FLEXURAL EXTENSION, REACTIVATION OF BASEMENT WEAKNESSES, AND INCIPIENT INVERSION IN THE FORELAND OF THE LATE PALEOZOIC OUACHITA OROGENY: BUFFALO NATIONAL RIVER REGION OF NORTHWESTERN ARKANSAS

The Buffalo National River (BNR) area in northwestern Arkansas on the southern flank of the Ozark dome is located 100 km north of the frontal thrusts of the late Paleozoic Ouachita orogenic belt. Faulting and folding of Ordovician through Pennsylvanian rocks records flexural extension and incipient inversion that developed on the uplifted foreland flank of the Laurentian continental margin. Geologic mapping and fault-slip analyses indicate that most faulting in the BNR area accommodated north-south (NS) extension that developed as the southern margin of Laurentia was flexed beneath the encroaching load of the Ouachita belt, probably during Middle Pennsylvanian. East-west-trending normal faults are common and many formed grabens, with down-to-south faults typically having greater offset. Transversely oriented northeast- and west-northwest-trending structural zones also formed and host arrays of discontinuous, often en echelon normal and strike-slip faults and linking monoclinal folds. These transverse zones likely reactivated underlying basement fault zones to laterally transfer flexural extension. Extension directions determined from faults in the northeast- and west-northwest-trending zones are systematically rotated counterclockwise or clockwise, respectively, from the overall NS extension direction, consistent with refraction of transtensional instantaneous strains over reactivated basement weaknesses.

Sparse, small reverse and strike-slip faults are recognized that accommodated NS shortening and overprint older structures. Reverse faults were only recognized in two areas (Sneeds Creek dome and north of St. Joe, AR) that also include open folding. Both areas are located near the intersections of northeast- and west-northwest-trending structural zones and are interpreted to reflect incipient inversion of older structures due to stresses concentrated at fault-block corners during final closure of the Ouachita orogenic belt. Other areas with N-S shortening are recorded by a younger phase of strike-slip faulting localized in northeast-trending zones.

The geology of the BNR area demonstrates how forelands can record the evolution of a plate-margin orogeny and how preexisting basement weaknesses may influence foreland deformation.