东北亚中生代洋陆过渡带的研究及启示

Research on the Mesozoic ocean-continent transitional zone in the Northeast Asia and its implications

从中生代起,亚洲大陆作为一个统一的大陆岩石圈板块,开始了大陆边缘的组建和改造。本文采用构造地层-地体观点,依据生物地层学和碰撞造山带的不同特征,将东北亚洋陆过渡带从西到东分成了7个带:(1)受郯庐断裂系改造的华北克拉通东缘带;(2)以近陆缘物质为主的增生带I;(3)以异源混杂堆积为主的增生带II;(4)新西伯利亚-楚科奇-阿拉斯加陆缘增生带III;(5)陆缘火山-深成岩带;(6)科里亚克增生带IV;(7)堪察加-萨哈林-东北日本增生带V。其中自早白垩世末至古新世初形成的楚科奇海-东锡霍特阿林的火山-深成岩带作为太平洋板块开始正向俯冲并导致弧岩浆活动的重要标志。此前晚三叠世至早白垩世末,在转换大陆边缘活动背景下,大量低纬度的外来地体以左旋平移断裂作用向北迁移并斜拼贴在陆缘。时空格局的分带性和阶段性清晰地展示了东北亚大陆边缘洋陆演化的关系。作者基于上述研究,并结合其他学科近期研究成果,对中国东部中生代岩浆作用与太平洋板块俯冲作用的关系进行了讨论,认为中国东部晚侏罗世-早白垩世大规模岩浆活动的高峰期正值东北亚洋陆过渡带转换大陆边缘活动和地体拼贴增生的阶段。然而太平洋板块正向俯冲主要发生在早白垩世末-晚白垩世,此时我国东部的大规模岩浆活动业已结束。因此难以将中国东部的岩浆活动与太平洋板块的正向俯冲作用相联系。以年轻陆壳组成的大兴安岭为例,作者提出晚侏罗世-早白垩世不同深度的两种地质作用同时控制着中国东部岩浆活动的源区特征和侵位的空间:即深部软流圈底辟上涌与中-上部地壳受到的洋陆之间的剪切走滑作用形成的变形。

A continental margin has been composed and transformed on the Asian continent as a unitary continental lithosphere plate since the Mesozoic. The Northeast Asia ocean-continent transitional zone is identified by the authors, according to the viewpoint of teetonostratigraphie terrane, as consisted of seven belts with different characters of biostratigraphy and collision orogenes ： （ 1 ） the east margin of the North China Craton. reconstructed by Tanlu fault system; （ 2 ） accretion zone I dominated by nearly terrigenous materials ; （3） accretion zone II dominated by heterologous mrlanges; （4） accretion zone III of the New Siberian-Chukotka-Alaska continental margin; （5） the voleano-plutonic zone of the continental margin; （6） accretion zone IV （the Koryak accretion zone ） ; and （7） accretion zone V （ Kamchatka-Sakhalin-Northeastern Japan accretion zone）. Among those the Chukotka sea-East Sikhote-Alin volcano- plutonic zone, which is mainly of the Late Cretaceous, especially marks beginning of normal subduction of the Pacific plate and subsequent arc-magrnatie activities. Early in the Late Triassic-Early Cretaceous, a large number of terranes migrated northward and joined together obliquely to the continental margin in a sinistral transcurrent faulting of the active transform margin. The evolution of the relative movements between the ocean and the continent in the Northeast Asia continental margin is clearly demonstrated by such a spatiotemporal pattern with its special zonation and periodicity. This study is used, jointly with recent results in related disciplines, to discussions of the relation between the Mesozoic magmatism in the eastern China with subduction of the Pacific plate. It is concluded that the peak period of the large-scale magmatism in the Late Jurassic-Early Cretaceous is in coincidence with period in which the ocean-continent transition zone is active and terranes accretion occurs in the Northeast Asia continental margin. As mentioned in this work, however, the normal subduction of Pacific Plate occurs in the end of the Early Cretaceous to the Late Cretaceous, when the large- scale magmatism has ended. It is, therefore, difficult to link the magmatic activities in eastern China with the subduction of the Pacific plate. Taking the Da Hinggan Mts. composed of young continental crust as an example, the authors suggest that the source characteristics of eastern China magmatism and its intrusive space are controlled by two geological processes in different depths in the Late Jurassic-Early Cretaceous simultaneously, i.e. the upwelling of deep diapers of asthenosphere and the deformation of the middle- upper crust subjected to shear strike-slip between the ocean and the continent.