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

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.