Anatomy of a Curved Orogen: The Broken River Orocline in the Northeastern Tasmanides (Australia)

Oroclines record crustal and lithospheric‐scale processes in active plate margins. In northeastern Australia, the Broken River Orocline is marked by the curvature of structural fabrics and tectonic elements. However, the geometry, structural evolution, and tectonic implications of this orocline are poorly understood. To address these issues, we conducted structural investigations in the inner zone of the orocline. We found that pre‐oroclinal structures (isoclinal folds, axial‐surface cleavage, and reverse faults) are bent around a NE–SW axial surface. Syn‐oroclinal NE‐trending folds are present but are not accompanied by an axial‐plane fabric. Post‐oroclinal deformation includes N–S folds, NW‐striking normal faults, and quartz veins. Based on field relationships, we suggest that the orocline formed during the late Devonian, and that pre‐oroclinal structures correspond to contractional deformation within a Devonian accretionary complex. The absence of syn‐oroclinal axial surface cleavage supports the idea that bending, rather than buckling, was the dominant mechanism of orocline formation. Post‐oroclinal extensional deformation occurred during the latest Carboniferous–early Permian, and was likely associated with backarc extension that affected eastern Australia during this period. The broader expression of the Broken River Orocline is evident by plan‐view curvatures of other tectonic features (e.g., basement blocks, Silurian–Devonian granitoids, and the lithospheric architecture), suggesting it reflects lithospheric‐scale processes likely driven by the mobility of the plate boundary. The curvature also coincides with the transition between non‐retreating (northern) and retreating (southern) subduction segments, raising the possibility that it formed to accommodate kinematic contrasts arising from along‐strike variations in trench migration rates.