THREE-PHASE LATE PALEOZOIC DEFORMATION ON THE SOUTHERN FLANK OF THE OZARK DOME, NORTHERN ARKANSAS

Geologic mapping in the Buffalo River region of northern Arkansas documents three phases of deformation that affected this foreland area of southern Laurentia. The oldest phase, D1, accommodated ENE shortening in the 4-km-long, east-vergent Kyles Landing anticline. A latest Mississippian-earliest Pennsylvanian growth of this fold is documented by strongly eroded Chesterian strata at its crest beneath unfolded Morrowan strata. The NNW-trending Carrollton dome, with a thrust(?) thickened section of Mississippian Boone Formation in a 1957 gas well, also probably formed during D1; flanking lower Pennsylvanian strata are much less deformed. Younger deformation, D2, formed widespread W- to WNW-striking normal faults, NE-striking dextral strike-slip faults, and variably oriented monoclinal folds. These structures accommodated N-S extension on the southern flank of the Ozark Dome. Phase D2 is principally Middle-Late Pennsylvanian in age because most faults are younger than Morrowan strata but older than probable Early Permian lead-zinc mineralization localized along D2 fault zones. A growth thickening of basal Morrowan strata in the east-west Braden Mountain graben marks a transition between D1 and D2.

Final deformation, D3, reactivated selected D2 fault zones during N-directed shortening. Partial inversion of the S-dipping Compton normal fault during D3 is indicated by overprinting, small conjugate strike-slip faults and by the presence in its footwall of the WSW-trending Sneeds Creek dome cored by a small thrust fault. Silicified D3 faults observed along the NE-trending Ponca lineament are probably synchronous with Early Permian lead-zinc mineralization. Regionally, D3 is indicated by N-S calcite-twin shortening documented in published studies and by a prominent joint set.

In a plate tectonic context, D2 and D3 phases record an evolution of the southern Laurentia margin from Middle Pennsylvanian flexural extension, caused by a growing thrust load of the Ouachita orogeny to the south, to shortening during final Permian closure of this orogenic belt. The origin of D1 shortening is speculative but may reflect strain caused by an Early Pennsylvanian collision of a southwestern crustal promontory of Laurentia, as has been suggested to explain early NE shortening in the Anadarko basin of southern Oklahoma.