This paper synthesizes the tectonic aspects of the results of Leg 124 drilling. We focus on the petrology, structure, stratigraphy, and Paleomagnetism of the cores to evaluate the initial tectonic settings of the basins and their later histories. The type of plate tectonic setting of the middle Eocene Celebes Sea cannot be determined unambiguously, but we do not favor an origin as a fragment of either the Indian Ocean or the west Philippine Sea plates. We cannot exclude an origin as a fragment

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... f the mostly subducted Molucca Sea Plate or a basin rifted from the edge of the east Asian mainland. The Sulu Sea, on the other hand, seems very likely to have formed by back-arc spreading behind the Cagayan Ridge during a short time interval in the early Miocene. Cessation of spreading in the Sulu Sea and volcanic activity on the Cagayan Ridge were coeval, possibly related to collision between the Palawan and Cagayan ridges. 1 Silver, E. A., Rangin, C, von Breymann, M., et al., 1991. Proc. ODP, Sci. Results, 124: College Station, TX (Ocean Drilling Program).