FIELD-BASED CONSTRAINTS ON THE STRUCTURAL EVOLUTION OF THE BENTON UPLIFT, OUACHITA MOUNTAINS

Understanding the structural development of basement-cored uplifts is critical for advancing our understanding of poorly exposed basement and subsurface terranes in south-central North America. We report on new mapping of uplifts in the Ouachita orogen. The Mississippian-Pennsylvanian Ouachita orogeny occurred when the Sabine arc terrane collided during south-facing subduction, during closure of the Cambrian passive margin of the North American plate. Folds and thrust faults generally verge north due to tectonic transport of an accretionary wedge from south to north. The Benton and Broken Bow Uplifts in the core of this wedge appear antithetic to this deformation, i.e., both features contain a sub-set of south-vergent structures. Field data from the western Benton Uplift in west-central Arkansas are most consistent with S-directed backthrusting, as indicated by asymmetric z-folds defined by the Arkansas Novaculite and interpreted as a record of wedge thickening. Steeply-dipping conjugate joints striking NNW and NNE represent N-S shortening, parallel to the maximum compressive paleostress direction and E-W extension, parallel to the Ouachita orogenic front (and the minimum compressive paleostress direction). These conjugate joints are interpreted as a record of orogen-parallel extension and a consequence of wedge thinning.