Kinematics of orocline and arc formation in thin‐skinned orogens

The regional traces of folds, faults, and foliations found in many fold‐thrust belts are bent in plan view. It proves valuable to distinguish between two types of bent orogens based on their kinematic evolution: oroclines (or rotational arcs) are bent orogens in which segments of the orogen change strike during the evolution of the bend, and nonrotational arcs are bent orogens in which segments of the orogen do not change strike during development of the bend. The kinematic evolution of a bend that forms in a thin‐skinned orogen can be described in terms of three parameters: the displacement path trajectories followed by points along the strike of the orogen, the magnitude and distribution of tangential extension along the strike of the orogen, and the change in position of the endpoints of the orogen with respect to a reference line. Only slight differences in the displacement path trajectory pattern determine whether an orogen evolves as a nonrotational arc or as an orocline; thus the distinction between these two types of bends is not always of major tectonic significance. Compressional deformation along irregular continental margins or the impact of indentors on continental margins during collisional orogenies more likely leads to formation of nonrotational arcs (as can be simulated with a sand wedge model). Interaction between a fold‐thrust wedge with obstacles in the foreland or with a wrench fault more likely leads to orocline formation. Orocline formation is also associated with noncoaxial reactivation of thrust faults. Tangential extension is a necessary consequence of certain displacement path trajectory patterns and can accompany development of either nonrotational arcs or oroclines. Two examples, one from the Umbrian Apennines and one from the Makran Range, demonstrate how movement on cross‐strike fault arrays can accommodate tangential extension in thin‐skinned bends.