东北印度洋85°E海脊的性质和起源:综述和新认识

Tectonic structure and origin of the 85°E ridge,Northeastern Indian Ocean:A review and new observations

85°E海脊是东北印度洋一条重要的线性基底隆起,形成于中生代印度板块北漂过程中的构造和岩浆活动.海脊的结构、性质和起源蕴含了东印度洋扩张和印度板块北漂过程的关键信息,然而目前对其构造属性和形成演化的认识存在较大争议.分析了85°E海脊及邻区的重磁异常特征,结合前人对海脊外部形貌、内部结构、深部构造以及东印度洋板块重建的研究成果,探讨了海脊的性质和起源.结果表明,85°E海脊的形成是热点活动、洋脊扩张、转换断层、扩张中心跃迁以及板块汇聚远程效应等多种地质过程综合作用的结果.海脊呈现明显的构造分段性,不同分段的结构、性质和成因机制不同.12°N以北的海脊形成于板内热点型岩浆作用;2°~12°N的海脊与NW-SE向和N-S向两期海底扩张的边界高度吻合,是白垩纪东印度洋扩张中心调整和板块重组的产物;2°N以南的阿法纳西-尼基廷海山是随着海底扩张逐渐侵位的热点型海脊,可能与2°N以北的海脊不存在成因上的关联.分析认为,2°~12°N的海脊中段是未来部署地球物理测量、进一步确认海脊性质和成因的关键区域.通过深海钻探揭示海脊不同分段的物质组成和形成时代,是破解85°E海脊的性质和起源、白垩纪印度洋板块重建事件以及热点-洋中脊相互作用机制等重大地质问题的关键途径.

The 85°E ridge in the Northeast Indian Ocean is a prominent linear basement uplift,formed by the tectonic and magmatic activities during the Mesozoic northward drift of the Indian plate.This ridge is a key unit for understanding the East Indian Ocean spreading and the northward drifting of the Indian Plate.However,its structural characteristics and formation and evolution process are controversial.In this paper,gravity and magnetic anomalies around the 85°E ridge are mapped and analyzed.We discussed the nature and origin of the ridge upon integration of previous works on morphology and deep structure of the crust under the ridge and the reconstruction models of the East Indian Ocean plate.The results show that the 85°E ridge is generated by the integrated multiple processes including hot spot activity,spreading,transform fault,transition of spreading center and long-distance effect of plate convergence.Different segments are dominated by different structures,natures and genetic mechanisms.The segment north of 12°N was formed by intraplate hot spot magmatism.Between 2°N and 12°N the ridge is highly consistent with the boundary of NW-SE and N-S seafloor spreading,resulted from the Cretaceous plate reorganization.The Afanasy-Nikitin seamount to the south of 2°N is a near-axis hot spot trail emplaced along with the seafloor spreading,and may not be genetically related to the ridge to the north of 2°N.We suggest,therefore,that the middle part of the ridge between 2°N and 12°N is a critical area for further confirmation of the nature and origin of the ridge.Geophysical surveys are required and scientific drilling will help solve major geological problems,such as the nature and origin of the 85°E ridge,the reorganization of the Indian Ocean Plate in Cretaceous,and the interaction mechanism between hot spot and mid-ocean ridge.