NANICHITO THRUST: DEFORMED UNCONFORMITY IN THE CENTRAL ZONE OF THE OUACHITA MOUNTAINS?

Integration of remote sensing data, detailed mapping (1:12000), and GIS analysis has led to a refined interpretation of the east central section of the Ouachita Mountains. Dominant northerly verging structures of the thrust belt are apparent in the younger Carboniferous flysch Less apparent are the complex, tighter folds documented in the older Mississippian sequence. Down plunge views reveal a contrast in folding above and below a very narrow discontinuity. Along the Eagle Fork River, OK, this discontinuity is a sub horizontal (~5 m wide) fault zone (Nanichito thrust) with several imbrications and a complex kinematic history. In the hanging wall, folds are very open (interlimb angle, ia., ~120-160°) and plunge 10-15° to the west. Northerly vergence is apparent in the synclines while the younger Nanichito anticline folds both the Boktukola and Nanichito thrusts. Four structural domains are identified in the footwall. Folds in the northern domain are open (ia. 71-102°), upright, and westerly plunging (8-19°). The central domain folds are tight (ia 35-60°), upright, and sub horizontal. In the southern domain, folds are open (63-106°), southerly verging, and gently plunging (1-17°) with variable plunge directions. Folds within the sub thrust imbrication are tight (ia. 10-54°), northerly verging, and moderately plunging (9-40°). The footwall folds were restored, assuming constant volume and flexural folding in the hanging wall. 20-30% shortening is apparent below the Buffalo thrust. In addition, three of the domains contain either upright or southerly verging folds; while the imbrication and hanging wall reveal primarily northerly vergence. These data argue for deformation during the Mississippian followed by a deformational hiatus. 700-1000 m of the Upper Stanley Group were involved with this early deformation which ended just before deposition of the Moyers Formation (Late Meramecian). No erosional surface has been documented. It is inferred that subsequent deformation within this shale sequence obliterated the primary structures. Approximately 20-30 My later northerly directed thrusting translated the entire sequences over the continental margin successions.