Structural geologists have long debated the tectonic significance of the sinuous map patterns of mountain belt trend lines. The term orocline was originally defined by Carey (1955) to denote map-view curves that developed by bending of an existing linear orogenic belt about a vertical axis of rotation. Although considerable evidence has thus been reported for oroclines, the mechanisms by which these belts acquired their arcuate shape remains disputed.
An arcuate shape can be produced by bending or buckling of a linear object. Bending (flexure) characterises the deformation of a linear object subjected to an external load applied perpendicularly to its long axis, while buckling is the deflection caused by an external load applied parallel to the long axis. A common view is that oroclines develop in response to an along-strike gradient of tectonic forces oriented at a high-angle to the long axis of the orogen. Such bending about a vertical axis can be generated in response to a horizontal pull produced by a sinking, negatively buoyant, lithosphere, or in response to a horizontal push (compression) due to the arrival of an obstacle or indenter in a subduction zone, or a combination of pushing and pulling (e.g., Rosenbaum and Lister, 2004).
An alternative proposition is that oroclines develop by horizontal buckling in response to a tectonic force oriented parallel or sub-parallel to the long axis of an orogen. Suggested tectonic scenarios for such buckling about a vertical axis include escape or extrusion out of a collisional orogen, attempted subduction of a
continental ribbon or orogen oriented at a high angle to the subduction zone, or by margin-parallel drag (e.g., Johnston, 2001; Offler and Foster, 2008; Cawood et al., 2011).
Here we employ three-dimensional analogue laboratory experiments to explore how such buckling may produce an orocline and the geodynamic conditions required for it to occur.
A first series of experiments demonstrates that a crustal ribbon carried by a subducting plate cannot buckle and detach from its mantle root because it weakens and deforms when entering the subduction zone, such that little compressive stress is transferred through the ribbon.
A second series of experiments shows that the aspect ratio of the ribbon impacts the wavelength of buckling and that the experimental tank employed is too small (maximum equivalent length is < 1500 km) to generate multiple buckles.
Finally, a third series of experiments shows that if the plate boundaries surrounding the ribbon resist its horizontal lateral motion, thrusts or strike-slip fault systems may be generated in the ribbon thereby preventing buckling.
We conclude that oroclinal buckling is favoured when a crustal ribbon is pulled by subduction, causing backarc extension. Hence, buckling and bending models for orocline formation are not mutually exclusive but reinforce each other.
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