ANALYSES OF DEFORMED ZONES IN THE MILLBORO SHALE, HIGHLAND COUNTY, VA: IMPLICATIONS FOR ALLEGHANIAN TECTONICS

The bed of the Cowpasture River in Highland Country, VA contains complexly deformed zones, some of which do not parallel the regional, northeast-striking structural trend. Shales in the riverbed display multiple events of deformation, as shown by several joint sets, disaggregated calcareous beds, small-scale folds, and slickenlines. Within the riverbed there are also uniformly bedded sections that are not folded or faulted, but do contain joint sets and occasional calcareous concretions. Investigation of the relative timing of the deformation events and the tectonic implications are the main issues explored in this research project. Ongoing field data collection includes orientation measurements of joint sets and concretions, precise mapping of deformed and undeformed sections in the riverbed, and analyses of relevant small-scale structural features, such as folds and slickenlines. Preliminary results suggest that deformation within the riverbed is partly composed of parasitic folds and faults of the regional, northeast-striking fold pattern. This is demonstrated by alternating sections of deformed and undeformed strata that imply larger-scale folding, as well as kink bands and centimeter- to decimeter-scale plunging anticlines and synclines. However, several sub-meter-scale folds and meter-scale faults are oblique to the northeast structural grain, and may have resulted from semi-chaotic, localized adjustments to diminished space in the nose of a regional fold. Rose diagrams of the principal joint sets predominantly show a northwest strike in both the deformed and undeformed zones, although some joint sets are overprinted and obscured within highly folded and faulted zones. The northwest strike is roughly parallel to the direction of regional compression during the Alleghanian Orogeny, which suggests that most of the joint sets are related to this event. Thus, the Cowpasture River region exhibits deformation of multiple styles and at multiple scales, which can be interpreted as an series of events that collectively comprised much of the contractional sequence of the Alleghanian Orogeny in present day western Virginia.