Eighty-Five East Ridge possible extinct ridge

ID: 2-13

Ocean: Indian
Spreading center type: Large-scale extinct MOR
Time of cessation: Uncertain, possibly ca. 120 Ma (Mishra, 1991)
Subsequent active spreading centre: Southeast Indian Ridge
Cessation style: Not constrained by available data
Later deformation or volcanism: Multiple magmatic events are suggested, possibly separated by ~15 Myr (Desa et al., 2006).

Review of previous studies:

The Bay of Bengal has limited age constraints due to extremely thick sediment cover that prevents direct sampling of the basement or reliable identification of magnetic anomalies. Mishra (1991) analyzed four profiles across the Bay of Bengal and reported that a zone of parallel, alternately magnetized crust was present along the profiles. They inferred that thois represented oceanic crust and that a region of elevated magnetization may represent a failed spreading centre (Mishra, 1991). They were unable to identify the magnetic isochrons or to provide a precise estimate of the age of the possible spreading centre.

Since the work of Mishra (1991) there have been numerous alternative explanations that were summarised by Desa et al., (2013) in their study of the structure. Desa et al., (2013) report that the magnetic field either side of the 85 east ridge is symmetric and mirrored, with lower magnetized material at the ridge and higher magnetization on the flanks. This magnetic signature is proposed to correspond with a broader negative gravity anomaly in the region of the ridge (Desa et al., 2013). Their observations are not inconsistent with an extinct ridge formation for at least part of the 85 E Ridge, yet they also do not confirm this and we find insufficient evidence to conclude that the ridge is an extinct spreading ridge.

Suggested reasons for cessation:

A ridge reorganisation related to the Kerguelen plume activity has been suggested at the approximate time that this ridge may have been active (Desa et al., 2013).


Desa, M. A., Ramana, M. V., Ramprasad, T., Anuradha, M., Lall, M. V. and Kumar, B. J. P., 2013, Geophysical signatures over and around the northern segment of the 85 E Ridge, Mahandi offshore, Eastern Continental Margin of India: Tectonic Implications. Journal of Asian Earth Sciences, v. 73, p. 460 - 472.

Mishra, D. C., 1991, Magnetic crust in the Bay of Bengal, Marine Geology, v. 99, p. 257-261.

Maps showing the location of extinct ridge segments

Depth profiles across the axes of extinct ridge segments

Gravity profiles across the axes of extinct ridge segments