Arc-continent collision as recorded in modern stream sand from Oecusse (Timor Island)

Abstract

The small territory of Oecusse in Timor Island has extremely complex geology, influenced by the collision of the Australian continent with the Banda Arc. In Oecusse, sedimentary, volcanic and metamorphic units associated with the lower plate Australian continental margin, the upper plate Banda Terrane, and synorogenic sedimentary successions are exposed. In this study, geochemistry, petrography, and heavy-mineral data on fluvial bedload sand transported by first- and second-order streams are used to better understand the status of arc-continent collision in Timor and the geological nature of the terranes involved in this tectonic process. The presence of upper mantle outliers in Oecusse is readily revealed by serpentinite grains, a heavy mineral assemblage with enstatite, olivine and Cr-spinel, and by Cr, Fe, Sc, Ti, and V concentrations comparable to the Primitive Mantle standard. Volcanic sources occur both in the Banda Terrane and Australian continental margin. The Manamas Formation (Miocene, Banda Terrane) is the most prominent volcanic source, shedding detritus dominated by mafic volcanic rock fragments, plagioclase, and clinopyroxene, with a geochemical signature similar to the Primitive Mantle. The volcanic units of the Barique Formation (Eocene-Miocene, Banda Terrane) and the Maubisse Formation (Permian, Australian continental margin) comprise compositionally heterogenous suites and are intercalated with sedimentary successions, delivering more felsic material, as indicated by the diversified assemblages of lithic fragments and heavy-minerals. Geochemically, they are characterised by lower contents of most compatible elements, higher contents of Th, Y and REE, steeper REE profiles, and stronger negative Eu anomaly than Manamas-derived detritus. These petrographic and geochemical features locally reveal a major felsic volcanic component in the Barique Formation, which cannot be related to the ongoing orogeny because of its proposed late Eocene to early Miocene age. With the exception of coastal sectors where the Manamas Formation is widely exposed, volcanic, sedimentary and low-rank meta-sedimentary rocks partially derived from the Australian lower plate tend to prevail, confirming that arc-continent collision is still at an early stage in the western part of Timor Island.