Publications

Gorontalo Bay is a deep inter-arm basin in northern Sulawesi. It is surrounded by mountains up to 2-3 km high formed by metamorphic complexes and granitoid intrusions that can be traced from the Central North Arm to the Neck and further south into mid Central Sulawesi. These contrast with the bay which reaches depths of 2 km with sediment accumulations beneath the seabed of several kilometres thickness. Most of the metamorphic rocks have previously been interpreted as Mesozoic or Palaeozoic basement. Late Miocene to Pliocene granitic igneous and volcanic rocks crop out in the North Arm and in Central Sulawesi. Combination of several geochronometers (U-Pb, Ar-Ar, (U-Th)-He) shows rapid, near-isothermal Pliocene exhumation of these granitoids in the Neck region. Almost all metamorphic rocks are Neogene. Neogene metamorphic core complexes have been identified in the North Arm and to the south of Gorontalo Bay in the Tokorondo and Pompangeo Mountains and suggest an extensional setting. Low-temperature thermochronology on apatites from granitoid rocks in the Neck reveals rapid exhumation rates of 0.75–0.9 mm/yr that indicate removal of c. 2 km of upper crust since the Middle Pliocene. Thermochronological systems with higher closure temperatures (Zr-He, Ar-Ar) suggest even higher exhumation rates, and therefore also higher erosion rates, of c.1-4 mm/yr. Young and rapid uplift is supported by sediment sequences on land including the Plio-Pleistocene Celebes Molasse on the west side of the Neck and the Middle to Upper Miocene Dolokapa Formation in the central North Arm. These sediments are suggested to be correlated with much thicker sequences offshore in Gorontalo Bay. Rapid uplift on land with contemporaneous subsidence offshore is interpreted to have resulted from extension driven by clockwise rotation of the North Arm associated with northward rollback of the North Sulawesi subduction zone.