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
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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 Ltd
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