Journal of the Geological Society; April 1989; v. 146; no. 2;
p. 357-358; DOI: 10.1144/gsjgs.146.2.0357
© 1989 Geological Society of London
Discussion on the Southern Uplands as an accretionary prism: the importance of analogues in reconstructing palaeogeography
John Milsom, M. Husen and Helen Kallagher write:
A note on the Sumatran-fore-arc. The validity of the use of modern fore-arcs as analogues for the Palaeozoic Southern Uplands of Scotland has recently been discussed by Leggett (1987). His paper made only passing reference to the Sumatran fore-arc but a subsequent discussion by Rock (1987) was concerned almost exclusively with that area. The geological picture accepted as the basis for his discussion differs from that described in published papers and from our own field observations. We would like here to try to limit the dissemination of some common myths.
Contrary to the statements made by Rock, the two largest islands in the fore-arc chain have not been mapped either ‘exclusively’ (Siberut) or ‘dominantly’ (Nias) as polymict melange. The only readily accessible studies of Siberut, quoted in van Bemmelen’s (1970) compilation of the geology of Indonesia, describe a number of Tertiary formations and agree that the pre-Tertiary is absent. The oldest rocks are apparently both folded and intensively faulted, but there is no suggestion that the term ‘melange’ would be in any way appropriate. The island was further mapped in 1983 by a team from the Indonesian Geological Research and Development Centre. They worked mainly in the south-eastern quarter, where they found only a single small outcrop of melange. All the remaining exposures were of muddy lithic sandstone, siltstone and mudstone, much of which was tuffaceous. These are commonly disturbed by small-scale faults and thrusts, and in places a cleavage developed in
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