Australian–Antarctic depression from the mid-ocean ridge to adjacent continents

Abstract

A morphotectonic depression, some 15 × 10⁶ km² in area and centred on the Australia–Antarctic discordance¹, extends across the southern half of Australia through the south-east Indian Ocean to Wilkes Land in Antarctica (Fig. 1a, b). The depression is co-extensive with a negative satellite free-air gravity anomaly² which suggests that the region is underlain by downward convecting asthenosphere. Here I outline the depression and explore its age. In a pre-breakup (>55 Myr) reconstruction (Fig. 1c), the Eastern Highlands of Australia and the Transantarctic Mountains are colinear; because the Eastern Highlands were notably high during the late Cretaceous, and shared with the Transantarctic Mountains a history of volcanism in the Jurassic, both highlands probably constituted the eastern flank of the past depression, in the centre of which thick (>8 km) late Cretaceous sediment in the Ceduna Saddle³, and a possible mirror-image in offshore Antarctica, mark its depocentre. A broad area of late Carboniferous marine sediments in southern Australia⁴ may represent the earliest sign of the depression.