LISTRIC-THRUST KINEMATIC MODEL FOR THIN-SKINNED, HIGHLY ASYMMETRIC, WILLS VALLEY ANTICLINE, SOUTHERN APPALACHIANS

Highly asymmetric folds with short, steep forelimbs and long, gentle, backlimbs are well known above basement-involved listric thrusts where they are associated with block rotation of the backlimb. Recently Suppe and others (2004) presented kinematic models for developing the same fold style, including rotating limbs, above thin-skinned ramp-flat faults. Although listric thrusts have been widely inferred to be present in thin-skinned thrust belts, as yet no example has been shown to fit a quantitative kinematic model. Do primary thin-skinned listric thrusts even exist? A primary listric thrust forms with a listric shape, whereas a secondary listric thrust owes its shape to deformation, as in a duplex. This paper applies a kinematic model for the hangingwall geometry above a primary listric thrust to a well-defined thin-skinned structure, the outcropping and seismically well-imaged Wills Valley anticline (WVA), one of the frontal structures of the southern Appalachians. The width of the structure at regional and the fault cutoff angle are sufficient to predict the fault shape and depth to detachment. The angular displacement must match the dip of the backlimb. The listric model and both shear-fold models of Suppe are applied to the structure. The constructed listric fault fits the observed shape and the angular displacement. Both shear-fold models can match the fault cutoff angle, backlimb dip and detachment depth, but neither model produces a backlimb that is long enough, leading to the conclusion that the listric model is best for this structure. To maintain a vertical trailing-edge pin line behind a rotated block, second-order shortening structures must be present within the block or near its trailing edge. The shortening required for balance in the WVA is accommodated by frontal imbricates in the massive carbonate section and in the younger section by forelimb folding in the overlying section. The WVA may be the only kinematically modeled listric-fault example, thick-skinned or thin-skinned, for which the predicted lower detachment has been observationally confirmed.