SEISMIC STRATIGRAPHY AND SUBSIDENCE EVOLUTION OF THE NORTHWEST SUMATRA FORC-ARC BASIN

New seismic reflection data collected with the Indonesian RV Baruna Jaya III under a French-Indonesian cooperative program allowed to study the geodynamic evolution and the seismic stratigraphy off northern Sumatra within the Sumatra fore-arc domain. The direction of subduction of the Indian oceanic crust below the Sunda arc is normal in front of Java and oblique along Sumatra. As a result, parts of the Sumatra margin are moving northwards along two large strike-slip faults parallel to the subduction trench, the Sumatra fault in Sumatra and the Mentawai fault at sea which separates the accretionary prism from the fore-arc basin. We describe the relationships between the seismic stratigraphy and the sedimentary history and we propose a tectonic evolution of the fore-arc domain. After widespread uplift and erosion during the Paleogene, the fore-arc basin becomes subsident during the Miocene as evidenced by shelf deposits organized in two transgressive-regressive sequences 2a-2b produced by tectonics and eustatism. This is interpreted as the result of a limited E-W extension linked to the incipient play of the Sumatra and Mentawai faults. In Pliocene-Quaternary times, the fore-arc basin is segmented into several sub-basins (Aceh, Simeulue and Nias basins) by compressional zones or by strike-slip faults. The subsidence rate increased strongly producing sequence 3 with two or four subunits for the Pliocene, and sequence 4 for the Quaternary. During this period, local variations of the sediments thickness and of the number of sequences indicate that tectonics prevails over eustatism in the fore-arc basin. The Sumatra margin is one of the best examples in the world to study the relationships between sedimentary and tectonic processes in an oblique subduction zone.