Dating of the 85°E Ridge (northeastern Indian Ocean) using marine magnetic anomalies

Abstract

The 85°E Ridge is a typical aseismic ridge in the northeastern Indian Ocean in view of the fact that it associates with exceptional gravity and magnetic signatures. The ridge possesses two different gravity anomalies: the north part (up to 5°N latitude) is associated with negative gravity anomaly, whereas the south part coincides with positive gravity anomaly. In contrast to this, the ridge consists of alternate streaks of positive and negative magnetic signatures distributed for asymmetrical extents. With the consideration of ridge seismic structure and geomagnetic polarity reversals, we modelled both positive and negative magnetic anomalies of the ridge. This shows that the 85°E Ridge was formed during the period of rapid changes in the Earth's magnetic field, earlier to that, the underlying oceanic crust was created in the Cretaceous superlong normal polarity phase. The results further reveal that the positive and negative magnetic signatures of the ridge have been created, in general, by a relief of the ridge and polarity contrast between the ridge material and adjacent oceanic crust, respectively. On correlation of the ridge's magnetization pattern to the geomagnetic polarity timescale, we believe that the 85°E Ridge volcanism started at anomaly 33r time (~80 Ma) in the Mahanadi Basin by a short-lived hotspot, thereafter the process continued towards south and finally ended at ~55 Ma in the vicinity of the Afanasy Nikitin Seamount.