THE STRESS FIELD DURING CONTINENT-CONTINENT CLOSURE INFERRED FROM JOINT DISTRIBUTION IN THE OUACHITA BELT AND ARKOMA BASIN

Three structural domains developed in the foreland of the Ouachita orogeny in Oklahoma and Arkansas during closure of South America into North America. To the south, a region of large, internally coherent, monoclinally-dipping thrust sheets known as the central thrust belt abuts a middle, internally-faulted, complexly-folded section known as the frontal imbricate zone, that is, in turn, bounded on the north by the gently folded cover rocks of the Arkoma basin. Fracture data collected from the three structural provinces indicate that in the relatively uniformly-dipping Arkoma basin and central thrust belt, the regional cross-fold joint sets are consistently oriented among outcrops. In contrast, joints in the frontal imbricate zone, where bedding dip domains are of limited extent, are less organized and occupy a larger range of orientations.

Joints in the Pennsylvanian section of the Ouachita belt and Arkoma basin document both regional and local stress fields present during the Ouachita orogeny. Most regionally traceable joints are sub-parallel to the transport direction within the Ouachita fold belt. In the Arkoma basin, cross-fold joint orientations range from 340-360°, in the central thrust belt, 355-010°, and in the frontal imbricate zone, 340-025°. Fringe cracks found within these strike ranges identify a clockwise evolution of stress, except in the southernmost central thrust belt, in which they suggest anticlockwise rotation. This clockwise trend is found on such a large scale that it reflects the convergence of South America upon North America. An initial, pre-folding compression is preserved in fractures throughout the autochthonous Arkoma basin. In the central thrust belt, these signature joints propagated early but were rotated 10-15° clockwise during thrust emplacement. Therefore, we infer that the fit between North and South America during continent-continent closure favored the regional realignment of the crustal-scale stress field in a clockwise manner.

Continued jointing accompanied faulting and folding in the frontal imbricate zone, where joint strike-domains change from the hanging wall to the footwall of faults. In addition, outcrops on fold noses and near thrust faults display a dominant joint set that is normal to the transport direction, indicating a local structural influence.