Tectonic Processes of the Australian-Antarctic Ridge

The Australian-Antarctic Ridge (AAR) has been remained uncharted until 2011 because of its remoteness and harsh weather conditions. From 2011, the multidisciplinary ridge program initiated by the Korea Polar Research Institute (KOPRI) surveyed the little explored eastern segments of the Australian-Antarctic Ridge (AAR) to characterize the tectonics, geochemistry, and hydrothermal activity of this intermediate spreading system. In this study, we present a detailed analysis of a 300-km-long KR1 supersegment of the AAR to quantify the spatial variations in ridge morphology and axial and off-axis volcanisms constrained by high-resolution shipboard bathymetry and gravity. The ridge axis morphology alternates between rift valleys and axial highs within relatively short ridge segments. To obtain a geological proxy for regional variations in magma supply, we calculated residual mantle Bouguer gravity anomalies (RMBA), gravity-derived crustal thickness, and residual topography for neighboring seven sub-segments. The results of the analyses revealed that the southern flank of the AAR is associated with shallower seafloor, more negative RMBA, thicker crust, and/or less dense mantle than the conjugate northern flank. Furthermore, this north-south asymmetry becomes more prominent toward the KR1 supersegment of the AAR. The axial topography of the KR1 supersegment exhibits a sharp transition from axial highs at the western end to rift valleys at the eastern end, with regions of axial highs being associated with more magma supply as indicated by more negative RMBA. We also compare and contrast the characteristics of the AAR with that of other ridges of intermediate spreading rates, including the Juan de Fuca Ridge, Galápagos Spreading Center, and Southeast Indian Ridge west of the Australian-Antarctic Discordance, to investigate the influence of ridge-hotspot interaction on ridge magma supply and tectonics. In addition, we estimate spreading rates and its temporal changes along the ridge segment based on two 400-km long magnetic lines acquired in 2015. The half-spreading rates computed using MODMAG range mostly between 28~34 mm/yr. The southern flank of the KR1 supersegment, which has been added to the Antarctic plate as a trailing side between the Australian and the Antarctic plate, tends to have faster spreading rates compared to the northern flank. According to the previous studies, the Macquarie plate, which is regarded as the intra-plate inside the Australian plate and bounded by the northern flank of the KR1 supersegment, has been rotated relative to the Antarctic plate since about 6 Ma. We revised rotation poles between the Macquarie-Antarctic plates for the chron C2Ay (2.58 Ma), chron C3Ay (6.04 Ma) using the Hellinger modeling tool. We examine relevant tectonic adjustments, if any, near the AAR and Macquarie plate using the revised rotation poles.