Ductile flow in the metamorphic rocks of central Sulawesi

Metamorphic rocks exposed along the Palu-Koro Fault of west-central Sulawesi, Indonesia, show abundant evidence of non-coaxial ductile deformation. The deformed rocks include gneisses, amphibolites and schists, that form part of a regionally metamorphosed basement complex of Mesozoic–Precambrian Australian (Gondwanan) origin. In the Palu and Neck regions of Sulawesi, ductile shear fabrics record low-angle westward extension. Further south in the Palu valley, extension is directed towards the south and SW, along with gently-dipping ductile thrust fabrics. Vergence exceptions are common at both outcrop and kilometre scale. Cross-cutting granitic dykes place some constraint on the timing of ductile foliation formation. In the neck region of Sulawesi, it occurred before c . 44–33.7 Ma. In the central and northern Palu valley, to the south, it occurred before 5–3.5 Ma. The timing and orientation of non-coaxial strain precludes its origin as a result of Palu-Koro Fault activity. Instead, ductile flow occurred during either Eocene–Miocene mid-crustal extension above a metamorphic core complex, Cretaceous subduction-related deformation in the over-riding plate, or intracontinental deformation within Gondwana.