Abstract

Dr N. M. S. Rock writes: the importance of modern analogues is asserted by Leggett (1986) as support for his interpretation of the Southern Uplands as an accretionary prism and he cites Sumatra as a good analogue (see also Leggett et al. 1983, p. 201). Three similarities are inferred: (1) both accretionary complexes have become emergent; (2) both accretionary/upper slope terrane boundaries are fundamental reverse faults; (3) both fore-arc basins may be underlain by a thin prong of continental crust.

I fear that these similarities may be accepted more by default than by consensus, since few geologists have worked extensively in both areas. As one who has, I suggest the similarities are misleading, for they take account only of data (e.g. seismic) which are largely intangible and subjective. Kieckhefer (1980) presented various other interpretations of (3) for Sumatra, while (2) for Southern Scotland is equally questionable since the Southern Uplands Fault (SUF) may show major normal or transcurrent movement.

I suggest that better evidence is the nature of the respective rock successions, and this presents a very different picture. The Mentawai islands forming the non-volcanic Sumatran fore-arc are mapped either exclusively (e.g. Pulau Siberut) or dominantly (e.g. Oyo Complex of Pulau Nias) as polymict melange (Hamilton 1979). Clasts in this melange include amphibolites, basalts and spilites, cherts, greenschists, limestones, siltstone/sandstone interbeds, mica-schists, scaly clays, gabbros, serpentinites and peridotites (Hamilton 1979, pp. 20-1; Moore & Karig 1980). Neither melange itself nor any of these particular lithologies is a significant component of

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