J. geol. Soc. London, Vol. 136, 1979, p. 587. Printed in Northern Ireland,

Discussion on magmatism and tectonics inSE Asia

DR D. STEPHENSONdrew attention to work recently DR H. G. READINGsaid that: Listening to several of completed by an IGS team on late-Tertiaryand the talks andthe subsequent discussions, especially Quaternary volcanoes in Burma, which strongly sug- about the Andaman Sea, Sumatra, the Andes and the gests the presence of an eastward-dipping subduction Hercynian fold belt,perhaps the solution to many zonebeneath continental crust throughout Central problems may be reached if moreconsideration is Burma. Three distinct N-S trending volcanic lines may given to the effects of strike-slip motion in these areas. be recognized, both spatially and in petrographic and For example, the NW-W trend of folds in northern chemical trends. The volcanics in general are markedly Sumatra and the NE-trending spreading centres in the potassic with both K,O/SiO, and (Na,O + K,O)/SiO, ‘back-arc’ basins of the Andaman Sea are compatible increasing from W to E. There thus exists a ‘classical’ with dextral strike-slip motion within the over-riding progression from ‘calc-alkaline’ characteristics in the SE Asian continental plate rather than the result of western line (Narcondam andBarren Islands) to ‘al- easterly subduction. The comparison of SE Asia and kaline’ in the eastern line (along the Sagaing-Hninzee the northern Andes (where, unlike the centraland fault system). This evidence gives strong support to the southernAndes, strike-slip motion is the dominant model, proposed by Beckinsale et al., of a subduction feature) with the Hercynian fold belt is particularly origin for the Tertiary ‘western peralkaline granites’ of pertinent. All these areas lack calc-alkaline volcanics Burma and Thailand. It seems reasonable to suggest andpaired metamorphic belts, andperalkaline gra- that this subduction zone is a northern continuation of nites are adominant feature of SE Asia andthe the one presently active beneath Sumatra, where the Hercynides. The origin of peralkalinegranites still younger volcanics have petrographic and geochemical seems obscure, but there is little evidence that there is similarities with those in Burma of comparable age. any causal connection with subduction. Complications do arise in this model owing tothe Whilst agreeing that subduction is the only way by presence of the NE-SW trending Andaman Sea which substantial amounts of oceanic crust can be lost spreading ridge between Burma and Sumatra. Trans- and that it explains most tectonic features in the fore- formfaults which cut this ridge probablycontinue arc region, we need to look for other explanations for northwards into Burma as active, dextral, transcurrent the features of back-arc regions of the over-riding faults,some of which have acted as channels for plate, especially if this is continental as in SE Asia and magmatic activity. the Andes. Strike-slip motion sometimes suggests an explanation.

0016-7649/79/0900-0587$02.00 @ 1979 TheGeological Society

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