Neogene Extension of the Central North Arm of Sulawesi, Indonesia

Abstract:

The peculiar K-shape of Sulawesi has been the subject of geological interest for many years. The east-west trend of the North Arm is commonly considered to have formed by crustal shortening of an intra-oceanic arc built on Eocene ocean crust. However, recent onshore and offshore geological studies suggest an important role for Neogene extension.

Field mapping and dating of the pre-Neogene basement indicates that Sulawesi’s North Arm constitutes more than a simple oceanic arc. Paleogene granites in this region suggest the basement includes either evolved arc crust or continental crust. LA-ICP-MS U-Pb dating of zircons shows that Paleogene granites have few inherited ages whereas Neogene granites contain Palaeozoic and Proterozoic inherited zircon cores, suggesting melting of Australian continental crust. Biostratigraphic dating of Neogene shallow marine sedimentary rocks on land indicates two distinct periods of sedimentation: Middle Miocene and Late Miocene–Pliocene. Field observations and remote sensing interpretation identify two major fault trends: E–W Neogene basin-bounding faults and young NW-SE strike-slip faults.

Offshore seismic surveys show several major sequences separated by regional unconformities in Gorontalo Bay, south of the North Arm, where there was deposition of more than 6 km of sediments. We correlate the thickest package south of the central North Arm, up to 5 sec TWT, with thick marine reworked volcanic ash deposits of Pliocene age mapped on land in the North Arm, on the Togian Islands in Gorontalo Bay, and in the East Arm of Sulawesi.

We interpret the land and offshore record to indicate arc-continent collision and underthrusting of Australian crust in the Early Miocene (c.22 Ma). There were several subsequent episodes of extension. A metamorphic core complex formed on land in the Middle Miocene (c.15 Ma), and later renewed extension was linked to initiation of southward subduction of the Celebes Sea in the Late Miocene-Early Pliocene (c.5 Ma).