PALEOGENE SEA-FLOOR SPREADING IN THE SOUTHEAST TASMAN SEA

Magnetic anomalies 11-18 have been identified in the southeast Tasman Sea, the area of ocean crust southwest of Fiordland, New Zealand, west of the Puysegur Trench (the present plate boundary) and southeast of the older (80-58 Ma) ocean crust of the Tasman Sea basin. Structures a ssociated with two changes in spreading direction are preserved in the area. Spreading between the Australian and Pacific plates began in th e Eocene, about 40 Ma in this area, orthogonal to Cretaceous-Paleocene spreading between them in the Tasman Sea. The boundary between the tw o ages of ocean crust is abrupt and associated with what we have inter preted as marginal uplift blocks and rift basins of the Resolution Rid ge system. Rifting appears to have propagated northeast along a fractu re zone. The northern blocks of the Resolution Ridge system may be iso lated fragments of continental crust of the Campbell Plateau. Between anomalies 18 and 11 (40-30 Ma) the spreading rate was about 1.5 cm/yr. Swath mapping and satellite altimetry data show that a second change in relative plate motion direction began shortly after anomaly 11, abo ut 30 Ma, and led to a change from tension to transcurrent motion alon g the plate boundary. The timing of the changes in spreading direction agrees with tectonic events interpreted from onshore data.