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Abstract

We present a new stratigraphy for the Togian Islands, Sulawesi, and interpret the age, character and evolution of Gorontalo Bay. At its western end the bay is underlain by continental crust. The central part is underlain by Eocene to Miocene oceanic and arc rocks, although the area south of the Togian Islands could have continental crust of the Banggai-Sula microcontinent thrust beneath this and the East Arm ophiolite. Gorontalo Bay was not a significant deep bathymetric feature before the Miocene. Field relationships indicate a latest Miocene to Pliocene age for inception of the basin. Medium-K to shoshonitic volcanism in the Togian Islands is not due to subduction but reflects crustal thinning and extension in the Pliocene and Pleistocene, causing the underlying mantle to rise, decompress and melt. Extension is continuing today and is probably the cause of volcanism at Una-Una. Volcanic activity migrated west with time and volcanic products have been offset by dextral strike-slip displacement along the Balantak Fault. Extension and subsidence was driven by rollback of the subduction hinge at the North Sulawesi Trench with a possible contribution due to flow of the lower crust.

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