Mantle downwelling beneath the Australian-Antarctic discordance zone: evidence from geoid height versus topography

Details Mantle downwelling beneath the Australian-Antarctic discordance zone: evidence from geoid height versus topography Mantle downwelling beneath the Australian-Antarctic discordance zone: evidence from geoid height versus topography

Apr 1991

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1991e%26psl.103..325m&link_type=abstract

Earth and Planetary Science Letters, Volume 103, Issue 1-4, p. 325-338.

Mathematics

Functional Analysis

17

Scientific paper

The Australian-Antarctic discordance zone (AAD) is an anomalously deep and rough segment of the Southeast Indian Ridge between 120° and 128°E. A large, negative (deeper than predicted) depth anomaly is centered on the discordance, and a geoid low is evident upon removal of a low-order geoid model and the geoid height-age relation. We investigate two models that may explain these anomalies: a deficiency in ridge-axis magma supply that produces thin oceanic crust (i.e. shallow Airy compensation), and a downwelling and/or cooler mantle beneath the AAD that results in deeper convective-type compensation. To distinguish between these models, we have calculated the ratio of geoid height to topography from the slope of a best line fit by functional analysis (i.e. non-biased linear regression), a method that minimizes both geoid height and topography residuals. Geoid/topography ratios of 2.1 +/- 0.9 m/km for the entire study area (38°-60°S, 105°-140°E), 2.3 +/- 1.8 m/km for a subset comprising crust <= 25 Ma, and 2.7 +/- 2.0 m/km for a smaller area centered on the AAD were obtained. These ratios are significantly larger than predicted for thin oceanic crust (0.4 m/km), and 2.7 m/km is consistent with downwelling convection beneath young lithosphere. Average compensation depths of 27, 29, and 34 km, respectively, estimated from these ratios suggest a mantle structure that deepens towards the AAD. The deepest compensation (34 km) of the AAD is below the average depth of the base of the young lithosphere ( ~ 30 km), and a downwelling of asthenospheric material is implied.
The observed geoid height-age slope over the discordance is unusually gradual at -0.133 m/m.y. We calculate that an upper mantle 170°C cooler and 0.02 g/cm3 denser than normal can explain the shallow slope. Unusually fast shear velocities in the upper 200 km of mantle beneath the discordance, and major-element geochemical trends consistent with small amounts of melting at shallow depths, provide strong evidence for cooler temperatures beneath the AAD.

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