Abstract

Rocks around the western plunge of the Benton uplift in the Ouachita Mountains of western Arkansas show multiple periods of deformation during the Ouachita orogeny. Seismic-reflection interpretations and surface geology are consistent with a thick section of highly deformed Paleozoic rocks that are separated as thrust sheets by north-vergent regional-scale thrust faults. North-vergent folds develop in such a setting; however, south-vergent folds with the axial planes dipping opposite to the direction of underthrusting are also observed on the Benton uplift. Development of such folds has been explained by models such as mechanical decoupling along zones of low shear strength in trenches, backthrusting, and backfolding, but none explains the south-vergent folds of the Benton uplift, mostly because of lack of adequate field data. Geometrical analyses show that reactivation of thrust faults during a secondary phase of deformation tightened and reoriented open folds of an initial phase and, as a result, developed the macroscopic and mesoscopic antivergent folds in the Benton uplift. Curvilinear map traces of the thrust faults and broad open folds that refold earlier structures indicate that there was continuous deformation after the development of antivergent folds.

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