TECTONIC SETTING OF THE TRIASSIC TO EARLY JURASSIC TRANSANTARCTIC BASIN – IMPLICATIONS FROM SANDSTONE ANALYSIS FROM NORTH VICTORIA LAND, ANTARCTICA

The Transantarctic Basin formed in between the East Antarctic craton and a magmatic arc related to subduction of the proto-pacific ocean. It was filled with clastic sediments from the Devonian to the Jurassic, but was subject to substantial changes with regard to shape, depositional environments, and sediment transport directions during this time. Based on few widely spaced outcrops its Mesozoic part is regarded as a retro-arc foreland basin.

In north Victoria Land, Triassic to Early Jurassic sandstones of the Section Peak Formation (Beacon Supergroup) overlie the deeply eroded Palaeozoic Ross Orogen. They reach about 200 m thickness and are intruded by Early Jurassic Ferrar sills. In the present tectonic model, the sandstones in north Victoria Land are thought to represent the most distal part of the foreland basin.

The petrographic composition of these sandstones and U-Pb age data of detrital zircons allow differentiating several sandstone types. For many of these, a local source from the Ross Orogen within north Victoria Land or adjoining areas can be identified, and the contribution of material from contemporaneous volcanic rocks of the magmatic arc is evident. The abundance of detritus from the Ross Orogen indicates that it was not covered by sediments but subject to erosion. This stands in contrast to the present basin model, which implies the burying of the Ross Orogen under a pile of sediments thickening towards the magmatic arc. Further evidence against a foreland-type basin are the lack of sedimentary recycling, the fining upward trend, and the lack of progradation. Instead, we propose an epicratonic setting of the north Victoria Land part of the Transantarctic Basin during the Mesozoic.

For the co-existence of an epicratonic basin in north Victoria Land and a foreland basin in other parts of the Transantarctic Mountains, a different subduction angle may be the most intriguing explanation. A foreland basin formed due to flat-plate subduction in the Cape Fold Belt (South Africa), the Ellsworth-Whitmore-Mountains, and possibly in the Central Transantarctic Mountains. In north Victoria Land and possibly in Australia, we assume a higher subduction angle and a narrower magmatic arc. Here, the Transantarctic Basin formed as a relatively narrow, belt-shaped basin in an epicratonic setting.