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The Jalan Wangsa Thrust in eastern Kuala Lumpur; a marker for a major crustal boundary between the leading edge of the Sibumasu Block and the Sukhothai Arc?
Kuala Lumpur and Singapore lie on opposite sides of the Bentong-Raub Suture Zone: the locus of amalgamation and consolidation of the Sibumasu Block with the southern part of the Sukhothai Arc and Indochina-East Malaya Block. These continental blocks separated sequentially from the supercontinent of Gondwana during the Permian and Lower Devonian respectively, with the Sukhothai Arc system developing between them during the Permian to Triassic (Metcalfe, 2011). Analysis of new data from the Bukit Timah Centre magmatic rocks of Singapore show that the final amalgamation of Sundaland into Mesozoic proto-Southeast Asia, succeeded a long record of then broadly north-directed subduction of Palaeo-Tethys Ocean crust, from at least the early Permian (Gillespie et al., 2019). Cessation of I-type magmatism in this sector of the Sukhothai Arc occurred around 230–225 Ma. Pervasive and forceful upward release of hydrothermal fluids followed pluton assembly in Singapore, probably triggered by slab break-off processes that led to rapid crustal rebound and resultant uplift. Alteration of granitic rock, associated with that fluid release, is recorded by significant development of widely distributed tuffisite in the Bukit Timah Centre rocks (Gillespie et al., 2019). A ‘top-to-the-east’ ultramylonite zone now identified in eastern Kuala Lumpur, named here as the ‘Jalan Wangsa Thrust’, is more than 300 m thick, east-verging, and superimposed on the later stages of assembly of the c. 200 Ma S-type granite plutons assigned to the Main Range Granite province that were generated by crustal thickening (Ghani et al., 2013). Younger bodies of S-type granitic rocks cut mylonitic rocks in the field outcrop. Final amalgamation after 198 ±2 Ma thus apparently involved east-directed (080°N present day) over-thrusting of at least parts of the leading edge of a now relatively buoyant Sibumasu onto the Sukhothai Arc/Indochina-East Malaya margin. The accretionary Bentong-Raub Suture Zone was probably over-ridden at this time. In Kuala Lumpur, Kenny Hill Formation strata intruded by Main Range granite plutons are disposed in the hanging wall of the Jalan Wangsa Thrust and preserve widespread evidence of a dominant phase of E- or ENE-verging, upward-facing fabrics and folds. Mesozoic strata in Singapore now assigned to the Middle Triassic Jurong Group, Upper Triassic Sentosa Group and Upper Triassic to earliest Jurassic Buona Vista Formation (Dodd et al., 2019), were folded and thrust (towards 050-060°N) after 209 Ma and would lie in the shear zone footwall (Fig. 1). This early-Mesozoic structural framework is likely to have dominated the Peninsular Malaysia region of proto-Southeast Asia assembly, prior to superposition of dextral strike-slip tectonics in the later Mesozoic (most likely from c. 90 Ma onwards). The engineering and hydrogeological properties of rock volumes are profoundly affected by geological structures (Hoek, 2007; Gattinoni, 2012). Due to the fractal distribution of these structures (cf. Kruhl, 1994; Barton and La Pointe 1995), understanding the nature and geometry of these features at a continental/orogenic-scale can also inform geoscience practitioners of the likely style of deformation and discontinuities that will be encountered in site-scale construction developments. Subsurface development across Peninsular Malaysia, in particular Kuala Lumpur and Singapore, should therefore take into account the record of Mesozoic tectonics presented here.