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 Ze aland has evolved from a spreading system into a transform boundary. S wath 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 Tr ench, striking successively N60 degrees E, N85 degrees E and N120 degr ees E. This reflects the reorganisation of the plate boundary in respo nse to changes in relative plate motion. A comparison of these orienta tions with the positions of the Pacific-Australia relative poles of ro tation enables us to estimate the age of STOC, where there are no iden tified magnetic anomalies. The youngest age of the oceanic crust is ca . 12 Ma at the south end of the Puysegur Trench. This age is consisten t with spreading rates and the amount of crust generated since 31 Ma. Curved fracture zones on either side of the Macquarie Ridge complex su ggest a continuous reorientation of transform faults, between 31 Ma an d ca. 15 Ma. Small-scale seafloor morphology shows a 13 degrees change of orientation in the L'Atalante Fracture Zone, that indicates increm ental, rather than continuous, changes in azimuth of the transform fau lts. Patterns of fanning ridges indicate that periods of asymmetric sp reading accompanied the spreading segment reorientations. Using swath data and plate reconstruction models we infer that between 31 and 12 M a the plate boundary reorganisation resulted in a continuous increase in the ratio of the cumulative length of transform faults over the cum ulative length of spreading segments, along the whole plate boundary. This indicates that, since 14-15 Ma, the plate boundary has become pro gressively predominantly transcurrent, allowing strike-slip motion to develop along a line of merging transform faults that connected to the intracontinental Alpine Fault.