The Oligocene-Miocene Pacific-Australia plate boundary, south of New Zealand: Evolution from oceanic spreading to strike-slip faulting
Since the Eocene, the Pacific—Australia plate boundary south of New Zealand has evolved from a spreading system into a transform boundary. Swath data acquired in the Southeast Tasman oceanic crust, between the Macquarie Ridge complex and the Resolution Ridge system, show that the spreading fabric changes orientation southwards along the Puysegur Trench, striking successively N60°E, N85°E and N120°E. This reflects the reorganisation of the plate boundary in response to changes in relative plate motion. A comparison of these orientations with the positions of the Pacific-Australia relative poles of rotation enables us to estimate the age of STOC, where there are no identified magnetic anomalies. The youngest age of the oceanic crust is ca. 12 Ma at the south end of the Puysegur Trench. This age is consistent with spreading rates and the amount of crust generated since 31 Ma. Curved fracture zones on either side of the Macquarie Ridge complex suggest a continuous reorientation of transform faults, between 31 Ma and ca. 15 Ma. Small-scale seafloor morphology shows a 13° change of orientation in the L'Atalante Fracture Zone, that indicates incremental, rather than continuous, changes in azimuth of the transform faults. Patterns of fanning ridges indicate that periods of asymmetric spreading accompanied the spreading segment reorientations. Using swath data and plate reconstruction models we infer that between 31 and 12 Ma the plate boundary reorganisation resulted in a continuous increase in the ratio of the cumulative length of transform faults over the cumulative length of spreading segments, along the whole plate boundary. This indicates that, since 14-15 Ma, the plate boundary has become progressively predominantly transcurrent, allowing strike-slip motion to develop along a line of merging transform faults that connected to the intracontinental Alpine Fault.