Ophiolite emplacement by collision between the Sula Platform and the Sulawesi Island Arc, Indonesia

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Abstract
Much of the tectonic complexity displayed in eastern Indonesia results from a series of Neogene collision events between island arcs, continental fragments, and the Australian continent. Here we examine the emplacement of a large ophiolite belt, resulting from the Miocene collision between the Sulawesi island arc and a continental fragment, the Sula platform. We present the results of several marine geophysical expeditions to the SW Molucca Sea and the NW Banda Sea, plus gravity and geology on the east arms of Sulawesi. The Batui thrust separates the ophiolite from sedimentary rocks deformed along the leading edge of the Sula platform. We mapped this thrust eastward from Sulawesi along the southern margin of the Gorontalo basin. The latter is floored by oceanic crust, and its south edge is uplifted against the thrust. Thus the Sulawesi ophiolite can be traced offshore to its origin as basement of the Gorontalo basin. The ophiolite is composed of harzburgite in the Southeast Arm and passes upward through a complex of gabbros and diabase dikes in the East Arm. Ophiolite melange underlies the harzburgites on the Southeast Arm beneath low‐angle thrust contacts where seen. Our local observations show the melange to be composed of thrust packets of both serpentine and red shale matrix varieties. The packets are several hundred meters thick, and the melange, where studied, has a moderate north to northeast dipping foliation. This orientation, if regionally representative of the melange fabric, is consistent with a significant northward component of movement of the lower plate, probably the Sula platform or its margin. Where the ophiolite is in contact with rocks of the central schist belt, it dips under the schist, but where it encounters melange, or Mesozoic or Paleogene sedimentary rocks, the ophiolite is thrust over them. The tectonic overlap sequence, from west to east, is schist over ophiolite over older sediments or melange. The ophiolite appears to have been emplaced by oblique convergence of the Sula platform along the southern edge of the Gorontalo basin. We suggest that the Gorontalo basin represents a forearc basin and the ophiolite is its basement, analogous to a number of other forearc settings. Deformation of the ophiolite may have occurred in part on the seafloor prior to emplacement, but we feel that much of the deformation occurred during emplacement. The Sulawesi ophiolite is only one of a number of ophiolites in the Indonesian region, each of which has a very different origin and tectonic history.