Kinematics of thrust sheets within transverse zones: a structural and paleomagnetic investigation in the Appalachian thrust belt of Georgia and Alabama

Abstract

Deformation styles of orogenic belts change along strike across transverse zones (TZs); hence, the kinematics of TZs is indispensable for three-dimensional restoration of thrust belts. We investigate the causes of deviation in strike of fold axes and fault surfaces in two TZs of the southern Appalachians using structural cross-sections and paleomagnetism. In the Rising Fawn TZ of Georgia, hanging-wall lateral-ramp folds yield paleomagnetic rotations and variation in magnitude of thrust translation. Plunge of folds, differential slip, and rotations are associated with lateral-ramp geometry and along-strike variation in the rheology and thickness of units that host lower and upper detachment levels. In Alabama, the internal geometry, strike, and paleomagnetic rotations of the Helena thrust sheet change across the Anniston TZ. These differences resulted from movement of the Helena thrust sheet over different footwall blocks and sub-décollement basement graben structures, and from differential slip along an oblique ramp. Rotations of thrust sheets within TZs occur where a thrust sheet was translated over: (1) an oblique/lateral ramp with contrasting rock strengths at lower and upper detachment levels, (2) a transverse basement fault that separates contrasting basement structural domains, and (3) intersections between frontal ramps and transverse structures. Because of the local causes of rotations in lateral structures, paleomagnetic and structural analyses of TZs are necessary for understanding the kinematics and restoration of both single thrust sheets and large-scale curves in thrust belts.