The nature and origin of the 85°E Ridge at 2°N. A hotspot track?

The evolution of the 85°E Ridge in the Indian Ocean remains enigmatic. Different possibilities of its formation, e.g. by a hotspot track, a weak plume tail or by lithospheric flexure, are under discussion. Unfortunately, understanding the crustal nature of the ridge is complicated by the fact that it is almost completely covered by sediments. Furthermore, the location of the 85°E Ridge below the seafloor is characterized by a negative free-air anomaly, which is fairly uncommon for magmatic ridges. \nIn 2017 a scientific cruise investigated the southern part of the 85°E Ridge and the surrounding oceanic crust to examine the genesis and crustal architecture of its southernmost part. Magnetic and gravity measurements were conducted in a tight grid supplemented by a 340 km long refraction and reflection seismic profile, which extends in a NW-SE direction and crosses the southern end of the 85°E Ridge at ~2°N. The location of the profile was chosen to investigate the southernmost 85°E Ridge close to the onset of the Afanasy Nikitin Rise (ANR), a magmatic basement high rising above the seafloor, whose formation is believed to be linked to the evolution of the 85°E Ridge. \nOur P-wave velocity and gravity models show that the crustal structure of the 85°E Ridge does not differ significantly from that of the adjacent normal oceanic crust. In contrast to the northern part of the 85°E Ridge, no magmatic structure on top of the oceanic crust was found. The investigated part of the 85°E “Ridge” is characterized by a slight depression and not an elevated basement, covered by up to 4.5 km thick sediments. Within this depression, the upper crustal layer is modelled with P-wave velocities of 5.8-6.2 kms-1, which are slightly higher than in the surrounding uppermost oceanic crustal layer (5.5-6.0 kms-1). The lowermost crustal layer with P-wave velocities of 6.7-7.1 kms-1 is thickened over a length of 60 km, resulting in a Moho depression of up to 2 km and causing the observed gravity low. The upper mantle has velocities of 8.0 kms-1 and does not show any signs for underplating in the area of the Moho depression. We will discuss our results with regard to possible origins and formation scenarios of the 85°E ridge.