Geology of the terminus of the St Clair fault: a study across the central and southern Appalachian juncture, Virginia-West Virginia
Geologic mapping along the northeasternmost 25 miles of the St. Clair fault, which traverses the Central and Southern Appalachian juncture at about the middle of this segment, was accomplished to observe the changes in geologic style and structural trend that occur across the juncture in this northwesterly Valley and Ridge strike belt. The study area is an excellent smale scale area for observations on the nature of the juncture over the larger scale as it includes most of the features that characterize this juncture such as change of physiographic and structural trend and transition from thrust fault-dominated deformation to fold-dominated deformation. The juncture or recess is prominent in the study area as a sharp bend in Gap-Moss Mountains and is in fact one of the sharpest bends occurring at the junction. Southwest of this recess, the stratigraphic units on the northwest of the St. Clair fault are overturned and a broad syncline is formed on the northwest, known as the Hurricane Ridge. The axis of this fold closely parallels the strike of the St. Clair fault. As the junction is approached the Hurricane Ridge syncline changes axial strike to the north and passes northwest of the Browns Mountain anticlinorium which is plunging out into the junction. Just northeast of the recess the St. Clair fault bifurcates and its strike is slightly changed when the segments rejoin. Northeast of this point, the St. Clair fault loses stratigraphic displacement and instead of a single broad fold northwest of the fault and parallel to it, there are numerous small folds developed that strike 20-30° more northerly than the fault. The fault does not, however, truncate any of the folds. The St. Clair fault extends 13 miles into the Central Appalachians where it dies out down the plunge of an anticline at Morning Knob in Alleghany County, Virginia. A strong shear zone is evident in the core of the structure at Morning Knob but is lost as the competent Silurian units plunge under Devonian shales.
Integrating this information with other recent work on the juncture it may be concluded that: 1. The change of topographic and structural trend across the Central and Southern Appalachian juncture does not require any significant difference in timing of deformation. 2. There is probably no significant difference in orientation of principal stress across the juncture but rather the change of trend and differences of geologic "style" reflect contrasts in thickness and facies within the sedimentary column and contrasts involving basement topography which indirectly influence cover structure. Thus the changes evident across the juncture are seen to be the result of differential physical response to the applied stress.