Structural evolution of the Texas Orocline, eastern Australia

The Texas Orocline, with a half wavelength of ~ 120 km, is the largest and most obvious orocline in the southern New England Orogen and is clearly recognised in geological maps and geophysical images. In the area of the orocline, there is a major unconformity between Devonian–Carboniferous metasedimentary rocks (Texas beds) and the overlying Early Permian rift-related basins. Detailed structural mapping shows that units both above and below the unconformity are folded around the orocline, indicating that at least part of the oroclinal deformation has occurred after the deposition of the Early Permian rocks. In addition, Early Permian (298–290 Ma) granitoids are aligned parallel to the oroclinal structure, further indicating that deformation partly occurred during or after the Early Permian. Pre-oroclinal isoclinal folds (F1) and related axial plane cleavage (S1) are well developed in the Texas beds and are curved around the oroclinal structure. Syn-oroclinal structures are characterised by minor kink folds and disharmonic folding (F2) in the core of the orocline. A later phase of kink folding corresponding to a ~ N–S shortening direction does not seem to be related to the orocline, and is interpreted as post-oroclinal deformation. A secondary penetrative fabric parallel to the axial plane of the orocline was not observed, indicating low contractional shortening across the orocline (< 30%). We demonstrate that the observed strain is too low to account for oroclinal bending during dextral transpression, as previously proposed. We suggest an alternative model involving an initial curved structure, probably related to subduction rollback or a pre-existing curvature in the palaeomargin of eastern Australia, which was amplified by dextral transpression and subsequent E–W contraction.