EPISODIC OROGENESIS IN THE OUACHITA FOLD AND THRUST BELT

Polyphase deformation has long been recognized within the Broken Bow uplift of the Ouachita Mountains. One of these deformational phases preceded northerly directed thrusting. Timing of this event has been uncertain as the youngest involved units are the Mississippian age Stanley Group. Recent study north of the uplift has documented 20-30% shortening in the Stanley Group not observed in the overlying 10-15 km Carboniferous flysch sequence. To the west, late Mississippian foreland deformation is documented within the Criner Hills near the Arbuckle Mountains. A multistage tectonic model is proposed incorporating Mississippian age shortening followed by 20-30 my of flysch deposition. Initial shortening involved the lower Paleozoic sequence which was deposited outboard of the continental margin on an extended continental crust . Southerly directed subduction and reactivation of basement blocks led to areas of southerly verging structures commonly observed in the uplifts. This shortening was oriented in a north-south direction and suggests a “soft collision” along the southern margin of North America. During the remaining Carboniferous, a thick submarine fan developed from east to west, filling the trough. Northerly directed thrusting developed during the middle to late Pennsylvanian with shortening nearly coaxial with the earlier stage. The Ouachtia sequence was detached and translated inboard 100-200 km. Early folds were deformed and younger northerly verging folds were developed in the overlying flysch sequence. The final stages of deformation involved counter clockwise rotation of the shortening direction towards the northwest and the transition to local thick skin deformation These stages reflect the collision events along the southern margin of North America. We interpret the initial stage as the collision of a small continental block or possibly island arc with southern margin of North America. Large scale northerly directed thrusting was driven by the collision of South America incorporating the intervening smaller blocks. The SE-NW shortening observed in the basement uplifts is believed to be related to final adjustments along the southeastern margin of North America.