Katili 1975 [PDF]

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0 ElsevierScientific PublishingCompany, Amsterdam -Printed



VOLCANISM ARCS



in The Netherlands



AND PLATE TECTONICS IN THE INDONESIAN



ISLAND



JOHN A. KATlLI ~~n~~~



of Mines, Jakarta ~~~do~esia~



(Accepted



for publication



December



6, 1974)



ABSTRACT Katili, J.A., 1975. Volcanism physics, 26: 165-188.



and plate tectonics



in the Indonesian



island arcs. Tectono-



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 Halmabera 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-no~hwest 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 counterclockwise 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.



166 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 talc-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.



INTRODUCTION



In a previous paper, attempts have been made by the present author to fit certain geological and geophysical features of the Indonesian island arcs to the new global tectonics (Katili, 1973a). The ideas have been further expanded to explain the tectonic evolution of western Indonesia by utilizing new data on radiometric age dating of granitic rocks (Katili, 1973b). The dispositions presented in those papers seem to justify the assumption that temporal changes of location and differing direction and rate of dips of Benioff zones, as well as the existence of parallel opposing subduction zones may be held responsible for the tectonic features of western Indonesia. The purpose of this paper is to investigate the distribution, composition and ages of volcanic rocks in western as well as eastern Indonesia and to see whether these features are in harmony with the synthesis already proposed for western Indonesia. The ideas regarding the relationship between volcanism and tectonics in Indonesia have been drawn heavily from Van Bemmelen (1949), whose understanding and synthesis regarding these two features in the pre-plate-tectonics days are without parallel. Using plate tectonics as a basis, modification, improvements and necessary changes will be made in the classical theories, substantiated by newly acquired geological and geophysical data from onshore and offshore areas. The present paper will discuss first the refinement of the existing Indonesian plate-tectonic models, based on data obtained from deep seismic profiles which industry has made available to us and on recent publications specifically dealing with areas considered here as paleo-subduction zones (Haile, 1972; Soekendar, 1974). To understand the present and paleogeographic distribution of the Indo-



167



nesian volcanoes, it is necessary to dwell in more detail on the present tectonic framework specifically in eastern Indonesia, and to find out how these complicated structures came into being. This is very important as the present tectonic configuration is considered to be anomalous, caused by the interaction of at least three major lithospheric plates (Katili, 1971). By restoring the eastern Indonesian arcs into their original position before the interaction of plates, we will notice that the paleo-tectonic pattern is a relatively simple one and forms an important clue in unravelling the tectonic evolution of this region. REFINEMENT OF THE INDONESIAN PLATE-TECTONIC



MODEL



Plate-tectonic models in a convergent juncture with examples from Indonesia have been constructed by Hamilton (1970), Katili (1971) and later on improved by Hamilton (1973) and Katili (1973a, b). Three profiles across the arc-trench system of Sumatra, Java and Timor are presented here for the purpose of refining the plate-tectonic model in a convergent juncture. These profiles are based on deep seismic profiles carried out by oil companies and some modifications were made to show the relationship between the oceanic volcano of the Christmas Island and the islandarc volcanoes of Java. The section across the Sumatra arc-trench system (Fig. 1) shows that this arc was formed solely by subduction of oceanic crust under continental crust. The continental crust is thick and old here as it comprises volcanic arcs



ARC



-



TRENCH



GAP



t.4AGM.AGENERATED ABOVE BENIOFF 20~~



LEGEND



Fig. 1. Schematic section across Sumatra (data from Mobil Oil Corp., modified by the author).



168 ARC -



TRENCH



GAP



VOLCANO I PLUTONIC



ASTHENOSPHERE MAGMA GENERATED ABOVE BENIOFF ZONE



LEGEND



B



Cenozoic



v v v m



Young volcanic ot Java



m



Trrlirry volcanic rocks of Chrlrtmas Island



IBZ!



sediments



Deformed



Fig. 2. Schematic author).



rocks



Tcrbary



lT%za



Tertiary granitic rocks 01 Java



m



;=&$$p”$=;,



l!3za



Oceanic



basement



en



crust



rocks



section across Java (data from Mobil Oil Corp., modified



by the



of the Permian, Cretaceous and Tertiary ages (Katih, 1973b). The magmatic rocks formed above the Benioff zone are mostly silicic and intermediate in character. Large ignimbrite deposits occur in this island. Very thick abyssal elastic sediments are subducted in the Sumatra trench (Hamilton, 1973) and the thick sediment pile was pushed up into an actual island chain. The section across the Java arc-trench system (Fig. 2) demonstrates that this arc was also formed by subduction of oceanic plate under continental crust. The crust is thin and relatively young as it consists mostly of a volboo-plutoni~ arc of Terti~ age (Katili, 1973a). Few i~imb~te deposits occur in Java. The magmatic rocks are mostly intermediate. The oceanic crust south of the trench is covered by 200 meter of Tertiary pelagic sediments (Hamilton, 1973) and it could be seen that the volcanic rocks of the Christmas Islands will be subducted in the future in the Java trench. A similar situation will be described from Timor where Permian volcanic rocks of oceanic origin were found in the Tertiary Timor subduction zone. The section across the Timor arc-trench system (Fig. 3) shows an entirely different character. Two distinct phases can be discerned in the development of the Banda arc. In an earlier phase, oceanic plate of the Indian-Australian plate was subducted under the Banda oceanic plate, and in a later phase followed by subduction of the Australian continental crust into the Banda arc subduction zone as Australia drifts continuously northward. The result is decrease or cessation of the active subduction zone, as evidenced by the lack of active volcanoes in Alor, Wetar and Romang. The absence of under-



169 ARC -



INDIAN-AUSTRALIAN



TREKH



. x



x



x



t



GAP



+



VOLCANO



t



l



I PLUTONIC



l



x ABOVE BENWF



INCLINED



BENIOFF



ZONE



ZONE



LEGEND



Cenozoic m rv’,‘I



sediments



Young volcanic rocks ot Florcs



lza m



l!%zl



Fig. 3. Schematic



section



across



Upper Paleozoic rocks as lllochlonou~



nappcs



Highly defor,mcd Paleozoic and Mesqo~c rocks beneath Timor Tertmry granitic of Florcs



Timor



(data



basement



rocks



from Mobil Oil Corp.,



Oceanic



modified



crust



by the



author).



thrusting in this area might also be ascribed to the cessation of the active subduction zone as outlined above. If this assumption is correct one should find predominant mantle material (ophiolites) in the older deposits of Timor and thick terrigenous sediments in the Plio-Pleistocene deposits. The magmatic rocks formed above the Benioff zone in Timor is intermediate and mafic. The crust here is thin and young and flanked on both sides by oceanic crust. The thickness of the sediments in the present Timor subduction zone is about 8000 feet, the sediments are relatively undisturbed and tensional faults can be observed, presumably caused by bending of the crust. In conclusion it can be said that the formation of the Sumatra-Java arc and the Banda arc (Lesser Sunda Islands) shows differences which could be ascribed to the crustal elements involved, as described above. Where thick and old continental crust is involved, we find large islands with silicic and intermediate volcanics, such as Sumatra and Java, and where thin and young crust is involved, a string of small islands with intermediate and mafic volcanics will be formed, such as Sumbawa, Flores, Alor, Wetar, Romang etc. In the Tertiary subduction zone of northwest Borneo, the scarcity of the traditional eugosynclinal elements such as ophiolites and cherts, abyssal red clays are apparent. The main flysch development (Upper Cretaceous to Upper Eocene) contains little or no chert or ophiolites and shows extremely regular dips (Haile, 1972). The Cretaceous subduction zone of Central Java, however, displays characteristic features of a Fransiscan-type melange (Soekendar, 1974) and the



170



same can be said regarding the melange of the Meratus Range in southeast Borneo. In the non-volcanic outer arc of Indonesia which is interpreted as a Tertiary subduction zone (Hamilton, 1970; Katili, 1973a), different types of petrotectonic assemblages can be discerned. The islands off the west coast of Sumatra are characterized by thick flysch-type deposits with few ophiolites. In the islands of Timor, Ceram, Buru and presumably Buton, large amounts of terrigenous material are encountered. The Plio-Pleistocene sediments display a truly sedimentary character and little ophiolite is present. These islands are potential areas for hydrocarbon accumulation as is evidenced from petroleum deposits in Ceram and asphalt deposits on Buton. The Tertiary subduction zone of East Celebes indicates that thin layers of pelagic sediments supplied to the trench were higly deformed and show much involvement of mantle. The same can be said about Halmahera and the small islands of this region which together with Celebes contain Indonesia’s largest lateritic nickel and iron-ore deposits. It is contended that in open-ocean island arcs, where little pelagic sediment is supplied to a subduction zone, deformation is apparent and mantle material is the dominant factor such as in Celebes and Halmahera. Minor occurrence of ophiolites and less deformation characterized the outer arcs where a large amount of terrigenous sediments is being fed to the subduction zone, like in Timor, Ceram and Buru. TECTONIC



FRAMEWORK



OF THE INDONESIAN



ISLAND



ARCS



Extending from the northwestern tip of Sumatra to Flores and the islands east of it, this island arc - usually named the Sunda arc - most clearly exhibits the mechanics and effects of plate tectonics. It is convex towards the Indian ocean and shows the regular and zonal physiographic, geologic and geophysical features which have been interpreted in terms of plate tectonics by Hatherton and Dickinson (1969), Fitch (1970,1972), Hamilton (1970,1973) and Katili (1971,1973a, b). The loopshaped Banda arc and the peculiar form of Celebes and Halmahera is the result of the northward drift of the Australian continent with New Guinea attached to it, combined with the westward thrust of the Pacific Plate (Katili, 1973a). A similar explanation, although with variations has already been advanced by Visser and Hermes (1962), Audley-Charles and Carter (1972), and Gribi (1973). The explanation which will be given below differs slightly in the sense that East Celebes and Halmahera, before collision, are here considered as two north-south trending arc-trench systems which accommodated the westnorthwest movement of the Pacific Plate in Tertiary time. We regard Timor, Ceram, Buru and Buton as belonging to the same southfacing island-arc system in view of their geologic similarities and we ascribe



171



their formation to the subduction of the Indian Ocean-Australian Plate. It has already been mentioned that the difference between Celebes and the Banda arc is clearly expressed in the difference between the geology of Buton, Buru etc. and that of the southeastern arm of Celebes. The Mesozoic in Celebes is strongly metamorphosed, while it is not in the islands of Buton and Ceram. Southeast Celebes contains abundant ophiolites derived from the oceanic-type crust with nickel and chromium deposits, while Buton, Ceram and Timor are composed of sedimentary sequences with hydrocarbon shows. The original position of the Celebes arc-trench system before collision can be reconstructed rather easily. The interruption of the continuous island arc-trench system occurs at Buru, but resumes its normal course in the Sangihe volcanic inner arc and the Talaud non-volcanic outer arc which trend parallel in a north--south direction. The Talaud ridge can be extended southward to the Mayu ridge where it bends westward and continues into the eastern arm of Sulawesi. The submarine Mayu ridge in the Moluccas Sea shows a very strong gravity minimum associated with an immense accumulation of “opaque” sediments and it might be interpreted as a recently filled remnant of an old trench. What is now the eastern part of Celebes could have been located at the southern continuation of the Talaud-Mayu ridge, approximately 600 km east of its present position and the same can be said about the western arc of Sulawesi which at that time acted as the corresponding volcanic arc. For the Halmahera arc-trench system a similar origin during a younger phase of crustal movement (Burdigalian) could be advocated. The arc west and north of Buru (Fig. 4) has been termed the SulawesiMoluccas “collision-zone” by Mobil Oil Corp. geologists (written communication, 1972). The shape of two eastern arms of Celebes and Halmahera is being compared with an “arrowhead” pointing westward and the two larger slightly arcuate western arms as a “wave front” proceeding from a projectile. It has already been known for a long time that the eastern arcuate arms, convex westwards, consist of ophiolites, and that the western arcs comprise active volcanoes, which in western Celebes have ceased in Quatemary times. Thus Celebes and Halmahera were once north-south trending island arcs convex towards the Pacific with westward-dipping subduction zones. The spear-heading westward movements of the Pacific Plate along the Sorong transform-fault system could thus easily explain the geologic complexities and also similarities of the Sulawesi-Halmahera region and can also account for the interruption of the island-arc system between the Banda arc and the Sangihe and Talaud bridge leading to the Philippines. During this movement the Banggai and Buton Islands were swept against the northeastern and southeastern arms of East-Celebes respectively. The east-west trending Banda arc only underwent a westward bending as the motion here was not transformed into a megashear such as in the area between West New Guinea and Celebes.



172



Fig. 4. Celebes-Moluccas collision zone and the Banda subduction zone. Note the two eastern arms of Celebes and Halmahera resembling arrowheads pointing westward, and the two slightly arcuate western arms comparable to a wave front proceeding a projectile. The subduction zone is characterized by Plio-Pleistocene basins while in the collision zone ultrabasic rocks predominate. LATE CENOZOIC TO RECENT VOLCANISM



It has already been pointed out by many authors that in erogenic regions three phases of magmatic evolution can be distinguished, well known in the literature as Stille’s “initialer Vulkanismus”, “synorogener Plutonismus und subsequenter Vulkanismus” and “finaler Vulkanismus”. This concept however cannot be rigorously applied when studying the relationship between volcanism and tectonics in Indonesia (Katili, 1969). Whereas Stille’s concept only referred to the evolution of one erogenic area, Van Bemmelen expanded the idea by applying it to parallel structural zones of the Sunda mountain system, in accordance with his undation theory. His cross-section from the Christmas Island passing through Java up to the island of Karimunjava in the Sunda shelf illustrates the different petrographic provinces in relation to the foreland, geosynclinal foredeep, volcanic inner arc, backdeep and hinterland. The lateral variation in composition of basaltic magma across the Indonesian island arcs might - according to Kuno (1966) - be due to the production of different magmas at different depths. Hatherton and Dickinson (1969) demonstrated that in Indonesia there exists a correlation between the increase of KaO content in recent volcanic products and the depth of the Benioff zone.



173



D.J. Whitford and I.A. Nichols (personal communication, 1973) concluded that when the KsO content of rocks from the individual volcanoes of Java are normalized to a given SiOs content and plotted against the depth of the Benioff zone, an approximately straight-line relationship is obtained. The same relationship is also observed for most of the large cation trace elements. The slight discrepancy between the geological, geophysical and volcanological features of Sumatra and Java have been explained in terms of differential northward movement of the Indian-Australian Plate, a different evolutionary stage of the downgoing slab presently descending in the Sumatra and Java trench and by the fact that past magmatic zones in Sumatra and Java had a different arrangement (Katili, 1973a). The distribution and composition of Late Cenozoic and Present volcanoes in relation to the present plate boundaries are depicted in Fig. 5. The distribution of volcanoes and the different petro~phic provinces is taken from Van Bemmelen (1949), while the plate boundaries are derived from Katili (1973a). Minor adjustments of plate boundaries have been made, based on recent publications (Tjia, 1973) and on data received from oil companies, such as the southern extension of the Palu fault zone to an area east of Sumba (M. Kontz, personal communication, 1973) and the branching-off of the Sorong transform fault (Hutton, tectonic map of Indonesia, in preparation). The dormant trench northwest of northern Celebes is after Hamilton (1973), while the Gorontalo transform fault was described earlier by the author (Katili, 1973a). Based on radar imagery and geological mapping in the region, Trail et al. (1974) suggest that the right-lateral movement has occurred not along a single fault as postulated by the author {Katili, 1970), but along several faults within a zone at least 15 km across. The talc-alkaline volcanoes of the Sunda and Banda arcs demonstrate clearly the effect of the present subduction process. Potassic volcanoes are present north of Java and north of Flores and Sumbawa, while they are absent east of Sumatra. This can be explained by the fact that the deepest penetration of the lithosphere in Sumatra reaches only 200 km while north of Java and Flores it reaches 400 and 700 km, respectively. No active volcanoes are encountered in Alor, Wetar and Romang, and this has been explained by the cessation of subduction in the Timor trench (Katili, 1973b). The volcanoes have apparently been shifted northwards (Van Bemmelen, 1949) and if these volcanoes can still be related to the Timor subduction zone one cannot help but conclude that dormant subduction zones can still cause volcanic activity far behind the island arc. As will be seen later, the anomalous position of the Una-Una volcanic island in the Gulf of Gorontalo, Central Celebes, can also be explained in this way. Fitch (1970) has pointed out that although there is no evidence from focal mechanism to support the existence of underthrusting along the



Hiah



alk~~rie



m&s



rocks



polemic



YOlePnic



Fig. 5. Late Cenozoic to Present volcanism L = ‘Lampung, Mt = Miut, Md = Midai.



*



‘otaruc



Calc-alkalIno? volcanicrocks



@a



a



ana theu: relatianship



to tectanics.



A = Alor,



W 3 Wetar, U = Una-Una,



K = Karimunjawa,



175



eastern end of the Sunda arc (east of 115”E), a well-developed inclined Benioff zone exists beneath the arc in this region. Decrease or cessation of the subduction zone as evidenced by lack of underthrusting does not necessarily imply cessation of movements of the deepest part of the downgoing lithosphere. The &c-alkaline volcanoes of the Banda arc trend parallel to the Timor Ceram trench, and end abruptly south of Ceram. The subduction zone terminates suddenly north of Buru and goes over into a no~he~t~outhwest trending transform fault which south of Celebes was cut off by the southward extension of the Palu-Koro fault zone. No active volcanoes are present between Buru and the southeastern arm of Celebes as the plate boundary here is represented by transform faults. In the western arm of Celebes, Late Cenozoic and active volcanoes exist in the southern end of the island, in the Gulf of Gorontalo tuna-Una volcano) and in the Minahassa and Sangihe region. The tectonic position of the potassic volcanoes in southern Celebes is not clear, as the Benioff zone dipping in the Java trench does not extend beneath these volcanoes. The Una-Una volcanic island situated in the Gulf of Gorontalo, Central Celebes, produces a medium alkaline series of rocks, and cannot be related to the talc-alkaline volcanoes of the Minahassa region in northern Celebes (Katili et al., 1960). By accepting the plate boundaries as seen in Fig. 5 one can relate this volcano easily with the southeastward-dipping dormant subduction zone postulated by Hamilton (1970) in the Celebes Sea. The relatively high potassic content of this volcano is compatible with its position relative to the trench. The active talc-alkaline volcanoes of the Minahassa-Sangihe group can be related to the westward-dipping subduction zone as the Minahassa trench trends parallel to this volcanic row. The Halmahera volcanoes run parallel to the Halmahera trench and their position is in agreement with an eastw~d-dipping subduction zone already postulated by Hatherton and Dickinson (1969). Complications in this region arise because of the overriding of a west-dipping subduction zone by an east-dipping one (Hamilton, 1973). Another interesting volcanic feature already described by Van Bemmelen (1949) is the occurrence of “plateau basalts” in Lampung, South Sumatra, Karimunjava (Java Sea), in Miut (West K~irn~t~) and Midai (Sunda Shelf), Such highly alkaline basalt has also been described by Hutchison (1973) from Kuantan and Segamat in the Malay Peninsula. The distribution of these highly alkaline basalts could perhaps not be related to the present inclined Benioff zone. They exist in South Sumatra, where the potassic volcanoes are absent and where the lithosphere does not penetrate deeper than 200 km. Neither can their occurrence in West Borneo, in the islands of the Sunda Shelf, or in the Malay Peninsula be related to another subduction zone as none exists at the southern end of the South



176



China Sea. They are not encountered in areas east of the Makassar Strait. Hutchison (1973) has related these basalts to deep-extension faulting due to the interaction of the Eurasian, Indian Ocean-Austr~ian and Pacific Plates. It is of interest to note that the highly alkaline basalts of Karimunjava and Sukadana are situated in elevated basement rocks. The Karimunjava arch consists, according to Nayoan (1973), of a coarse elastic complex, more than 1000 m thick, of slightly metamorphosed quartzsandstone of pre-Tertiary age, capped by basaltic rocks. The Lampung high which is capped by the Sukadana basalts consists also of pre-Tertiary gneisses and amphibolite intruded by granitic rocks of Cretaceous age (Katili, 197310). A speculative thought arises that these alkaline basalts could be interpreted as volcanically active hot spots as they cannot be related to the present subduction zones and occur in an uplift, marked by elevated basement rocks. If this assumption is correct, then one has to accept the theory that the Sunda land should be considered as fixed for at least tens of millions of years (Wilson, 1972). TERTIARY



VOLCANIST



Figs. 6 and 7 depict the distribution of the Tertiary talc-alkaline volcanic rocks compiled mainly from Van Bemmelen (1949) and Katili and Marks (1963). The corresponding subduction zones have been drawn based on Hamilton (1970) and Katili (1971). The geographic location of the islands in eastern Indonesia before the interaction of the Eurasian, Indian-Australian and Pacific Plates has been reconstructed based on the kinematic analysis already discussed in the preceding pages regarding the tectonic framework of the Indonesian island arcs. The Oligo-Miocene andesitic rocks west of Sumatra and south of Java and their relationship to the Early Tertiary subduction zones have been treated by the author in previous publications (Katili, 1971,1973a). The granites in Sumatra, which belong to this arc, have been dated radiometrically (Katili, 1973b). Tertiary volcanic rocks of the southwestern arm of the Celebes comprise trachytic, andesitic and dacitic pyroclastic rocks, lavas and lahar deposits which are partly consolidated. These rocks occur near Pare-Pare and locally along the Palu fault zone. Intermediate volcanic and basic volcanic rocks consisting of volcanic breccia, lava, pillow lava, and tuff, are present in the southwestern arm, in the western part of the Central Sulawesi and at the northeastern end of the island. K/Ar dating on rocks yielded 7.5-17.7 m.y. (Indonesia Gulf Oil, written communication, 1972), that is from Upper to Middle Miocene. Basic volcanic rocks are also present in the form of basaltic and spilitic



177



2



z Ei i



:A



j



:::.



.,.



.‘.’.:.:



:,



.:.,,



., .,..



.,.,.’



S3a



;,.



_



.



1.;



.,.‘,‘.‘,‘. ”



;’



1.;



::::



:,:,:.



fj



5



Fig. 7. Volcanism



in Indonesia



during Middle-Upper



Miocene



KALIMANTAN



time (Burdigalian).



1



BUTON



Direction ai pli(e llwmnent



LEGEND



179



rocks. The age is not known with certainty but volcanics south of Donggala, Central Celebes are considered to be the volcanic facies of the Eocene Tinombo Formation (R. Sukamto, personal communication, 1973). Granitic rocks in the southern part of the Celebes yielded ages ranging from 5 - lo6 m.y. to 8.6 * 10’ m.y., that is, from Early Pliocene to Late Miocene (Indonesia Gulf Oil, written communication, 1972). The corresponding Tertiary subduction zone situated in the eastern arm of Sulawesi has been described by Hamilton (1970) and will not be dealt with in this paper. The younger Tertiary volcanic rocks in the northern arm of Celebes have been investigated in detail by Trail et al. (1974). The Dolokapa formation of Early to Late Miocene age consists of andesite interlayered with grauwacke, and limestones. Early Miocene to Pliocene, Bilungala volcanics near Gorontalo consist of andesite, dacite, basalt and rhyolite. The Late Miocene to Pliocene Wobudu breccia consists of andesitic agglomerate, tuff, ash and some dacite and basalt. The Pani volcanics, which are supposed to be Pliocene, are composed of dacite, rhyolite and andesite while the youngest volcanic rocks, named the Pinogu volcanics, of Pliocene to Pleistocene age, consist of andesite, dacite tuffs and agglomerate. No radiometric age dating has been carried out on the granitic rocks in this region but field-relationships indicate, according to Trail et al. (1974), ages ranging from Pliocene (Bumbulan granodiorite) to Miocene (Bolihuto and Bone diorites). The corresponding subduction zone of the Miocene volcanics in the northern arm of Celebes might also be the one in the eastern arm of Celebes which was mentioned previously (Fig. 6). The younger Pliocene volcanics and granitic rocks in the Gorontalo section, however, might be caused by the subduction of a minor trench northwest of Sulawesi which occurred after the spearheading westward movements of the Sorong transform-fault system (Fig. 5). For the description of the younger Tertiary volcanic rocks in Halmahera and New Guinea (Fig. 7) the reader is referred to Van Bemmelen (1949), Katili and Marks (1963), and Visser and Hermes (1962). PRE-TERTIARY



VOLCANISM



An ~po~nt occurrence of Cretaceous volcanic rocks in the Gumai Mountains of South Sumatra has been described by Musper (1937). The rocks occur in two different facies, the Saling Series consisting of tuffs, coarse volcanic breccias, lava flows of basaltic and andesitic composition and reef limestones, and the Lingsing Series comprising monotonous formations of thin-bedded silicious marly and clayey shales with radiolarian cherts. Volcanic rocks of andesitic and basaltic composition also occur in the Lingsing Series. Other occurrences of Upper Cretaceous volcanic rocks have been de-



180



LEGEND



Fig. 8. Volcanism



in Indonesia



during Cretaceous.



scribed by Koolhoven (1935) from the Meratus Mountains in southeastern Borneo. They consist of volcanic breccia, tuffs and lavas of andesitic composition. Cretaceous granitic rocks occurring within the same belt as the volcanic rocks and the corresponding subduction zones have been reported by the author in a previous publication (Katili, 1973b). Beside this Sumatrayoutheast Borneo arc--trench system another Cretaceous arc-trench system with an opposing subduction zone has been postulated by the author (Katili, 1973a) north of the arc described above. The radiometric ages of the plutonic rocks have been described in detail (Katili, 1973b; Pupilli, 1973) and within this zone Pupilli (1973) mentioned grey to greenish-grey effusive rocks of andesitic origin possessing an age of about 92.4 m.y. (Fig. 8). The occurrence of a Triassic-Jurassic arc-trench system with a Benioff zone dipping towards the Asiatic continent was also postulated by the author (Katili, 1973b) based mostly on the ages and composition of the Malayan-Indonesian tin granites. Volcanism in West Malaysia was essentially rhyolitic. Another Triassic arc-trench system passing through West Borneo and Natuna with an opposing subduction zone was described by Hutchison (1973) and Pupilli (1973) (see Fig. 9). The Late Triassic Serian volcanic series of Serawak ranges in composition from basalt to rhyolites. Similar volcanic rocks were found in offshore wells in the South China Sea, consisting of andesite, basalt, trachyte, dacite, breccia and tuffs with radiomet-



Fig. 9. Volcanism



in Indonesia



during Triassic and Jurassic.



ric ages ranging from 169 +_7 m.y. to 1’71 +_3 m.y. For a detailed description the reader is referred to Pupilli (1973). The corresponding subduction zone of this north-facing minor volcanic arc has been discussed by Hutchison (1973) and Pupilli (1973). Turning to the Permian volcanism (Fig. 10) we observe that it occurs along the whole length of Sumatra. The occurrence in the Padang Highlands, Central Sumatra and in Djambi, South Sumatra has been described in detail by Klompe et al. (1961). In Central Sumatra, the volcanic rocks consist of flows of hornblende andesites, augite andesites and tuffs with intercalation of silicified shales and limestones containing Permian fossils (Katili, 1969). The plate-tectonics model requires the existence of Permian granites in Sumatra and this has been reported by the author (Katili, 197313) based on recent radiometric age dating. Late Paleozoic granites possessing ages of ca. 276-298 m.y. are present in South and Central Sumatra. Abundant andesitic and basaltic volcanic rocks have been described by Klompe et al. (1961) in West Borneo and West Malaysia. Widespread andesitic to rhyolitic volcanic and pyroclastic activity characterized the region of the Malay Peninsula east of the Main Range (Hutchison, 1973). The corresponding subduction zone of the Sumatran Permian volcanics dips towards the Asiatic continent while the paleo Benioff zone associated with the Malayan-Borneo volcanics is inclined towards the Indian Ocean. The occurrence of this double opposing arc-trench system was first described by Katili (1973a), and later on reinforced by Hutchison (1973) and



182



LEGEND



Fig. 10. Vofcanism



in Indonesia



during Permian.



Pupilli (1973). Abundant alkaline granites of different ages in western Borneo appear to sustain the existence of the postulated opposing subduction zones. Another occurrence worth mentioning here is the Permian volcanism in Timor. This has been discussed by De Roever (1941) and the rocks here consist of olivine basalts, trachy-basalt, alkali-trachytes and alkali-rhyolites. They are older than the ophiolites in Timor, and De Roever (1941) considers these volcanics as having been formed during what he called the “early geosynclinal stage”. The geologic occurrence, composition and age of these volcanics show that they cannot be regarded as belonging to a Permian volcano-plutonie arc (De Roever, 1941). The plate-tectonic concept makes it reasonable to assume that they were once Permian oceanic volcanoes subducted in the Tertiary trench of Timor. CONCLUDING



REMARKS



In Permian time a subduction zone dipping towards the northeast in the direction of the Asian continent, must have existed in or west of Sumatra, indicating one of the earliest episodes of lithospheric descent in this region. Andesitic volcanism and granitie emplacement in Sumatra accompanied this subduction process. At the same time a minor subduction zone dipping towards the southwest is believed to have been operating at the northeastern margin of the conti-



183



nent. Andesitic, basaltic and granitic rocks, encountered in the eastern part of West Malaysia and West Borneo can be considered as the co~esponding vol~~o-plutoni~ arc. In Triassic-Jurassic time the subduction zone at the southwestern continental margin shifted towards the Indian Ocean. The Benioff zone dipping towards the continent is believed to have been shallower than the Permian one as evidenced by the well-developed broad volcano-plutonic arc of the Malayan Peninsula and the Indonesian tin islands which occupied a belt situated closer to the Asian continent than the Permian one. Another minor subduction zone with opposing dip developed at the same time, presumably along the Lupar line in Serawak (Hutchison, 1973), indicating a migration of the northeastern subduction zone towards the South China Sea. The corresponding volcanic arc consists of the Serian volcanic rocks and the Triassic volcanics encountered in drill holes in the Sunda Shelf, as mentioned earlier. In Cretaceous time both the southwestern and northeastern subduction zone became larger as they moved towards the directions of the Indian Ocean and the South China Sea, respectively. For a detailed description of this Cretaceous arc-trench system the reader is referred to Katili (1971, 1973a, b). During Tertiary time (Aquitanian) the development of the arc-trench system in Indonesia reached its highest point. A spreading center which had its origin in the Indian Ocean generated an arc-trench system stretching from the northwestern tip of Sumatra, Java, the Lesser Sunda Islands, Timor, Tanimbar, Kai, Ceram, Bum and Buton. The Banda arc at that time exhibited an east-west trend and together with Nias-Mentawai-South Java submarine ridge comprises an approximately six thousand kilometer long Tertiary subduction zone dipping at a relatively steep angle towards the continent. Intensive volcanism occurred simultaneously with this renewed subduction of which the products are now well exposed along the west coast of Sumatra, the south coast of Java and the Lesser Sunda Islands. Granitic rocks found in Java, Flores, Alor and Ambon also belong to this Tertiary volcano-plutonic arc. At more or less the same time a new pattern of subduction in the form of a north-south trending east-facing island arc broke through east of Borneo, ori~nating from a spreading center situated in the Pacific Ocean. This emergence of the Sulawesi-Philippine island-arc system coincided with the change in movement of the Pacific Plate which since Eocene+ligocene time was directed west-northwestward (Ben-Abraham and Uyeda, 1973). In the Middle to Upper Miocene (Burdigalian) time this north-south trending S~awesi-Mind~au subduction zone migrated farther eastward and created the eastward-facing Halmahera island arc. This arc could not be developed farther south as its growth was hampered by the northwardadvancing Australian continent with New Guinea attached to its northern border.



184



Subduction ceased, presumably at the end of the Miocene, and caused uplift creating the Indonesian non-volcanic outer arc such as Mentawai-Nias, Timor, Tanimbar, Kei, Buru, Ceram and Buton. The most dramatic event in the geologic history of Indonesia took place in Pliocene time when the northward-advancing Australian continent coupled with the counter-clockwise rotation of New Guinea and accompanied by the spearheading westward thrust of the Sorong fault system, caused the westward bending of the east-west trending Banda arc. This movement, while severely transforming the east-facing Sulawesi and Halmahera arcs into their present K-shape configuration, also pushed Sulawesi hundreds of kilometers back towards the Asian continent (compare Fig. 7 and Fig. 5). In Plio-Pleistocene time the subduction zone west of Sumatra and south of Java shifted oceanward to the present Sumatra-Java trench. Late Cenozoic to Recent volcanism, however, migrated in opposite directions as the dip of the Benioff zone is much shallower than the previous one. The continuing northward movement of the Australian continent and the westward thrust of the Pacific Plate is being accommodated respectively by the loop-shaped Banda trench and the Sorong transform-fault system, along which the islands of Sula Spur and Buton were swept against Celebes. The volcanoes in eastern Indonesia which developed after the collision, followed more or less the same pattern as the Tertiary ones but are absent in areas where subduction has been replaced by transcurrent movements. They are, however, active in the northern part of Celebes, where minor subduction zones have developed to accommodate the differential movements of this island. They are also present in Halmahera where as a consequence of the collision a subduction zone with reverse polarity developed west of this island. The tectonic evolution of the Indonesian Archipelago as depicted above (Fig. 11) demonstrates that since Paleozoic time the subduction zones have spread themselves systematically in ever-widening areas away from the continent towards the Indian Ocean, and later on during the Tertiary in the direction of the Pacific Ocean. Older subduction zones occur closer to the continent, while the younger ones are situated nearer to the ocean. The corresponding volcano-plutonic arcs also exhibit a zonal structure but volcanics and granites of different ages can occur in one erogenic belt as their locations depend largely upon the rate of the dip of the Benioff zones. The well-developed zonal structure and the existence of volcanic hot spots in western Indonesia are difficult to reconcile with anything but the existence of a fixed continent since Late Paleozoic time in this region. In western Indonesia, the subduction zones have not shifted very far towards the Indian Ocean, so that younger volcanism penetrates the older erogenic belts. In eastern Indonesia the migration occurred at distances which could be expressed in hundreds of kilometers (Fig. 11). This discrepancy might presumably be ascribed to the divergent behaviour of the spreading centers situated in the Indian and the Pacific Ocean respectively.



186



The regular zonal outgrowth of the subduction zone towards the oceans continued undisturbed in western Indonesia but was severely interrupted in the eastern part during Pliocene time when the collision took place between the Indian-Australian, Asian and Pacific Plates, as has been described earlier (Figs. 4 and 5). Another result of the collision is the development of minor subduction zones with reverse polarities, such as in Halmahera and northwestern Celebes. The synthesis put forward in this paper appears to favour the idea that the Banda Sea be considered as an oceanic crust trapped between younger arcs, rather than a basin which came into existence by a diapiric pull-apart mechanism as suggested by Karig (1971) and prematurely used by the author as a basis to construct a plate-tectonic model of eastern Indonesia (Katili, 1973b). Further gravity, seismic and heat-flow investigations in the small deep-sea basins of eastern Indonesia are needed before this intriguing problem can be satisfactorily solved. ACKNOWLEDGEMENTS



I wish to acknowledge Dr. Ismet Akil from Pertamina for giving me the permission to publish Mobil Oil Corporation’s data on the sections across Sumatra, Java and Timor. My deepest appreciation goes further to Mr. Michael Kontz from Gulf Indonesia for releasing radiometric data of the volcanic rocks of Celebes and for the long discussions I have had with him on the geology and the fascinating seismic profiles across the deep-sea trenches and basins of eastern Indonesia. My further thanks go to P.T. Tropic Endeavour Indonesia for showing me the Gorontalo transform-fault system from their radar imagery. Finally I wish to thank Dr. F. Hehuwat, Director National Institute of Geology and Mining in Bandung in assisting me to prepare the figures accompanying this paper.



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