对弧后扩张作用的探讨

A STUDY OF BACK-ARC SPREADING

俯冲之后的大洋板块继续向陆运动,导致大陆地壳和上地幔内部各圈层在深部发生多层次的水平剪切,引起弧后宽达几千公里地区的裂陷扩张,并形成盆地。其形成的盆地平行于俯冲带。弧后扩张作用的强度和存在与否,取决于仰、俯冲板块双方运动速度矢量的组合类型。离散型产生扩张,聚敛型则以挤压为主。

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).