渤海新生代盆地浅部构造迁移特征及其深部动力学机制探讨

Cenozoic shallow tectonic migration and the deep dynamic mechanism in the Bohai Sea

渤海湾盆地的海域部分与渤海湾盆地其他部分一样,新生代为断陷-拗陷盆地.中渐新世以来,渤海湾盆地的沉积沉降中心迁移至渤海海域,并且拗陷期出现普遍的加速沉降现象.综合分析渤海各坳陷新生代构造样式和断裂时空展布特征,表明断裂发育总体受伸展控制,只是在中渐新世后受走滑影响.选取基本垂直渤海构造走向的6条大剖面,进行伸展量估算,并在剖面上选取虚拟井计算沉积沉降速率,结果表明,在断陷阶段,孔店组-沙四段沉积期,各坳陷沉积沉降速率较低且相差不大,发育多个沉积沉降中心;至沙三段沉积期沉积沉降速率达到最快,东营组沉积期沉积沉降速率变慢,但沉积沉降中心已从四周逐步迁移至渤中坳陷;拗陷阶段,以渤中坳陷为中心,出现的强烈沉积沉降,明显高于正常拗陷阶段沉积沉降速率.结合最新的深部探测资料,认为渤海新生代盆地的断裂展布,受中生代末先存的构造格局影响;断陷盆地的形成,则是太平洋板块俯冲的持续后退和印藏碰撞远程效应持续东进共同作用的结果;中渐新世以来的加速沉降现象,则是太平洋俯冲板片前缘向东迁移引起的岩石圈迅速冷却的最直接表现.

The Bohai sea area is developed with Cenozoic faulted-depressed basin.Since Oligocene,the depositional or subsiding centers of the Bohai Bay had moved to the Bohai sea area,and the subsidence was accelerated during the depressing period.The characteristics of structural styles in each Cenozoic depression and temporal-spatial variation faults were analyzed based on previous studies.It shows that the development of faults is generally controlled by extension,and influenced by slip-faults only after Oligocene.Choosing 6sections which were basically perpendicular to the tectonic trends of the Bohai sea,we made an estimation of the extension proportions,and the rate of subsidence of the Bohai sea was also calculated.The results suggested that in the uplifting period during the 4th Member of the Shahejie Formation and the Kongdian Formation,the Bohai sea area has several depositional or subsiding centers,and the rates of subsidence were slow and barely the same to each other;during the third Member of Shahejie Formation,the rate of subsidence were the fastest,and during the Dongying stage,they became slower again,and the depositional or subsiding centers had transited to the Bozhong Depression.In the depressing period,intensively subsidence or deposition around the Bozhong Depression was obviously higher than normal.Combined with the latest data,it was concluded that the development of faults was influenced by the Mesozoic tectonic framework;the formation of the faulted basin was the joint result of the Pacific plate＇s continuous backmoving and the India-Tibet collision.Accelerated subsidence since Oligocene,is the most direct result of the rapid cooling of the lithosphere after the subduction frontier of the Pacific plate moved to the east.