Magnetic lineations constraints for the back-arc opening of the Late Neogene South Banda Basin (eastern Indonesia)

The South Banda Basin is located within eastern Indonesia near the triple junction between the Eurasian, Pacific and Indo-Australian plates. It is underlain by oceanic crust, but its origin and age were not well established. It has been interpreted as a Mesozoic trapped piece of Indian ocean or as a Cretaceous-Eocene basin related to the Celebes and Sulu basins, but a Neogene back-are origin was also considered. Recent geochemical and geochronological studies strongly support the latter hypothesis. In this paper we present a new analysis of the magnetic data from the eastern part of the South Banda Basin, the Damar Basin. We used magnetic field measurements collected during nine oceanographic cruises from various institutions. Looking for magnetic correlation in the time span given from recent geochronological data, the comparison between measured profiles and theoretical profiles deduced from the reversals of Earth's magnetic field during Neogene time allows us to infer an opening of South Bands Basin during Late Miocene-Early Pliocene time, from 6.5 to 3.5 Ma. Magnetic lineations 2An, 2Ar, 3n, 3r, 3A, and possibly 3Ar are recognized, with an extinct spreading centre trending ENE-WSW. At least five segments are identified, each segment being separated by inactive transform faults perpendicular to the extinct spreading centre. A half spreading rate of about 3 cm/yr is calculated, based on spacing of magnetic lineations. The opening history of the basin is discussed. The cause of cessation of spreading is likely the are-continent collision dated at about 3 Ma. However the onset of opening is less determined. We suggest that Damar Basin began to open at about 6.5 Ma during magnetic period 3An as an intra-are basin, separating the Banda volcanic are to the south from the incipient Lucipara volcanic are to the north. The latter was probably created at the beginning of rifting of Damar Basin, as shown by both magnetic and geochronological data. The young age of the South Banda Sea Basin is contradictory with its great depth. We discuss this problem in view of thermal and tectonic considerations. We finally conclude that our magnetic model allows us to more precisely describe the opening of the basin. (C) 2001 Elsevier Science B.V. All rights reserved.