Slab Rollback Versus Delamination: Contrasting Fates of Flat-Slab Subduction and Implications for South China Evolution in the Mesozoic

Growing evidence supports that the widely developed Mesozoic folds and magmatism in the South China Block are attributed to the flat-slab subduction, followed by slab steepening, of the Paleo-Pacific Plate (PPP). However, the dynamics of the transition from flat- to steep-slab subduction, as well as the relationship between tectonic processes and surface deformation and magmatic events, are poorly constrained. Using a 2-D thermomechanical numerical model, we systematically investigate the effects of slab rollback or delamination following the eclogitization of a subducted oceanic plateau on the surficial topography, structural deformation, and magmatism. Our results indicate that the subduction of an oceanic plateau drives flat-slab subduction, which further leads to the development of a wide thrust fault zone, an uplifted and magmatically active foreland, and the foreland basin subsidence. This model is further complicated by the transition from flat- to steep-slab subduction through slab rollback or delamination which is controlled by the subduction rate and the timing of oceanic plateau eclogitization. The comparisons between our numerical results and the geological data from South China reveal that the slab delamination model reproduces the regional evolution of the Mesozoic South China. Based on our models, we suggest that the eclogitization during flat-slab subduction is not instantaneous, which is instead at least 18 Myr after the oceanic plateau reaches the ideal conditions of phase change.