The Conrad Rise Revisited: Eocene to Miocene Volcanism and Its Implications for Magma Sources and Tectonic Development

Abstract

The Conrad Rise (CR), located midway between Antarctica and the Southwest Indian Ridge (SWIR), remains one of the least explored submarine large igneous provinces (LIPs) in the Indian Ocean to date. Relying on only seafloor paleomagnetic records, early studies hypothesized that the formation of the CR occurred during the Late Cretaceous. Here, we present new geochemical and geochronological data, including Sr‒Nd‒Pb‒Hf isotopes and ⁴⁰Ar/³⁹Ar data. Our results indicate that the uppermost part of the CR (Ob and Lena seamounts) unexpectedly formed later than previously predicted, at approximately 40 Ma in an intraplate setting. Another small seamount north of the Ob seamount formed later, at 8.5 Ma. The isotopic composition of lava from the small seamount north of the Ob seamount overlaps with that commonly defined by the Indian plume component. Overall, the isotopic variations defined by the volcanic suite from the CR could be accounted for by a three-component mixing model involving the common component, lower continental crust, and depleted mantle endmembers. The newly obtained ⁴⁰Ar/³⁹Ar ages imply that the CR volcanism might have been triggered by major regional plate reorganizations during the middle to late Eocene and the late Miocene, inducing the release of a small upwelling rising from the African large low-velocity province.

Key Points

• This is the first report of geochronological lava data from the Conrad Rise, an aseismic ridge in the southwestern Indian Ocean

• Contrary to previous hypotheses, late volcanism at the Conrad Rise occurred in the middle to late Eocene and the late Miocene

• The isotopic features of the Conrad Rise result from a mixture of common Indian Ocean plume components and lower continental crust

Plain Language Summary

The Conrad Rise is a large oceanic volcanic plateau in the Indian Ocean. Previous studies have provided only vague estimates of when it formed because very few direct studies of the area have been performed, and no geochronological studies have been conducted. This study presents new geochemical data for the Conrad Rise, including isotopes and radiometric ages. The results suggest that the uppermost part of the Conrad Rise formed approximately 40 million years ago and that the small seamount in the northern part of the Conrad Rise formed 8.5 million years ago in an intraplate setting. The Sr‒Nd‒Pb‒Hf isotopic signature of lavas from the Conrad Rise might be derived from a ternary mixture between the common mantle component present in other lavas from the Indian Ocean, lower continental crust remnants, and the depleted mantle. The new age data indicate that the late volcanic activity might have been triggered by the release of a small plume from the African superplume caused by regional plate reorganization.