Abstract

In a number of curved fold and thrust belts worldwide, such as the Cantabrian arc and the Wyoming-Idaho salient, paleomagnetic data indicate vertical axis rotations inconsistent with structural findings. This apparent conflict is especially pronounced in the Pennsylvania salient, where the paleomagnetically defined vertical axis rotations are opposite of the rotation sense indicated by structural studies. We resolve this apparent disparity by developing an integrated and kinematically admissible model that has implications for other curved mountain belts. The curvature of the Pennsylvania salient may be explained by deformation partitioning in an initially laterally tapered basin between the Cambrian-Ordovician carbonates and the overlying siliciclastic rocks. Earliest deformation occurred in the lowest strata and progressed toward the foreland and up section with time. Lateral differences in initial layer-parallel shortening in the Cambrian-Ordovician carbonates caused differential translation and rotations about a vertical axis in the cover sequence. Because of initial basin geometry, the orogenic wedge developed a lateral taper. With further shortening in the wedge and involvement of the cover rocks in the fold and thrust deformation, gravitational driving forces became more important, and the paleostress trajectories diverged in opposite directions on the two arms of the salient in response to the lateral taper.

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