Structural Profiles of Ouachita Mountains, Western Arkansas

ABSTRACT The Ouachita Mountains of Oklahoma and Arkansas are the largest exposure of the Pennsylvanian-age orogen rimming the southern margin of North America. The exposure consists of a thick Carboniferous flysch sequence overlying a thin early Paleozoic deep-water sequence and is generally interpreted to have been deformed in a south-dipping subduction zone. Two balanced cross sections (~ 40 km apart) of the Ouachita Mountains in western Arkansas are presented here, illustrating the regional structural style. Major features of the cross sections include (from north to south) (1) triangle zones along the northern border of the frontal thrust zone produced by imbrication at depth, (2) large-scale (~ 10-km wavelength) fault-propagation folds in the frontal thrust zone, formed primarily above normal faults that offset the basement and act as buttresses at depth, (3) a late-stage basement uplift along the reactivated Johns Valley normal fault system, resulting in the antif ormal structure of the Benton uplift and backthrusts in the northern Benton uplift, and (4) small-scale (1-3 km) heavily faulted folds in the early Paleozoic deepwater rocks exposed in the Benton uplift. Greenschist metamorphism in these rocks is attributed to the estimated 13 km of Carboniferous overburden, which was later eroded. Reconstructions of the late Paleozoic continental margin are made from the two cross sections. The reconstructed shelf-to-slope transition is interpreted to underlie the southern flank of the Benton uplift. Using modern analogs for the across-strike width of the shelf-to-slope transition, a minimum regional shortening estimate of 30-50% (110-155 km) is obtained for deep-water rocks currently exposed in the Benton uplift.