Crustal P-wave velocity structure of the Dabie-Sulu ultrahigh-pressure metamorphic belt and mechanisms of its subduction and exhumation

Abstract：Analysis of 3-D P-wave velocity structure shows that the crustal velocity structure below the Sulu-Dabie ultrahigh-pressure metamorphic belt has the following basic features: the upper crust has a markedly high velocity and is upwarped, the middle crust thickens, the lower crust is deeply buried and the Moho is downwarped. Compared with the Dabie area, the upper crust below the Sulu-Dabie ultrahigh-pressure metamorphic belt has a high P-wave velocity，and the area of the high-velocity region at the surface and volume of the high-velocity body of the upper crust are large. However, the Moho beneath the Dabie orogen is more deeply downwarped than that beneath the Sulu UHPM terrain, and the mountain root of the crust beneath the Sulu region gradually wore away. All these indicate that more intense subduction and exhumation took place in the Sulu terrain, and more UHP metamorphic rocks with higher velocity and density were exhumed to the crust and surface in the Sulu orogen than in the Dabie region. However, in Sulu, orogeny and UHP metamorphism started later but ended earlier. The orogeny in Sulu probably underwent a relatively short but intense evolution process. The formation and evolution of the orogeny and UHP metamorphic belt in Sulu may probably be related to the large-scale sinistral strike-slip motion of the Tanlu fault belt and the effects of the NW-SE extensional stress field in North China. Especially, the effects of the regional-scale extensional stress field in the North China region since the Mesozoic is probably an important tectonic cause for the exhumation of voluminous UHP metamorphic rocks subducted into the mantle to the upper crust or the surface.