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Geologists have long recognized the importance and sought the meaning of curved orogenic belts. Over the last few decades, several classification schemes have been proposed for curved orogens that have been largely based on the relationship between, and geometry of, displacement and strain trajectories. However, determination of strain trajectory paths and the complete displacement field of an arcuate orogen is difficult at best. It is often possible to measure only one component of the total strain field, but virtually impossible to measure the complete displacement field. In addition, many of these classification schemes have changed or modified original definitions for some key terms, most notably the word orocline, which has propagated confusion in the literature (e.g., the Alaskan and Bolivian oroclines).

To avoid some of the ambiguity and confusion associated with classifying curved belts, we propose a new classification scheme based on the angular relationship between structural trend or grain (orientation of major thrusts and folds) and secondary imposed curvature (rotations acquired subsequent to initial thrusting and folding). In this manner, classification of curved belts can be simplified into three broad categories: (1) oroclines, (2) progressive arcs, and (3) primary arcs. Oroclines are those orogens that were originally linear and were curved during a subsequent deformation event. Progressive arcs develop their arcuate nature contemporaneously with growth of the belt. Primary arcs are those orogenic systems that inherit curvature during initial deformation and experience no appreciable tightening during subsequent deformation. The one criterion for ascertaining whether a curved orogen developed through a primary, secondary, or progressive mechanism is the temporal and spatial relationship between the deviation in structural trend and the vertical-axis rotation that took place within the belt. At present, the most useful geologic technique for determining such a relationship, and hence the kinematic classification (primary, progressive, or secondary), of a curved orogen is the combination of paleomagnetism and detailed structural analysis. Following identification of the appropriate kinematic classification for a given curved belt, emphasis should be placed on qualifying that classification with the mechanism by which curvature was attained (e.g., indenter, buckling, wrenching, etc.). In this way, the kinematics of deformation is separated from the mechanics of the process, which can often be described by more than one kinematic model.

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