尼日尔三角洲的重力滑动构造

The gravity-slide tectonics in the Niger Delta

位于南大西洋东岸被动大陆边缘之上的尼日尔三角洲发育一大型重力滑动构造。自非洲大陆向南大西洋方向,该重力滑动构造可以划分出前缘挤压构造变形区和后缘拉张构造变形区。前缘挤压构造变形,形成褶皱冲断带;后缘拉张构造变形,形成堑垒构造;两者之间的过渡带发育滑脱褶皱。三角洲沉积的时代为始新世—第四纪,地层剖面自下而上依次是海相页岩(Akata组)、近海三角洲相砂岩—页岩互层(Agbada组)和陆相冲积砂岩(Benin组)。3个岩石地层单元都是穿时的,向大洋方向变新。重力滑动构造的主滑脱面位于Akata海相页岩中。根据生长地层、不整合以及卷入变形的地层时代,重力滑动构造起始于约12 Ma,伴生有广泛发育的泥构造。重力滑动构造的形成可能与新生代喀麦隆火山带(Cameroon volcanic line)的火山活动有一定的关系。

There exist a large scale gravity-slide tectonics in the Niger Delta located on the eastern passive continental margin of the South Atlantic Ocean.From the African Continent to the South Atlantic Ocean,the gravity-slide tectonics can be divided into the front compressional deformation province and the trail extensional deformation province.The front compressional deformation formed a fold-thrust belt.The trail extensional deformation formed horst-graben structures.The detachment folds are well developed in the transitional zone between these two deformation provinces.The delta sequence was formed in Eocene-Quaternary,constituted with the marine shale(Akata Fm.),paralic sand and shale(Agbada Fm.)and continental alluvial sands(Benin Fm.)from the bottom to the top.These three lithostratigraphic units are diachronous.All of them are younger seaward.The main detachment fault of the gravity-slide tectonics lies in the Akata marine shale.According to the growth strata,unconformity and the age of the strata involved in the deformation,the gravity-slide deformation onset at 12 Ma.Accompanied with the gravity tectonics,shale tectonics are widely developed in the Niger Delta.There is possibly some relation between the gravity-slide tectonics and the Cenozoic volcanic activities of the Cameroon volcanic line.