Neogene basin history and paleomagnetism of a riftedand inverted forearc region, on- and offshore Sumba, Eastern Indonesia

Abstract

The island of Sumba is located in the arc/trench gap at the junction of the Sunda and Banda Arcs. To elucidate its enigmatic forearc position, stratigraphic and paleomagnetic data were collected along three transects in central and east Sumba. Sumba represents the emerged part of a terrane of SE Asian origin. Results of stratigraphic and paleomagnetic investigations on central and east Sumba are presented and discussed in order to identify patterns of horizontal and vertical crustal movements. A synthetic seismogram of the onshore record enables correlation with the offshore Neogene E of Sumba, in order to interpret the data in a regional context. The Neogene basin history of Sumba started when overall subsidence took place around the transition from Early to Middle Miocene. On east Sumba, above a basal condensed interval, up to 600 m of Middle-Late Miocene volcaniclastic submarine fan deposits occur. The sediments reflect island arc volcanism and are interpreted as parts of a prograding submarine fan system on a north-dipping paleoslope, fed by a volcanic source to the south of Sumba. Biostratigraphic data indicate a high volcanic supply during the middle Miocene and early-middle Tortonian, a waning supply during the late Tortonian and renewed supply during the Messinian. These patterns probably reflect variation in volcanic activity. During the Messinian calcilutites and foraminiferal chalks became dominant and deposition of these pelagic sediments continued into the Early Pliocene. Using the pattern of magnetic polarity, changes in a selected interval the position of the MioceneePliocene boundary is better constrained. Seismic studies in the offshore area E of Sumba (Van der Werff 1995a, b) indicate the presence of an extensive tract of N-prograding turbidites on Sumba Ridge (Savu Basin). The proposed correlation with the onshore record suggests lateral continuity from the island into the offshore area, where the volcaniclastic submarine fan system presumably had its largest development, Subsidence proceeded so fast ( - I m/ka) on east Sumba, that before the beginning of the Late Miocene, sediments were deposited below the CCD. This pattern suggests that the Sumba region underwent strong crustal attenuation due to rifting. As this was coupled with the arrival of volcaniclastics we envisage intra-arc rifting, coupled with the growth of a short lived volcanic arc further south. Its initiation at the transition of Middle-Late Miocene suggests interaction with the then approaching Banda Arc subduction system. The paleomagnetic data indicate that during this process Sumba was at a paleolatitude of - 12 ’ South and underwent a counter-clockwise rotation o f 5 The reappearance of planktonic foraminiferal assemblages during the latest Miocene and their present elevation above sea level suggest an average rate of post-Miocene uplift of - 0.7 mm/a. Emergence of Sumba probably did not take place before 3 Ma. This inverted motion of the Sumba terrane is probably coupled with the change to the present tectonic situation, in which volcanic activity was resumed in the present inner arc after a non volcanic interval during the spreading stage. 1: 1997 Elsevier Science Lt