Abstract: Abstract: Based on the seismic S-wave tomography, the authors carried out a systematic study of the speed structures of the lithosphere and asthenosphere of Eurasia and the West Pacific, and found a gigantic low-velocity anomaly zone between East Asia and the West Pacific . According to this discovery, combined with the tectonic, mantle petrological and geochemical characteristics, as well as other geophysical features, a mega-rift system is defined in the region. The 3D seismic Vs speed structures of the lithosphere and asthenosphere are very similar to those of the Mid-Pacific, Mid-Atlantic, and Mid-Indian ridges and their neighboring regions, but notably different from those of the present plate subduction zone on the East Pacific margin. On the basis of a further discussion on the dynamic characteristics of this region, it is concluded that the rift system resulted from the massively active extensional deformation and strong delamination and thinning of the lithosphere, formation of large rift basins, and upwelling and heating of a large amount of the asthenospheric substances since the Mid-Late Mesozoic. Marginal seas developed on the basis of the continental rift system, and the main extension stage is determined to be from the middle Oligocene to middle Miocene (32-13Ma). The extensions of these marginal seas had ended after the period from 17 to 15Ma; as a result not all of the seas were connected with the mid-ocean ridges. Four stages of the tectonic-deformational dynamic evolution may be distinguished, and it is considered that the tectonic superposition of interactions of various neighboring plates resulted in the formation of the continental rifts, marginal seas, and trench-arc-basin systems between present East Asia and the West Pacific since the Mid-Late Cenozoic.