南海区域岩石圈的壳-幔耦合关系和纵向演化

CRUST MANTLE COUPLING MODEL AND VERTICAL EVOLUTION OF LITHOSPHERE OF SOUTH CHINA SEA REGION

南海区域岩石圈由地壳层和上地幔固结层两部分组成。具典型大洋型地壳结构的南海海盆区莫霍面深度为９～１３ｋｍ，并向四周经陆坡、陆架至陆区逐渐加深；陆缘区莫霍面一般为１５～２８ｋｍ，局部区段深达３０～３２ｋｍ，总体呈与水深变化反相关的梯度带；东南沿海莫霍面深约２８～３０ｋｍ，往西北方向逐渐增厚，最大逾３６ｋｍ。南海区域上地幔天然地震面波速度结构明显存在横向分块和纵向分层特征。岩石圈底界深度变化与地幔速度变化正相关；地幔岩石圈厚度与地壳厚度呈互补性变化，莫霍面和岩石圈底界呈立交桥式结构，具有陆区厚壳薄幔—洋区薄壳厚幔的岩石圈壳－幔耦合模式。南海区域白垩纪末以来的岩石圈演化主要表现为陆缘裂离—海底扩张—区域沉降的过程，现存的壳－幔耦合模式显然为岩石圈纵向演化产物，其过程大致可分为白垩纪末至中始新世的陆缘裂离、中始新世晚期至中新世早期的海底扩张和中新世晚期以来的区域沉降等三个阶段。

According to the buried depth of Moho discontinuity,velocity structure of upper mantle and the computed results of tectonic subsidence and thermal evolution,the crust mantle coupling model and vertical evolution of lithosphere of the South China Sea region was discussed in the present paper.The lithosphere of South China Sea region consists of the crust and solid layer of mantle.The buried depth of Moho discontinuity gradually deepens from the South China Sea Basin,slope and shelf to the continent and is 9 13 km in the basin with the typical oceanic crust,15 28 km in the margin (30 32 km locally)and 28 30 km in the southeast China coast area with the deepest to be more than 36 km.The seismic S wave velocity structure of upper mantle in the South China Sea region is characteristic of the regional blocking and vertical layering.The depth change of the bottom of lithosphere is positively correlated with that of the velocity of mantle.The buried depth of Moho discontinuity generally shows the mirror relation to the topography.The thickness of mantle lithosphere presents the complementarity with that of the crust.The bottoms of Moho and lithosphere are of flyover structure and form the crust mantle coupling model with the thick crust,thin mantle in the continent and thin crust,thick mantle in the ocean.The lithospheric evolution of South China Sea region since the end of Cretaceous was mianly a process of marginal rifting,seafloor spreading and regional subsidence.The present crust mantle coupling model clearly resulted from the vertical lithospheric evolution.Three stages were mostly recognized from the evolution history:(1)the marginal rifting from the end of Cretaceous to middle Eocene;(2)the seafloor spreading during the late of middle Eocene and the early of Miocene;and (3)regional subsidence since the late of Miocene.