Gravity anomalies over the AAD.
Abstract.
The crenulated geometry of the Southeast Indian ridge within the
Australian-Antarctic discordance is formed by numerous spreading ridge segments that are offset,
alternately to the north and south, by transform faults. Suggested causes for these offsets,
which largely developed since ~20 Ma, include asymmetric seafloor spreading, ridge jumps, and
propagating rifts that have transferred seafloor from one flank of the spreading ridge to the
other. Each of these processes has operated at different times in different locations of the
discordance; here we document an instance where a small (~20 km), young (<0.2 Ma), southward
ridge jump has contributed to the observed asymmetry. When aeromagnetic anomalies from the
the Project Investigator-1 survey are superposed on gravity anomalies computed from Geosat GM and
ERM data, we find that in segment B4 of the discordance (between 125 and 126 E), the roughly
east-west-trending gravity low, correlated with the axial valley, is 20-25 km south of the ridge
axis position inferred from the center of magnetic anomaly 1. Elsewhere in the discordance, the
inferred locations of the ridge axis from magnetics and gravity are in excellent agreement. Ship
track data confirm these observations. We speculate that the ridge axis in B4 has recently jumped
to the south, from a ridge location closer to the center of the young normally magnetized crust,
to that of the gravity anomaly low. The position of the gravity low essentially at the edge of
normally magnetized crust requires a very recent (<0.2 Ma) arrival of the ridge in this new
location. Because this ridge jump is so young, it may be a promising location for future detailed
studies of the dynamics, kinematics, and thermal effects of ridge jumps.
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