Early opening of Australia and Antarctica: New inferences and regional consequences

• Gravity anomaly on conjugate Australian–Antarctic margins signs continental breakup.
• Precise reconstructions with Polda Block and Tasmania as transient micro-continents
• Transform motion of Zelandia wrt Australia, Tasmania, W Antarctica between 128–83 Ma
• Convergent motion between E and W Antarctica accommodated in TAM between 128–94 Ma
• Extension between Australia and E Antarctica triggered by Kerguelen hotspot inception

Both continental margins of Australia and Antarctica exhibit a very clear gravity anomaly on the satellite free-air gravity data. The detailed sinuosity of these first-order conjugate features matches perfectly, suggesting that they are the signature of the initial continental breakup and mark the ocean–continent boundary. Another weaker, still clearly deciphered, pair of symmetrical gravity anomalies is identified oceanward. These anomalies are considered as pseudo-isochrons F and G and tentatively dated 128 and 94 Ma. Precise reconstructions of pseudo-isochron F are achieved over three sections of the margin, denoting the relative motion of Australia and East Antarctica, the Polda Block and East Antarctica, and Tasmania and West Antarctica. The Polda Block and Tasmania are transient micro-continents. Tasmania and Australia are reconstructed to align their linear eastern margin. The eastern margins of reconstructed Australia, Tasmania, and West Antarctica on one hand, the western margin of reconstructed Lord Howe Rise and Campbell Plateau on the other hand, fit a small circle of radius 15°, which suggests a transform motion between 128 and 83 Ma along this plate boundary. The reconstruction predicts a gap between East and West Antarctica, probably filled by non-cratonic continental crust compressively deformed and thickened by the SW motion of East Antarctica and participating to the formation of the Trans-Antarctic Mountains. The initial extension between Australia and East Antarctica may be related to the inception of the Kerguelen hotspot, ~ 1000 km to the west. The different rheology of cratons and orogenic terranes has played a role in the style and localization of both extensional and compressional deformations.