The structural evolution of the Bentong-Raub Zone and the Western Belt around Kuala Lumpur, Peninsular Malaysia Jorien L.N. van der Wal MSc Thesis 2014 Supervisors: Dr Ernst Willingshofer, Dr Thomas Francois, Dr Liviu Matenco ABSTRACT To determine what deformed the inherited Bentong-Raub suture zone in western Peninsular Malaysia, a detailed field study in the area around Kuala Lumpur was conducted focussing on the various structural, metamorphic and kinematic features of the Bentong-Raub Zone and the surrounding Western- and Central Belts. The in-depth knowledge on large-scale structures and the tectonic origin of the study area was used to develop a tectonic model for the evolution of western Malaysia. Four deformation phases were recognized in the field, of which the most prominent one, D1, is a progressive deformation phase comprising two foliations, two folding geometries and coeval shear, and can be related to NE-SW shortening. Presumably, this deformation corresponds to accretionary wedge formation during northward subduction of the Paleo- Tethys Ocean beneath Indochina. Burial of sediments in this accretionary wedge led to the growth of biotite, muscovite, chlorite, garnet, quartz and feldspar under greenschist facies metamorphic conditions. Sedimentation and subsequent deformation and burial-related sub- greenschist facies metamorphism of a Triassic forearc basin recorded a second deformation event D2. Intrusion of the Triassic S-type Main Range Granites caused contact metamorphism in the surrounding metasediments and was followed by steep normal faulting (D3) and a later, regional strike-slip shearing event (D4). The combined effect of D3 and D4 is thought to have played a major role in the exhumation of western Peninsular Malaysia during Paleogene times. It is also suggested that this shear is related to Eocene formation of offshore sedimentary basins due to Indian-Eurasian collision and subsequent clockwise rotation of south-east Asia. i Table of Contents Abstract ....................................................................................................................................... i 1. Introduction ......................................................................................................................... 4 2. Geological Setting ............................................................................................................... 6 2.1. Sibumasu Terrane ............................................................................................................ 8 2.2. Sukhothai Arc .................................................................................................................. 8 2.3. Oceanic rift basins ........................................................................................................... 9 2.4. The Bentong-Raub Zone ................................................................................................. 9 3. Methods ............................................................................................................................. 11 4. Results ............................................................................................................................... 13 4.1. Field observations .......................................................................................................... 13 4.1.1. Lithologies & Metamorphism ................................................................................. 13 4.1.2. Ordovician-Silurian, Devonian & Bentong-Raub metasediments .......................... 17 4.1.3. Triassic (meta-) sediments ...................................................................................... 25 4.1.4. Main Range Granites .............................................................................................. 28 4.1.5. Kinematics .............................................................................................................. 30 4.2. Thin section and SEM analysis ..................................................................................... 32 4.2.1. Metasediments ........................................................................................................ 33 4.2.2. Granites ................................................................................................................... 37 4.2.3. Shear senses ............................................................................................................ 38 5. Discussion ......................................................................................................................... 39 5.1. Interpretation of field & thin-section analysis ............................................................... 39 5.2. Cross section .................................................................................................................. 45 5.3. Tectonic implications .................................................................................................... 51 5.4. Is Bentong-Raub a suture? ............................................................................................. 54 6. Conclusions ....................................................................................................................... 55 Outlook ................................................................................................................................. 56 Acknowledgements .................................................................................................................. 57 References ................................................................................................................................ 58 Appendix 1 GPS Coordinates and orientation measurements ............................................... 60 Bentong-Raub zone .............................................................................................................. 60 Ordovician-Silurian east of Kuala Lumpur .......................................................................... 67 Ordovician-Silurian west of Kuala Lumpur ......................................................................... 73 ii Granites east of Bentong-Raub ............................................................................................. 75 Granites west of Bentong-Raub ............................................................................................ 77 Triassic .................................................................................................................................. 81 Devonian ............................................................................................................................... 85 Appendix 2 Thin section descriptions and scans ................................................................... 92 Appendix 3 SEM-results ...................................................................................................... 102 Sample M9 .......................................................................................................................... 103 Sample M15 ........................................................................................................................ 104 Sample M24 ........................................................................................................................ 107 iii 1. INTRODUCTION Peninsular Malaysia comprises two tectonic terranes (fig. 1), which both originated from the supercontinent Gondwana and subsequently assembled in Late Triassic times (Metcalfe, 2013). The Sibumasu Terrane, now representing west Malaysia, separated from Gondwana in Early Permian (Sakmarian) times, during the opening of the Meso-Tethys. The Sukhothai Arc, a volcanic arc system that formed on the margin of the much larger Indochina Block/Terrane, represents Central and Eastern Peninsular Malaysia (see figure 1). The Indochina Block separated from Gondwana in Devonian times and Sibumasu was separated from Australia/Gondwana by Late Permian times. The volcanic-plutonic Sukhothai (island) Arc formed during northward subduction of the Paleo-Tethys Ocean beneath the Indochina Block in Late Palaeozoic (Permian) times. The subduction of the Paleo-Tethys Ocean was followed by the collision of the two tectonic terranes and subsequent initiation of orogenic deformation in Lower Triassic times. In Late Triassic-Early Jurassic times, as a consequence of the A-type subduction of the Paleo- Tethys Ocean beneath Indochina, S-type syn- to post-orogenic granites were emplaced (pink coloured units in Fig. 2). These granites, representing the main topographical highs of Peninsular Malaysia, are known as the Main Range granites. They separate accretionary wedge deposits of the Bentong-Raub Zone, from the foredeep deposits of the Semanggol Formation in NW Peninsular Malaysia that formed on the depressed Sibumasu margin (Metcalfe, 2013). Fig. 1 Overview of the various tectonic terranes which together form the geology of Peninsular Malaysia in its large-scale tectonic setting (François, 2015). Suture zones are highlighted by red lines. The dotted red line going through Peninsular Malaysia (blue outline) is the Bentong-Raub zone (Wakita & Metcalfe, 2005; in Hutchison, 2007). 4 The boundaries between the north-south trending belts that characterise the Palaeozoic and Early Mesozoic geology of Peninsular Malaysia are thought to represent the outlines of paleo- subduction sutures (Tan, 1996; red lines in fig. 1). Specifically, the narrow zone separating the Western Belt (i.e. Sibumasu Terrane) from