Halmahera Arc Research Articles

Tectonic evolution of eastern Indonesia and its bearing on the occurrence of hydrocarbons

The evolution of the eastern Indonesian island arcs over the last 30 million years has been strongly affected by the northward drift of the Australian continent and the westward thrust of the Pacific Plate. As Australia drifted northward, New Guinea first collided with the Sepik island arc about 30 million years ago. About 20 million years ago, a major reorganization in the subduction pattern of the Indonesian region took place, resulting in the formation of an 8000 km long, east-west trending arc-trench system stretching from the western tip of Sumatra to Buru and even further to the Indonesian arc by way of Java, the Lesser Sunda Islands, Timor, Tanimbar, Kai and Seram. Prior to the arrival of the Australian continent at the Southeast Asian continental margin, a north-south oriented Sulawesi-Mindanao volcanic arc existed about 800 km east of Borneo. Further to the southeast, New Guinea and Sepik, now welded into a bigger microcontinent, collided with the Inner Melanesian island arc. This opened the Australian Plate to the influence of the WNW moving Pacific Plate. About 10 million years ago a south dipping subduction zone broke through north of Irian Jaya but no volcanism accompanied this process. The most dramatic event in the geologic history of Indonesia took place about 5 million years ago when the anticlockwise rotation of Irian Jaya and continuous northward movement of Australia trapped the Banda Sea and caused ‘tectonic shaving’ of the Birdhead of New Guinea along the Sorong transform fault system. Subsequently, Buton and Sula and other microcontinents collided with Sulawesi and Halmahera transforming the double island arcs into a K-shaped form. A small west-dipping subduction zone developed in northern Sulawesi accompanied by active volcanoes in Minahasa and the Sangihe island. Other small subduction zones with reverse polarities subsequently developed in northwest Sulawesi and Halmahera, which can be held responsible for the generation of, respectively, the active Una-Una volcano in the Gulf of Gorontalo, Central Sulawesi, and the volcanoes in the western Halmahera arc. Active collision accompanied by emplacement of ophiolites is presently taking place between the west-facing Halmahera arc and the east-facing Minahasa-Sangihe arc. The tectonic evolution of eastern Indonesia, which clearly envisaged a structural continuation of the Sunda-Banda arc and the existence of the north-south trending Sulawesi-Mindanao arc during the Tertiary period, could conveniently be used to explain the hydrocarbon occurrence in complex paleosubduction or collision zones of Timor, Seram and eastern Sulawesi. Whereas oil and gas accumulation in the pull-apart basins of Irian Jaya are found mostly in Tertiary deposits, the source rocks in the collision zones are likely to be of Mesozoic age. Main exploration target areas in eastern Indonesia are the intracratonic basins of the Arafura Shelf, the marginal (rift) basins skirting the southern and eastern section of the Banda arc, the collision zones of Timor, Seram, East Sulawesi and the thrustbelt of Irian Jaya.

The Molucca Sea Revisited: Evolution of an Arc-Arc Collision

The Molucca Sea is the site of the collision between the opposed Sangihe and Halmahera arcs and is floored by the coalesced accretionary wedges to the two subduction zones. In the north, the sea is bounded by the southern islands of the Philippine Archipelago, while in the south it abuts on the Sula Spur Continental Fragment. Free air and Bouguer anomaly values in its central part are among the lowest recorded offshore anywhere on the globe and provide severe constraints on structural and evolutionary models for the area. Further constraints are provided by the patterns of regional seismicity, which show earthquakes to be virtually confined to the two Benioff zones. Recently obtained geological data from the Halmahera Arc, together with results from the 1988 cruise of the RRS Charles Darwin and reflection seismic data from the southern part of the sea allow refinements to be made to previous interpretations. An understanding of the area is of importance from an exploration point of view because the Mesozoic sediments of the Sula Spur, which are believed to have been laid down at the margin of Gondwanaland, are generally considered to have hydrocarbon potential.

Boron-isotope systematics of Halmahera arc (Indonesia) lavas: Evidence for involvement of the subducted slab

Dehydration of sediments and oceanic crust within the subducting slab at convergent plate margins is probably a ubiquitous feature. This leads to fractionation of elements between fluids and solids so that the slab-derived component of island-arc lavas is modified from the originally subducted material. Sediments and altered oceanic crust are enriched in boron and cesium relative to uncontaminated mantle products, and these elements are highly mobile during fluid-rock interaction. The combination of B-isotope systematics and Cs concentrations in lavas from the Halmahera arc (Indonesia) suggests that they have been influenced by fluids derived from dehydration and/or melting of the subducted slab.

Late Palaeogene–Quaternary geology of Halmahera, Eastern Indonesia: initiation of a volcanic island arc

The Late Palaeogene–Quaternary stratigraphy of Halmahera is described, and new formation names are proposed, based on recent field investigations of the NE and central part of the island. This stratigraphic information provides new insights into the Neogene history of Halmahera and the development of the present island arc. The Late Palaeogene and younger rocks rest unconformably on an ophiolitic Basement Complex which formed part of a Late Cretaceous–Early Tertiary fore-arc. After volcanic arc activity ceased in the Eocene the former fore-arc terrane was uplifted and deeply eroded in the Late Palaeogene. Some of the Late Palaeogene–Early Miocene river valleys are currently being re-excavated by the present rivers. Slow subsidence began in the mid–late Oligocene and by the end of the Miocene all eastern Halmahera was the site of shallow-water carbonate deposition. There is no evidence for arc volcanism in central Halmahera at this time and the reported Oligo-Miocene volcanism in nearby regions is interpreted as volcanism related to the Sorong Fault system. The Miocene shallow water region subsided rapidly in the Early Pliocene and the sedimentary basin formed was filled with marls succeeded by siliciclastic turbidites, with increasing amounts of calc-alkaline volcanic debris from a Pliocene volcanic arc built on the western arms of Halmahera, probably on the eroded Early Tertiary arc. This phase of rapid subsidence in the Pliocene back-arc region resulted from the initiation of subduction of the Molucca Sea lithosphere eastwards beneath Halmahera. Differential subsidence on NW–SE and NE–SW sets of faults in the region immediately behind the active arc led to the formation of deep sediment-filled basins adjacent to the eastern arms. A major deformation event in the Pleistocene resulted in folding and local thrusting at the junction between eastern and western Halmahera and volcanism ceased in the Pliocene arc. The third Halmahera arc, the Quaternary arc, currently active in the northern part of the islands, began activity within the last 1 Ma and is built upon the deformed and partly eroded Pliocene arc. The Pleistocene deformation event and shift in position of the arc are interpreted as the result of the interaction of the eastward-dipping Molucca Sea plate with adjacent plates, either with a fragment of the Australian continent in the Sorong Fault zone and/or with the Philippine Sea plate beneath northern Halmahera.

Sediment incorporation in island-arc magmas: Inferences from 10Be

The radioisotope 10Be has been used as a tracer to evaluate subduction and recycling of sediments in island arcs. As a cosmogenic isotope strongly enriched in oceanic sediments, it is especially suitable for monitoring sediment subduction. We report here 10Be results for 106 arc volcanic rocks and 33 basalts from mid-ocean ridges, oceanic islands, continental rifts, and continental flood basalt provinces. The 33 basalts from non-arc environments all contain < 1 × 10 6 atom/gram (g −1) and form a reference or “control group” against which the arc data may be compared. Lavas from the Sunda, Mariana and Halmahera arcs, each with 12 samples, are indistinguishable from the control group. Nine samples from Japan range from 0.1 × 10 6 to 13.5 × 10 6 g −1 of 10Be. Lavas from the Aleutians and Central America (excluding Costa Rica) consistently have 10Be contents greater than those of the control group, extending to 15.3 × 10 6 g −1 for the Aleutians and 24 × 10 6 g −1 for Central America. Both the Japan and Central American arcs show a geographic variation in 10Be abundances; the Aleutian arc does not. Conceptually, four models may be used to explain the incorporation of 10Be in island arcs. Model 1 is the limiting case calculation presented in Brown et al. (1982). Physically, it corresponds to an assumption that all 10Be in the uppermost sediment layers is carried to depth, the 10Be is mechanically decoupled from the sediment pile and is added to the arc magma source region. This model results in the highest calculated 10Be contents in arc lavas. In the more physically reasonable Model 2, the 10Be-rich upper sediments are assumed to mix with the deeper, 10Be-poor, sediments and the sediments are subsequently incorporated into the source region. Model 3 assumes 10Be to be incorporated from sedimentary layers encountered during magma ascent. In Model 4 only sediments contained within grabens in the downgoing slab may be subducted. As yet insufficient data exist to permit conclusive evaluation of these models, but correlations between 10Be contents in arc volcanic rocks and recent sedimentation rates and sediment thickness, measured outboard of the trench, suggest that Model 2 may be the most important mechanism for 10Be incorporation in many island arcs.

An introduction to the Sangihe arc: Volcanism accompanying arc—Arc collision in the Molucca Sea, Indonesia

In the Molucca Sea region of northeastern Indonesia, the Sangihe and Halmahera arcs are presently in the process of colliding, the earth's only example of a collision between facing volcanic arcs. The collision event is more advanced in the northern Molucca Sea where back-arc thrusting occurs along the Cotobato and Philippine trenches and volcanic centers are inactive and dissected. This paper describes the Sangihe arc, the western arc of the collision. The Sangihe are is approximately 500 km in length extending from the NE tip of Sulawesi to Mindanao, Philippines. The arc comprises over 25 Quaternary volcanic centers, the eight presently active volcanoes being confined to the southern half of the arc. The active volcanic front lies 100–110 km above the top of the west-dipping Benioff zone and volcanoes extend up to 70 km behind the front, thereby defining a volcanic arc overlying earthquakes 100 to 180 km deep. Rock types range from basalt to rhyolite, andesites are dominant. Pyroxene-andesites are common on Sulawesi and at or near the volcanic front from Ruang to Buhias-Pahepa and Buang to Mamanuk. Hornblende-andesites are common at the volcanic front between Mahengetang and Sangihe Island and in centers located well behind the volcanic front. Tholeiitic (TH) suites, defined on the basis of relative degrees of iron enrichment, are confined to the southern volcanic front, lack hornblende, are low-K (K55 < 1.05), have low Rb/Zr ratios (0.10–0.17), low Ba/Y ratios (3.4–4.9), low Rb/Cs ratios (8–14) and have the highest 87Sr/86Sr ratios (0.7039–0.7042). Calcalkaline (CA) suites occur throughout the arc, are low- to high-K (K55 ≅ 0.6–2.0), have variable Rb/Zr ratios (0.15–0.65) and Ba/Y ratios (4.2–12.4), high Rb/Cs ratios (14–82) and low 87Sr/86Sr ratios (0.7035–0.7040). Zr and Y distribution coefficients are lower in TH suites (DZr = 0.0, DY = 0.2–0.5) than in CA suites (DZr = 0.0–0.8, DY = 0.3–1.2). Ti initially increases with increasing SiO2 in some TH suites while decreasing with SiO2 in all CA suites. With the exception of Bukide (Ti/Zr = 138), Ti/Zr ratios of basalts exceed 100 only in TH suites (76–130). LIL-element contents and LIL/HFS-element ratios increase 2 to 3 times across the are away from the volcanic front and 2 to 3 times from south to north along the arc. 87Sr/86Sr ratios decrease away from the volcanic front. The south to north increase in LIL-element contents and LIL/HFS-element ratios, without corresponding changes in Sr-isotopic compositions are interpreted as resulting from decreasing degrees of partial melting northward. This reflects the gradual cessation of volcanism in the north due to collision.

Past and present geotectonic position of Sulawesi, Indonesia

Sulawesi with its peculiar K-shaped pattern is situated in an area where the Eurasian, Indian—Australian and Pacific plates interact and collide. Complex geological processess in this area resulted in the transformation of a normal island-arc structure into an inverted one, deformation of an already tectonized belt, sweeping of fragments against unrelated terrain, thrusting of oceanic and mantle material over the island arc, closing of deep-sea basins behind the arc, trapping of old oceanic crust caused by the rolling up of an island arc, formation of a marginal basin by the spreading of the sea floor behind the arc, development of small subduction zones with reverse polarities etc. Small deep-sea basins surrounding Sulawesi such as the Gulf of Bone and the Gulf of Gorontalo originally formed the arc—trench gap of the Sulawesi island arc. The Banda Sea is considered as an oceanic crust trapped by the bending of the east—west trending Banda arc due to the northward drift of Australia combined with the westward movement of the Pacific plate. Similarly the Sulawesi Sea consists of an old Pacific crust trapped by the westward bending of the Sulawesi island arc, caused by the spearheading westward thrust along the Sorong transform-fault system, in which later a minor spreading center became active in its central part. The Molucca Sea comprises tectonic mélange in which presumably a small spreading center developed between the two colliding arcs of northern Sulawesi and western Halmahera. While the Benioff zones dip under the northern Sulawesi and Halmahera arcs in normal fashion, the mélange thrusts over them. The Strait of Makassar is a marginal basin which was brought into existence by the spreading of the sea floor between Kalimantan and Sulawesi. The evolution of Sulawesi started in Miocene time or even earlier when 800 km east of Kalimantan a north—south trending east-facing island arc came into existence, originating from a spreading center located in the Pacific Ocean. Volcanism and plutonism accompanied this subduction process. Collision between Sulawesi and the Australian—New Guinea plate which occurred in early Pliocene time severely transformed Sulawesi into an island with its convex side turned towards the continent, at the same time causing obduction of ophiolite in the eastern arc of this island. The movement of the Pacific plate continued and gradually pushed Sulawesi towards the Asian continent, resulting in the closing of the sea between Kalimantan and Sulawesi islands separated by small straits and deep seas resembling the complicated pattern of the Philippine Archipelago, in which the original double island-arc structure can no longer be recognized.

Volcanism and plate tectonics in the Indonesian island arcs

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.