A LATE MIOCENE PLATE BOUNDARY REORGANIZATION ALONG THE WESTERNMOST PACIFIC-ANTARCTIC RIDGE

Magnetic and bathymetric measurements carried out in the southwest Pac ific between New Zealand and Antarctica south of 60 degrees S, combine d with the satellite-derived gravity map, add new information on the s tructural fabric and tectonic development of the westernmost portion o f the Pacific-Antarctic plate boundary. The plate boundary includes in its northern part a series of short spreading centres offset by NNW-S SE-trending fracture zones. Towards the south, it becomes structurally more complicated and appears to be formed by extension along an ancie nt strike-slip lineation, as a consequence of a change in the Pacific- Antarctic relative motions that resulted in a geometric readjustment o f the plate boundary. The identification of marine magnetic lineations crossing this portion of the boundary testifies that this readjustmen t is relatively recent; the newly created crust is not older than Late Miocene and is presently surrounded by oceanic regions where the magn etic anomalies range from 19 to 24 (similar to 41-53 Ma) northeastward , and from 7 to 9 (similar to 24.5-28 Ma) southwestward. This segment of the plate boundary formed along an older fracture zone that was pul led-apart by the new plate motions, and the segmented ridge-transform system then evolved.