Mécanismes de l'extension continentale au Mésozoïque en Asie de l'Est

Abstract : Continental lithosphere can be stretched according to three modes (wide rift, narrow rift, Core Complex). In East Asia, a continental extension occurred during the Late Mesozoic to Cenozoic times and seems to do not correspond to any of three modes currently defined. This period is characterised by an exceptional lithospheric thinning (> 100 km) with the presence of MCC, sedimentary basins and a huge magmatic activity. Based on a multi-scale approach, this work aims to better understand the mechanisms of this lithospheric deformation (never addressed) and the engine of the extension (yet highly debated). This study provides new multidisciplinary constraints from the analysis of finite strain (ductile or brittle), rock magnetism (AMS, palaeomagnetism), geochronology (U/Pb on zircon and 40Ar/39Ar on single crystals) and gravity. Different objects have been recognised, revealing different amounts of extension (MCC vs. sheared pluton), and show that the continental crust is locally highly deformed, with emplacements of large MCCs between "rafts" or "boudins" domains which are weakly strained to unstrained. By comparison of available crustal and mantle data (seismic tomography, geochemistry), this study shows that the lithospheric thinning recognised for the Mesozoic is mainly related to a major mantle heat flux, the extension plays a limited role in this thinning (<20%). In addition, given the exceptional high geothermal gradient in the region at the end of the Mesozoic, it seems very likely that MCC may have developed without pre-thickened crust. Comparative analysis of stretching directions within the crust and mantle highlights that the subduction of the (palaeo) Pacific plate along the East Asian margin may play an initial and major role during Late Mesozoic extensional event. A geodynamic model has been proposed to show the role of the successive retreat of subducting slabs coupled to a thermal erosion of the lithosphere.
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Nicolas Charles. Mécanismes de l'extension continentale au Mésozoïque en Asie de l'Est. Sciences de la Terre. Université d'Orléans; Institute of geology and geophysics, Chinese academy of sciences, 2010. Français. ⟨NNT : 2010ORLE2055⟩. ⟨tel-00604173⟩

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