Thermal anomalies beneath the Australian-Antarctic discordance

Abstract

Significant variations in shear wave velocity along the Southeast Indian Ridge are delineated by SS-S differential travel times. The Australian-Antarctic Discordance shows SS-S residuals that are 3–8 s faster than the adjoining segments 1500–2000 km away. The large variability suggests that thermal structure in the upper mantle, as opposed to crustal structure, is the major cause of the velocity anomaly. To quantify this inference, we invert both geoid and topography into temperature perturbations beneath the ridge within specific depth extents, assuming that the thermal sources and the geophysical observations are dynamically related in a viscous earth. The joint inversion of these two sets of data is robust, demanding a negative temperature anomaly of 80 and 250°C beneath the discordance, for layers extending from the surface to depths of 300 and 100 km, respectively. In a forward sense, the temperature so constrained predicts well the trend of the along-axis variations in SS-S residuals, but with a smaller amplitude, especially for the 8 s contrast between the discordance and the eastern end of the ridge where unusually high residuals sample a broad, shallow region of ridge segments. Considering the proximity of the SS bounce points to the ridge axis and the possibility of sampling a partial melt zone, the discrepancy in magnitude may be partially reconciled by deducting a presumed 2–3 s melt effect from the high residuals in the eastern end. The resolved thermal perturbations drive asthenospheric flow along the ridge, the flow converging and descending in the vicinity of the discordance. This simple thermal-viscous flow fmodel is consistent with the idea of ‘pipe’ flow. For the Southeast Indian Ridge, the source of the pipe flow may be the mantle plumes at the two ends of the ridge.