Was the Indosinian orogeny a Triassic mountain building or a thermotectonic reactivation event?

The underlying cause of Indosinian thermotectonism remains unclear, in part because the term has also been adopted to explain Triassic orogenesis across southern China. This paper puts forward the case that use of the term Indosinian should be confined to Vietnam where deformation is linked to continental accretion as opposed to southern China where Triassic igneous activity, metamorphism and deformation are linked to the development of an active plate margin through north-directed subduction of the Pacific oceanic plate. A review of the regional palaeogeography, as well as palaeontological and thermochronological data, highlights the lack of evidence to support the Indosinian as a major mountain building event. There is no definitive evidence for Triassic collision between the Indochina and South China blocks. Preference is given to a plate tectonic model that explains the Indosinian as a reactivation event driven by accretion of Sibumasu block to Indochina.