THE INDIAN CREEK FAULT ZONE: A LATE PALEOZOIC OBLIQUE RIFT IN NORTHERN ARKANSAS

Faults have long been recognized in the southern Ozark dome of northern Arkansas, but their detailed geometry and kinematics have received less study. New geologic mapping in Ordovician-Pennsylvanian strata of the western Buffalo River area has identified numerous structures that accommodated N-S extension associated with late Paleozoic flexure of the foreland in response to loading of the southern continental margin by the Ouachita orogenic belt. As an example, the Indian Creek fault zone is a 2-km-wide, 11-km-long, N60°W-trending zone composed of seven, right-stepping en echelon faults that are well exposed in the western Buffalo River area. Individual faults within this zone are 1.5- to 6-km long, strike N70°-75°W, and have dips ranging from 50° to 85° north or south. The faults are downthrown toward the center of the zone and have throws as great as 100 m. Within the zone, maximum down drop has occurred in the Jim Bluff graben, whose strata are about 200 m and 140 m lower than correlative Paleozoic strata on adjacent northern and southern flanks of the zone, respectively.

En echelon fault patterns in plan view are commonly associated with strike-slip fault zones, yet for the Indian Creek zone, kinematic indicators such as fault-plane slickenlines and conjugate small faults within adjacent strata document normal, dip-slip motion for the individual faults. These en echelon normal faults mimic geometries produced in physical models of oblique rifts extended at low angles (15°-30°) to their normal. The Indian Creek fault zone coincides with a WNW-trending magnetic gradient in regional aeromagnetic surveys that probably reflects a lithologic contrast in buried Precambrian basement. Together, these relations suggest that the Indian Creek fault zone formed as an oblique rift over a WNW-trending basement weakness in response to N-S extension.