KINEMATIC VORTICITY ANALYSIS OF THE LINVILLE FALLS THRUST SHEET NEAR BOONE, NC

The Blue Ridge thrust complex in western North Carolina is a stack of thrust sheets that were emplaced during the late Paleozoic Alleghanian orogeny. The shear zones associated with these thrust faults are commonly hundreds of meters thick and characterized by mylonitic rocks that have been deformed under greenschist-facies conditions. Although there are qualitative estimates of the shear zone thicknesses, there have been no quantitative estimates or detailed studies that examine the influence of rock type on the nature of strain partitioning.

In this study, we collected oriented samples along a 2.5 km map transect across the Linville Falls thrust sheet near Boone, North Carolina, from the Linville Fall thrust at its base to the Fries thrust at its top. Using an average dip of the mylonitic foliation of 35 degrees, this corresponds to a structural thickness of about 1.5 km. The rocks that make up the Linville Falls thrust sheet at this location have been mapped as the Cranberry Gneiss but our recent, more-detailed mapping shows that there are distinct mappable units within the Cranberry Gneiss that broadly vary from granitic gneiss to mica-rich gneiss. We used a combination of kinematic vorticity, W_k, and petrographic analyses in oriented thin sections to assess the intensity of shearing and variation in non-coaxial deformation across the thrust sheet.

As expected, the most intense shearing occurs near the major faults. Between the major bounding faults there exist zones of intense shearing in both the granitic and micaceous units but these zones tend to be concentrated in the more micaceous rocks. In general, the kinematic vorticity numbers indicate general shear of the rocks within the thrust sheet (W_k ~ 0.5), which is consistent with the interpretation that displacement of the thrust sheet was accommodated by penetrative non-coaxial deformation throughout the thrust sheet as well as strong simple-shear-dominated deformation closer to the bounding faults.