Coral terrace surveys and U-series ages of coral and mollusk shells yield a surface uplift rate of ~0.6 m/ka for Kisar Island. The small island is located NE of Timor in the active Banda Arc of Indonesia. Based on this rate, Kisar first emerged from the ocean as recently as ~450 ka. Terrace surveys show warping that follows a pattern of east-west striking folds, which are along strike of thrust-related folds of similar wavelength imaged by a seismic reflection profile just offshore. This deformation shows that the emergence of Kisar can be attributed to forearc closure along the south-dipping Kisar Thrust. Terrace morphology and coral ages are best explained by recognizing major terraces as mostly growth terraces and minor terraces as mostly erosional into older growth terraces. All reliable and referable coral U-series ages are marine isotope stage (MIS) 5e (118-128 ka), which encrusted the coast up to 60 m elevation. All coral samples are found below 6 m elevation, but a tridacna (giant clam) shell in growth position at 95 m elevation yields an age of 195 +/- 31 ka, which corresponds to MIS Stage 7. Loose deposits of coral fragments found on top of low terraces between 8 and 20 m elevation yield ages of < 100 years and may represent paleotsunami deposits from previously undocumented seismic activity in the region. The metamorphic rocks of Kisar, Indonesia, which correlate with the Aileu Metamorphic Complex of East Timor, record the breakup of a supercontinent with associated rifting, metamorphism from arc-continent collision, and the growth and exhumation of a new orogenic belt. The protoliths of these rocks are mostly psammitic with minor basaltic and felsic igneous rocks. Geochemical analyses of mafic meta-igneous rocks show rift affinities that are likely related to rifting of Gondwana and later breakup in the Jurassic Period. The Aileu Complex is overlain by younger sedimentary rocks deposited on the northern passive margin of Australia, which collided with the Banda Arc in latest Miocene time. This collision caused metamorphism of the distal edge of the continental margin rocks at conditions of 600-700°C at 6-8 kbar and up to 700-850°C at 8-9 kbar locally, corresponding to depths from 25 to 30 km. These rocks were then rapidly uplifted and exhumed. U-Pb analysis of detrital zircons indicates a Permian to Late Jurassic age of the sedimentary sources and confirm an Australian provenance. The timing of metamorphism of the Aileu Complex is poorly constrained by previous studies, of which only a white mica cooling age of 5.36 +/- 0.05 Ma proved reliable. Prior apatite fission track studies show that all tracks are partially to completely annealed suggesting recent rapid cooling. A domal geometry of the island above the sea floor is expressed in the pinnacle shape. Foliations on Kisar Island generally strike parallel to the coastline, which is may be suggestive of doming. The Kisar Thrust, which is imaged in offshore seismic reflection data, may indicate that the doming corresponds to diapirism into the hinge of an active thrust-related anticline or diapirism of buoyant continental material along the thrust itself.



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Physical and Mathematical Sciences; Geological Sciences



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Kisar, Banda Arc, Indonesia, tectonics, coral, tridacna, uranium series, arc-continent collision

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