THE PROBLEM OF PALINSPASTIC RECONSTRUCTION AROUND A THRUST BELT RECESS: AN EXAMPLE FROM THE APPALACHIAN THRUST BELT IN GEORGIA

The characteristically sinuous map traces of orogenic thrust belts include salients (broad, sweeping curves, convex in the thrust transport direction) and recesses (more angular bends, concave in the thrust transport direction). Although some recesses result from bending of strike in a thrust belt propagating around a foreland obstacle, many recesses are the intersections of the distal arms of two adjacent salients, implying that a recess is a consequence of curvature of the salients.

A distinct structural recess in the Appalachian thrust belt in Georgia comprises an intersection between two elements of regional strike at an approximately 45° angle. Folds and faults from both structural trends intersect and include clearly defined interference folds, enabling the tracing of both strike directions through the intersection. The intersection and fold interference exemplify a well recognized problem in balancing palinspastic reconstructions of sinuous thrust belts.

Palinspastic restorations around bends in thrust belts commonly present difficulty in volume balancing. Cross sections generally are constructed perpendicular to structural strike, which is the line of assumed slip direction. An array of cross sections around a structural bend may be restored and balanced individually; however, restorations perpendicular to strike along intersecting thrust faults yield an imbalance in the along-strike lengths of frontal ramps. At a recess, where the curvature is concave toward the foreland, the restored strike length of a ramp exceeds that of the map trace in translated position. The restoration leads to a similar imbalance in the surface area of a stratigraphic horizon, reflecting a volume imbalance in three dimensions.

Three alternative solutions are suggested: (1) treating the ends of the fault segments as fault tips, so that displacement diminishes to zero toward the tip; (2) restoring a trapezoidal block (bounded by cross sections perpendicular, respectively, to two intersecting structural trends), so that higher order compression and/or extension are accommodated within the block by superposed folds and/or faults, diffusive mass transfer, etc.; or (3) incorporating successive episodes of deformation with contrasting slip directions that correspond to the two intersecting structural trends.