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

: 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 granitic aligned foliated granitic mylonite, K-feldspar; lineated granitic mylonite, dips 60°