Structural Evidence for a Palaeozoic Orogeny in North-west Malaya

Abstract

AbstractLower Palaeozoic rocks in the Langkawi Islands have been subject to three generations of deformation. In the slightly dynamo-meta-morph slates in the south-east two generations of cleavage folding have been developed with a N.N.W. trend. The first resulted in a recumbent folding with a subhorizontal slaty cleavage, whereas during the second generation the slaty cleavage was deformed in open folds and a crenulation cleavage developed in the steep limbs. The two deformations were probably not formed at exactly the same time, but seem to be related with each other. Boudinage of granite sills was contemporaneous with the development of the slaty cleavage.In the unmetamorphosed northern part of the area cross-folding is found. Flexure folds with a N.W. trend are superimposed on folds with a N.N.E. trend. In the Upper Palaeozoic sediments such deformations are missing. The Permian and Carboniferous strata show only a slight warping around the granite batholith which was intruded during Jurassic time (Pacific orogeny). A similar warping is found in the large low-angle overthrust, in which Silurian was thrust over Permo-Carboniferous. Consequently this thrust plane is dated as pre-granitic and post Permo-Carboniferous. Contact metamorphism by the granite intrusion has resulted in hornfelses in the Permo-Carboniferous sequence and in the development of oriented minerals, mainly clinozoisite, in the Ordovician-Silurian slates. Knick zones and intense jointing parallel with the mineral orientation (third generation structures) are also a result of the granite intrusion.The first two generations of folding, which are not present in the Upper Palaeozoic beds, are products of an erogenic phase, “the Langkawi folding phase,” during late Silurian-Devonian time. The lack of Devonian strata in Thailand and the incompleteness of the Devonian sequence in Malaya (recently a few outcrops of probably Middle-Upper Devonian strata are found along the west coast of Malaya) can be an indication that the Langkawi folding phase is not restricted to the Langkawi Islands.