New Views on Frontal Thrust Belt of Ouachita Mountains, Arkansas and Oklahoma
Michael B. Underwood
The Ouachita frontal thrust belt is comprised of several north-verging thrusts such as the Y-City, Ti-Valley, Ross Creek, and Choctaw faults. Even though the deformed strata appear to be coeval throughout (upper Morrow to middle Atoka), structural fabrics change markedly along the strike of the thrust belt. Early phase (prethrust) deformation in Arkansas is evident, but it is not pervasive. Instead, the structural style is dominated by regional-scale upright folds and smaller mesoscopic folds, which verge parallel with the thrusts. West of the Arkansas-Oklahoma border, however, a broad zone, termed the "Holson Valley corridor," resembles the older Maumelle chaotic zone (Morrowan) of Arkansas, as well as melange terranes of the circum-Pacific. Structural fabrics include we structure, scaly argillite, isoclinal folds, pinch and swell, and dismembered bedding. Many folds display steeply plunging hinge lines and/or axial planes that are strongly discordant to regional thrust trends. The melange-style deformation is tectonic in origin but clearly predated regional thrusting.
Overall, the frontal thrust belt typifies a transition from subduction-accretion to collisional-foreland tectonics. An abrupt structural boundary (Y-City fault) separates melange style from foreland style through most of Arkansas; this fault likely served as the master decollement along which the Ouachita accretionary complex was abducted onto the southern edge of North America. A more gradual transition in tectonic regime is preserved across the Holson Valley corridor; this zone probably represents the final gasp of subduction-accretion prior to suturing.
AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.