Oroclines are some of the largest structures affecting Earth's continental crust. However, their mode of formation and 3-dimensional geometry remain poorly understood. Two end-member orocline types are recognized. Progressive oroclines are syn-orogenic, are thin-skinned, characterize thrust belts, and form open, convex toward the foreland, lobate bends. Secondary oroclines are larger, and are thought to be thick-skinned. They can be isoclinal, and occur as coupled orocline pairs. Analogue modelling shows that progressive oroclines develop as a function of indenter shape and vector of motion, as well as reflecting variations in the depth and character of the sedimentary sequence being deformed. Problematically, realistic analogue models have not been able to reproduce the development of secondary oroclines: lithospheric beams are too weak to buckle when placed within a plate tectonic framework with realistic boundary conditions. Given that there are well-described and mapped secondary oroclines, resolving how they form and the plate-tectonic setting that gives rise to them remain key outstanding goals of tectonic analysis. If secondary oroclines are thin-skinned, a reassessment of the geological considerations that are the basis of the thick-skinned interpretation of secondary oroclines is required. It would also call into question the relationship between primary and secondary oroclines.
Stephen T. Johnston (Mon,) studied this question.