Review of past and present geotectonic concepts of eastern Indonesia
In: Netherlands Journal of Sea Research. Netherlands Institute for Sea Research (NIOZ): Groningen; Den Burg. ISSN 0077-7579; e-ISSN 1873-1406, more
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| Abstract |
By the turn of the last century Dutch geo-scientists already were comparing the Indonesian island arcs to the complicated structures of the European Alps and recognized that the Indonesian Archipelago possessed a dual character, both as the intersection of two of the largest and youngest mountain systems, and as an intercontinental zone between the Asiatic and Australian continents. About half a century ago they discovered in Indonesia the largest negative gravity anomalies at sea, and established that the depth of earthquake epicenters increases landward from the trenches. Despite the limited marine technology then, they discerned that the Indonesian island arcs represent a mountain belt in statu nascendi, exhibiting a systematic relationship of active tectonic and magmatic features to the deep submarine trenches. The geological and geophysical findings at sea by the first Snellius Expedition were integrated with the theories born out of the results of geological mapping on land. This is why the tectonic theories proposed by Dutch and other European geologists before the second World War were superior to those proposed by others. Though most of these theories can no longer be accepted without modification or refinement, they constitute part of the basis of the new global tectonics. Since the advent of the plate-tectonic concept, active subduction zones, transform faults and spreading centers have been recognized in Indonesia with reasonable confidence, by their physiographic, geologic and geophysical characteristics. In contrast to this, in much of the interior of the Eurasian continent the structural complexity of similar rock assemblages which have been folded, thrust and crumpled together by nearby subduction and collision is far more difficult to unravel. Consequently, the sort of geologic events deduced from the Indonesian Archipelago are of a type that should be recorded in older tectonic belts around the globe. The modern tectonic setting of the whole equatorial Indo-Pacific region, for example, has recently been compared to the terrane map of the North American Cordillera. The position of eastern Indonesia within the plate-tectonic framework is the key to resolving contradictory views on the tectonics of the Banda Sea. For example, did the Indonesian orogeny take place at the Gondwana margin or the Asian margin, are Timor and Seram a tectonic melange and thus part of the Tertiary Indonesian island arcs, or are these two islands a part of the passive Australian margin? Oceanic magnetic stripes from the Sulu, Celebes and Banda Seas all trend NE-SW. These new data suggest that the Sulu, Celebes and probably the Banda Sea represent areas of trapped Indian Ocean crust. Deep sea drilling in the Banda Sea can resolve much controversy. The Banda Sea occupies a critical position in the complex convergent zone between Australia, Southeast Asia and the Philippine Sea Plate. The determination of the stratigraphy and basement ages of the Banda Sea will constrain evolutionary models which have been proposed. Another unsolved question of key importance in our understanding of the evolution of Sulawesi and the Moluccas is the function and timing of events of the Birdhead 'bacon slicer', or the tectonic shaving in Irian Jaya. Once this mechanism is understood, the development and timing of the various structural features of Sulawesi, Halmahera and the Banda Arc will be classified. Opinions still differ regarding the 'birthplace' of the micro-continents in the Banda Sea. Some regard them as a result of Jurassic rifting of Gondwana in northwestern Australia while others consider them displaced westward from northern Irian Jaya along the Sorong transform fault. Several authors suggested that the eastern parts of Sulawesi, Buru and Seram represent micro-continents which originated from Irian Jaya, while others considered East Sulawesi and North Sulawesi remnants of ophiolite belts or fragments of island arcs that originate from the Pacific Ocean. Contrary to those who positioned Sulawesi close to Kalimantan in Miocene time or who separated the eastern and western arms of Sulawesi and placed them around continental Australia during its drift northwards, I maintain the view that in Miocene time Sulawesi emerged as a double island arc east of Kalimantan. For the Halmahera arc-trench system a similar origin during a younger phase of crustal movement could be advocated. The shape of the two eastern arms of Sulawesi and Halmahera can be compared with an 'arrowhead' pointing westward, with two larger slightly arcuate western arms as a 'wave front' proceeding from it. Thus Sulawesi and Halmahera were once north-south trending island arcs convex towards the Pacific with westward-dipping subduction zones. After collision with the Irian Jaya plate, a reversal of polarity occurred as demonstrated by the trenches which developed northwest of Sulawesi and west of Halmahera. This controversy cannot be solved without determining the absolute ages of the eastern Sulawesi subduction complex. Marine research should also focus on the Sorong transform fault system between Sulawesi and Irian Jaya to elucidate its role in the westward displacement of the Sula-Banggai - Buton continental fragments. The nature, structure and history of the ridges in the Central Banda Sea, and their relationship to the oceanic crust of the adjacent North and South Banda Basin, should be investigated in more detail. Seram, Buru and Ambon require detailed studies to determine whether the arc-trench system predominates or whether micro-continent tectonics played the more significant role in their evolution. In the geological future, eastern Indonesia will be squashed between Australia and Asia, and the region will resemble the complex terrains now observed in the Alps and the Hercynian regions, a conclusion already drawn by the Dutch pioneers several decades ago. |
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