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Cenozoic Tectonics of SE Asia and Australasia INDONESIAN

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Cenozoic Tectonics of SE Asia and Australasia INDONESIAN Cenozoic tectonics of SE Asia and Australasia 47 INDONESIAN PETROLEUM ASSOCIATION Proceedings of the Petroleum Systems of SE Asia and Australasia Conference, May 1997 CENOZOIC TECTONICS OF SE ASIA AND AUSTRALASIA Robert Hall SE Asia Research Group, University of London ABSTRACT quency and rapidity of changes in regional tecton- ics. A plate tectonic model for the development of the region of SE Asia and Australasia is presented and INTRODUCTION its implications are summarised. The complexity of the present-day tectonics of the region and the The region of SE Asia and Australasia includes observable rates of plate motions indicate that ma- examples of almost every plate tectonic configura- jor oceans, or multiple small oceans, have closed tion at different stages in the Wilson cycle between during the Cenozoic, and that the configuration of rifting and continental collision. It is the only place the region has changed significantly during this on Earth where we can observe arcs in collision, time. Despite the long-term convergence there has one of the few places where an oceanic spreading been frequent opening of marginal basins, and ex- centre is actively propagating into continental tension related to strike-slip faults resulting from crust, and includes areas with the highest global partitioning of oblique convergence at plate rates of plate convergence and separation. But how boundaries. Present-day plate motions, based for useful is plate tectonics in describing the evolution example on GPS measurements and seismicity, il- of the region? It is good at describing interaction lustrate the complexity of processes but appear to between slowly moving, large plates with rela- have little relevance in understanding the long- tively simple geometries but its application to the term kinematic development of the region. There SE Asia-Australasian region is more difficult be- are three important periods in regional develop- cause of the number of small plates required to ac- ment: at about 45 Ma, 25 Ma and 5 Ma. At these count for the region’s development. Furthermore, times plate boundaries and motions changed, prob- many ideas about orogenic evolution through the ably as a result of major collision events. Indenta- Wilson cycle are based on the early, simple and tion of Asia by India may have modified the Eura- powerful plate tectonic concepts reproduced in sian continent but there is little indication that In- text-books but some of the axioms have changed or dia has been the driving force of tectonics in SE advances in knowledge have made them invalid. Asia. The movement of Australia northwards has For example, ophiolites are rarely formed in nor- caused rotations of SE Asia blocks and accretion mal ocean basins, continental crust is now known of microcontinental fragments to SE Asia. Since to be subductable, convergence may continue long 25 Ma the oceanic region east of Eurasia has been after continent-continent collision, and much de- driven by motion of the Pacific plate. To improve formation of continents may be distributed rather our tectonic models, detail is needed which can be than concentrated at plate margins. compiled from proprietary data, such as coastline, shelf edge, age and lithofacies information, held In discussing regional tectonics it is necessary to by companies. Improved dating of events is re- be aware of the many problems. How reliably and quired in order to identify regional events and their for how long can present plate motions, for exam- consequences and identify the processes that cause ple from GPS measurements, be projected into the the effects. Few sedimentary basins in the region past? How much of the record is missing? Subduc- will fit into simple basin models because of the fre- tion leads to destruction and formation of marginal From: Petroleum Systems of SE Asia and Australasia. pp. 47-62. Edited by J. V. C. Howes & R. A. Noble 1997, Indonesian Petroleum Association, Jakarta. 48 R. Hall basins at the major plate boundaries during short structions which keep Eurasia fixed in its present time intervals and these basins may be impossible position (Rangin et al., 1990; Lee and Lawver, to reconstruct. There are considerable difficulties 1994). in identifying the importance of strike-slip mo- tions. What is the relevance of plate tectonics to Reconstructions of SE Asia and Australasia (Fig. smaller (e.g. basin) scale reconstructions? Distrib- 1) shown on a global projection are presented at 10 uted deformation, for example of Sundaland, may Ma intervals for the period 50-10 Ma (Figs. 2 to 6). not be amenable to simple plate tectonic analysis. More than 100 fragments are currently used, and There are important problems in identifying verti- most retain their current size in order that they re- cal axis rotations on a regional scale. main recognisable. During the 50 Ma period frag- ments represented may have changed size and A logical parsimony is necessary in reconstruc- shape or may not have existed, both for arc and tion, limiting the number of plates and their detail, continental terranes. Thus, the plate model can and observations of the present-day tectonics of only be an approximation. Some of the elements of the region show us that any plate model must be an the model are deliberately represented in a stylistic over-simplification. Nonetheless, accepting its manner to convey the processes inferred rather limitations, analysis of the region as rigid plates than display exactly what has happened, for exam- can still give important insights. It is important to ple, the motion of the terranes of north New work progressively back in time, recognising the Guinea. The reconstructions presented here extend increasing uncertainty in older reconstructions; re- the model developed previously for SE Asia and alistic Mesozoic reconstructions are not currently the reader is referred to Hall (1995, 1996) for a possible. 2-D plate tectonic cartoons are no longer more complete account of the assumptions and adequate descriptions or tools for understanding. It data used in reconstructing that region. Yan and is essential to test plate tectonic models by using Kroenke (1993) have produced an animated recon- maps which can be examined at short time inter- struction of the SW Pacific. Some important dif- vals. Animations (e.g. Yan and Kroenke, 1993; ferences between this model and theirs results Hall, 1996) can expose flaws in models, and major from the choice of reference frames; they used the gaps in our knowledge, but also help identify truly hotspot frame whereas this reconstruction uses a regional events. Compilations, for example of palaeomagnetic reference frame. Other differences lithofacies, do not make sense unless regional plate result from different interpretations of geological movements are considered. This paper attempts to data. The model presented here shows the move- summarise the regional tectonic development of ment of India and new interpretations of the SW SE Asia and Australasia based on such a plate tec- Pacific; a few important references are cited in the tonic model which has been animated using 1 Ma text but in the space available here the reasons for time-slices. For the petroleum industry these re- the differences from previous models and the justi- constructions may help in understanding the devel- fication of the reconstruction must await a more opment of sedimentary basins, and the distribution complete account which will be published else- of petroleum resources, by identifying important where. What follows below is a brief account of controls on their tectonic setting and the timing of the model and its major features, with discussion regionally important events. of its principal implications. Basis for the Model RECONSTRUCTIONS The reconstructions were made using the ATLAS Configuration at 50 Ma computer program (Cambridge Paleomap Serv- ices, 1993). In the ATLAS model the motions of At 50 Ma (Fig. 2) India and Australia were sepa- the major plates are defined relative to Africa and rate plates although their motions were not greatly its movement is defined relative to magnetic north. different. Transform faults linked the slow-spread- There has been little Cenozoic motion of Eurasia ing Australia-Antarctic and the fast spreading In- and it remains in a similar position in all the recon- dia-Australia spreading centres. Older parts of the structions, although there are small movements of East Sulawesi ophiolite probably formed at the In- Eurasia due to the plate circuit used in the ATLAS dia-Australia ridge. India collided with Asia in the model, particularly for the last 5 Ma. Therefore early Tertiary but there remains considerable con- there are minor differences compared to recon- troversy about the exact age of collision, and its Cenozoic tectonics of SE Asia and Australasia 49 consequences (Packham, 1996; Rowley, 1996). margins on all sides. To the west the passive mar- The position of the Eurasian margin and the extent gin was formed in the Late Jurassic and Fig. 2 pos- of Greater India are major problems. The recon- tulates a failed rift, possibly floored by oceanic struction shown in Fig. 2 shows a conservative es- crust on the site of the present-day Banda Sea, par- timate, and since India-Asia collision began at tially separating a Bird’s Head microcontinent about 50 Ma this implies that the Asian margin ex- from Australia. Mesozoic lithosphere was present tended south to at least 30°N. Many of the tectonic north of the Bird’s Head south of the active Indian- events in SE Asia are commonly attributed to the Australian spreading centre. Further east in the Pa- effects of Indian indentation into Asia and the sub- cific, Indian and Australian oceanic lithosphere sequent extrusion of continental fragments east- had been subducting northwards beneath the wards along major strike-slip faults. Despite the Sepik-Papuan arc before about 55 Ma.
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