洋-陆转换带类型与成因机制

Formation mechanism and types of Ocean-Continent Transition Zones

洋-陆转换带是被动陆缘的特殊构造,是伸展背景下大陆岩石圈与大洋岩石圈相互作用的关键区域,对于理解和认识大洋和大陆的地球动力过程、机制尤为关键。基于不同的被动陆缘类型,本文总结了不同类型被动陆缘的洋-陆转换带分类及特征,同时探讨其成因机制。根据大陆边缘类型,洋-陆转换带可以划分为四类,火山型被动陆缘中以海倾反射层和下地壳高速体为特征的洋-陆转换带、非火山型被动陆缘蛇纹石化地幔橄榄岩出露的洋-陆转换带、异常减薄洋壳组成的洋-陆转换带、强烈减薄陆壳为主的洋-陆转换带。洋-陆转换带成因模式取决于不同类型被动陆缘的伸展破裂过程,火山型被动陆缘起源于主动的火山裂谷,通过热作用来减薄岩石圈的底部进而发生地幔熔融,产生溢流玄武岩,浅表形成海倾反射体,下地壳表现为高P波速度异常且巨厚的高速体。非火山型陆缘的岩石圈横向伸展与深度相关,岩石圈变形既有均一纯剪切变形(均匀伸展)也有不对称的简单剪切变形(拆离断层)。

Ocean-Continent Transition Zone（OCT）is a special structure of passive continental margins,which is the key area of interaction between continental and oceanic lithospheres under an extensional tectonic setting.It is especially important for understanding the geodynamical process of the ocean and the continent.Based on the different types of passive continental margins,the classification and characteristics of the OCTs are summarized,and the formation mechanisms are also discussed in this paper.The OCTs can be classified into four types：OCT characterized by seaward dipping reflectors and a high-velocity lower crust in the volcanic passive continental margins;OCT with exhumation and exposure of serpentinized continental mantle;OCT with the anomalously-thinned oceanic crust,and OCT with the strongly-thinned continental crust in the nonvolcanic passive continental margins.The origin of the OCT depends on the extension of passive continental margins.The volcanic passive continental margins originated from the active volcanic-type rift which caused the decrease in the strength of the lithospheric bottom by thermal action,and then the mantle melting occurred,which resulted in the seaward dipping reflectors in the shallows and the thick high-velocity lower crust with high-P wave velocity anomalies.The lithospheric extension of the non-volcanic passive continental margins was the depth-dependent extension,and the lithospheric deformation had both pure shearing（uniform extension）and asymmetric simple shearing（detachment-related faulting）.