The subducting sinking oceanic plate in the subduction zone is melted as thedepth and temperature increase, yet it does not stop moving. Both the meltedand melting oceanic slabs continue to move landward together with the asthenos-phere and mantle. As a result, various layers may slip horizontally along weakplanes or zones in continental crust and upper mantle. Shearing between the lay-ers results from differential horizontal movements of the layers. Consequenly, hor-izontal extension vastly occurs in the upper layers of the continental lithosphere,thus leading to the development of spreading basins in weak zones behind theisland arcs (Fig. 3). Such basins can be classified into three types: marginal sea basins (e. g. theJapan Sea), continental basins near the margin (e. g. the Bohai Bay Basin), andcontinental basins far off the margin (e. g. the Fenhe-Weihe Basih). All thesebasins extend parallel to the trenches and, on the whole, become younger land-ward. The style of back-arc spreading, different from that of sea floor spreading,is commonly characterized by asymmetry, multicentres, microtaphrogeny and weakopening. Whether the spreading may occur and be strong or weak depends on thetype of combination of the velocity vectors of obducting and subducting plates(Fig. 4). The convergence is marked by intense compression in the trench-are-back are region, no back-are spreading basins and fast consumption of oceanicplates, whereas the divergence is characterized by weak coupling in subductionzones, pronounced spreading behind island ares and slow consumption of oceanicplates. As a result of different types of combination of the velocities, there aremany marked differences between the western and eastern Pacific in respect togeological features on the margins, the age and dip angles of the subducting pla-tes (Fig. 2).