ALONG-STRIKE VARIATION IN STRUCTURAL STYLES ACROSS THE OUACHITA MOUNTAINS: EFFECT OF STIFF LAYER VS. WEAK LAYER RATIO AND MAJOR DETACHMENT SURFACES

Map patterns illustrate a significant change in structural styles from east to west within the Ouachita Mountains. The frontal zone changes abruptly westward from broad faulted folds to narrow fault imbricates. Structural style of the central Ouachita Mountains changes westward from a core area of highly deformed pre-Meramecian rocks (Benton uplift) to broad northwest-vergent faulted folds. Morrowan to Atokan turbidites crop out in the cores of the synclines. In contrast to the central Ouachita Mountains, the southern Ouachitas change eastward from a core area of deformed pre-Meramecian rocks (Broken Bow uplift) to north-vergent broad faulted folds. The Mississippian Stanley Group turbidites which separate the eastern and western parts of the central and southern Ouachita Mountains encompasses as a wavelength transition zone. Differences in observed fold wavelengths for different formations result from variation in thickness ratio of stiff-layer versus weak-layer strata across the Ouachita Mountains. The shortest fold wavelengths are in the core areas of the central and southern Ouachita Mountains where thin stiff-layer cherts and sandstones are interbedded with thicker weak-layer shales. Longer wavelength folds are in the eastern part of the frontal and southern Ouachita Mountains, and in the western central Ouachita Mountains. These longer wavelength folds are restricted to the upper Stanley Group through Atoka Formation where thick units of stiff-layer sandstone and interbedded shale turbidites predominate. Several detachment horizons within the Paleozoic sequence affect the overall structure of the Ouachita Mountains. For example, along the eastern frontal belt, the Morrowan Johns Valley Shale (Y-City fault) detachment serves as a south-vergent delamination backthrust, where north-directed allochthonous strata are underplated and overturned southward beneath an extremely thick (> 8500 m) passively uplifted Atoka Formation. In the western part of the frontal zone of the Ouachita Mountains, where the Atoka Formation is much thinner (<1600 m), a lower Atoka Formation shale serves as a backthrust detachment.