Pattern of slow seafloor spreading (<4 mm/year) from breakup (96 Ma) to A20 (44.5 Ma) off the southern margin of Australia

Australia separated from Antarctica by continental extension between the mid-Jurassic (>160 Ma) and mid-Cretaceous (96 Ma), then by slow seafloor spreading (half-rate < 4.4 mm/ year) on a separation azimuth of 335° until A21 time (49 Ma), at an intermediate half-rate (10 mm/ year) until A20 time (44.5 Ma), and then at a fast rate (20 mm/ year) on a separation azimuth of 360o to the present. A compilation of seafloor spreading magnetic data for the entire southern margin, including critical data collected during the cruise of the R/ V Rig Seismic in 1986, confirms the previous work except for the re-interpretation of the oldest anomalies. The phase of slow spreading is characterised by (a) jumps of the spreading ridge to Australia between 131.25° E and Tasmania to accommodate the southeastward offset of the line of separation between Tasmania and Antarctica, and (b) variable azimuths of spreading isochrons within individual spreading segments, as, for example, from 090o±5° between 129° and 130oE (an angle of 65° between separation and spreading azimuths) and 075° and 080° between 130° and 131.75°E (an almost orthogonal 80°). The variable azimuth of the spreading isochrons, oblique to the separation azimuth, is similar to that found in the young (