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The Plate Tectonics of Cenozoic SE Asia and the Distribution of Land and Sea

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The Plate Tectonics of Cenozoic SE Asia and the Distribution of Land and Sea Cenozoic plate tectonics of SE Asia 99 The plate tectonics of Cenozoic SE Asia and the distribution of land and sea Robert Hall SE Asia Research Group, Department of Geology, Royal Holloway University of London, Egham, Surrey TW20 0EX, UK Email: robert*hall@gl*rhbnc*ac*uk Key words: SE Asia, SW Pacific, plate tectonics, Cenozoic Abstract Introduction A plate tectonic model for the development of SE Asia and For the geologist, SE Asia is one of the most the SW Pacific during the Cenozoic is based on palaeomag- intriguing areas of the Earth$ The mountains of netic data, spreading histories of marginal basins deduced the Alpine-Himalayan belt turn southwards into from ocean floor magnetic anomalies, and interpretation of geological data from the region There are three important Indochina and terminate in a region of continen- periods in regional development: at about 45 Ma, 25 Ma and tal archipelagos, island arcs and small ocean ba- 5 Ma At these times plate boundaries and motions changed, sins$ To the south, west and east the region is probably as a result of major collision events surrounded by island arcs where lithosphere of In the Eocene the collision of India with Asia caused an the Indian and Pacific oceans is being influx of Gondwana plants and animals into Asia Mountain building resulting from the collision led to major changes in subducted at high rates, accompanied by in- habitats, climate, and drainage systems, and promoted dis- tense seismicity and spectacular volcanic activ- persal from Gondwana via India into SE Asia as well as cre- ity$ Within this region we can observe collision ating barriers between SE Asia and the rest of Asia Contin- between island arcs, between island arcs and ued indentation of Asia by India further modified Sundaland continents, and between continental fragments$ and created internal barriers affecting biogeographic pat- At the same time ocean basins are opening terns From a biogeographic and tectonic viewpoint, the major Cenozoic tectonic event in SE Asia occurred about 25 within this convergent region$ SE Asia includes million years ago, resulting in major changes in the configu- areas with the highest global rates of plate con- ration and character of plate boundaries, and caused effects vergence and separation$ which propagated westwards through the region This event It is clear from the geology of the region that led to the progressive arrival of Australian microcontinental the snapshot we see today is no less compli- fragments in Sulawesi, providing possible pathways for mi- gration of faunas and floras between Asia and Australia, but cated than in the past$ The region has devel- also creating new barriers to dispersal oped by the interaction of major lithospheric Tectonic reconstruction maps of lithospheric fragments plates, principally those of the Pacific, India- cannot be translated simply into maps of land and sea which Australia and Eurasia (Fig$1), but at the present are of greater value to biogeographers Determining the pal- day a description only in terms of these three aeogeography of the region is not yet possible, but an at- tempt is made to outline the main likely features of the ge- plates is a very great oversimplification$ Many ography of the region since the late Oligocene minor plates need to be considered, and in Evidence from all fields of biogeography is required to some parts of the region the boundaries be- test different tectonic models and identify the origin of tween these smaller plates are very uncertain$ It present biogeographic patterns but there must be a focus on is also clear that some of the deformation cannot plants and animals which have difficulty in dispersing, and be described in simple plate tectonic terms$ for which non-geological controls are unimportant The present distribution of plants and animals in SE Asia may Lithosphere has deformed internally, and mate- owe much more to the last one million years than the pre- rial has been added by arc volcanic processes, ceding 30 million years which means that at least one of the axioms of Biogeography and Geological Evolution of SE Asia, pp 99-131 Edited by Robert Hall and Jeremy D Holloway © 1998 Backhuys Publishers, Leiden, The Netherlands 100 R* Hall 20oN PHILIPPINE South SEA China EURASIA Sea PACIFIC 10oN Sulu Sea ▲ ▲ Celebes Sea CAROLINE 0o Bismarck Sea Banda Sea Solomon Sea 10oS Woodlark Basin INDIA INDIA-AUSTRALIA 90oE 100oE 110oE 120oE 130oE 140oE 150oE 20oS Fig1 The larger tectonic plates of SE Asia and the SW Pacific The many small ocean basins and the major strike-slip fault systems at the margins of SE Asia and Australia are manifestations of the complexity of plate tectonics in the region which requires a description in terms of many more plates than those shown plate tectonics, of rigid fragments moving on a of the region, into which large volumes of sphere, cannot be assumed$ sediments have been shed, removed from rising The complexity of the present-day tectonics mountains$ Thus the distribution of land and sea of the region and the observable rates of plate has changed during the Cenozoic, and many motions (e*g$, Hamilton, 1979; McCaffrey, 1996) parts of the region have seen dramatic vertical indicate that major oceans, or multiple small movements of several kilometres, with moun- oceans, have closed during the Cenozoic$ Sev- tains where once there were oceans, and deep eral major island arcs have certainly formed dur- marine regions where mountains had existed$ ing this time and some may have completely The abrupt division between Asian and Aus- disappeared$ At some plate boundaries strike- tralian floras and faunas in Indonesia, first recog- slip faulting has dismembered previously coher- nised by Wallace in the nineteenth century, has ent regions, and along these boundaries there its origin in the rapid plate movements and reor- can be both major crustal subsidence and uplift ganisation of land-masses in SE Asia$ Wallace due to deformation$ During the past 50 million realised that the region had changed dramati- years the configuration of the region has there- cally in the past without knowing the cause, and fore changed significantly in plate tectonic since his work there has been a general aware- terms$ Accompanying these large scale move- ness that the present distribution of land and sea ments have been equally significant vertical is not the same as that of the past, and that the movements, recorded in the sedimentary basins changes are in some way implicated in biogeo- Cenozoic plate tectonics of SE Asia 101 graphic patterns$ We are now confident that the culty, make reconstructions; the known motions geological changes are the result of plate move- of major plates do impose limits on possibilities; ments and are not the consequence of an ex- and the resulting interpretations do offer a panding Earth$ However, although the very means of identifying important tectonic events large-scale motions of major plates have been and highlighting key problems$ This paper ex- reasonably well known for the last 20 years or plains the background to a plate tectonic model so, the detail necessary to reconstruct SE Asia of SE Asia and the SW Pacific and discusses its has been lacking$ Furthermore, because of the implications for biogeography$ complexity of the geology of the region and be- cause so much of it has been remote and diffi- cult of access, geologists have not been able to Mesozoic to Cenozoic background clearly describe its long term development and it is only in the last few years that models ex- In very general terms, the region owes its origin plaining the development of the region have to the pre-Cenozoic break-up of the Gondwana been produced$ super-continent (Fig$2), the subsequent move- Making tectonic reconstructions of SE Asia ment of Gondwana fragments northwards, and becomes more difficult as the age of the recon- their eventual collision with Eurasia$ Metcalfe struction becomes greater, and examination of (1998 this volume) provides an account of the present tectonics of the region shows why present knowledge of the Palaeozoic and Meso- this is so$ Projecting motions that are known to- zoic development of SE Asia$ It is clear that day into the past is very problematical; our ob- many fragments separated from Gondwana and servations of the present tectonics indicate that amalgamated in SE Asia over a considerable pe- plates, plate boundaries and motions can be riod of time$ The process of rifting led to forma- geologically ephemeral features$ In some parts tion of new oceans, and the northward motion of the region, for example the Philippines and of Gondwana fragments required subduction of east Indonesia, it is not even certain that plate older oceanic crust at the edges of the growing tectonics provides a suitable model for a de- Eurasian continent$ By the Mesozoic, a region tailed understanding of the development of the composed of fragments derived from Gondwa- region$ Despite these difficulties, a plate tectonic na formed a Sundaland core surrounded by model does have value and by working back subduction zones$ from the present-day we can, albeit with diffi- Subduction meant that the Sundaland margins were complex$ Island arcs at the margins may have been underlain by continental and oceanic crust, and there were probably many small ocean basins behind the arcs and above the subduction zones$ The widespread ophiolites are fragments of oceanic lithosphere now found Avalonian- on land, and much of this lithosphere was Cadomian terranes formed in subduction-related
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