Abstract

The Luxi–Nujiang suture, which extends from Yunnan into Myanmar, is shown to continue into Thailand and the Malacca Strait. It forms the eastern boundary of a discrete block, here called the Irrawaddy Block, which was formerly considered to be part of Gondwana-derived Sibumasu. It has a distinct Palaeozoic succession, with thick Lower Permian, glacially influenced, diamictite-bearing, marine mass-flow intervals deposited at the Gondwana margin. The rest of Sibumasu, east of the suture, also has local, thin, Lower Permian diamictite units; however, they have been interpreted as ice-rafted deposits, laid down at a greater distance from the Gondwana margin. In NE Sumatra a diamictite-bearing (?)Carboniferous–Lower Permian succession has also been described as of mass-flow origin and it is suggested that it is also part of the Irrawaddy Block. It requires that the bounding suture is aligned SE, beneath the Malacca Strait. In the Late Cretaceous–Palaeogene, the dextral India–Australia oceanic transform propagated onshore as a strike-slip fault, bounding on the east the north-going India Plate. It disrupted the Irrawaddy Block and for much of its length coincided with the line of the earlier suture. Insofar as the acronym Sibumasu was coined to include Sumatra, consideration should be given to renaming this block Sibuma.

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