Structural evolution of the early Permian Nambucca Block (New England Orogen, eastern Australia) and implications for oroclinal bending

The Paleozoic to early Mesozoic southern New England Orogen of eastern Australia exhibits a remarkable ear-shaped curvature (orocline), but the geodynamic processes responsible for its formation are unclear. Oroclinal bending took place during the early Permian, simultaneously with the deposition of the rift-related Sydney, Gunnedah, and Bowen basins, which bound the oroclines to the west. The Nambucca Block is another early Permian rift basin, but it is situated in the core of the oroclinal structure. Here we present new stratigraphic, structural, and geochronological data from the Nambucca Block in an attempt to better understand its tectonic history and relationships to the formation of the oroclines. We recognized four phases of folding and associated structural fabrics (S1-4), with the second phase (S-2) dated at 275-265 Ma by 40Ar/39Ar geochronology of muscovite. This age overlaps with independent constraints on the timing of oroclinal bending, suggesting that the earlier two phases of deformation in the Nambucca Block (F-1 and F-2) were associated with orocline formation. We propose that oroclinal bending involved three major stages. The first stage (<300 Ma) was associated with variations in rates of trench rollback and formation of rift basins in a hot extensional back-arc setting. This was followed by a second stage of oroclinal bending, possibly linked to dextral wrench faulting, which involved similar to N-S contraction (F-1). Subsequent deformation at 275-265 Ma involved formation of nappe-style structures (F-2). This phase of contractional deformation may have resulted from an increased plate coupling that was possibly linked to flat-slab subduction.