The geotectonic features of the Southeast Indian Ridge and its current research progress

Abstract

The Southeast Indian Ridge (SEIR) is the fastest spreading ridge in the Indian Ocean, with an average half spreading rate of 25−50 mm/a. The crust produced by the SEIR accounts for more than half of the total Indian oceanic crust. It represents a pivotal element in shaping the current tectonic settings of the Indian Ocean. Compared with the Southwest Indian Ridge (SWIR) and Northwest Indian Ridge (NWIR), the temporal and spatial evolution of the SEIR has been more complex, as reflected by the present-day tectonic and geomorphological diversity of its segmentation pattern. In this study, we integrate the topographic and geomorphological features, the gravimetric and geomagnetic signatures, and the MORB geochemical characteristics from the SEIR and the adjacent areas, in order to define the segmentation of the ridge, to trace its evolution through time, to characterize the heterogeneity degree of the mantle source beneath the ridge and to explore potential genetic links between regional intraplate volcanisms and the spreading ridge. The study shows that the axial bathymetry of SEIR undulates along the ridge, with the shallowest depth at the Amsterdam-St. Paul Plateau (ASP) and the deepest ones at the Australian-Antarctic Discordance (AAD). Using bathymetric, gravity and geomagnetic data, thirty-one first-order ridge segments can be distinguished. The boundaries of each segment coincide with transform faults. The eastern Indian Ocean expanded owing to three main successive phases of oceanic accretion each characterized by a distinct orientation: northwestward phase before 84 Ma, a north-south one between 84 and 38 Ma and a northeastward phase after 38 Ma. The mantle source of the SEIR is heterogeneous, especially in ASP and AAD regions. While the ASP results from a hotspot-ridge interaction, the AAD represents the isotopic boundary between Indian and the Pacific Ocean mantle domains. In the eastern Indian Ocean, there are many off-axis oceanic plateaux, such as the Kerguelen Plateau, Broken Ridge, Ninetyeast Ridge, Naturaliste Plateau and Wallaby-Cuvier Plateau. The buildings of these off-axis oceanic plateaux are either related to plume activity, and/or to presence of remnants of continental crust material embedded in the shallow mantle. The SEIR integrated with the NWIR at ~38 Ma, while the SWIR wedged into the NWIR-SEIR sometime after or before 38 Ma at the Rodrigues Triple Junction. The heavy isotopic compositions of SEIR are similar to NWIR and the northeast end of SWIR. Compared with Pacific-Antarctic Ridge, the Indian ridge systems show obvious Dupal anomaly, indicating the larger-scale mantle heterogeneities. This study can help better understand the evolution of the eastern Indian Ocean, the ridge systems and tectonic patterns of the entire Indian Ocean. It can also enhance our knowledge on Gondwana continent rifting and the evolution of Indian Ocean.