Cenozoic Arc Processes in Indonesia: Identification of the Key Influences

The Geological Society of America Special Paper 436 2008

Cenozoic arc processes in Indonesia: Identiﬁ cation of the key inﬂ uences on the stratigraphic record in active volcanic arcs

Robert Hall Helen R. Smyth SE Asia Research Group, Department of Geology, Royal Holloway University of London, Egham, Surrey, TW20 0EX, UK

ABSTRACT

The Indonesian region includes several volcanic island arcs that are highly active at the present day, and also contains a record of Cenozoic volcanic activity owing to subduction of oceanic lithosphere at the margins of SE Asia. As a result of long-term subduction, there is a high regional heat fl ow, and a weak crust and lithosphere, as identifi ed in other subduction zone backarcs. The stratigraphic record in the Indo- nesian region refl ects a complex tectonic history, including collisions, changing plate boundaries, subduction polarity reversals, elimination of volcanic arcs, and extension. The arcs have not behaved as often portrayed in many arc models. They mark subduction but were not continuously active, and it is possible to have subduction without magmatism. Subduction hinge retreat was accompanied by signifi cant arc volcanism, whereas periods of hinge advance were marked by reduction or cessation of volcanic activity. Growth of the region occurred in an episodic way, by the addition of ophiolites and continental slivers, and as a result of arc magmatism. In Indonesia, relatively small amounts of material were accreted from the downgoing plate during subduction, but there is also little evidence for subduction erosion. During collision the arc region may fail, resulting in thrusting, and the weakest point is the position of the active volcanic arc itself. Volcanic arcs shift position suddenly, and arcs can disappear during collision by overthrusting. Arcs are geologically ephemeral features and may have very short histories in comparison with most well-known older orogenic belts. The stratigraphic record of the basins within arc regions is complex. Because of a weak lithosphere the character of sedimentary basins may be unusual, and basins are commonly very deep and subside rapidly. There is a high sediment fl ux. The volcanic arc itself infl uences the stratigraphic record and basin development. The load imposed by the volcanic arc causes fl exure and provides accommodation space. The volcanic arc thus can form the basin and supply most of its sediment. Tropical processes infl uence the mineralogy and apparent maturity of the sediment, especially volcanogenic material. A complex stratigraphy will result from the waxing and wan- ing of volcanic activity.

Keywords: Cenozoic, Sunda Arc, Banda Arc, Sangihe Arc, Halmahera Arc, Indone- sia, stratigraphic record.

Hall, R., and Smyth, H.R., 2008, Cenozoic arc processes in Indonesia: Identiﬁ cation of the key inﬂ uences on the stratigraphic record in active volcanic arcs, in Draut, A.E., Clift, P.D., and Scholl, D.W., eds., Formation and Applications of the Sedimentary Record in Arc Collision Zones: Geological Society of America Special Paper 436, p. 27–54, doi: 10.1130/2008.2436(03). For permission to copy, contact [email protected]. ©2008 The Geological Society of America. All rights reserved.

27 28 Hall and Smyth

INTRODUCTION background on the region, and a summary of the history of several of the most important Cenozoic arcs in Indonesia. We then dis- The size of Indonesia, the abundance of volcanic activity, cuss particular features of the record of arc activity and attempt to and its long volcanic history make it an important area for under- identify those that may be of general interest in understanding arc standing arc processes and infl uences of volcanic arcs on the processes and those that may be useful in interpreting the record stratigraphic record. Hamilton (1988) commented that the arcs of activity in older arcs elsewhere in the world. Our discussion of Indonesia–southern Philippines–western Melanesia are in “the of the stratigraphic record in Indonesian arcs is based mainly on region of greatest modern variety and complexity,” but the region studies carried out by the SE Asia Research Group in several dif- remains understudied and often overlooked. The long history of ferent arcs over many years, especially in Java and Halmahera, subduction beneath Indonesia has infl uenced arc development. but also in arcs of Sumatra, Sulawesi, and Borneo. These studies Subduction has produced a thin and warm lithosphere beneath have included extensive fi eld work and related work documented the upper plate, which is unlike that of older stable continents and in theses, but some of the results are unpublished and in places their margins. These features have infl uenced the development of we may appear to make statements unsupported by easily acces- the Indonesian arcs, which differ in many ways from textbook sible literature; we hope this will be forgiven, and we felt it was examples of arcs described from other parts of the world. They justifi ed in our attempt to give an account of the region in a paper were formed at the margins of a large, weak subduction backarc of reasonable length. region and consequently seem to have been unusually responsive to plate boundary forces. Because of their youth it is not pos- Indonesia and Large Explosive Volcanic Events sible to study the deep levels of the arcs, but the arc history that is revealed by their stratigraphy offers insights into the develop- Indonesia is a large country that includes >18,000 islands ment of older orogens and older volcanic arcs. and stretches >5000 km from west to east (Fig. 1). Wallace (1869) In this paper we identify some important features of Indo- tried to draw attention to its size in his book The Malay Archi- nesian volcanic arcs, particularly those that infl uence the strati- pelago by comparing the island of Borneo to the British Isles, graphic record of arc activity. Because the geology of Indonesia is and subsequently Umbgrove (1938) and van Bemmelen (1949) not familiar to many readers, we begin our discussion with some compared the region to areas more familiar to North American

Andaman 10°N Sea South China Sea Philippines

Sunda 5°N Malay Peninsula Shelf Sangihe Celebes Sea

Borneo Bird’s Nias Halmahera Head 0° Molucca

traits Sea

Indian Sulawesi Ocean Sumatra Makassar S Java Sea 5°S Banda Flores Sea Sea Java

Timor 10°S 0 500 1,000 Northwest Shelf Australia kilometers 100°E 110°E 120°E 130°E

Figure 1. Indonesia shown on a digital elevation model (DEM), based on Shuttle Radar Topography Mission (SRTM) data merged with the Sandwell and Smith (1997) bathymetry. Cenozoic arc processes in Indonesia 29 and European readers. Figure 2 is a modern attempt to show the island of Sumatra, was the largest in the last 2 m.y., and its effect size of the region by comparing Indonesia to the conterminous on climate must have been even more devastating. It has been United States. According to the Smithsonian Global Volcanism speculated that the eruption led to accelerated glaciation and a Program (Smithsonian, 2006), Indonesia includes 150 Holocene bottleneck in human evolution (Ambrose, 2003; Rampino and volcanoes; 95 of these could be considered active because they Self, 1992, 1993a, b). have erupted since A.D. 1500. Indonesia has a long record of For the last 36 m.y., 42 large explosive events are recorded volcanic activity, recording subduction related to northward in a recent global compilation (Mason et al., 2004); 33 of movement of the Indian-Australian plate and westward motion these large explosive events are recorded from North America, of the Pacifi c plates. Cenozoic subduction of Indian-Australian whereas Toba alone is situated in a humid tropical climate. The oceanic lithosphere beneath most of Indonesia began after Aus- record has been interpreted to indicate that there were two pulses tralia separated from Antarctica and moved rapidly northward in Cenozoic global volcanism: one between 36 and 25 Ma, and from ~45 m.y. ago (Hall, 2002). the other since 13.5 Ma. However, Indonesia remains a rela- With a long history of volcanic activity, and a well-known tively remote area, which is diffi cult to explore because of the explosive character in historical times, it is surprising that the absence of infrastructure, diffi cult terrain, and rain forest cover. only exceptionally large eruptions known from Indonesia in the We suggest that the absence of exceptionally large eruptions Cenozoic are those of Toba. Of the Holocene volcanoes, 32 have in the stratigraphic record from Indonesia has more to do with records of very large eruptions with a volcanic explosive index preservation and sampling than with volcanic activity. As we (VEI) >4; 19 of these have erupted in the past 200 yr, includ- report below, there is indeed evidence of large explosive events ing Tambora in 1815 (VEI = 7) and Krakatau in 1883 (VEI = in Indonesia during the Cenozoic, and no doubt more of these 6). There are only 4 volcanoes in the Smithsonian global data remain to be discovered. The lack of information on large erup- set of large-volume Holocene explosive eruptions with a VEI of tions in Indonesia is not dissimilar to the general lack of infor- 7; the other 3 erupted in prehistorical times and pre-date 4000 mation on Indonesia’s volcanic history. This partly refl ects the B.C. Tambora, on the island of Sumbawa, is known for its impact absence of modern detailed studies, but ideas about arcs may on global climate, and its 1815 eruption resulted in the North- be unduly infl uenced by results from more accessible or bet- ern Hemisphere’s “year without a summer” in 1816, when crops ter exposed regions, from ocean drilling, and from geochemi- failed, causing famine and population movements (Harington, cal studies. However, as with large eruptions the stratigraphic 1992; Zeilinga de Boer and Sanders, 2002). The 74 ka eruption record in Indonesia has some valuable insights to reveal; this of Toba (Chesner and Rose, 1991; Chesner et al., 1991), on the paper identifi es some of them.

5000 km

Figure 2. Indonesia and Malaysia compared to the USA. The box is 60° from east to west and 25° from north to south. 30 Hall and Smyth

ARC ACTIVITY IN INDONESIA active since the Eocene (e.g., Garwin et al., 2005; Hall, 2002; Hamilton, 1977) and is the product of subduction of the Indian- The distribution of modern-day volcanoes in Indonesia Australian plate beneath the Sundaland margin. Sundaland shows clearly that most volcanic activity is related to subduction (Fig. 4) is the continental core of SE Asia, comprising Indochina, (Fig. 3). This is also emphasized by the distribution of seismic- the Thai-Malay Peninsula, Sumatra, Java, Borneo, and the shal- ity (Cardwell and Isacks, 1978; Cardwell et al., 1980; Engdahl low marine shelf (Sunda Shelf) between them, and formed by et al., 1998; England et al., 2004). Global positioning system amalgamation of continental blocks during the Triassic. There (GPS) observations (Bock et al., 2003; Kreemer et al., 2000; is a Proterozoic basement (Liew and McCulloch, 1985; Liew Rangin et al., 1999), seismicity, and volcanic activity indicate and Page, 1985) intruded by Permian–Triassic granites formed rapid plate movements and considerable tectonic complexity. by subduction and postcollisional thickening of the continental Present-day Indonesian volcanic activity can be regarded as the crust (Hutchison, 1989, 1996). In Borneo the oldest rocks known products of four separate volcanic arcs (Fig. 3): the Sunda Arc, are Paleozoic metamorphic rocks intruded by Mesozoic granites. stretching from Sumatra through Java to the east, the horseshoe- From Sumatra to Borneo the continental core is surrounded by shaped Banda Arc of eastern Indonesia, the Halmahera Arc Mesozoic ophiolitic and arc igneous rocks, and possible frag- of the North Moluccas, and the Sangihe Arc, extending from ments of continental material, accreted during the Cretaceous. Sulawesi into the southern Philippines. Here we give ﬁ rst a brief In Sumatra and West Java the Sunda Arc is built in part on the summary of the Cenozoic history of each of these arcs and the continental margin of Sundaland. In East Java it is constructed character of the crust on which they are constructed to provide on arc and ophiolitic rocks accreted at the Mesozoic active mar- the background to our commentary on Java, east Indonesia, and gin and in part on a continental fragment that was added by a the Moluccas. More detailed discussion of each of these arcs, Cretaceous collision. literature references, and animated reconstructions of the tec- In Sumatra a record of subduction-related magmatism tonic development are in Hall (2002) and Garwin et al. (2005). extends back into the early Mesozoic or late Paleozoic (McCourt et al., 1996), but in Java the oldest record of subduction is Cre- Java: The Sunda Arc taceous. The Sundaland Cretaceous active margin is interpreted to have run the length of Sumatra into West Java and then turned Indonesia is situated on the southern margin of the Eurasian northeast into SE Borneo (Hamilton, 1979). In Central Java and plate at the edge of Sundaland (van Bemmelen, 1949; Hamilton, SE Borneo there is evidence for Cretaceous high pressure–low 1979; Hutchison, 1989; Katili, 1975). The Sunda Arc has been temperature subduction-related metamorphism (Parkinson et

5°N

Sangihe Talaud Arc Morotai Mayu Halmahera Borneo Arc 0° Sunda Sumatra Shelf Kalimantan

Sulawesi Obi Sunda Arc

Strait Seram 5°S Buru Java Sea Banda Arc Makassar Java Wetar Bali Flores Sunda Arc 10°S Lombok Timor

95°E 100°E 105°E 110°E 115°E 120°E 125°E 130°E

Figure 3. Present-day volcanoes in Indonesia (Smithsonian, 2006) and the principal volcanic arcs discussed in the text: Sunda Arc, Banda Arc, Halmahera Arc, and Sangihe Arc. Indonesia is shaded in gray, and the area of Malaysia is unﬁ lled. Cenozoic arc processes in Indonesia 31

EURASIAN PHILIPPINE PLATE SEA

Manila Trench PACIFIC Philippin PLATE OCEAN South China Philippines Sea e Andaman Trench 10°N Sea PACIFIC

MESOZOIC Sulu PLATE Borneo Sea

SUNDALAND CORE Suture

Molucca Sunda Celebes Suture CAROLINE Shelf Sea Molucca Sea Sorong PLATE 0° S SUNDALAND Suture unda

Seram Trench Bismarck Trough Sea Makassar Strait Java Sea Sulawesi Banda Sea Banda Suture Suture New Guinea ADDED BY EARLY CENOZOIC Arafura 10°S Java Shelf INDIAN Trench Trough OCEAN Timor Sahul ADDED BY Shelf EARLY MIOCENE Coral Sea 0km 1000 INDIAN-AUSTRALIAN PLATE AUSTRALIA

90°E 100°E 110°E 120°E 130°E 140°E 20°S

Figure 4. Wide plate boundary zone of Indonesia. The light shaded area is the zone of collision between the Eurasian, Indian-Australian, and Pacifi c–Philippine Sea plates, which can be considered as a wide suture zone, within which there are smaller sutures shown in darker shading. Lines show incremental growth of the Indonesian region at different stages by addition of fragments of continental, arc, and ophiolitic rocks. Modifi ed from Hall and Wilson (2000). al., 1998). In Indonesia there is almost no Paleocene record, and plate. In the Java-Sulawesi sector of the Sunda Arc, volca- it appears that most of Sundaland was emergent. All published nism greatly diminished during the early and middle Miocene, plate tectonic reconstructions show subduction beneath Java although northward subduction of Indian-Australian lithosphere before 45 Ma, but there is no record of volcanic activity in Java continued (Fig. 5). Following Australian continental collision in the early Cenozoic (Smyth et al., this volume). We suggest in east Indonesia, the Sunda subduction hinge advanced north- there was a cessation of subduction beneath Java in the Late ward as a result of counterclockwise rotation of the Borneo-Java Cretaceous following the Cretaceous collision of a continental part of the Sundaland margin, and this is interpreted to have led fragment at the Sundaland margin (Smyth et al., this volume). to termination of magmatism, despite continued subduction, Our recent and current work in Java suggests that subduction because of the absence of fresh material replenishing the mantle resumed in the middle Eocene, forming a volcanic arc in the wedge (Macpherson and Hall, 1999, 2002). At the end of the Southern Mountains that ran the length of Java. At this time, middle Miocene at ca. 10 Ma, volcanic activity resumed in the ca. 45 Ma (Fig. 5), the rate of Australia-Antarctica separation Java-Sulawesi sector of the Sunda Arc, after the termination increased, and Australia began to move northward relatively rap- of Borneo-Java rotation. Magmatism began again in Java with idly. Since then there has been continuous subduction of Indian increased vigor, but in a position north of the Paleogene arc, Ocean lithosphere beneath the Sunda Arc. During the Eocene forming the modern arc (Fig. 5). During most of the Cenozoic and Oligocene the position of the volcanic arc remained broadly since the Eocene, Java was a chain of volcanic islands, and only fi xed, and from Sumatra to Sulawesi abundant volcanic activ- in the last 5 m.y. has the island emerged as an extensive area ity accompanied northward subduction of the Indian-Australian of land. 90°E 120°E 30°N 90°E 120°E 30°N 45 Ma 15 Ma Shikoku PACIFIC Basin PLATE

PACIFIC PLATE

Luzon Parece Proto-South Vela Iz China Sea u Palawan Basin Boni Sulu n Sea

Marianas Sumatra

Sumatra Celebes Sea West 0° Borneo 0° Borneo Philippine Sunda Basin Arc Molucca Jav Sulawesi Sangihe a East Sea Ar West Java c Phil Halmahera Sulawesi ippine s– Halmahera INDIAN-AUSTRALIAN Banda INDIAN-AUSTRALIAN PLATE Embayment New Solomon PLATE Halmahera Guinea Sea Arc

20°S 20°S

90°E 120°E 30°N 25 Ma 5Ma

PACIFIC PLATE South China Sea Mariana Trough

h a Se

Proto-Sout China Parece Celebes Vela Sea Basin a Ayu Se 0° Molucca Trough

Celebes Bismarck Sea Caroline Sea Sea Banda Sea New Guinea Bird’s Head INDIAN-AUSTRALIAN Sula INDIAN-AUSTRALIAN Spur PLATE PLATE New Guinea 20°S Cenozoic arc processes in Indonesia 33

Figure 5. Reconstructions of the Indonesian region, based on Hall (2002). 45 Ma: The Paciﬁ c plate was moving broadly NW and subducting beneath the east Asian margin and the eastern margin of the Philippine Sea plate, forming the Izu-Bonin-Marianas Arc. Spreading centers linked the West Philippine Sea Basin and the Celebes Sea. Australia was moving NNE on the Indian-Australian plate, and northward subduction produced the Sunda Arc and the East Philippines–Halmahera Arc. The proto–South China Sea was subducting southward beneath the north Borneo–Luzon margin. The parts of the Paciﬁ c and Indian-Australian plates shown in blue without anomalies have been subducted since 45 Ma. 25 Ma: The East Philippines–Halmahera–South Caroline Arc collided with the Australian margin in New Guinea. Collision between Australia and SE Asia began with initial contact of the Sula Spur, the continental promontory east of the Bird’s Head of New Guinea, and East Sulawesi. These events caused major reorganization of plate boundaries. Active spreading in the South China Sea was caused by southward subduction of the proto–South China Sea. 15 Ma: Subduction continued beneath the Sunda Arc, but arc volcanic activity ceased in Java between ca. 20 and 10 Ma as the hinge advanced, owing to collision of the Australian margin in Sulawesi. Arc terranes in New Guinea moved westward in a wide left-lateral strike- slip zone. Locking of splays at the western end of the fault zone induced eastward subduction of the Molucca Sea; westward subduction of the Molucca Sea beneath the Sangihe Arc had begun at ca. 25 Ma. 5 Ma: Volcanic activity resumed in Java at ca. 10 Ma and has continued until the present. Hinge rollback into the Banda Embayment south of the Bird’s Head produced the arcuate subducted slab beneath the Banda Arc and led to backarc spreading in the Banda Sea. The Molucca Sea was in the process of elimination by subduction at its west and east sides beneath the Sangihe and Halmahera Arcs.

Eastern Indonesia: The Banda Arc longer be accommodated by orogenic contraction. At this time the oldest oceanic lithosphere in the Indian Ocean, of Late Jurassic East of Java and Borneo, Indonesia can be considered to age, arrived at the eastern end of the Java Trench. This area of be a wide and complex suture zone (Fig. 4), and even today old crust north of the NW Shelf of Australia formed an embay- it can be tectonically described only in terms of several small ment in the Australian margin, the proto–Banda Sea. Because of plates and multiple subduction zones (Hall, 2002; Hall and Wil- its age and thickness, the Jurassic ocean lithosphere fell away rap- son, 2000; Hamilton, 1979). Eastern Indonesia is the product of idly, causing the Banda subduction hinge to roll back rapidly to a long period of extension, subduction, and collision and has the south and east, inducing massive extension in the overlying a complex basement; there is some continental crust but also plate. In western Sulawesi this fi rst induced extensional magma- much more arc and ophiolitic basement in comparison with tism, which began at ca. 11 Ma (Polvé et al. 1997). As the hinge western Indonesia. The Banda Arc is the horseshoe-shaped arc, rolled back into the Banda Embayment it led to formation of the which today extends east from Flores to Buru (Fig. 3), pass- Banda Arc and the opening of the North Banda Sea, as described ing through Timor and Seram and includes both an outer non- by Hamilton (1988). Further rollback caused the opening of the volcanic arc and an inner volcanic arc. Although there was a Flores Sea and later the South Banda Sea (Fig. 5). South of the volcanic arc in eastern Indonesia during the Paleogene, and Bird’s Head microcontinent the rollback of the subduction hinge parts of this and possibly older arcs may be found in the highest resulted in collision of the Banda volcanic arc at ca. 3 Ma with nappes of Timor and other Banda islands, we do not term them the Australian margin in the region of Timor and the cessation of the Banda Arc. The older volcanic arc rocks formed part of the volcanic activity in this segment of the arc. After collision, conver- Paleogene Sunda Arc. The Banda volcanic arc is young and has gence ceased south of the volcanic arc, and new plate boundaries been active only for ~10 m.y. (Abbott and Chamalaun, 1981; developed north of the arc between Flores and Wetar and to the Honthaas et al., 1999), and the horseshoe-shaped Banda Arc is a north of the South Banda Sea. Within the Bird’s Head microconti- phenomenon of the same time interval. It formed by subduction nent there has been signifi cant shortening and probable intraconti- of an embayment within the northward-moving Australian plate nental subduction within the last 3 m.y. at the Seram Trough. (Charlton, 2000; Hall, 1996, 2002; Hamilton, 1979). The arc developed within the collision zone after the Australian margin Moluccas: The Halmahera and Sangihe Arcs collided with the former active margin of the Sunda Arc. From the Eocene the Sunda Arc can be traced east through The Halmahera and Sangihe Arcs both have long volcanic Sulawesi into the East Philippines and Halmahera Arcs in the histories. The modern Halmahera Arc is constructed on older Pacifi c. During the Paleogene (Fig. 5) subduction was northward arcs, of which the oldest known is an intraoceanic arc formed beneath Sulawesi and the East Philippines until the fi rst collision in the Pacifi c in the Mesozoic (Hall et al., 1988a, 1995) pre- of Australian crust, the Sula Spur, with SE Asia in the early Mio- sumably built on older oceanic crust. The modern Sangihe Arc cene ~25 m.y. ago (Fig. 5). The collision caused the long subduc- is constructed on arcs formed at the Pacifi c margin in the early tion system to separate into two parts. West of Sulawesi, northward Cenozoic (Hall, 2002); again the deepest parts are built on older subduction continued in the Sunda Arc, but to the east subduction oceanic crust (Evans et al., 1983). Both of the currently active ceased, and the Australia–Philippine Sea plate boundary became arcs formed during the Neogene. They are unusual in that they a strike-slip system. Between the two, the Australia–SE Asia col- are the only arcs in the world that are currently colliding. lision formed a mountain belt in East Sulawesi. By the late middle The pre-Eocene history of these arcs is not well known. At Miocene, at ca. 12 Ma, convergence in East Sulawesi could no 45 Ma (Fig. 5) the Halmahera Arc was far out in the western 34 Hall and Smyth

Pacifi c but was situated on the southern margin of the Philip- Molucca Sea has been eliminated by subduction at both its east- pine Sea plate beneath which there was northward subduction ern and western sides (Fig. 5). The Sangihe–North Sulawesi Arc of Indian-Australian lithosphere. Between 45 and 25 Ma the is now thrusting over the Halmahera Arc in the Molucca Sea, Philippines-Halmahera Arc remained at approximately the same as discussed below. The central Molucca Sea mélange wedge latitude above a north-dipping subduction zone north of Austra- and ophiolites represent the forearc basin and basement of the lia. At ca. 25 Ma (Fig. 5) there was the most important Cenozoic Sangihe Arc, which will soon have completely overridden the change in plate boundaries in the region. Arc-continent collision Halmahera Arc (Hall, 2000). between the East Philippines–Halmahera Arc and the New Guinea margin terminated northward subduction of oceanic lithosphere RESULTS OF LONG-TERM SUBDUCTION north of Australia, and a major left-lateral strike-slip boundary developed in northern New Guinea. Arc terranes were translated There is a great contrast between present tectonic activity, westward within this strike-slip system. The arc terranes at the manifested by seismicity and volcanism at the margins, and the southern edge of the Philippine Sea plate moved in a clockwise apparent interior stability of Sundaland, which has led some direction along the northern New Guinea margin within the left- authors to describe the core as a shield or craton (e.g., Ben- lateral Sorong strike-slip zone. At the western end of the left- Avraham and Emery, 1973; Gobbett and Hutchison, 1973; Tjia, lateral fault system there was subduction beneath the Sangihe Arc, 1996). It is true that most of the areas of extreme relief, high and shortening and uplift in East Sulawesi. The Philippine Sea elevations, and actively rising mountains are in eastern Indonesia, plate moved northward as it rotated clockwise, accompanied by but the core is certainly not a craton. Most of the Sunda Shelf is complex strike-slip faulting and minor subduction at its western fl at and close to sea level (Fig. 1), and even the large island of edge in the Sangihe Arc and Philippines (Fig. 5). The Molucca Borneo has generally low elevations with the exception of the Sea double subduction system was initiated at ca. 15 Ma (Fig. 5). isolated 4 km peak of Mount Kinabalu in the north. However, the Subduction on the west side of the Molucca Sea in the Sangihe picture of Indonesia as an active volcanic margin surrounding a Arc had started soon after the 25 Ma plate reorganization, but stable continental region is misleading. There has been signifi cant the oldest Neogene volcanic rocks in the Halmahera Arc have deformation within Sundaland during the Cenozoic. Heat fl ow, ages of ca. 11 Ma (Baker and Malaihollo, 1996). Initiation of seismic tomography, and geological observations indicate that east-directed Halmahera subduction probably resulted from the the Sundaland continental core north of Indonesia is unusual. locking of one of the strands of the left-lateral Sorong fault zone Sundaland has high surface-heat-fl ow values (Fig. 6), typi- at the southern edge of the Molucca Sea. In eastern Indonesia the cally more than 80 mW/m2 (Artemieva and Mooney, 2001; Hall

Figure 6. Contoured heat-fl ow map for SE Asia, based on the database of Pollack et al. (1990, 1993) and oil company compilations (Kenyon and Beddoes, 1977; Rutherford and Qureshi, 1981). Cenozoic arc processes in Indonesia 35 and Morley, 2004). At the margins of Sundaland, high heat fl ow is notably at the Indonesian arcs. These features have infl uenced arc related to subduction-related processes and magma rise, but in the development in some ways not identifi ed in many arc models. interior this is not a result of arc magmatism as sometimes suggested (e.g., Nagao and Uyeda, 1995). High heat fl ow values are ISLAND ARC MODELS recorded in the interior Sundaland basins from the Gulf of Thai- land to west Borneo >800 km from the active volcanoes of the Many models of island arc development were formulated in Sunda Arc and similar distances from oceanic crust of the South the 1960s and 1970s during the rapid period of development of the China Sea. The hot interior of Sundaland appears to be the conse- plate tectonic theory (e.g., Dewey, 1980; Dewey and Bird, 1970; quence of high upper-crustal heat fl ow from radiogenic granites Dickinson, 1973, 1974, 1977; Dickinson and Seely, 1979; Karig, and their erosional products, the insulation effects of thick sedi- 1971), and they are still commonly reproduced in textbooks. ments, and a high mantle heat fl ow (Hall and Morley, 2004). Much of the subsequent work on island arcs in the past 30 yr has P and S wave seismic-tomography models (Fig. 7) show focused on geochemistry and magmatic processes, as it seems that Sundaland is an area of low velocities in the lithosphere to be widely thought that our knowledge of tectonic processes and underlying mantle (e.g., Bijwaard et al., 1998; Lebedev and in volcanic arcs is complete. A common arc model (Fig. 8) sug- Nolet, 2003; Ritsema and van Heijst, 2000; Widiyantoro and gests that after the initiation of subduction (a problem in itself), van der Hilst, 1997) in contrast to Indian and Australian conti- a volcanic arc develops, typically between 70 and 100 km from nental lithosphere to the NW and SE, which are colder, thicker, the subduction trench (Dickinson, 1973, 1977; Karig and Shar- and stronger. Low mantle velocities are commonly interpreted man, 1975); this width increases with the age since arc initiation. in terms of elevated temperature, and this is consistent with An accretionary prism formed by progressive transfer of material regional high heat fl ow, but they may also partly refl ect elevated from the downgoing slab to the upper plate develops near the mantle volatile contents, partial melting, or seismic anisotropy trench (Scholl et al., 1986). As time proceeds the accretionary (Lebedev and Nolet, 2003). The high heat fl ows seen region- prism becomes larger and wider (Dickinson, 1973, 1977), under- ally across Sundaland, and the generally low mantle velocities thrusting leads to elevation of the arcward part of the prism which observed in tomographic models, suggest mantle heat-fl ow val- may form a forearc high, and the arc-trench gap progressively ues on the order of 40 mW/m2 (Hall and Morley, 2004), which widens. At the same time, debris from the volcanic arc is depos- are at the high end of the range estimated globally (Artemieva ited in the forearc in basins in front of the arc, and the size and and Mooney, 1999). Thus they suggest that the region is under- width of these basins increase with time. Continued arc magma- lain by a thin and weak lithosphere. tism is interpreted to lead to thickening of the crust beneath the Similar lithosphere has been identifi ed in other subduction arc. The backarc region is usually not shown in the tectonic mod- zone backarcs (Hyndman et al., 2005). Subduction has produced els; in some arcs backarc basins are formed, fl oored by oceanic a different mantle beneath the upper plate, associated with a lith- crust, whereas in others no oceanic backarc basins are formed. osphere that is not like that of older stable continents and their The reasons for the formation of backarc basins, which are found margins. The high heat fl ow and thin, weak lithosphere are the mainly in the western Pacifi c, are still not clear. However, these consequence of long-term subduction at the Sundaland margins, basins are some distance from the arc, typically >100 km. Little

Figure 7. Depth slices through S20RTS S-wave tomographic model (Ritsema and van Heijst, 2000) for SE Asia. High shear velocities are represented by blue, and low shear velocities by red, with an intensity that is proportional to the am- plitude of the shear velocity perturbations. The range in shear velocity variation is given below each map. 36 Hall and Smyth

Volcanic arc Forearc basin Structural in Oceanic crust A B folding (kneading) Trench

crystalline

Offscraped oceanic deposits

SEDIMENT sedimentary SUBDUCTION C

Subduction complex ocean crust crystalline

Lithosphere small Forearc basin accretionary D body Asthenosphere

crystalline Forearc basin SUBDUCTION EROSION

SEDIMENT E SUBDUCTION

km CRATONIC MASSIF 30

SUBDUCTION 50 km 100 km EROSION

Figure 8. Some models for arc development. (A) The arc-trench gap increases in width as progressive accretion occurs at the trench (modiﬁ ed from Dickinson, 1977). As discussed in the text, this type of model is based in part on observations from Indonesia, but new mapping suggests a less important role for subduction accretion. (B, C, D, E) The range of tectonic processes at subduction margins, from accretion to erosion, modiﬁ ed from Scholl et al. (1980).

attention has been given to the area directly behind the volca- knowledge acquired by hydrocarbon exploration in Indonesia to nic arc, and most arc-derived debris is usually shown to move interpret arc history and tectonic processes. into the forearc rather than into the area behind the arc. In recent The Indonesian arcs differ from these models in a number years, in many accounts of arcs in other regions, and in modeling of ways. Arc magmatism may be discontinuous despite continu- of arcs, the arc model has become more sophisticated. Partly as ous subduction. In many Indonesian arcs there is little evidence a result of discoveries made by the Ocean Drilling Program, fea- of major addition of new material by accretion at the trench, but tures such as subduction erosion (e.g., Kay et al., 2005; Ranero equally there is little evidence for subduction erosion. There are and von Huene, 2000; von Huene and Scholl, 1991) and chang- sudden movements of the position of the volcanic arc, and there ing slab dips (e.g., Funiciello et al., 2003; Kay et al., 1987; Van- may be evidence of signifi cant thrusting within the arc itself, nucchi et al., 2004) have been interpreted as important infl uences not always with obvious cause; collisions, subduction accretion, on arc development (Fig. 8). However, there has been no ocean subduction erosion, or changing angles of subduction do not drilling of the Indonesian arcs, and few offshore investigations offer solutions. In general, in the Indonesian arcs there is evi- of the forearcs; we are still dependent on fi eld studies and the dence of a staccato development, possibly related to processes Cenozoic arc processes in Indonesia 37 of arc-continent collision but in some cases with no clear reason. cessation of arc activity. Hamilton (1988, 1995) pointed out that Observations of seismicity on the basis of well-located hypo- subduction zones are often portrayed as static systems with the centers (Engdahl et al., 1998; England et al., 2004) show that subducting slab rolling over a stationary hinge and sliding down along the modern arc there are sectors that are aseismic or signif- a slot fi xed in the mantle, but this is not a good model. He empha- icantly less seismically active in comparison with other sectors. sized the necessity of viewing subduction in a dynamic way and This may indicate differences in coupling of the downgoing and argued that it is most commonly driven by gravity acting on the overriding plate at the subduction zone, and variations in cou- subducting slab, and most subduction zones are characterized by pling and decoupling in time as well as space could contribute retreat of the hinge with time, or rollback. Extension of the over- to a discontinuous tectonic history. This could also be related to riding plate is known to be common in arc settings (e.g., Chase, dynamics of fl ow in the mantle wedge (S. Lamb, 2006, personal 1978; Dewey, 1980; Elsassar, 1971; Hamilton, 1988, 1995) and commun.; Oncken et al., 2006). may be the normal condition for a volcanic arc. For the period Large-scale dynamic processes have undoubtedly infl uenced since 25 Ma, tectonic models (Hall, 1996, 2002) suggest that the stratigraphic record of the Indonesian arcs. However, smaller hinge retreat (Fig. 9) in most subduction zones in SE Asia and scale effects are also observed. The growth of arc volcanoes has the western Pacifi c was accompanied by signifi cant arc volca- in some parts of the region contributed to basin development by nism (Macpherson and Hall, 1999, 2002) and in some zones by fl exural loading. Furthermore the products of the Indonesian vol- marginal basin formation. canic arcs have been modifi ed by processes related to eruption and Extension in arcs may be essential at regional and local weathering in a tropical setting. Both these features may have rel- scales to induce melting, to provide pathways for magmas, and evance to the interpretation of ancient arcs and their stratigraphic to vary the rate of supply of volatile components to the mantle record, as discussed below. We begin with the larger scale effects wedge. In particular, hinge retreat allows replenishment of the and move on to more subtle features of arc stratigraphy. mantle wedge by an infl ow of hot, undepleted mantle (Andrews and Sleep, 1974; Furukawa, 1993). This can also cause ablation Magmatism and Subduction of the base of the overriding plate (Furukawa, 1993; Iwamori, 1997; Rowland and Davies, 1999), and the mantle that upwells The association between subduction and magmatism is well may melt by decompression. These processes can maintain known. Early plate tectonic models (e.g., Oxburgh and Turcotte, steady-state arc magmatism at a relatively constant distance from 1970) attributed magmatism to frictional heating at the Benioff the trench over prolonged periods. zone, melting of the subducting slab, and compressional shorten- Where subduction is characterized by a fi xed hinge, or ing of the overriding plate in the arc-trench gap. Melts are now by hinge advance, magmatism may decline or cease, although considered to originate primarily in the mantle wedge above subduction continues. A fi xed hinge may result from coupling the subduction zone and are thought to result from the input of of the subducting and overriding plate or from collision, caus- volatiles that lower the mantle solidus. In Indonesia, almost all ing the two plates to move in the same direction as the slab is Cenozoic magmatism seems to have been infl uenced by subduc- being subducted. In these cases, magmatism may cease, although tion, although it has not always been the direct result of melting subduction continues, simply because the mantle wedge is not of the mantle wedge above the subducting slab. In some cases being replenished, becomes depleted, and can no longer melt. For a subduction signature is inherited from previous subduction the period since 25 Ma (Fig. 9), hinge advance in SE Asia and events. The geochemistry of igneous rocks (e.g., Pearce and the western Pacifi c was accompanied by reduction in magma- Peate, 1995) is often used to infer the tectonic setting in ancient tism (Macpherson and Hall, 1999, 2002). The Sunda Arc pro- arcs, but if a subduction signature is inherited, volcanic arcs may vides a good example. From Java eastward the hinge advanced be mistakenly identifi ed in ancient orogenic belts as subduction from ca. 20 to 10 Ma, and magmatic activity declined. In East related when they are not. Macpherson and Hall (1999, 2002) Java, Paleogene volcanic activity culminated with a short-lived highlighted areas in Indonesia where there has been arc-like explosive phase (Smyth et al., this volume) and then declined sig- magmatism but subduction is not active, such as south Sulawesi nifi cantly or terminated; magmatism resumed at ca. 12–10 Ma. (Polvé et al., 1997) or northern Java, where volcanoes are very far There is no indication that subduction ceased or slowed during from the active arc and have an ultra-potassic character (Edwards the period of reduced magmatism, and the Indian-Australian et al., 1994; Macpherson, 1994). In these places the character of plate continued to move northward relative to Eurasia at a similar magmatism refl ects older subduction events that have enriched rate to that during the Paleogene. The reduction in magmatism the mantle. can be understood as the result of hinge advance driven by the However, the association between magmatism and subduc- collision of Australia in eastern Indonesia (Hall, 1996, 2002) and tion is commonly not straightforward. Some arc models portray manifested by counterclockwise rotation of the Borneo region compressional arcs with active magmatism, and contraction is (Fuller et al., 1999). When rotation of Borneo ceased, the hinge often thought to be typical of active arcs, probably because the retreated again and magmatism resumed, especially in the Banda subduction model causes us to think of plate convergence. In Arc, where dramatic retreat of the subduction hinge required a the Indonesian arcs, contraction is commonly associated with massive infl ux of fertile mantle beneath the overriding plate. 25-5 Ma

Japan 25-5 Ma

Izu-Bonin Ryukyu 25-5 Ma

Philippines Hinge Advance Hinge Mariana Retreat 25-15 Ma Sangihe Mariana Sunda 15-5 Ma 25-5 Ma

Halmahera 15-5 Ma Solomons New Sunda-Banda Hebrides 20-10 Ma 10-5 Ma

A: Andaman Sea BS: Banda Sea 5-0 Ma Bi: Bismarck Sea M: Mariana Trough Wd: Woodlark Basin Japan

Izu Ryukyu Bonin

Philippine Trench Hinge Manila Advance Trench M Hinge Sangihe Mariana A Retreat

Halmahera North Sulawesi New Britain Bi Solomons Sunda BS Wd Banda

Figure 9. Movements of subduction hinges in the Neogene, modiﬁ ed from Macpherson and Hall (2002). Regional plate reorganizations occurred at ca. 25 Ma and 5 Ma. Major coastal outlines are shown for reference. For subduction zones shown without shading, there was no signiﬁ cant movement of the hinge. Bold letters indicate areas of young marginal basin formation. Cenozoic arc processes in Indonesia 39

Subduction Accretion and Subduction Erosion Near the trench there is some evidence for a young accretionary complex constructed against an older backstop (Fig. 10), A feature of many early arc models is an increase in the width interpreted to be an older accretionary complex (Kopp et al., of the arc-trench gap with increasing age, commonly attributed to 2002, 2006; Schlüter et al., 2002), but this is only ~50 km wide. accretion, and some of these models were based on observations Schlüter et al. (2002) suggest there was a progressive growth of from Indonesia (e.g., Dickinson, 1973; Moore and Karig, 1980). the arc-trench gap south of Sumatra since the Paleocene, but the In contrast, in many arcs the role of subduction erosion in the age control on this development is poor. In contrast, Kopp et al. development of the arc has become widely recognized as impor- (2006) suggest there has been subduction erosion at the Sunda tant (e.g., Scholl et al., 1980; von Huene et al., 2004). In Indo- margin south of Java, and if this is correct, the arc-trench gap nesia these contrasting processes are diffi cult to assess, because must have been wider in the past. The seismic lines on which the age and nature of the crust between the arc and the trench in these interpretations are based do not show a progressive growth Indonesian arcs is unexplored or poorly known. There is a great of the accretionary zone but rather a young accretionary com- need for detailed exploration of these parts of all arcs. However, plex juxtaposed against an older one (Fig. 10). This relationship observations on land suggest that neither subduction accretion could equally be interpreted in terms of an abrupt change in the nor erosion has had a signifi cant impact on arc history. subduction history, and we note that on land there is evidence for such a change; the volcanic arc ceased activity at ca. 20 Ma and Change in Width of Arc-Trench Gap with Age resumed activity ~10 m.y. later, 50 km north of its previous posi- There is good evidence of Early Cretaceous subduction and tion. Why this happened is not clear. At present the available data widespread evidence from Sumatra to SE Borneo of middle to are simply not adequate to distinguish different models. Late Cretaceous collisions accompanied by ophiolite emplace- However, investigations of the Sumatra forearc in the Nias ment (Miyazaki et al., 1998; Parkinson et al., 1998; Wakita, 2000). region do provide the basis for testing the interpretation of arc As noted above, there is almost no Paleocene stratigraphic record growth by continuous accretion. Nias, at the north end of the in Indonesia, and the whole region seems to have been emergent. Sumatra forearc (Fig. 1), is commonly cited as the classic exam- In many parts of Indonesia, poorly dated, typically terrestrial ple of a forearc high elevated by accretion, following the work of rocks interpreted to be Paleocene or Eocene rest unconformably Moore and Karig (1980) on the basis of their mapping of part of on older basement rocks. In East Java the oldest sedimentary the island. Later mapping of the whole island and other islands of rocks that rest unconformably on ophiolitic basement are middle the forearc does not support this model (Samuel, 1994; Samuel Eocene or older and lack volcanic debris (Smyth, 2005; Smyth et al., 1995, 1997; Samuel and Harbury, 1996). There is little evi- et al., this volume). There is little evidence for latest Cretaceous dence on Nias for the progressive addition of material. The ophi- to early Eocene volcanic activity in most of the Sundaland mar- olitic basement includes Cretaceous rocks and locally is overlain gin between Sumatra and Sulawesi. The Cenozoic stratigraphic unconformably by a thin Eocene marine cover. Most of the island record, commonly volcanogenic, begins in the middle Eocene, consists of a thick sequence of Oligocene to lower Miocene deep and in Java the current phase of subduction along the arc began marine clastic sedimentary rocks and a thick sequence of lower to after 45 Ma, when Australia began to move rapidly northward. upper Miocene shallow marine clastic sedimentary rocks. There Although it is commonly assumed that subduction at the Sunda are a few limestones and some tuff layers in the sequence. The Trench in the Late Cretaceous (e.g., Metcalfe, 1996; Hall, 2002) island became emergent in the Pliocene. The ophiolitic base- continued into the Cenozoic, we question this assumption. Ham- ment was in place by the Eocene, and the deep marine sedimen- ilton (1988) stated that the modern subduction system in Java was tary rocks above the basement were deposited on it. However, inaugurated no earlier than late Oligocene time. We, too, consider the sedimentary rocks are not material scraped off the downgo- that the present subduction began only in the early Cenozoic, ing plate but represent the fi ll of a forearc basin, later inverted although we now know of evidence for subduction beneath Java (Fig. 11), in which most material was carried from the direction before the late Oligocene (Smyth, 2005; Smyth et al., this vol- of Sumatra. The Oligocene to early Miocene development of the ume). However, there is no evidence in Java to support the sugges- outer forearc was characterized by extension, followed by early tion of late Paleogene southward subduction or an arc collision in Miocene inversion. Mélanges are not subduction accretion com- or with Java in the Paleogene (Hamilton, 1988). plexes but are diapiric in origin and formed by mud volcanism In the Sunda Arc between Sumatra and Java the arc-trench that continues today. The forearc islands have a complex history gap is rather large, between 300 and 350 km (Fig. 1), and subduc- of local uplift and subsidence to which addition of material at the tion began in the early Cenozoic. If the arc-trench gap increased trench has contributed, but the Sumatra forearc does not fi t very progressively with age, there should be evidence of movement well to the model of continuous widening and accretion inter- of either the volcanic arc or the trench during the Cenozoic. In preted from the early land-based studies. East Java the volcanic arc remained in the same position from the Eocene to the early Miocene (Smyth, 2005; Smyth et al., this vol- Mélanges ume). It then ceased activity, and a new arc formed ~10 m.y. later, Mélanges were formed at numerous stages in the develop- 50 km to the north, where it has remained since the late Miocene. ment of arcs in Indonesia (Hall and Wilson, 2000; Hamilton, 40 Hall and Smyth

SW NE 0 JAVA A SEA Outer Arc High Mentawai 2 Forearc Basin INDIAN B OCEAN C 4

(s) 10°S Java Trench Mentawai

T Wrench 6 100°E 110°E Accretionary Wedge I Fault

TW Accretionary Wedge II Sunda Trench 8 Continental crust

10 A Oceanic crust of Indian Ocean Plate 50 km

SW ACCRETIONARY DOMAIN NE 0 Forearc Strata ACTIVE FOSSIL Trench infill FRONTAL PRISM -10 OUTER HIGH OCEANIC-TYPE SHELF CRUST

(km)

Depth Oceanic mantle Subductin Shallow Mantle -30 g Slab

B 50 km -40 S OUTER FOREARC HIGH FOREARC BASIN N 0 Basin Sediments: 1 Slope Deposits Hemipelagic / Pelagic sediment Normal and Volcanic Ashes Faulting 2 Trench Vertical Movement Post-sedimentary 3 and Doming buckling

TWT (s) 4

5 C 40 km 6

Figure 10. Cross sections across the Sunda forearc, based on marine geophysical investigations. (A) At the south end of Sumatra the accretionary wedge is interpreted to be ~300 km wide and constructed against continental crust beneath the forearc basin (Schlüter et al., 2002). (B) Interpret- ed tectonic units of the Sunda margin south of West Java (Kopp et al., 2002), where the accretionary domain is interpreted to have an active part and an older part, juxtaposed against oceanic-type crust beneath the forearc basin. (C) Section south of Central Java (Kopp et al., 2006) shows an uplifted forearc high, covered by thin sediments interpreted as evidence of subduction erosion. The age of the unconformity is unknown, but the thin sediments above the unconformity suggest a young age. A similar unconformity on land is late Miocene to Pliocene, which developed after northward thrusting within Java.

1979). Marine geophysical studies suggest that mélanges are and seismically incoherent volume of sediment in the central formed at Indonesian subduction zones (Kopp et al., 2002; Molucca Sea. Mélanges have also formed far from subduction Schlüter et al., 2002), but they are also formed at later stages dur- zones without collision (Samuel et al., 1997). In Timor, mélange ing collision (Silver and Moore, 1978). In the central Molucca formation is attributed by some authors mainly to thrusting Sea (Fig. 12) mélanges were formed at two stages. Mélanges related to collisional processes (Harris et al., 1998) and by oth- reported from Talaud (Moore et al., 1981) and present on Mayu ers mainly to diapiric processes expressed as modern eruption of were not formed during the present arc-arc collision but are older mud volcanoes (Barber et al., 1986). Active mud volcanism in rocks forming part of the pre-Neogene basement of the Sangihe East Java (Smyth, 2005), behind both the Paleogene and mod- forearc. Presumed mélanges of the present collision complex are ern arcs, erupts rock samples from deeper parts of the basin all submarine and constitute part of the bathymetrically shallow directly north of the modern arc and brings to the surface blocks Cenozoic arc processes in Indonesia 41

Lahewa Sub-basin Mujoi Sub-basin Gomo Sub-basin Mola Basement High

MID-MIOCENE– SEDIMENT & OLIGOCENE– LATE PLIOCENE– EARLY PLIOCENE MELANGE EARLY MIOCENE COMPLEX PLEISTOCENE Coast SEDIMENT Coast

MID-MIOCENE– EARLY PLIOCENE

BASEMENT SW NE

0 V=H 10 Km

SW NE Accretionary Forearc Sumatran fault zone, Indian Ocean Trench Nias wedge basin magmatic arc & backarc

Continental crust

0 V=H 60 Km Moho

+ Overriding continental lithosphere

Figure 11. Cross sections across the Sumatra forearc, modiﬁ ed from Samuel and Harbury (1996). Mapping of Nias by Samuel (1994) shows that the area between Sumatra and Nias was in extension for much of the Cenozoic and that most of the island’s sediments were derived from Sumatra. The island has not emerged as a forearc high as the result of progressive subduction accretion. V=H—vertical and horizontal scales are equal. of the oldest parts of the basin sequence entrained in overpres- of the two arcs (Fig. 13). However, on Talaud a post–middle sured muds. This is clearly not related to subduction because the Miocene sedimentary sequence rests unconformably on ophio- Benioff zone is >100 km below the site of mud volcanism. The lites that are middle Eocene or older (Moore et al., 1981). The mélanges at the surface, and the presumed equivalents beneath middle Miocene to Pleistocene rocks are tuffaceous sandstones, the surface, are the result of overpressures related to rapid depo- siltstones, and shales with intercalations of limestone, marl, and sition of thick sedimentary sequences, hydrocarbon generation, conglomerate in which the sediments are dominated by volcanic and young thrusting very far from the subduction zone. debris. They were deposited in deep water by turbidity currents. On Talaud, Neogene strata are in many places little deformed, Mélanges and Ophiolites and they are moderately to strongly deformed locally and typi- The emplacement of ophiolites is often linked to the pro- cally in narrow zones adjacent to mélange rocks or ophiolites. cesses of mélange formation and accretion. Between the Sangihe On Mayu, coastal exposures of highly indurated mélange yield and Halmahera Arcs (Fig. 12) is a collision complex in which Eocene ages (Baker, 1997). All of the mélange is probably older there are ophiolites and mélanges. The Talaud Ridge at the center than middle Miocene, and none appears to be related to the pres- of the Molucca Sea was interpreted on the basis of marine geo- ent collision. physics as a mélange wedge (McCaffrey et al., 1980; Silver and The similarity in character and structure of the ophiolitic Moore, 1978) including slices of the Molucca Sea lithosphere rocks to those of the basement complex on Halmahera (Hall (Fig. 13). These rocks are exposed on Talaud and on two tiny et al., 1988a) suggest that much of the mélange formed dur- islands along the ridge, Mayu and Tifore. The ophiolites were ing Eocene or older deformation events. The interpretation of interpreted by McCaffrey (1983, 1991) as part of the subducted the ophiolites as part of the subducted Molucca Sea lithosphere Molucca Sea plate, which has been emplaced by the collision requires some complex arguments (McCaffrey, 1991) to account 42 Hall and Smyth

Philippine Fault Philippine Trench EURASIAN PLATE

Sulu Sea PHILIPPINE SEA Mindanao PLATE

1800 km Celebes Sea Cotobato Trench Sangihe Arc Halmahera Arc

Halmahera

MOLUCCA SEA PLATE

SORONG FAULT ZONE Sulawesi

New Guinea Banda Sea

AUSTRALIAN PLATE

1800 km

Figure 12. Molucca Sea region, showing the converging Halmahera and Sangihe Arcs, displayed in a three-dimensional (3D) diagram that represents, in simplifi ed form, the geometry of the converging plates in east Indonesia. Modifi ed from Hall et al. (1995). for their elevation instead of their disappearance by subduction. gap is just as wide. The role of the upper plate in providing mate- In contrast, if the ophiolites are interpreted as part of the Eocene rial is well illustrated at the eastern edge of the Philippine Sea or older basement (Fig. 13), as they are in Halmahera, and they plate in the Nankai margin, where classic fold and thrust struc- formed the basement to the forearc, the gravity model is much tures are formed in the thick “accretionary wedge” (Mikada et easier to understand. Where the Halmahera forearc and arc have al., 2005; Taira et al., 1991). The sediment in the upper part of the been signifi cantly overthrust the Sangihe forearc has been jacked prism is dominated by clastic material carried along the trough up. The wide Molucca Sea collisional complex is composed from the Japan–Izu-Bonin Arc collision zone, and relatively of the accretionary wedges of both arcs. The basement of the small amounts of material are contributed from hemipelagic Sangihe forearc is exposed where it thrusts over this wedge. The sediments on the subducting slab (Karig and Ingle et al., 1975; ophiolitic rocks of the central Molucca Sea are not part of the Klein and Kobayashi et al., 1980; Taira et al., 1991). In Indonesia Molucca Sea plate but are the basement of the Sangihe forearc the role of the structurally upper plate is well illustrated by the (Hall, 2000). The ophiolites were the basement before the mid- eastern Makassar Strait, where there has been no subduction, yet dle Miocene. accretionary-style structures have developed in sand-dominated sedimentary wedges that have built out from Sulawesi (Puspita et Accretion or Erosion? al., 2005). These structures closely resemble the structures devel- In many Indonesian arcs, there seems to have been relatively oped in subduction accretionary complexes, and major eastward little growth of the forearc region by the addition of material subduction during the Cenozoic has been suggested by some from the downgoing plate. Many so-called accretionary com- authors (e.g., Charlton, 2000; Guntoro, 1999). However, most plexes are in fact dominated by material derived from the arc. At authors have interpreted the West Sulawesi margin to be either the northwestern end of the Sunda Arc there is undoubtedly sedi- a passive margin with oceanic crust to the west (e.g., Bergman ment at the distal end of the Bengal Fan (Curray, 1994) that is et al., 1996; Cloke et al., 1999; Hall, 1996; Hamilton, 1979) or a being added to the overriding plate, although most of this is prob- rifted margin with the thinned continental crust to the west (e.g., ably mud-dominated sediment (Stow et al., 1989). Farther south, Calvert, 2000; Situmorang, 1982). Recent offshore seismic data south of south Sumatra and Java, the amount of sedimentary show that there has been no subduction (Fraser et al., 2003; Pus- cover on the subducting plate is much thinner, yet the arc-trench pita et al., 2005) and suggest that the Makassar Strait is fl oored Accretionary complex Accreted SANGIHE TALAUDsediments and crust SNELLIUS RIDGE MINDANAO Forearc basin of Halmahera forearc Backarc basin 0 v v v v v v v v v v v v v v v v v v 10 v v v v v v v v v v v v v v v Arc crust v v v v v v Backarc crust v v v v v Forearc crust v v Arc crust v v v v v v v v v v v v v v v v v 20 v v v v v v v v v v v Talaud 30 EURASIAN PLATE PHILIPPINE SEA PLATE A 40 Sangihe Arc Halmahera Arc 1 Forearc crust

meters 50 Snellius Sangihe Ridge

Kilo 60 Sangihe mantle Halmahera mantle 70 B 80 MOLUCCA SEA PLATE Morotai 90 A Mayu 100 WESI C SULA s HALMAHERA

s Accretionary complex NW SANGIHE MOROTAI SE Backarc basin Forearc basin Forearc basin 0 v v v v v v v v v v v v v v v v v v v v v v v 10 v v v v v Forearc crust Forearc crust v v v v v v v Arc crust v v Arc crust v v v v v v v v v Backarc crust v v v v v v v v v v v v v v v v v v 20 v v v v v v v v v v v v 30 40 EURASIAN PLATE PHILIPPINE SEA PLATE Sangihe Arc Halmahera Arc

eters

m 50

Kilo 60 Sangihe mantle Halmahera mantle 70 MOLUCCA SEA PLATE 80 90 B 100 100 50 Calculated 0 MGAL Observed -50 -100 Talaud-Mayu -150 0 50 100Ridge 150200 250 -0.59 -0.59 -1.83 km -0.43 -0.43 10 0.40

0.44 C 30

WNW Sangihe Arc Halmahera Arc ESE 100 0 km 100 200 300 0 10 20 2.86 30 40

eters 3 m 50 3.40 g/cm

Kilo 60 70 +0.20 +0.20 80 3.34 +0.05 +0.05 90 Mantle density D 100

Figure 13. Cross sections across the Molucca Sea (Hall, 2000) drawn at the same vertical and horizontal scales (A, B) in comparison with those based on gravity modeling (C, D) by McCaffrey et al. (1980). In section A at the latitude of Talaud, the entire arc and forearc of the Halmahera Arc has been overridden by the Sangihe forearc. Ophiolites interpreted as part of the Sangihe forearc basement are exposed in the Talaud Islands. Farther south (section B), only part of the forearc has been overridden, but the Halmahera Arc in Morotai was overridden by its own backarc in an earlier thrusting episode. In contrast, McCaffrey et al. (1980) interpret the ophiolites as part of the subducted Molucca Sea plate. 44 Hall and Smyth by extended continental crust (Nur’aini et al., 2005; Puspita et the whole of the forearc and volcanic arc have disappeared. This al., 2005). The structures are foreland-type fold and thrust belts, process is not complete, and it is likely that much, if not all, of and developed as westward thrusting has progressed since the the Halmahera Arc will disappear beneath the Sangihe Arc. This Pliocene, all the material having been derived from Sulawesi. process is certainly not subduction erosion, as intended by most There certainly has been addition of material to the continen- authors (Scholl et al., 1980; von Huene and Scholl, 1991; von tal margin of Sundaland during the Cenozoic (Fig. 4). However, Huene et al., 2004). However, neither is it subduction accretion, the additions occurred during relatively short periods of time, at as usually envisaged. The arc-trench gap of the Sangihe Arc has widely spaced intervals, and were related to the collision of con- increased since the two arcs came into collision, but the arc- tinental fragments at the Sundaland margin. The material added trench gap of the Halmahera Arc has diminished. included arc and ophiolitic rocks, which may in some cases represent backarc basins within the continental margin or the rem- Arc Movements and Changing Slab Dips nants of material subducted during the arc-continent collision. In Indonesia, arc growth seems to have occurred discontinuously Observations in Indonesian arcs raise the question of whether rather than by continuous steady-state addition of material at the there is such a thing as a compressional arc. Arcs may certainly trench that led to gradual widening of the arc-trench gap. be compressed, but when contraction occurs volcanic arc activity ceases. Magmatism may resume when compression ceases, but Disappearance of Arcs the stratigraphic record suggests it does not persist during con- Westward subduction of the Molucca Sea beneath the traction. In the Banda Arc, magmatic activity terminated in the Sangihe Arc probably began in the early Miocene. Eastward sub- Wetar sector of the volcanic arc at ca. 4 Ma, when arc-continent duction of the Molucca Sea plate beneath Halmahera began in collision began in Timor, and was never resumed during stacking the middle Miocene. The double subduction zone was initiated of nappes (Audley-Charles, 2004). In the Halmahera Arc, vol- at this time, forming a new plate, the Molucca Sea plate, separate canic activity ceased as arc-arc collision began but resumed in a from the Philippine Sea plate. The oldest volcanic rocks dated new location shortly afterward. The repeated failures in different from the Halmahera Arc are 11 Ma in Obi at its southern end, and places in the entire arc system in Halmahera suggest that inter- they become younger to the north (Baker and Malaihollo, 1996). mittent thrusting was followed by periods of relaxation in which The Molucca Sea was eliminated from south to north, and the arc magmatism resumed. two forearcs began to collide. The result of the collision was that Such abrupt movements in the position of the volcanic arc the Halmahera Arc system (meaning the entire region between are not uncommon and may follow periods when an arc is com- the trench and backarc region) failed repeatedly, with thrusting in pressed, but they may be due to other tectonic causes. Sudden different directions at different stages in the collision. First, the shifts in position of the volcanic arc could be related to shorten- backarc was thrust over the volcanic arc, and later the forearc was ing of the entire arc system (forearc, arc, and backarc), which thrust toward the volcanic arc. In south Halmahera the backarc may involve thrusting of the backarc or the forearc toward the region was thrust onto the forearc, in places entirely eliminating arc, the magmatic arc itself being the weakest point. Field obser- the Neogene arc. At the southern end of the Halmahera Arc on vations in Halmahera show that the site of thrusting was close Obi the arc was thrust onto the forearc (Ali and Hall, 1995; Ali et to the former active arc. Experimental and numerical modeling al., 2001). After west-vergent thrusting, volcanism in the Halma- studies also show that the active arc is the weakest point in the hera Arc resumed between Bacan and north Halmahera. At the whole arc system (Shemenda, 1994; Tang and Chemenda, 2000; south end of the arc on Obi, and at the north end from Morotai Tang et al., 2002). These studies show that failure occurs directly northward, volcanism ceased. In the northern Molucca Sea the beneath the arc. It can take place on faults dipping toward the Sangihe forearc was then thrust east onto the Halmahera forearc trench or arc, and this depends on the thickness of subducted and arc. In the region between Morotai and the Snellius Ridge, crust and fl exural rigidity of the upper plate. parts of the Neogene Halmahera Arc and forearc have now dis- Since the middle Pliocene (Fig. 14) the Halmahera Arc sys- appeared. Farther south, this east-vergent thrusting carried the tem failed more than once close to the site of the active volcanic Halmahera forearc onto the fl anks of the active Halmahera Arc, arc, presumably refl ecting its weakness owing to mineralogy and and pre-Neogene rocks of the Halmahera forearc basement are magmatism. Active volcanism has most recently resumed in a now exposed in islands of the Bacan group and off the coast of restricted area at the center of the arc chain, over a distance of northwest Halmahera. ~200 km, in comparison with the 700 km length of the early Plio- Cross sections drawn across the present-day collision zone cene arc. The arc-arc collision explains why the arc volcanism from south to north can also be considered to display the sequence may have ceased and then resumed as stresses changed, but this of events in time, and a series of sections illustrating the earlier does not really account for the change in position of the arc. The stages in the collision can be inferred from the geology of the Quaternary volcanoes are situated ~50 km west of the Pliocene Halmahera Arc (Fig. 14). An interesting consequence of the col- centers (Fig. 15). Hall et al. (1988b) speculated that the shift lision is that the Halmahera Arc is being progressively overrid- may be related to a steepening dip of the subducted slab beneath den by the Sangihe Arc, and in the northern Molucca Sea almost Halmahera. A highly complex geometry of subducted slabs lies Talaud ridge 0Ma

Philippine Sea plate

Halmahera Arc and forearc overridden by Sangihe forearc Molucca Sea plate sinks deeper Emergence of ophiolitic basement of Sangihe forearc

0Ma

Halmahera forearc overridden by Sangihe forearc Molucca Sea plate sinks Local emergence of islands in ‘collision complex’

0Ma

Halmahera Arc fails again Overthrusting of forearc region: detachments deep within basement Minor backthrusting at back of Sangihe forearc

Sangihe Arc Halmahera Arc 2Ma

Molucca Sea plate

Volcanic activity ceases in Halmahera arc Arc completely overridden by backarc region

Figure 14. Cross sections across the Molucca Sea drawn at same vertical and horizontal scales to illustrate the sequence of convergence of the Halmahera and Sangihe Arcs since 2 Ma. The lowermost section is inferred from geological mapping on land. The upper three sections are drawn at different latitudes across the Molucca Sea from south (bottom) to north (top). They can be considered to represent the sequence of events in the last ~2 m.y. that have led to the structure at the latitude of Talaud, where convergence is most advanced. Collision has resulted in the almost complete elimination of the Halmahera Arc and forearc at the latitude of Talaud. A 127o E 129o E

MOROTAI

2No

Modern volcanoes of the Halmahera Arc Pleistocene to modern Molucca Sea Halmahera Arc

Miocene to Pliocene Arc

HALMAHERA

0o Weda Bay

050100 BACAN Km

110o E 112o E B Modern volcanoes of the Sunda Arc

Oligo-Miocene volcanic centers of the Southern Mountains Arc

7So

EAST JAVA

SUNDA ARC

SOUTHERN MOUN TAINS ARC 8So

0 50 100

Figure 15. Abrupt movements of the volcanic arcs in (A) Halmahera (ca. 2 Ma) and (B) Java (ca. 10 Ma) took place, as discussed in the text. Cenozoic arc processes in Indonesia 47 beneath the Halmahera and Sangihe Arcs and the southern Philip- considered backarc basins. They are found behind the arc but pines (Fig. 12). The west-dipping Philippine slab has now arrived are much closer to it than typical backarc basins. They are not at depths where it would hit the subducted Halmahera slab and underlain by newly formed oceanic crust, nor are they charac- increase its dip, thus shifting the arc to the west if melting occurs terized by obvious extension. They are fi lled with volcanic arc at a constant depth. Macpherson et al. (2003) showed that dif- material, mainly reworked as sediments. The basins formed in ferences in the geochemistry of Quaternary lavas in comparison a setting that may have been extensional (as noted above, pos- with Neogene lavas of the Halmahera Arc are consistent with an sibly the typical condition of a volcanic arc) or neutral, using increased sediment fl ux interpreted to indicate an increase in dip the terminology of Dickinson (1995) and Busby and Ingersoll of the subducted slab. (1995), but certainly not compressional. They are not retro-arc In East Java, activity in the Southern Mountains Arc termi- basins, and although there may be evidence of thrusting, this nated in the early Miocene after a period of >20 m.y. at the same occurred after the basin formed. location. About 10 m.y. later a new arc formed, 50 km north of East Java and Halmahera provide two examples. In Java the the older arc, that has remained at the same position since the deep Kendeng Basin behind the arc is fi lled with a thick sequence late Miocene (Fig. 15). Why the volcanic arc moved north is not of volcanic and sedimentary rocks mostly derived from the arc known. There is no evidence for a collision at the Java Trench. itself. The basin formed in the middle Eocene, when the volca- There was Neogene contraction in Java, although it is not well nic arc began its activity. Modeling by Smyth (2005) suggested dated. It is possible that the slab dip remained at the same angle, that at least part of the accommodation space was created by the the depth to the Benioff zone remained constant, and therefore load of the volcanic arc. The basin in Weda Bay (Nichols and contraction of the arc meant that when volcanism resumed the Hall, 1991) behind the Halmahera Arc has been investigated as new arc formed at the same distance from the trench. Several part of oil exploration, and seismic lines close to the arc show no possible explanations are suggested by Smyth et al. (this vol- indication of fault control on subsidence (see Waltham et al., this ume), but all are speculative. volume). Again, a likely important contribution to subsidence is The abrupt change in position of the volcanic arc is a feature fl exural loading by the volcanic arc. This suggestion is explored of several Indonesian arcs, and the causes are not known. How- in more detail by Waltham et al. (this volume) in a mathematical ever, we do not know of examples of gradual movement of the model. They conclude that volcanic loading can make a contri- arc with increasing age of the magmatic arc, as might be expected bution to basin subsidence in arc settings within ~200 km of the from many arc models. This could mean that slab dip gradually arc, which may be the primary cause. In continental margin arcs declines as the arc-trench gap increases, so that the arc remains in such as Java, and long-lived intraoceanic arcs such as Halmahera, the same position. Obviously this idea is diffi cult to test, as there there is a signifi cant density contrast between the deeper crust is no way of measuring slab dip in the past. The dip of slabs in and the eruptive arc products. In these cases the volcanoes form most Indonesian arcs is typically steep where the slab reaches a load that can produce or contribute to the formation of fl exural >~200 km in depth. The depth to the Benioff zone beneath active basins close to the arc. Elsewhere the absence of deep basins in arcs is also much more variable than previously thought (England arcs may be due to the small density contrast between volcanoes et al., 2004). Perhaps magmatic activity essentially pins the arc in and the underlying crust. one position until it ceases, and then there are a number of factors which could control the location of the resumption of volcanic Sediment Character in Indonesian Arcs activity, including thickness or strength of the crust, preexisting weaknesses in the crust, or changing dip of the subducting slab. All the Indonesian arcs have been in an equatorial position At present, all are conjectures and illustrate how incomplete our throughout the Cenozoic (Hall, 2002), and as a result the arc knowledge of arcs remains. products have formed in a tropical climate. Tropical processes have several effects on rocks and grains within them. Deep tropi- Volcanic Arc Loading cal weathering of well-jointed rocks can lead to a high degree of in situ rounding of material that eventually falls out of the out- Volcanoes exert a load. Since most basic to intermediate crop (Fig. 16). The onion-skin type of weathering of outcrops arc volcanoes are denser than average continental crust, long- is common in Indonesia, and rounded clasts are formed at all lived volcanism should contribute to subsidence. Several of scales from huge boulders to pebbles. The rounded clasts may the Indonesian arcs have deep fi lled basins close to, and both be incorporated in volcaniclastic and sedimentary rocks without in front of and behind, the arc. Few of these basins have been any transport whatsoever. Weathering of material also leads to explored seismically, and their deep structure is unknown. How- rapid destruction of unstable rock fragments and minerals. This ever, there are indications that arc loading may have contributed is particularly important for volcanic-derived material that con- to their formation, because the basin sequences thicken toward tains many unstable minerals and rock fragments such as mafi c the arc, and the timing of basin development is closely linked to minerals, feldspars, and clays. The interpretation of transport his- activity in the arc. As discussed by Nichols and Hall (1991) and tory and maturity based on grain shape and light mineral modes Smyth et al. (this volume), the basins are not what are normally can be very misleading in tropical settings. Two recent studies of 48 Hall and Smyth

Figure 16. Tropical weathering of rocks, resulting in rounding of clasts before they are even removed from the outcrop. Rounded clasts are produced at all scales from small pebbles to boulders. Both photographs show rocks exposed by quarrying of the Pendul Diorite, Jiwo Hills, East Java. sedimentary rocks deposited close to active margins in the Indo- refl ects weathering rather than provenance. Apatite is stable dur- nesian region illustrate the effects of tropical weathering (van ing burial but is susceptible to acidic weathering, which is com- Hattum, 2005; Smyth, 2005). In both cases the tectonic setting mon in humid tropical settings (Morton, 1984). Plots of chemical is known, but if these examples had come from much older oro- indices of weathering and alteration (Fig. 17) show that very few genic belts the interpretation could well be very different. of the sandstones are similar in composition to the fresh source There was subduction of the proto–South China Sea beneath rock (van Hattum, 2005). The composition has changed between north Borneo (Hall, 2002; Hazebroek and Tan, 1993; Hutchi- erosion and deposition. The tropical setting of the sandstones son, 1996; Tongkul, 1991) between the Eocene and early Mio- needs to be considered before provenance can be interpreted. cene. A large sedimentary fan, the Crocker Fan, formed at this In East Java, Eocene to lower Miocene quartz-rich sand- active margin. All sandstones are quartz-rich, and their composi- stones plot on commonly used ternary diagrams as recycled oro- tions plotted on conventional ternary diagrams suggest they are genic or cratonic interior–derived sediments (Fig. 18), but this mature recycled orogenic products, but there are some anomalies interpretation is even more misleading than that for the Borneo (van Hattum et al., 2003; van Hattum, 2005). The oldest sand- sandstones. Examination of quartz grains shows that many have stones show the greatest compositional maturity, and although a volcanic origin, and a volcanic provenance is supported by the younger sandstones plot as recycled orogenic sediments they abundance of fresh euhedral zircons. The sandstones have previ- are less mature, which is inconsistent with their derivation by ously been interpreted as eroded from a continental Sundaland recycling of older sedimentary rocks as suggested by Hutchison source because they are rich in quartz and well sorted, but careful et al. (2000) and William et al. (2003). The sandstone textures examination reveals abundant evidence of their volcanic origin are in marked contrast to their apparent compositional maturity on the basis of textures, light mineral constituents, quartz charac- (van Hattum, 2005; van Hattum et al., 2006). They are immature, ter, clay mineralogy, and zircon character and ages (Smyth, 2005; typical fi rst-cycle sandstones; grains are angular to subangular, Smyth et al., this volume). These quartz-rich rocks are not the and there are few clasts that suggest recycling. The sandstones product of recycling of material derived from continental source are poorly sorted and have a muddy matrix and very low poros- regions but are the result of explosive volcanic events such as the ity, and they contain abundant euhedral and subhedral zircon eruption of crystal-rich magma, followed by air-fall sorting and grains typical of fi rst-cycle sandstones. Subrounded and (rare) subsequent epiclastic reworking (Cas and Wright, 1987; Walker, rounded grains are less abundant. Tourmaline also occurs pre- 1972). In tropical settings, intense weathering rapidly causes the dominantly as unabraded grains. The shapes and lack of abrasion breakdown of labile minerals, mineral aggregates, and lithic frag- of zircons and tourmalines indicate that long-distance transport is ments. The redeposited sediment will be rich in resistant minerals unlikely. Zircon and tourmaline typically make up >70% of the such as quartz and heavy minerals like zircon, and it has a higher heavy mineral assemblages in the Crocker Fan sandstones. Their percentage of quartz per unit volume than the source material. abundance indicates erosion from acid plutonic rocks, and their The high rates of weathering observed in tropical settings shapes suggest a nearby source area. Apatite would normally also can also have a signifi cant infl uence on the preservation of be abundant in material derived from acid plutonic rocks, but it unconsolidated volcanic deposits. The potential for preserving is commonly absent, and those grains present are typically pitted volcaniclastic deposits on steep terrestrial slopes is low unless or partially dissolved. This suggests that the abundance of apatite they are rapidly overlain by lavas or buried under younger arc Cenozoic arc processes in Indonesia 49

100 100

90 95 Increasing chemical maturity

80 90

weathered 70 85

+CaO+Na O)] 80 Strongly HUMID 50 2 75

SiO %

23 2340 2 70

30 65 Fresh (source) rock ARID 20 60

CIW (100)[Al O /(Al O 10 55 Strongly altered 0 50 0 102030405060708090100 0 5 10 15 20 25

CIA (100)[Al23 O /(Al 23 O +CaO+Na 2 O+K 2 O)] Al23 O +K 2 O+Na 2 O

Figure 17. (A) Chemical index of weathering (CIW; Harnois, 1988) plotted against chemical index of alteration (CIA; Nesbitt and Young, 1984).

(B) Chemical maturity, expressed as a function of percentage of SiO2 and total percentage of Al2O3 + K2O + Na2O. After Suttner and Dutta (1986) for Sabah sandstones from van Hattum (2005). deposits. The breakdown and recycling of soft, nonwelded ashes inaccessible areas. An extremely large eruption, possibly on the can be very rapid in terrestrial settings with intense precipita- scale of Toba, terminated the Eocene to early Miocene phase of tion. Lahars are common hazards on many of Java’s volcanoes arc activity in the Southern Mountains of East Java (Smyth, 2005) such as Merapi (Lavigne et al., 2000). Preservation is more likely at ca. 20 Ma, and other such events will surely be recognized in if the volcanic material is deposited in a marine setting below the future. The tropical setting means that some features may be the storm-wave base, but even this material may be apparently typical only of tropical arcs—for example, the rapidly enhanced more mature than would be expected in a nontropical environ- maturity of sandstones and enrichment in quartz in volcano- ment because of eruptive sorting of material in the ash cloud. genic debris deposited in arc basins. Nonetheless, in ancient arcs The East Java sandstones are well sorted, quartz-rich, and clearly it is still necessary to consider the likely climatic setting before deposited in shallow marine settings, but they were derived from interpreting provenance, both in terms of transport distances and volcanic material produced by explosive eruptions that occurred source regions. a short period before their deposition. We know of other quartz- Other features, such as the importance of extension in arcs, rich sandstones in Indonesia that we and others have previously the infl uence of volcanic arc loading on sedimentary basin devel- interpreted as having a continental provenance, and our experi- opment, the critical role of hinge movement in magmatism, and ence of sandstones in Borneo and East Java now causes us to the weakness of the arc, have more general applicability. Certain consider more carefully if some of these interpretations may have features of Indonesian arcs could also be of general relevance but been wrong. Quartz character and heavy mineral studies (Smyth, could simply refl ect particular differences between tectonic pro- 2005; Smyth et al., this volume) are of particular importance for cesses in SE Asia and those of other arc regions—for example, reassessment. In older orogenic belts, similar misinterpretations the absence of a relationship between arc-trench gaps and dura- could easily be made, especially if the ancient climate is not con- tion of magmatic arc activity, emplacement of ophiolites, and for- sidered or known. mation of mélanges. The long history of subduction in the region has resulted in a thin, warm, and weak lithosphere, and although CONCLUSIONS: LESSONS FOR ANCIENT ARCS the region is not typical of continental crust it does share many features with other subduction zone backarcs (Hyndman et al., The Indonesian arcs differ in many ways from those com- 2005). The behavior of Indonesian arcs may be more representa- monly portrayed in many arc models. Are the Indonesian arcs typi- tive of arcs in the upper plate of long-lived subduction margins cal, or are they one member of a spectrum of arcs? The absence of in which the lithosphere becomes unusually responsive to small well-documented major eruptions in the Cenozoic stratigraphic changes in plate boundary forces. record of Indonesian arcs is likely to refl ect their tropical setting Indonesian arcs appear to have had relatively short lives and the lack of detailed studies in diffi cult terrain and relatively in comparison with many ancient arcs, but this is probably a 50 Hall and Smyth

A Q B CRATON Continental Block INTERIOR Recycled Orogen TRANSITIONAL Magmatic Arc CONTINENTAL Q - Total free quartz F - Feldspar L - Total lithic RECYCLED fragments OROGEN DISSECTED ARC

T UPLIFT

TRANSITIONAL ARC UNDISSECTED BASEMEN ARC 1mm FL

C D

200µ m 200µ m

Figure 18. (A) Quartz sandstones from East Java plotted on a QFL ternary diagram, which could be interpreted as indicating a reworked continental orogenic provenance. The sandstones are actually ﬁ rst-cycle products of rapid reworking of volcanogenic material. Examples of volcanic quartz grains from the Jaten Formation, Pacitan, East Java. (B) Bipyramidal quartz grain. (C) Scanning electron microscope (SEM) image of a quartz shard. (D) SEM image of rounded melt embayments in quartz.

reﬂ ection of their young age and the greater ease with which time and may ultimately leave little trace in the stratigraphic events of different arcs can be resolved in the relatively recent record. Even in the young arcs of Indonesia, dating of events is past. Pre-Cenozoic arcs with lives of tens of millions of years not good, and more and better dating is required to understand may include several phases of arc activity, and several different and interpret arc processes. We also need to know more about arcs. The complexity of tectonic evolution in the region shows the deep structure of arcs. For example, very little is known that a great deal can happen in a 45 m.y. time span, and much about the crust between the arc and trench in any Indonesian of this complexity could be missed in the study of ancient arcs. arc, and almost nothing is known of the thickness and character This could be an explanation of the apparently compressional of the crust beneath the volcanic arc. Most arcs in the western arc; short-lived contractional events in older arcs may appear Paciﬁ c are similarly poorly known. At the very least, the record to be contemporaneous with arc magmatism when in fact they of the Indonesian arcs shows that some features of arc models punctuate arc activity. The history of the Neogene Halmahera need to be questioned and that our understanding of tectonic Arc shows that an arc can be formed and destroyed in a short processes in volcanic arcs needs to be improved. Cenozoic arc processes in Indonesia 51

ACKNOWLEDGMENTS Bijwaard, H., Spakman, W., and Engdahl, E.R., 1998, Closing the gap between regional and global travel time tomography: Journal of Geophysical Research, v. 103, p. 30,055–30,078, doi: 10.1029/98JB02467. Our knowledge of Indonesian arcs has been acquired through Bock, Y., Prawirodirdjo, L., Genrich, J.F., Stevens, C.W., McCaffrey, R., Sub- many projects carried out by the SE Asia Research Group at arya, C., Puntodewo, S.S.O., and Calais, E., 2003, Crustal motion in Indo- London University and Royal Holloway by many different nesia from Global Positioning System measurements: Journal of Geo- physical Research, v. 108, doi:10.1029/2001JB000324. people. We thank all of them. Our work has been supported at Busby, C.J., and Ingersoll, R.V., eds., 1995, Tectonics of Sedimentary Basins: different times by the University of London Central Research Cambridge, Massachusetts, Blackwell Science, 579 p. Fund, the Natural Environment Research Council (NERC), and Calvert, S.J., 2000, The Cenozoic geology of the Lariang and Karama regions, Western Sulawesi, Indonesia [Ph.D. thesis]: University of London, 353 p. the Royal Society, but mainly by a consortium of oil compa- Cardwell, R.K., and Isacks, B.L., 1978, Geometry of the subducted lithosphere nies whose membership has changed with time. We have been beneath the Banda Sea in eastern Indonesia from seismicity and fault fortunate with help and support from colleagues in Indonesia plane solutions: Journal of Geophysical Research, v. 83, p. 2825–2838. Cardwell, R.K., Isacks, B.L., and Karig, D.E., 1980, The spatial distribution at the Geological Research and Development Centre Bandung of earthquakes, focal mechanism solutions and subducted lithosphere in (now the Geological Survey of Indonesia), Lemigas, Indone- the Philippines and northeast Indonesian islands, in Hayes, D.E., ed., The sian Institute of Sciences (LIPI), and Institut Teknologi Band- Tectonic and Geologic Evolution of Southeast Asian Seas and Islands: American Geophysical Union, Geophysical Monograph 23, p. 1–35. ung. We thank Tim Charlton, Ron Harris, and Dave Scholl for Cas, R.A.F., and Wright, J.V., 1987, Volcanic Successions: London, Allen and helpful comments on an earlier version of the paper, and Wim Unwin, 528 p. Spakman for help in producing Figure 7. Charlton, T.R., 2000, Tertiary evolution of the Eastern Indonesia collision complex: Journal of Asian Earth Sciences, v. 18, p. 603–631, doi: 10.1016/ S1367-9120(99)00049-8. 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