孟加拉残留洋盆地形成过程与构造单元划分

Tectonic Evolution and Element Division of the Bengal Remnant Oceanic Basin

孟加拉盆地位于印度板块东北部,处于印度板块、欧亚板块和缅甸微板块汇聚区,其东临印缅造山带,西接印度古地盾,北隔西隆高地与喜马拉造山带前渊相望,向南进入孟加拉湾。孟加拉盆地西部和北部发育在印度古陆上,其余大部分地区分布在白垩系残留洋壳上,属于残留洋盆地。本文结合区域构造和剖面演化特征,还原了孟加拉盆地的形成过程,并据此将孟加拉盆地划分为:前裂谷-同裂谷期、大陆漂移期、软碰撞期和硬碰撞期共四个构造演化阶段。结合构造演化过程和盆地现今构造沉积特征,将孟加拉盆地划分为:西部陆架、陆架斜坡带、南部坳陷、中部隆起带、北部坳陷和东部褶皱带共六个次级构造单元。演化过程研究表明,孟加拉盆地残留洋部分形成于古新世,并自始新世开始范围不断减小;至上新世,盆地东部残留洋消失,残留洋局限于盆地南部地区。孟加拉盆地残留洋部分控制了盆地沉积中心的分布和迁移,决定了盆地油气的分布。

The Bengal bain lies in the northeast of the India plate,where the India, Eurasia plate and the Burma micro plate converge. It abuts the Indo -Burma orogen in the east,the India continent shield to the west, facing the Himalayas foredeep across the Shilong plateau in the north, and extending southward to the Bengal bay in the south. Both the western and northern Bengal basin developed on the Indian ancient land, while the other majority of it is on the Cretaceous oceanic ernst,belonging to a remnant oceanic basin. This paper studies its regional tectonic evolution and characterizes the cross ections to illuminate the forming processes of the Bengal basin. It is suggested that this basin has undergone four stages ： the pre rift - synrift, continental drift, soft collision and hard collision. According to the tectonic evolution and the current tectono - sedimentary characteristics, the Bengal basin is divided into six secondary tectonic elements ： （1） west shelf, lying on the northeast margin of the India continent shelf; （2） shelf slope,located in the transition zone of the eastern India continent shelf to the deep Bengal basin; （3） north depression,located in the north part of the Bengal basin, also known as ＂Surma sub basin＂ ; （4） south depression, south part of the basin, totally lying on the oceanic crust ; （5） central uplift, an east - west - trending low uplift zone, separating the north depression from the south one; and （6） east Fold belt,the east part of the basin,a nearly north - south strike fold zone,adjacent to the India - Burma orogen in the east. Studies of the evolution processes show that the remnant oceanic part of the Bengal basin came into being in the Paleocene, and began to shrink from Eocene. In Pliocene,the eastern part of the remnant ocean basin vanished because of the plate convergence,and the remnant ocean was confined to the southern basin. The remnant oceanic part of the Bengal basin exerts a key control on the distribntion and migration of the deposition center as well as the hydrocarbon distribntion.