ASYMMETRIC SEA-FLOOR SPREADING AND SHORT RIDGE JUMPS IN THE AUSTRALIAN-ANTARCTIC DISCORDANCE

The crenulated geometry of the Southeast Indian ridge within the Austr alian-Antarctic discordance is formed by numerous spreading ridge segm ents that are offset, alternately to the north and south, by transform faults. Suggested causes for these offsets, which largely developed s ince similar to 20 Ma, include asymmetric seafloor spreading, ridge ju mps, and propagating rifts that have transferred seafloor from one fla nk of the spreading ridge to the other. Each of these processes has op erated at different times in different locations of the discordance; h ere we document an instance where a small (similar to 20 km), young (< 0.2 Ma), southward ridge jump has contributed to the observed asymmet ry. When aeromagnetic anomalies from the Project Investigator-1 survey are superposed on gravity anomalies computed from Geosat GM and ERM d ata, we find that in segment B4 of the discordance (between 125 degree s and 126 degrees E), the roughly east-west-trending gravity low, corr elated with the axial valley, is 20-25 km south of the ridge axis posi tion inferred from the center of magnetic anomaly 1. Elsewhere in the discordance, the inferred locations of the ridge axis from magnetics a nd gravity are in excellent agreement. Ship track data confirm these o bservations: portions of Moana Wave track crossing the ridge in B4 sho w that a topographic valley correlated with the gravity anomaly low li es south of the center of magnetic anomaly 1; while other ship track d ata that cross the spreading ridge in segments B3 and B5 demonstrate g ood agreement between the axial valley, the gravity anomaly low, and t he central magnetic anomaly. Based on these observations, we speculate that the ridge axis in B4 has recently jumped to the south, from a ri dge location closer to the center of the young normally magnetized cru st, to that of the gravity anomaly low. The position of the gravity lo w 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.