TECTONIC DEFORMATION IN THE MID-CONTINENT OF THE UNITED STATES: EVIDENCE FOR PERICLINAL BUCKLE FOLDING IN THE OZARK PLATEAU

A series of NE-SW trending folds in sandstones of the Early Ordovician Roubidoux Formation are exposed in road cuts, over a distance of approximately 10 km, along US highway 63 in the northern part of the Salem Plateau, central Missouri. These folds are remarkable as they record a period of localized, moderate-strain, which is in marked contrast to the sub-horizontal, adjacent and nearby exposures of Ordovician strata (e.g., Gasconade Fm., Jefferson City Fm., Cotter Dolomite). Initial hypotheses proposed these folds are the result of either karst-related deformations or far-field tectonic forces. This study presents the results of an integrated detailed field analysis of 41 folds and geomechanical model to constrain their origin. Detail study of folds from three selected mapping areas display a “Basin & Dome” pattern. Stereographic data show two orientations where the major one is at 330^o and the minor one is 059^o in average. Those trends of the shortening directions are recorded in all those areas, which is consistent with the presence of “basin & dome”. Also, several patterns are noticed in stereograms: partial great circles, partial small circles, and hooks and maybe mistakenly interpreted as “conical folds”. However SCAT analyses in conjunction with traditional methods indicates these folds are best interpreted as periclines.

This study proposes that the Roubidoux folds are formed by a single shortening event driven by transpression. Two faults the Bradford Branch fault and the Licking fault bound the mapped area to the north and south. Finite element analyses with left-lateral strike-slip motion along these faults shows transpression in a restraining jog involving these faults results in a localized shortening direction at 335^o consistent with the orientation axial surfaces of the periclines. Numerical models also indicate that viscosity contrasts between formations are sufficient to form periclines. Detailed field work combined with numerical modelling is consistent with the formation of these folds as a result of reactivation of basement faults with left lateral slip during a single deformational event, possibly during the late Paleozoic, to form periclinal folds.