REVISED MACQUARIE PLATE MOTION AND ITS TECTONIC IMPLICATIONS ON PACIFIC-ANTARCTIC-AUSTRALIAN INTERACTION

We use the newly acquired shipboard magnetic data at the eastern segments of the Southeast Indian Ridge, west of the Macquarie Triple Junction, to revise the Pacific-Antarctic-Australian plate motions for the last 6 Myr. The plate motions at this region are complicated as the Macquarie plate has behaved as an independent plate since ~6 Ma. Reconstructing the kinematic history of the Macquarie Plate is important not only because of its interactions with the Pacific Plate across the Macquarie Ridge and Hjort Trench, but also to understand the formation of a new plate from the pre-existing plate. However, because of its remoteness and unfavorable sea conditions, the tectonic details of the Macquarie plate have been limited to a few observations. Between 2011 and 2015, the magnetic data across the KR1 and KR2 segments were collected using the transit time of RVIB Araon and M/V L’Astrolabe. The KR1 segment is located between the Balleny Fracture Zone and Macquarie Triple Junction, and the KR2 is located between the Tasman and Balleny Fracture Zones. Using the newly acquired and archival magnetic anomalies, we estimate the finite rotation parameters of the Macquarie-Antarctic and Australian-Antarctic motions for eight chrons (C1o, C2, C2Ay, C2Ao, C3y, C3o, C3Ay, and C3Ao). Compared to the previous analyses, the revised rotation poles for chrons C2Ay and C3Ay are closely located, whereas the corresponding confidence regions become much tighter. Our study also confirms that the Macquarie plate has been an independent rigid plate for the last ~6 Myr. Based on the revised Macquarie plate motion, we propose that the Macquarie plate is located between the Tasman fracture zone and the Hjort Trench and between a relatively diffuse zone along 50°S and only spreading boundaries of the KR1 and KR2 segment of the Southeast Indian Ridge. We then extend our analysis to the kinematic motion of the Macquarie plate relative to the Pacific plate by summing Macquarie-Antarctic and Antarctic-Pacific rotations. This approach enables us to explore the kinematic relations of Pacific-Macquarie plates over the Hjort Trench and of Pacific-Australia plates over the Alpine Fault Zone of New Zealand.