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Hall, R. 2000. Neogene History of Collision in the Halmahera Region

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1 PROCEEDINGS, INDONESIAN PETROLEUM ASSOCIATION 27th Annual Convention & Exhibition, October 1999 NEOGENE HISTORY OF COLLISION IN THE HALMAHERA REGION, INDONESIA Robert Hall SE Asia Research Group, Department of Geology, Royal Holloway University of London, Egham, Surrey TW20 0EX, UK; +44-1784-443592; Email: [email protected] ABSTRACT In the Molucca Sea the two arcs of Sangihe and Halmahera are in active collision. Results of study of The present day geology of the Molucca Sea region con- the Halmahera arc based on field mapping on land (e.g. tains a record of the stages in the collision between two Hall, 1987; Hall et al., 1988a, b; 1995; and unpublished arcs. Observations offshore and on land in the Halmahera results) and offshore mapping of the Molucca Sea based islands can be used to infer the sequence and timing of particularly on results obtained during the 1994 MODEC events which created the double subduction system and cruise (Rangin et al., 1996) can be combined to model the now complete subduction of the Molucca Sea plate. the evolution of collision and help understand the devel- In the northern Molucca Sea the Halmahera arc has opment of this arc-arc collision. Observations in the been entirely overridden by the Sangihe forearc and it Molucca Sea are interpreted in the light of the geology seems probable that in a few million years time the en- on land to the north and west, and from observations of tire Halmahera arc will have disappeared with almost the Halmahera arc to the south and east. At present, on no trace. the west side of the Molucca Sea the active volcanoes of the Sangihe arc can be traced from north Sulawesi into Mindanao. In contrast, on the east side of the INTRODUCTION Molucca Sea volcanic activity in the Halmahera arc ceases south of the latitude of Morotai. Eastern Indonesia includes the junction between the Australian and Philippine Sea Plates with a complex of small plates forming the Eurasian and SE Asian margins PRESENT DAY CROSS SECTIONS (Hamilton, 1979). The Philippine arcs, a complex of modern and ancient island arc and continental fragments, A cross section between Morotai and the Sangihe arc terminate southwards in the Molucca Sea collision zone (Figure 2B) shows the overthrusting of the Halmahera where the opposed Halmahera and Sangihe arcs are arc by its own backarc which occurred at the end of the actively converging (Figure 1). The southern boundary Pliocene. South of Morotai volcanic activity was renewed of both the Molucca Sea and the Philippine Sea plates is during the Quaternary and the present arc was built on the Sorong Fault system. The Molucca Sea plate has an the thickened arc crust. North of Morotai volcanic ac- inverted U-shaped configuration (McCaffrey et al., 1980) tivity ceased and at present the Halmahera forearc is and is dipping east under Halmahera and west under the being overridden by the Sangihe forearc. The Sangihe arc (Figure 2). Regional seismicity suggests that overthrusting of one forearc by the other has led to ma- approximately 200-300 km of lithosphere has been jor thickening of the accretionary complex producing the subducted beneath Halmahera. On the opposite side of large volume of low density material and associated grav- the Molucca Sea, the Benioff zone associated with the ity low of the central Molucca Sea. west-dipping slab can be identified at least to a depth of 600 km beneath the Celebes Sea. At the latitude of Talaud (Figure 2A) the process has 2 proceeded still farther. Here almost the entire former Snellius ridge parts of the Neogene Halmahera arc and volcanic arc and its forearc have been overridden by the forearc have now disappeared. Further south this east- Sangihe forearc. The Snellius ridge is interpreted to be vergent thrusting carried the Halmahera forearc onto the equivalent of the Halmahera backarc region where the flanks of the active Halmahera arc and pre-Neogene Mio-Pliocene carbonates rest unconformably on pre- rocks of the Halmahera forearc basement are now ex- Neogene ophiolitic and arc crust. The thickening of the posed in islands of the Bacan group and off the coast of collision complex by the accretion of the Halmahera northwest Halmahera. accretionary wedge and forearc crust, and by shorten- ing of the Sangihe forearc, has caused the uplift of forearc Where the Halmahera forearc and arc have been sig- basement in the Talaud islands (and locally in Mayu too) nificantly overthrust the Sangihe forearc has been jacked where ophiolites are exposed. The Molucca Sea plate is up. The wide Molucca Sea collisional complex is com- now entirely subducted and sinking more deeply into the posed of the accretionary wedges of both arcs. The mantle as one proceeds from south to north. forearc basement of the Sangihe arc is exposed where it thrusts over this wedge. The ophiolitic rocks of the central Molucca Sea are not part of the Molucca Sea LATE NEOGENE HISTORY OF COLLISION plate but are the basement of the Sangihe forearc. Me- langes reported from Talaud (Moore et al., 1981) and The cross sections drawn across the present-day colli- present on Mayu were not formed during the present sion zone from south to north can also be considered as collision but are older rocks forming part of the pre- representing the sequence of events in time and a series Neogene basement of the Sangihe forearc. Presumed of sections illustrating the earlier stages in the collision melanges of the present collision complex are all sub- can be inferred from the geology of the Halmahera arc marine and constitute part of the bathymetrically shal- (Figures 3 and 4). Westward subduction of the Molucca low and seismically incoherent volume of sediment in Sea beneath the Sangihe arc probably began in the early the central Molucca Sea. Miocene. Eastward subduction of the Molucca Sea plate beneath Halmahera began in the middle Miocene. The double subduction zone was initiated at this time forming CONCLUSIONS a new plate, the Molucca Sea plate, separate from the Philippine Sea plate. The oldest volcanic rocks dated from The Neogene Halmahera volcanic arc did not extend the Halmahera arc are 11 Ma in Obi at its southern end significantly north of the Snellius ridge and had no equiva- and are younger to the north (Baker and Malaihollo, lent in Mindanao. The northern Molucca Sea terminated 1996). at a major strike-slip zone, which reoriented during the Pliocene to its present NW-SE direction and passes The earliest indications of arc-arc collision are of through Mindanao into the Cotobato fault system (Fig- Pliocene age. The Halmahera arc failed at the site of ure 1). The double subduction zone never extended north the active volcanic arc, presumably reflecting its weak- of this strike-slip zone into the Philippines. ness due to mineralogy and magmatism, and there was westward thrusting of the region behind the arc towards Melanges may have been formed during collision but the forearc. In Obi the arc was thrust onto the forearc. they are not yet exposed onshore. Exposed melanges on In south Halmahera the backarc region was thrust onto islands of the central Molucca Sea are older rocks form- the forearc, in places entirely eliminating the Neogene ing part of the forearc basement. Ophiolites have also arc. been exposed by the collision process but they are not fragments of the subducted ocean crust of the Molucca After this episode of west-vergent thrusting the volcanism Sea plate but basement of the Sangihe forearc. The in the Halmahera arc resumed between Bacan and north Molucca Sea plate is now deep in the mantle and sinking Halmahera. On Obi and from Morotai northwards further with time. The present Halmahera trench or volcanism ceased. In the northern Molucca Sea the trough broadly represents the frontal thrust of the Sangihe Sangihe forearc was then thrust east onto the Halmahera forearc which is overriding the Halmahera forearc and forearc and arc. In the region between Morotai and the arc. Locally there is backthrusting of the Sangihe forearc 3 towards the Sangihe arc at the Sangihe trench or trough Asia. Geological Society of London Special Publication, but this is a relatively minor feature. 106, 153-184. During collision the active volcanic arc has repeatedly Hall, R. 1997. Cenozoic tectonics of SE Asia and Aus- proved to be the weak point of the entire forearc-arc- tralasia. In: Howes, J. V. C. and Noble, R. A. (eds.) backarc section, presumably reflecting its quartz-rich Petroleum Systems of SE Asia and Australasia. pp. 47- rheology at depth, and higher temperature at shallow 62. Indonesian Petroleum Association, Jakarta. depth. In the islands of Halmahera and Obi much of the Neogene arc has been overthrust by backarc and forearc Hall, R., Ali, J. R. Anderson, C. D. and Baker, S. J. crust. During arc-arc collision one of the arcs must be 1995. Origin and motion history of the Philippine Sea overridden and presumably by chance this has proved to Plate. Tectonophysics, 251, (1-4), 229-250. be the Halmahera arc. Ultimately the Halmahera arc is doomed to disappear. In less than 5 million years from Hall, R., Audley-Charles, M. G., Banner, F. T., Hidayat, today the evidence for the collision of two arcs will be S. and Tobing, S. L. 1988a. The basement rocks of the very difficult to find and the geology of the region will Halmahera region, east Indonesia, a Late Cretaceous- most likely be interpreted in terms of a single arc. This is Early Tertiary forearc. Journal of the Geological Society a very important conclusion for tectonic models and re- of London, 145, 65-84.
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