Geology of a Miocene collision complex, Buton, eastern Indonesia

The island of Buton in eastern Indonesia is part of a Neogene collision zone that encompasses much of the eastern margin of Sulawesi. Miocene collision of microcontinents with a west-dipping subduction zone emplaced the Tukang Besi platform (TBP) against Buton, and the Sula platform (SP) against the East Arm of Sulawesi, and thrust large ophiolite sheets eastward over the imbricated margins of the platforms. Reconnaissance observations of the resulting collision complex in Buton (Wolio Complex) indicate that imbricate west-dipping thrust sheets and overturned folds are the predominant structural element, although later steep faults offset the imbricate stack and largely control present map patterns.The Wolio Complex consists of the sedimentary Turumbia sequence, peridotite, and two distinct groups of metamorphic rocks. The Turumbia sequence makes up most of the eastern part of the Wolio Complex and consists mostly of deep-water limestone ranging in age from Late Triassic through late Eocene or Oligocene. It is interpreted as a deep-water facies of the western TBP margin. Ophiolitic rocks are represented in present outcrop only by massive peridotite in the western part of the Wolio Complex, but the compositions of clasts in conglomerates overlying the Wolio Complex indicate that a full ophiolite succession was exposed to erosion during the collision. Metabasite and metachert ranging in grade from greenschist to amphibolite facies are found locally in fault contact with peridotite and are interpreted as the remnants of a metamorphic sole developed at the base of the ophiolite. Pelitic phyllite and quartzite in northeastern Buton probably represent a slice of the continental basement of the TBP, for similar rocks have been dredged from the northeast margin of the TBP and also form the pre-Mesozoic basement of the SP.The middle to upper Miocene Tondo Formation in Buton consists of clastic strata derived from uplift and erosion of the Wolio Complex, placing an upper limit on the age of TBP collision. We infer a middle Miocene time of collision, with oblique convergence continuing into late Miocene time. In contrast, sparse biostratigraphic data place the age of the SP-East Arm collision in the late Miocene. Separate microcontinents may have collided successively with different parts of the Buton-Sulawesi convergent margin, or a single large microcontinent may have been fragmented during oblique collision.