TUNNEL VISION: KINEMATICS OF BRITTLE AND DUCTILE DEFORMATION IN THE HISTORIC BLUE RIDGE TUNNEL AND ROCKFISH GAP, VIRGINIA

Beneath Rockfish Gap, one of the lowest elevations along the crest of the Blue Ridge Mountains, sits the historic 1858 Blue Ridge Tunnel (BRT). The BRT cuts directly through foliated metabasalts and metasedimentary rocks of the Ediacaran Catoctin Formation and provides a rare 3-dimensional exposure of the Blue Ridge cover sequence on the western limb of the Blue Ridge Anticlinorium. The purpose of this study is to characterize brittle and ductile deformation features in the Catoctin Formation in the BRT to determine their kinematics and understand the timing.

The Catoctin Formation includes a thick sequence of metabasaltic greenstone with thin layers of meta-arkose, mudstone, and conglomerates that formed between 560 - 575 Ma. In the arkoses and conglomerates, quartz and perthitic feldspars are the dominant clasts and are typically surrounded by a sericite matrix. Interestingly, there is a dearth of basaltic detritus in the Catoctin sedimentary layers. Foliation strikes to the NE and dips moderately to the SE with down dip chlorite elongation lineations. Large epidosite-rich sandstone boudins began as fluvial channel or sheet elements, and typically display top-to-the NW asymmetry. Small, localized folds also occur in the sedimentary units and are tight, overturned NW-verging folds, consistent with NW-directed Neoacadian deformation. Foliation in the greenstone is cut by low-angle top-to-the west shear zones. Thin section analysis reveals stylolites, pressure shadows, and antitaxial veins from formed from dissolution, mass transfer, and volume loss processes under lower greenschist facies conditions.

The Catoctin Formation is cut by two dominant fracture sets with the older set striking WNW-ESE and the younger set striking NE-SW. The 40º range in the WNW-ESE set of fractures likely resulted from a change in principal stress direction between the Late Alleghanian Orogeny and Early Mesozoic Atlantic rifting.