The Alleghanian deformational sequence at the foreland junction of the Central and Southern Appalachians

A complex sequence of deformation produced the major central and southern trends of the Appalachian fold-and-thrust belt in the Roanoke recess, Virginia. The incipient recess first experienced the Appalachian-wide stress field, then shifted to far-field effects from incremental counterclockwise rotation of the shortening direction, which resulted in the Central Appalachian fold belt, and then shifted to incremental clockwise rotation, which produced the Southern Appalachian fold-and-thrust belt. We analyzed joints, veins, normal and reverse faults, stylolites, and paleoseismites from Mississippian strata at the structural front of the Southern Appalachian fold-and-thrust belt, and the adjacent Appalachian Plateau west of the recess. We distinguished seven deformational events using orientations, intersection relationships, fault-slip directions, and mineralization histories. Five of these sets represent late Paleozoic deformational events (A1-A5), with shortening directions that show an evolving Alleghanian fold-and-thrust belt in the recess. A1 (shortening trend 085 degrees -265 degrees ) is consistent with the previously determined Appalachian-wide stress field and incipient layer-parallel shortening strain in Middle Mississippian carbonates. A2 (trend 145 degrees -325 degrees ) is a newly recognized event, herein called the Princeton event, which is consistent with dominant orientations of previously determined layer-parallel shortening strain, clastic dikes in Upper Mississippian strata, and stylolites. These far-field effects may mark high-angle basement faulting associated with development of the foredeep bulge during incipient thrusting along the Pulaski thrust system far in the hinterland. A3 (trend 120 degrees -300 degrees ) corresponds to initiation of the major Central Appalachian deformation, which resulted in fold-and-thrust belt structures such as the North Mountain Fault and Wills Mountain Anticline, while A4 (trend 160 degrees -340 degrees ) is associated with Southern Appalachian (e.g., St. Clair thrust and Glen Lyn footwall-syncline) structures. A5 (trend 010 degrees -190 degrees ) represents late Alleghanian deformation of the Glen Lyn Syncline, likely associated with blind thrusting coeval with emplacement of the nearby Pine Mountain thrust sheet. Two post-Alleghanian fracture sets, PA1 (joint trend 150 degrees -330 degrees ) and PA2 (joint trend 060 degrees -240 degrees ), are orthogonal; PA2 is younger. These joint sets are associated with strike-slip and normal faults that are compatible with some fault-plane solutions from the nearby Giles County seismic zone.