Discussion on structural evolution of Mesozoic Peninsular Malaysia

W. Altermann writes: In their discussion of palaeotectonics and palaeogeography of Malay Peninsula, Harbury et al. propose an alternative interpretation to Sengor’s (1984) model of Triassic or early Jurassic collision of the Cimmerian (Sibumasu) block with the Indochina block in the east. Their interpretation depicts Malay Peninsula as part of Triassic and lower Jurassic Australian Gondwana. The Raub Bentong Fault Zone is considered as having originated through the Permo-Triassic southward destruction of ‘Tethys Ocean 11’ underneath the Malay Peninsula. For the Triassic this zone is described as a normal fault (see figs 11-13, Harbury et al .) . The Raub Bentong Zone, however, is generally accepted to be the southern extension of the Nan-Uttaradit suture in north Thailand (see Harbury et al . , fig. 2). This suture was described by Macdonald & Barr (1984) as having a volcanic arc setting. Macdonald et al. (1985) reported crossite blueschists associated with ultramafic and mafic rocks from this suture. Barr & Macdonald (1987) identified type I1 ophiolites, formed by a spreading process above a subduction zone in this suture, and also reported a K-Ar metamorphic age of 269 f 12 Ma for the crossite schists. Helmcke (1985) reported a K-Ar age of 344 f 22 Ma for the ultramafic and mafic rocks included in the suture. None of these references are cited in the palaeogeographic discussion by Harbury et al. Helmcke (1983, 1985, 1986a, 1986b), Helmcke & Kraikhong (1982), Helmcke & Lindenberg (1983) and Altermann (1983, 1986, 1987, 1989), Altermann et al. (1983), Winkel et al. (1983) proposed that the main orogeny in Southeast Asia was late Palaeozoic (Carboniferous and/or Permian) rather than Triassic. Although these models discuss the Sibumasu block as welded to Palaeoeurasia lready in the Permian, this possibility is completely ignored by Harbury et al., who still maintain that the Southeast Asia is generally thought to have experienced a major Triassic orogeny. In their palaeogeographic discussion these authors argue that there is no evidence for a major Triassic/Jurassic orogeny and therefore Peninsular Malaysia, Thailand and Burma were not attached to central and east Thailand and Indochina until after Middle Jurassic or Cretaceous. This shows only that the attachment was not Early Mesozoic. Data on Palaeozoic rocks, presented by Harbury et al. support Helmcke’s (1985) interpretation. Palaeontological evidence for a close Eurasian relationship of the terranes discussed, as presented by Bunopas & Vella (1984), Walchia piniforrnb from south Thailand; by Ingavat-Helmcke & Helmcke (1986), Permian foraminifera with Euramerican affinities; by Buffetaut & Ingavat (1985), Eurasian Triassic land vertebrates from east Thailand; by Fontaine (1986), Permian corals from Sumatra, known also from south China, Yunnan, Thailand and Laos; by Grant (1976), warm water Permian brachiopods from southern Thailand, are not considered by Harbury et al. Discussion of . such evidence as well as of tectonic, sedimentologcial and other arguments can be found, for instance, in Metcalfe (1986). Although these arguments may be equivocal, to ignore them in the discussion of palaeontological data is a mistake, while the hint that palaeomagnetic data may in future, help to solve the problem is little value in the present controversy. The suggestion by Audley-Charles (1987) that Cathaysian flora may not have been geographically isolated from Gondwana, but reflects warmer Tethyan conditions along eastern Gondwana is highly speculative and not sufficient argument for Jurassic drift and Cretaceous collision. The palaeontological problems generated by a Jurassic Gondwana provenance of Malay Peninsula as proposed by Harbury et al. are serious and their discussion would require a separate paper. In addition there are the questions to be answered: where is the suture of the post-Jurassic collision and what kind of Tethys was destroyed during this event? The publications cited by Harbury et al., selectively support the hypothesis presented. Thus Stauffer (1974) is cited, but there is no reference to Stauffer (1983), where a Triassic orogeny is proposed. The paper of Metcalfe (1986) seems forgotten, in which Audley-Charles’ (1983) plate tectonic reconstruction is judged to be untenable and the ‘change from cold water faunas with NW Australian affinities to warmer water faunas with Eurasian affinities in the Lower to Middle Permian transition on the Sibumasu Block’ is tough to support Helmcke’s hypothesis (Metcalfe,