Tectonic evolution of forearc nappes of the active Banda arc-continent collision: Origin, age, metamorphic history and structure of the Lolotoi Complex, East Timor

An integrated multidisciplinary investigation of the Lolotoi Complex of East Timor (Timor Leste) indicates that it is part of the Banda forearc that was metamorphosed and rapidly exhumed during the Eocene and accreted to the NW Australian continental margin during Late Miocene to present arc–continent collision. Greenschist, graphitic phyllite, quartz–mica schist, amphibolite and pelitic schist dominate metamorphic rock types. Mineral, whole rock, and trace element geochemical analyses of metabasites indicate protolith compositions consistent with tholeiitic basalt and basaltic andesite with mixed MORB and oceanic arc affinities. Metapelite schist is mostly composed of metasedimentary units derived from mafic to intermediate rocks with oceanic to continental volcanic arc provenance.Thermobarometric calculations show peak metamorphic conditions of 530 °C to 680 °C for garnet–biotite pairs and amphibole, and peak pressures of 5 to 10 kbar for garnet–aluminosilicate–quartz–plagioclase assemblages. Peak metamorphism occurred at 45.36 ± 0.63 Ma, as indicated by Lu–Hf analyses of garnet. Detrital zircon grains have a U/Pb age distribution with spikes at 663, 120 and 87 Ma, which is typical of detrital zircon ages throughout the Great Indonesian Arc of Asia, but is distinct from Australian affinity units. These data indicate deposition and later metamorphism occurred after 87 Ma.Structural analyses of the metamorphic rocks and their sedimentary and volcanic cover units reveals 5–6 deformational phases of alternating shortening and extension. There is little to no evidence of strike-slip deformation. Phases 1–4 are inferred as pre-Oligocene from age determinations. Phases 5 and 6 are most likely related to latest Miocene to Pliocene nappe emplacement and Pliocene to present collisional deformation. Kinematic indicators show mostly top to the SE directed shortening and top to the south and SE extension. Structural mapping indicates that the Lolotoi Complex and some of its cover units are in thrust contact with underlying Gondwana Sequence rocks. Asian affinity volcanic and sedimentary cover units are found mostly in normal fault contact with metamorphic rocks.These data indicate that the Lolotoi Complex of Timor Leste is correlative with the Mutis Complex of West Timor and both form part of the Banda Terrane, which is composed mostly of dispersed fragments of the eastern Great Indonesian Arc. The study demonstrates the complex nature and deformational history of forearc basement.