Evolution of the Australian-Antarctic discordance since Miocene time
K. Marks, J. Stock, and K. Quinn
Journal of Geophysical Research - Solid Earth, 104, 4967-4981, 1999
Present-day (a) Gravity anomalies and (b) Depth anomalies over the Australian-Antarctic discordance
In this exciting new research we chronicle the development of the Australian-Antarctic discordance
(AAD), the crenelated portion of the Southeast Indian ridge between ~120 and 128 E, since
anomaly 6y time (19 Ma).
Our gravity field reconstructions
reveal that the overall length of the Australian-Antarctic plate boundary within the AAD has been
increasing since 19 Ma, and that the number of propagating rifts and fracture zones in the
vicinity of the discordance has increased dramatically in recent times. Our
bathymetric reconstructions show that the regional, arcuate-shaped, negative (deeper than
expected) depth anomaly presently centered on the discordance began migrating westward before
anomaly 5ad time (~14.4 Ma), and that a localized depth anomaly low, which at time 5ad lay
beneath the ridge axis in spreading corridor B5, has been split apart by subsequent seafloor
spreading. The magnetic anomaly patterns suggest that the depth anomaly is not always
associated with a particularly contorted plate boundary geometry. Although the plate boundary
within the AAD has been getting progressively more crenelated with time, this effect shows little
to no migration along the ridge axis since 19 Ma. Thus, any geodynamic models of the evolution
of the discordance must account for the following observations: 1) the crenelation of the plate
boundary within the AAD has increased with time; 2) the center of the crenelated zone does not
appear to have migrated along the ridge crest; and 3) both the depth anomaly and the isotopic
boundary between Pacific and Indian mantle have been migrating westward along the ridge axis
but at apparently different rates. We suggest that both along-axis migration of the depth anomaly
and isotopic boundary, as well as temporal variation in the upwelling mantle material beneath the
AAD, and local tectonic effects, are required in order to explain these observations.
Magnetic anomaly data, finite rotations, and covariance values can be obtained by clicking on hightlighted data type.
Images and figures follow.
Magnetic anomaly identifications
Tectonic sketches of the evolving discordance zone at selected times
Best fit pole positions for reconstructing Australia to Antarctica
Gravity and Depth anomaly reconstructions at selected times
Chron 6y
Chron 5ad
Chron 5o
Chron 4
Chron 3y
Chron 2ay
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