Volcanism and plate tectonics in the Indonesian island arcs

Abstract

Studies of seismic and geologic profiles across the Sumatra, Java and Timor arc—trench systems reveal that large islands of the inner volcanic arc with silicic and intermediate volcanism, such as Java and Sumatra, were formed due to subduction of oceanic plate under a thick and old continental crust. The string of smaller volcanic islands east of Java, Sumbawa, Flores, Alor etc. with intermediate and mafic volcanism came into existence because of subduction of oceanic plate underneath a thin and young crust. The tectonic evolution of the Indonesian Archipelago from Late Paleozoic till Pliocene time, proceeded with subduction and accompanying volcanism spreading systematically in ever-widening areas away from the continent towards the ocean. The well-developed zonal structural belts of western Indonesia are generated by spreading centers situated in the Indian Ocean and the South China Sea. Paleo and active volcanism in this region exhibit a regular zonal pattern, although due to the changing dip of the Benioff zone the magmatic rocks do not necessarily become younger towards the ocean. In mid-Tertiary time there emerged east of Borneo, first the Celebes—Philippine and later the Halmahera island arcs, which originated from a spreading center in the Pacific Ocean. This new pattern of subduction broke through as the movement of the Pacific Plate changed into a west-northwest direction during Eocene—Oligocene time. The most dramatic event in the geologic history of Indonesia occurred in Pliocene time, when the northward advancing Australian continent coupled with the counter clockwise rotation of New Guinea and accompanied by the spear heading westward thrust along the Sorong transform-fault system, severely interrupted the regular zonal outgrowth of eastern Indonesia. The east—west trending Banda arc was bent westward while the north—south striking Celebes and Halmahera arcs were pushed back towards the Asian continent. The islands of Banggai and Buton, originating from New Guinea and the Banda arc respectively, were swept against Celebes. Minor subduction zones with reverse polarities developed west of Halmahera and northwest of Celebes. As a consequence of the collision, Late Cenozoic to Present volcanism in eastern Indonesia, which accompanied the latest subduction process, shows some peculiar behaviour. In Alor, Wetar and Romang, volcanism ceased as the Indian—Australian Plate moved continuously to the north and subsequently caused subduction of the Australian continental crust into the Timor trench. Between Buru and the southeastern arm of Celebes no active volcanoes are encountered as the subduction zone terminates suddenly and goes over into the Sorong transform fault. The isolated Una-Una volcano in the Gulf of Gorontalo, Central Celebes, could be related to the southeast-dipping dormant subduction zone situated in the Celebes Sea. The occurrence of volcanoes north of Alor and Wetar and in the Gulf of Gorontalo tends to support the opinion that dormant subduction zones can still be held responsible for the occurrence of active volcanoes far behind an island arc. Thus we conclude that the late Cenozoic to Present active calc-alkaline and potassic volcanoes of the Indonesian Archipelago are directly related to the process of lithosphere subduction as envisaged by the plate-tectonic concept. Late Cenozoic high-alkaline basalts in western Indonesia may mark the sites of volcanic hot spots implying that this part of the region is a stationary continental plate relative to the mantle since at least tens of millions of years. The beautifully concentric arrangement of the Phanerozoic arc—trench systems of western Indonesia, however, suggests that the continent, around which the younger subduction zones have developed themselves in a systematic way, had already reached its fixed position far back in Paleozoic time.