The Initial Flat Subduction of Paleo-Pacific Plate Beneath South China Block in the Mesozoic

The Mesozoic foreland cratonward migrated orogen and related synorogenic magmatism in the South China were induced by the flat subduction of Paleo-Pacific Plate (PPP). The PPP started flat subduction beneath South China Block (SCB) in ca. 270±5 Ma with a passive margin in southeastern China coast transferring to an active one. Compared with current flat subduction zones, the age of subducted PPP beneath SE China was relatively old (about 70 Ma). The dip angle of the initial weak zone between continental and oceanic domains usually plays a dominant role in dictating the initial angle of the subducted slab. Under what range of dip angle will flat subduction happen to such an old slab? Here, using 2-D thermomechanical numerical models, we investigate the range of the dip angle of the weak zone that can lead to the initial flat subduction of PPP beneath SCB in the Mesozoic. Our results indicate that the PPP subducted flatly only with the dip angle of the weak zone smaller than 12 degree. This small dip angle led to the occurrence of initial flat subduction and the associated strong interplate viscous coupling in PPP flat subduction, resulting in strong SCB deformation and seismic activity in the Mesozoic.