Geology of a Miocene collision complex, Buton, eastern Indonesia RANDALL B. SMITH Department of Geology, Sonoma State University, Rohnert Park, California 94928 ELI A. SILVER Earth Sciences Board, University of California, Santa Cruz, California 95064 ABSTRACT rived from uplift and erosion of the Wolio the Miocene collision resulted from oblique, Complex, placing an upper limit on the age of southwest-directed convergence between the The island of Buton in eastern Indonesia is TBP collision. We infer a middle Miocene Pacific and Australian plates. The oblique colli- part of a Neogene collision zone that encom- time of collision, with oblique convergence sion led to development of major, left-lateral passes much of the eastern margin of Su- continuing into late Miocene time. In con- strike-slip faults within the collision zone, such lawesi. Miocene collision of microcontinents trast, sparse biostratigraphic data place the as the Sorong fault system of northwestern Irian with a west-dipping subduction zone em- age of the SP-East Arm collision in the late Jaya. Along these faults, large continental frag- placed the Tukang Besi platform (TBP) Miocene. Separate microcontinents may have ments were removed from the northern margin against Buton, and the Sula platform (SP) collided successively with different parts of of New Guinea and transported westward into against the East Arm of Sulawesi, and thrust the Buton-Sulawesi convergent margin, or a the Banda Sea region. The largest such fragment large ophiolite sheets eastward over the im- single large microcontinent may have been is the Sula platform (Pigram and others, 1985; bricated margins of the platforms. Reconnais- fragmented during oblique collision. Hamilton, 1979), which collided with the Su- sance observations of the resulting collision lawesi arc during Miocene time. Hamilton complex in Buton (Wolio Complex) indicate INTRODUCTION (1979) and Bowin and others (1980) identified that imbricate west-dipping thrust sheets and additional continental fragments, including the overturned folds are the predominant struc- Geologists have long recognized the impor- largely submerged Tukang Besi platform adja- tural element, although later steep faults tance of collision events in the formation of cent to southeast Sulawesi, and a fragment en- offset the imbricate stack and largely control mountain belts. Early plate-tectonic models em- compassing western Seram and Buru in the present map patterns. phasized the role of collisions in which the col- northern Banda arc. On the basis of dredge haul and geophysical data, Silver and others (1985) The Wolio Complex consists of the sedi- liding elements (continental margins, island interpreted a series of northeast-trending subma- mentary Turumbia sequence, peridotite, and arcs) are structurally continuous for great dis- rine ridges in the central Banda Sea as a conti- two distinct groups of metamorphic rocks. tances along the length of the orogenic belt. The nental borderland terrane similarly displaced The Turumbia sequence makes up most of recent development of the terrane concept em- from western Irian Jaya. the eastern part of the Wolio Complex and phasizes that many collision zones have a more consists mostly of deep- water limestone rang- modest extent and commonly involve smaller, These microcontinents have clearly played a ing in age from Late Triassic through late less elongate crustal elements such as oceanic major role in the Neogene development of the Eocene or Oligocene. It is interpreted as a plateaus, microcontinents, and amalgamated arc Banda Sea region. Silver and others (1983b) deep-water facies of the western TBP margin. terranes. Eastern Indonesia (Fig. 1) provides a discussed the geologic effects on the island of Ophiolitic rocks are represented in present geologically recent example of the processes in- Sulawesi resulting from the Sula platform colli- outcrop only by massive peridotite in the volved in terrane displacement and accretion sion. Many aspects of the geology of these mi- western part of the Wolio Complex, but the (Silver and Smith, 1983). The Neogene tectonic crocontinents remain poorly understood, how- compositions of clasts in conglomerates over- evolution of this region has been dominated by a ever, including their original positions along the lying the Wolio Complex indicate that a full series of collision events of various scales. A col- New Guinea margin, the times of their detach- ophiolite succession was exposed to erosion lision is now in progress between the Banda arc ment, their subsequent movement histories, and during the collision. Metabasite and meta- and the northwest Australian margin (Bowin the nature and ages of the structures bounding chert ranging in grade from greenschist to and others, 1980; Karig and others, 1987), and them. amphibolite facies are found locally in fault the Neogene orogenic belt of Papua New Guin- The island of Buton lies between the Tukang contact with peridotite and are interpreted as ea and Irian Jaya has been interpreted as the Besi platform and southeast Sulawesi. Although the remnants of a metamorphic sole devel- result of a similar island-arc/continental-margin much of Buton is blanketed by little-deformed oped at the base of the ophiolite. Pelitic collision during Miocene time (Jacques and Pliocene and Quaternary sedimentary strata, phyllite and quartzite in northeastern Buton Robinson, 1977; Hamilton, 1979). These colli- older deformed rocks that crop out in many probably represent a slice of the continental sions have occurred on the leading edge of the areas provide evidence for a Miocene collision basement of the TBP, for similar rocks have Australian continent during its rapid northward between the Tukang Besi platform and the pre- been dredged from the northeast margin of movement away from Antarctica, which began Miocene accretionary terrane of southeast Su- the TBP and also form the pre-Mesozoic in early Eocene time. lawesi. In this paper, we describe the lithologic basement of the SP. On the basis of regional structural patterns assemblages and structure of the Miocene colli- The middle to upper Miocene Tondo For- within the New Guinea mobile belt in Irian sion complex of Buton. Our studies support mation in Buton consists of clastic strata de- Jaya, Dow and Sukamto (1984) postulated that Hamilton's (1979) inference that the Tukang Geological Society of America Bulletin, v. 103, p. 660-678, 15 figs., 1 table, May 1991. 660 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/103/5/660/3381214/i0016-7606-103-5-660.pdf by guest on 28 September 2021 MIOCENE COLLISION COMPLEX, EASTERN INDONESIA 661 Figure 1. Generalized tectonic map of the Banda Sea region, showing the location of Buton and the inferred continental fragments derived from the northern margin of New Guinea. Modified from Hamilton (1979), incorporating data from Silver and others (1983b, 1985). Besi platform is a continental fragment and indi- Katili (1978), and Sukamto (1978). The central plete, north-topping ophiolite is present (Silver cate that its collision with southeast Sulawesi belt appears to be an accretionary complex and others, 1983a). No radiometric ages are probably began during middle Miocene time. formed during Cretaceous and Paleogene time available for the ophiolite, and no oceanic sedi- This is substantially earlier than the late Mio- (Hamilton, 1979). In contrast, the eastern and ments have been found in depositional contact cene emplacement of the Sula platform against western belts record a younger, Neogene episode with it. As a result, the age of formation of the the East Arm of Sulawesi, and we examine al- of westward-dipping subduction. Neogene arc ophiolite is constrained only to be older than the ternative kinematic models for these diachro- magmatism in the western belt commenced dur- Miocene clastic strata that overlap it and thus nous microcontinent collisions. ing late early Miocene time, continuing into the postdate its structural emplacement. Gravity Pliocene and sporadically into the Pleistocene data over the ophiolite show it to thicken west- GEOLOGIC SETTING OF BUTON (Sukamto, 1978; Hamilton, 1979). The imbri- ward and dip beneath the central schist belt cated eastern belt is interpreted as a Neogene along a major thrust fault (Silver and others, Buton is part of a zone of late Tertiary defor- accretionary complex formed by westward- 1978,1983a). mation encompassing much of eastern Sulawesi. dipping subduction and by partial underthrust- In the eastern half of the East Arm, the ophio- Sulawesi can be divided into a western Creta- ing of the Sula and Tukang Besi platforms. lite is thrust over and intersliced with Triassic ceous and Tertiary volcanic arc (Fig. 1), a The eastern belt includes the East Arm and through lower Miocene sedimentary rocks in a central belt of schist and highly deformed sedi- eastern part of the Southeast Arm of Sulawesi, northwest-dipping imbricate thrust complex. mentary rocks, and an eastern belt dominated by and we infer that it extends southward across a This sedimentary sequence is made up of deep- an ophiolite complex imbricated with sedimen- submerged shelf to Buton. On Sulawesi, much water and shelf limestones and represents the tary rocks and mélange (Fig. 2). The tectonic of this belt is underlain by a tectonized ophiolite deeper marginal facies of the Sula platform, interpretation of these belts has been discussed complex. In the Southeast Arm, ultramafic which was underthrust beneath the ophiolite by Silver and others (1983a), Hamilton (1979), rocks dominate, but in the East Arm, a com- during the Miocene collision (Hamilton, 1979; Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/103/5/660/3381214/i0016-7606-103-5-660.pdf by guest on 28 September 2021 Figure 2. Geologic map of the eastern Sulawesi-Buton collision zone. The Sula and Tukang Besi platforms are interpreted as continental fragments that collided with the accretionary terrane of eastern Sulawesi during the Miocene. Land geology modified from Sukamto (1975). Offshore structural features and bathymetry from Silver and others (1983a).