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A ‘top-to-the-east’ ultramylonite zone is identified in north-eastern Kuala Lumpur and named here as the ‘Ukay Perdana Shear Zone (UPSZ). The UPSZ is at least 250–300 m thick, east-verging, and superimposed on the later stages of assembly of the c. 200 Ma S-type ‘Western Belt’ granite plutons generated by crustal thickening and assigned to the Main Range Granite Province. Younger bodies of S-type granitic rocks cut the shear zone. These intensely deformed quartzofeldspathic rocks contain distinctive relict porphyroclasts of perthitic K-feldspar (microcline), oligoclase and quartz, entrained within the ultramylonitic fabric. Migrating sub-grain boundaries in quartz indicate deformation occurred under moderate to high temperature conditions during ductile deformation (c. 500–600°C); undulose extinction suggests down-temperature evolution of the deformation history. This deformation event implies that the final stages of collisional interaction between Sibumasu and the Sukhothai Arc would have involved now east-directed (080°N, then generally northward) over-thrusting of the leading edge of Sibumasu onto the Sukhothai Arc/Indochina-East Malaya margin after 198 ±2 Ma. Kenny Hill Formation strata in Kuala Lumpur preserve widespread evidence of a dominant phase of Eor ENE-verging, upward-facing fabrics and folds; that deformation is judged to be a hanging wall response to deformation in the UPSZ. This early-Mesozoic structural framework is significant during assembly of the Peninsular Malaysia region of Sundaland; evidence form Kuala Lumpur is combined with new published Singapore data. Dextral strike-slip tectonics is superimposed on this collisional framework in the later Mesozoic and Cenozoic; subsurface development across Peninsular Malaysia, and in Kuala Lumpur and Singapore in particular, should account for deformation structures linked with this expanded record of Mesozoic tectonics.

The Ukay Perdana Shear Zone in Kuala Lumpur: A crustal-scale marker of early Jurassic orogenic deformation in Peninsular Malaysia