Numerical models of two-dimensional buckling and bending mechanisms and implications for oroclines

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The question of whether oroclines form by orogen-parallel buckling or orogen-perpendicular bending is a matter of debate. To address this problem, we conducted two-dimensional numerical models that investigate differences in the characteristics of oroclines caused by buckling, bending or a combination thereof. Our models consider a simple setup of a viscous layer embedded in a less viscous matrix and subjected to buckling and bending conditions. Models were run with varied layer thicknesses and viscosity contrasts. We applied a spectrum of deformation regimes, with pure buckling and pure bending as end members. Buckling models produced significant layer compression in inner fold hinges, whereas bending models produced widespread extension. Fold geometry also differed between buckling and bending models, as observed through relationships in fold amplitude, fold width, layer thickness, and interlimb angles. A comparison of the numerical results with three real-world examples (Kazakhstan, Gibraltar and Cantabrian oroclines) reveals significant similarities in geometrical features (particularly for the Kazakhstan and Gibraltar oroclines), and some discrepancies. The models show that geometric relationships, such as amplitude-width, thickness-width, thickening, and interlimb angles, provide limited information on the deformation. However, the distribution of stress across the fold hinge and limbs is a primary factor that might be used to understand whether oroclines formed by buckling or bending.