Stratigraphic constraints on suture models for eastern Indonesia
J. Milsom
Department of Geological Sciences, University College London, London WC1E 6BT, UK Received 10 May 2000; accepted 29 June 2000
Abstract
Although collision in eastern Indonesia is now accreting the Australian continent to Southeast Asia, the small North and South Banda oceanic basins within the suture zone are interpreted as Late Cenozoic extensional features. Stratigraphic columns from the surrounding
islands conform to one of three generalised patterns, two of which can be related to the margins of SE Asia Sundaland and the Australian continent, respectively. The third system, which is dominant in the outer Banda Arc and eastern Sulawesi, is associated with a microcontinent
that was rifted from Australia in the Jurassic, drifted northwards ahead of Australia in the Cretaceous and collided with the Sundaland Margin in the Paleogene. Subsequent collapse of the resulting collision orogen led to rapid extension and the formation of the Banda Sea behind the
Outer Banda Arc thrust belt. Eastern Indonesia thus duplicates a pattern familiar in the Mediterranean. The Tertiary compressional structures of the region cannot be explained solely in terms of the most recent collision, which began only in the Pliocene. q 2000 Elsevier Science Ltd.
All rights reserved.
Keywords : Eastern Indonesia; Banda Arc; Eurasian Plates
1. Introduction
Suturing of northern Australia to SE Asia takes place within a diffuse and still poorly understood region Fig. 1
where relative motions between the Indo-Australian and Eurasian Plates are absorbed by subduction beneath the
Sunda Arc and collision around the strongly curved Banda Arc. The plate boundary, which south of the Sunda Arc is
marked by a deep trench, is replaced in the Banda Arc by a series of relatively shallow troughs. There is no significant
offset in the line of active and recent volcanoes but the two forearcs are very different. Except in the area to the west of
Sumatra, the Sunda forearc ridge is entirely submarine, but the Banda forearc Outer Banda Arc is capped by large
islands such as Timor, Tanimbar, Seram and Buru Fig. 1. The origin of the back-arc Banda Sea is still unclear,
with some authors e.g. Silver et al., 1985 regarding the oceanic parts as trapped slices of Indian Ocean or Molucca
Sea crust, while others Hamilton, 1979; Re´hault et al., 1994 have argued in favour of Neogene extension. A
pattern of local extension in an overall collisional environ- ment suggests analogies with the Mediterranean, where
continental collision has produced deep basins floored by attenuated continental crust in the Alboran and Aegean seas
and by oceanic crust in parts of the Tyrrhenian Sea Dewey, 1988. The Mediterranean basins resemble the Banda Sea,
not merely in size, but also in being partly enclosed by orogens with total curvatures approaching 1808 Fig. 2.
Many of the allochthonous terranes which make up east- ern Indonesia are of Australasian origin. This is true not
only of the large landmass of New Guinea, which is still linked to Australia, but of many of the smaller islands on the
Asian side of the collision suture. Some of this material has been transferred from Australasia to Eurasia during the
Pleistocene and continuing Banda Arc collision, but other fragments must have been accreted earlier. Two important
‘Australian’ elements are Buton Island, southeast of Sula- wesi, and the Banggai and Sula Islands, which form the
Sula Spur Fig. 1. There is a wide measure of agree- ment Hamilton, 1979; Milsom, 1985; Silver et al.,
1985 that the Sula Spur is a fragment of New Guinea which was transported west along transcurrent faults
during the late Tertiary. Observations made in the course of oil exploration programmes have dated its
collision with the East Arm of Sulawesi to between 5.2 and 3.8 Ma Davies, 1990.
The history of Buton, which collided with the Sundaland Margin much earlier, in the Early or Middle Miocene, is
more controversial Davidson, 1991. One school of thought considers it to have also come from the Birdshead Smith
and Silver, 1991, but the correlations with New Guinea are less convincing, whereas the Mesozoic sediments are
Journal of Asian Earth Sciences 18 2000 761–779
1367-912000 - see front matter q 2000 Elsevier Science Ltd. All rights reserved. PII: S 1 3 6 7 - 9 1 2 0 0 0 0 0 0 3 5 - 3
www.elsevier.nllocatejseaes
E-mail address: j.milsomucl.ac.uk J. Milsom.
virtually identical to those on the northern Banda Arc islands of Buru and Seram.
The Australian-derived fragments listed above all contrast strongly with Sumba, the westernmost island in
the Outer Banda Arc Fig. 1, which resembles SW Sula- wesi. Thus, and in very simple terms, the geology of eastern
Indonesia can be summarised by three generalised associa- tions of sedimentary, metamorphic and igneous rocks, of
which two are related to the continental margins of South- east Asia Sundaland and Australasia, respectively. The
third, Banda, association is dominant in and around the Banda Sea. The stratigraphic data, while not defining the
entire history of the suture zone, can be used to constrain the range of acceptable hypotheses.
2. Sundaland Margin Association
