Splinter Tectonics and the Reworking of Forearc Crust in Timor, Indonesia

Timor is situated in the Lesser Sunda chain of islands in the non-volcanic outer Banda Arc of the Australia–Southeast Asia orogen. Diachronous collision of the irregular passive margin of Australia with the detached margin of SE Asia in the Miocene produced an assemblage of nappes, mélanges and metamorphic terranes that characterise the complex geology of Timor. This article has been developed from a keynote presentation delivered at the 6th International Geoscience Conference of the Institute of Geosciences of Timor-Leste (IGTL), held in Dili in February 2026. In keeping with that format, the paper is a synthesis and interpretive review of Timor’s metamorphic terranes, and a fuller ex-planation of the concept of ‘splinter tectonics’ that was introduced. Field, petrographic and geochemical evidence show that the unique features of the Boi metamorphic massif on West Timor can be explained by a process whereby blocks and slices of deeply-buried upper-plate crust became entrained in the subduction channel and juxtaposed against hot forearc mantle before being extruded by buoyancy forces. The Boi Massif is therefore interpreted as a case study that provides new insights into the tectonic evolution of the Banda Arc and possible processes operating within convergent margin systems elsewhere.