Late Jurassic rifting along the Australian North West Shelf: margin geometry and spreading ridge configuration

Abstract

The Argo and Gascoyne Abyssal plains in the easternmost Indian Ocean document the last stages of eastern Tethys evolution before the breakup of eastern Gondwana. Thus they provide crucial information not only for modelling the evolution of the eastern Tethys and Proto-Indian Ocean, but also to understand the complex geodynamic history of the North West Shelf. We have revisited the marine magnetic anomaly record of the Argo and Gascoyne Abyssal Plains in combination with other geological and geophysical data from the North West Shelf and southeast Asia. Based on the combined data, we have created a revised plate-tectonic model and a set of palaeogeographic reconstructions for the evolution of the North West Shelf for the early stages after the breakup. The main difference between this model and previously published models is that we have interpreted a complete section of anomalies, M25A – M22A, in the Gascoyne Abyssal Plain, northwest of the Exmouth Plateau. The magnetic anomalies have the same trend as in the Argo Abyssal Plain. Our new plate-tectonic reconstructions show that continental breakup in the Argo and northern Gascoyne Abyssal Plains, east and northwest of the Exmouth Plateau, respectively, started simultaneously in the Oxfordian with M25A identified as the oldest anomaly. In the Gascoyne Abyssal Plain, the oldest anomaly sequence, M25A – M22A (154.5 – 150.4 Ma) indicates that the ‘Argo’ spreading ridge continued around the northern margin of Greater India, and was probably linked with the Somali Basin. Sea-floor spreading continued until M14, separating the West Burma Block and possibly other smaller continental fragments like the Sikuleh Terrane of Western Sumatra from the northern Australian margin. A southward-directed ridge jump at M13 (134 Ma) transferred segments of Australian Plate oceanic crust to the West Burma Plate. Contemporaneously, an anticlockwise change in spreading direction fixed the West Burma Block relative to Greater India until its collision with the southern Eurasian margin.