OROCLINES, PROGRESSIVE OROCLINES AND PRIMARY CURVED ARCS: INTEGRATING PALEOMAGNETIC AND STRUCTURAL ANALYSIS

For over a century, geologists have recognized the importance and sought the meaning of curved mountain belts. It has become increasingly apparent that the map pattern of an orogenic system is just as critical for understanding fold-thrust belt development as cross-sectional and longitudinal perspectives. With the expansion of deformation analysis into three-dimensions, previous classification schemes have been proposed that are largely based on theoretical geometries of displacement and strain trajectories. While it is possible to measure some components of the total strain field, the complete displacement field is nearly impossible to document. On the other hand, the timing relationship between structural development and vertical-axis rotation of an orogen's "limbs" is often determinable. The most useful tool for estimating vertical-axis rotation to date is paleomagnetism.

We propose a new classification scheme for curved orogens based on the angular relationship between structural trend (orientation of major thrusts and fold axes) and secondary imposed curvature (rotations acquired subsequent to initial thrusting and folding). Oroclines (purely secondary curvature), such as the Cantabrian Arc, are systems in which the structural grain of the fold-thrust belt was initially linear and later bent by rotations about a vertical axis. Progressive oroclines (i.e. the Wyoming Salient) show vertical-axis rotations occurring contemporaneous to thrusting/folding. Finally, primary curved arcs formed with a curvature that was not produced by vertical-axis rotations. Following the identification of the appropriate kinematic classification for a given curved belt, emphasis is placed on qualifying the classification with the mechanism by which curvature was attained (e.g., indenter, buckling, wrenching, etc.).