Revisiting the magnetic anomalies along the West Australian margin identifies a new continental fragment that accreted to Sumatra during the Early Eocene

Plate models reconstructing the formation of the West Australian margin differ in their treatment of the section of the Australian margin extending from the Wallaby-Zenith Fracture Zone to the tip of the Exmouth Plateau. Some reconstructions model Greater India as the conjugate, while others do not model any conjugate plate at all. The formation of the passive margin on the Australian plate implies that there must have been a conjugate continental plate that rifted away. Our revised reconstruction that includes all the abyssal plains along the West Australian margin reveals that, apart from Greater India and Argoland, a third continental block (Gascoyneland) must also have rifted from Australia since the Jurassic. From 132 Ma, while initially moving about the same Euler pole as Greater India, it formed the stretched continental crust of the Exmouth Plateau and then the oceanic crust of the Gascoyne and Cuvier abyssal plains. At 115 Ma Gascoyneland began moving in a northerly direction while Greater India continued westward only later moving northward from approximately 95 Ma when it was located entirely west of Gascoyneland. Gascoyneland did not pass west of the Investigator Ridge, a north-south-oriented linear feature at 98°E marking the western limits of the curved fracture zones of the Wharton Basin. Gascoyneland’s change in direction of plate motion would have formed these curved fracture zones and, assuming the N-S orientation of the Investigator Ridge continued into now subducted oceanic crust, would have reached West Sumatra at around 60 Ma. Plate tectonic models indicate that Sumatra was derived from accreted continental fragments originating from Gondwana (Metcalfe, 1996), although the continuity of Triassic sediments in West Sumatra, Sibumasu and East Malaya contradict this (Barber and Crow, 2003). The Woyla Group, consisting of the Sikuleh, Natal and Bengkulu terranes located along the west coast of Sumatra, has been identified as an oceanic arc, which accreted during the Jurassic-Early Cretaceous after the formation of a short-lived, narrow marginal sea (Cameron et al., 1980). The same authors considered the Woyla Group to overly continental crust due to the presence of the Sikuleh granitoid batholith. We propose Gascoyneland is now buried beneath the Woyla Terrane. References Cameron, N.R., Clarke, M.C.G., Aldiss, D.T., Aspden, J.A. and Djunuddin, A. (1980) The geological evolution of northern Sumatra. Indones. Petrol. Assoc., Proceedings 9th Annual Convention, Jakarta, 1980, pp. 149-187. Barber, A.J., and Crow, M.J., 2003, An evaluation of plate tectonic models for the development of Sumatra: Gondwana Research, v. 6, p. 1-28. Metcalfe, I., 1996, Gondwanaland Dispersion, Asian Accretion and Evolution of Eastern Tethys: Australian Journal of Earth Sciences, v. 43, p. 605-623.