Tectonic implications of subthrust structures revealed by seismic profiling of Appalachian‐Ouachita Orogenic Belt

A growing body of deep seismic reflection profiles across the Appalachian-Ouachita orogenic belt reveals subthrust structures which have been previously interpreted as products of continental rifting, of later thrusting, or of some combination of rifting and thrusting. The subthrust features can be placed into three general categories: (1) small normal faults which offset lower plate shelf sequences and basement in the foreland and have been attributed to thrust loading, perhaps reactivating earlier rifting faults, (2) moderately dipping seismic events which lie entirely below lower plate shelf sediments in the foreland, interpreted in part as strata deposited during the earlier rifting, and (3) hinterland-dipping reflection sequences beneath more interior portions of the mountain belt, previously interpreted as thrust faults which imbricate sediments and basement related to the earlier rifted margin. These interpretations are evaluated through a comparison of reflection profiles across the orogenic belt to previously interpreted seismic lines across modern extensional and compressional settings. The comparisons suggest that although some normal fault offsets of lower plate shelf sequences may be original or reactivated rifting faults, the dipping events entirely beneath the shelf sequence are more diagnostic of the rifting process. These dipping sequences are therefore consistent with previous interpretations that large portions of the earlier rifted margin are preserved in an undeformed state beneath thrust sheets in the Appalachian foreland. The comparisons also suggest an alternative to interpretations of the hinterland-dipping sequences as thrust-related, in that the sequences are similar in seismic appearance to that of volcanic wedges commonly observed in the narrow zone separating continental from oceanic basement on modern passive margins. The implication of this new interpretation is that the actual continent/ocean boundary related to the earlier rifted margin may be preserved on the lower thrust plate beneath interior portions of the Appalachians and Ouachitas.