New field, geochemical, and geochronological data from the Segama Valley Felsic Intrusions (SVFI) of Sabah, north Borneo, shows them to be arc-derived tonalites; not windows or partial melts of a crystalline basement beneath Sabah. U-Pb zircon ages date emplacement in the Triassic and Jurassic: 241.1 ± 2.0 Ma, 250.7 ± 1.9 Ma, 178.7 ± 2.4 Ma, and 178.6 ± 1.3 Ma; contemporaneous with peaks in magmatism and detrital zircons in Sarawak and west Kalimantan (west Borneo). Isotopic data for Sr, Nd, and Pb from whole rocks, and for Hf and O from zircon all show mantle and/or MORB affinities indicating a mantle-derived origin. Enrichment of fluid mobile trace elements and trace element ratios indicate that the most likely setting for this is in a continuation of the Sundaland continental arc. There is no evidence in the field, geochemical, or zircon U-Pb data for continental basement in the Segama Valley region. The intrusive nature of the Segama Valley tonalites constrains the emplacement age of their supra-subduction zone host rocks to at least the Triassic. This new data expands the Triassic and Jurassic extent of Borneo and the Sundaland arc, and challenges models of Borneo's development predominantly through allochthonous terrane accretion in the Cretaceous. Instead, we propose a model of protracted autochthonous growth through supra-subduction zone crustal extension and associated magmatism.