The method of emplacement and sequential deformation of major thrust zones may be deciphered by detailed geologic mapping of these important structures. Thrust fault zones may have added complexity when horse blocks are contained within them. However, these horses can be an important indicator of the fault development holding information on fault-propagation folding or fold-to-fault progression. The North Mountain fault zone of the Central Appalachians, USA, was studied in order to better understand the relationships of horse blocks to hanging wall and footwall structures. The North Mountain fault zone in northwestern Virginia and eastern panhandle of West Virginia is the Late Mississippian to Permian Alleghanian structure that developed after regional-scale folding. Evidence for this deformation sequence is a consistent progression of right-side up to overturned strata in horses within the fault zone. Rocks on the southeast side (hinterland) of the zone are almost exclusively right-side up, whereas rocks on the northwest side (foreland) of the zone are almost exclusively overturned. This suggests that the fault zone developed along the overturned southeast limb of a syncline to the northwest and the adjacent upright limb of a faulted anticline to the southeast. 1. Introduction The North Mountain fault is a major structure that extends about 150？mi (240？km) from Rockbridge Co., central Virginia, northward to south-central Pennsylvania in the eastern U.S. Along the fault, Cambrian and Ordovician dominantly carbonate rocks occur in the hanging wall at the present erosion level and Silurian and Devonian clastic rocks occur in the footwall. The fault is a zone, which is as much as 1-mile wide, that contains rocks from Cambrian to Devonian age in a series of fault slices or horses. In the study area, the present leading edge lies on a footwall frontal ramp that rises from a lower décollement in Cambrian rocks. To the south, the North Mountain fault loses displacement in Rockbridge Co., where horizontal shortening is transferred to the Staunton-Pulaski thrust system located to the southeast [1]. Rader and Perry Jr. [2], Kulander and Dean [1, 3], and Dean et al. [4] interpreted fault genesis to a fault-propagation fold. In this model, the overturned limb of the fault-propagation fold is preserved in the footwall and total displacement is less than 10？mi (16？km). An alternative interpretation [5] based on seismic reflection data describes the North Mountain fault as the leading edge of a large thrust-sheet complex that duplicates Cambrian and Ordovician rocks
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