LATE PALEOZOIC OZARK PLATEAU COMPRESSION FOLLOWING DOCKING OF THE OUACHITA BLOCK: ANCESTRAL ROCKIES DEFORMATION IN THE MIDWEST?

Narrow zones of outcrop-scale deformation in Paleozoic strata follow both NE-striking fault/lineaments and NW-striking Late Proterozoic basement tectonic zones from central Missouri to northern Arkansas in the Ozark Plateau (a cratonic platform foreland to the Appalachian-Ouachita orogen). This outcrop-scale deformation includes abundant faults (normal, reverse, and strike-slip), joint sets, conjugate hybrid shear sets, calcite veins, clastic dikes, and folds (typically fault-propagation folds). Cross-cutting relationships and fold tests suggest at least four deformations (Events 1 through 4, chronologically) of Pennsylvanian and older strata in the study area.

Events 1 and 4 indicate a NW/SE greatest in-plane paleostress. This stress trajectory is consistent with reactivation of basement tectonic zones during late Paleozoic southern Appalachian convergence, thus suggesting Events 2 and 3 also occurred during late Paleozoic. Trajectories of greatest in-plane stress suggested by Event 2 structures change smoothly from NNW-SSE in the SW of the study area to NE-SW in the NE of the study area. This trajectory pattern is consistent with a slip-line field associated with docking of the NE corner of the Ouachita block with North America. The NE-striking fault/lineaments follow one set of these slip-lines and thus may have formed at this time. Reverse faults and folds are most common in Event 3, and greatest in-plane stress trajectories are uniformly NE-SW across the study area. Event 3 Ozark deformation links Ancestral Rockies deformation in the SW midcontinent and Illinois Basin deformation in the midwest that show similar NE-SW shortening directions. This compressional event may be related to late Paleozoic convergence along the SW North American plate-margin previously proposed by other authors.