A 3‐D dynamic model for the anomalous topography and geoid over the south‐east Indian Ridge
Journal
Geophysical Journal International
Journal Volume
121
Journal Issue
1
Date Issued
1995-01-01
Author(s)
Abstract
The anomalous sea‐floor topography and geoid height over the Australian‐Antarctic Discordance (AAD) are examined and attributed to dynamic mantle flow in this study. Because the subsidences of sea‐floor depth and geoid with age exhibit forms of isostatic response of cooling from the surface, they are removed from the data to retain the dynamic components. In removing the cooling effect we allow for different subsidence rates for different spreading corridors. The reduction yields primarily north‐south‐trending lows crossing the AAD in both the residual topography and the residual geoid. Assuming that the mantle behaves viscously, the residual data were inverted to thermal perturbations in the assigned depth extents. The obtained thermal perturbations exhibit mainly east‐west, or axis‐parallel variation and indicate that along the 3000 km long ridge, the mantle beneath the AAD is about 150 and 50°C colder than average for perturbation layers confined within 100 and 300 km from the surface, respectively. As a result, the 3‐D flow driven by the thermal anomaly is also predominantly east‐west oriented with a major downwelling near the AAD and its aged mantle. To independently constrain these results, we derive a simple formula to estimate the mantle temperature beneath the ridge from the subsidence rates of topography and geoid for different corridors. This simple estimation yields a variation of about 200°C with the low near AAD and high at the segment to the east. The trend of variation is consistent with that from the dynamic modelling, and the magnitude is relatively high but favours the model in which the dynamic source is concentrated within the upper asthenosphere. The thermal anomaly could be, however, eliminated by lateral advection in the along‐axis direction which is not considered in the inversion. The advection effect is estimated to cause a decay of the dynamic topography at a rate of the order of 0.005 km Myr −1 , making the downwelling die out very slowly with age. The actual 3‐D anomalous flow in the south‐east Indian Ridge area is probably still characterized more by a downwelling stretched across‐axis as depicted by our dynamic modelling than in the form of a focused plume sinking at the AAD. Copyright © 1995, Wiley Blackwell. All rights reserved
Subjects
along‐axis variation | Australian–Antarctic Discordance (AAD) | mantle dynamics | mid‐ocean ridge | viscous flow
SDGs
Type
journal article