Uplift rate of coral reef terraces in the area of Kupang, West Timor: Preliminary results

Abstract

A Th/U date of 152,000 years obtained from the prominent fifth step, at 44 m elevation, of a flight of 7 well defined raised reef terraces at Cape Namosain, 5 km west of Kupang, allows inference of a preliminary mean uplift rate of 0.3 mm/yr since the last interglacial. Such a slow uplift rate is supported by the observation of numerous large modern reef platforms and of very limited mid-Holocene emergence — if any — throughout the region. Subject to conformation through further dating, this suggests the existence of a more rapid uplift zone in central West Timor, by contrast with the area of Kupang to the west and of northern East Timor and Atauro Island to the east, possibly in relation with the NW-SE seismic tear zone which has been recently pointed out within the subducting plate north of West Timor. One of the main interests in the terraces of Cape Namosain, apart from its neotectonic aspect, resides in the fact that there may be an oppurtunity to define the 180,000 year old paleo-sea level (terrace VIII of Huon Peninsula): based on the geomorphological interpretation of the series, it could correspond here to the fourth or the sixth terrace. Field investigation and dating in that regard are currently in progress. Another Th/U date of 124,000 years, obtained on the lowest emerged terrace (7 m above low tide) in the southeast of the nearby island of Semau, indicates that this second area has been uplifting little during the last 125,000 years. This, together with the observation of tilts and other structural features, leads to the conclusion that the area of the southwest of Kupang is affected by differential uplifts and block-faulting.