TRANSITION FROM SEDIMENTARY TO TECTONIC FABRICS FROM AMS IN A TRANSECT FROM THE FORELAND TO FOLD AND THRUST BELT OF THE OUACHITA OROGEN IN WESTERN ARKANSAS

The Ouachita fold and thrust belt records shortening during late Paleozoic collision of Laurentia and Gondwana, but the nature of shortening strain within thrust sheets and how far blind thrusts propagated into the Arkoma foreland basin remain uncertain. We use anisotropy of magnetic susceptibility (AMS) from 42 sites in western Arkansas as a fabric proxy to assess the foreland extent of thrust-related shortening. AMS samples were collected from sandstone beds of (1) Middle Pennsylvanian Atoka Formation (n = 39) across the transect, (2) Desmoinesian (Middle Pennsylvanian) strata in the Paris syncline (n = 2), and (3) Early Pennsylvanian Jackfork Sandstone (n = 1) in the northern Y-City thrust plate. Following the model of Pares et al (1999), sites were classified into 3 groups whose AMS ellipsoids change from oblate (T factor > 0.5) to prolate (T < 0). The classes reflect progressive overprint of layer-parallel shortening on a preexisting sedimentary fabric. Class 1 oblate sites (T > 0.5) retain sedimentary fabrics and predominate in northern foreland basin sites from the Arkansas River Valley and southern Boston Mountains. At 2 test sites, the Kmax (maximum susceptibility) axes parallel paleocurrent directions determined from cross beds or asymmetric ripples. Class 2 (oblate-triaxial, 0 < T < 0.5) and Class 3 (prolate-triaxial, -0.5 < T < 0) sites characterize Ross Creek and northern Y-City thrust plates in the northern Ouachita Mountains and have generally E-W trending Kmax axes that parallel fold axes. Kmax axes record an intersection lineation formed perpendicular to shortening. Class 2 and 3 fabrics also extend to thrust-cored asymmetric anticlines in the southern Arkoma basin (Ranger, Washburn, Pine Mountain) and reflect the northern-detected limit of thrust-related layer-parallel shortening. A mean shortening azimuth of 359° ± 6° for the transect is estimated from 26 Class 2 and 3 AMS sites.

Pares et al., 1999, Tectonophysics, 307, 1-14