduction and a similar UHP metamorphism [ 2 , 5 , 6 ].…”