ENHANCED EROSION AND SLOPE INSTABILITY OF THE BLUE RIDGE ESCARPMENT ALONG FRACTURE-CONTROLLED TOPOGRAPHIC LINEAMENTS: A CASE STUDY FROM WESTERN NORTH CAROLINA, SOUTHERN APPALACHIANS

The Blue Ridge Escarpment is a NNE-trending steep landform that separates the high topography of the western North Carolina Blue Ridge physiographic province from the lower elevation Piedmont to the east. It intersects numerous E-W, NE-SW, and SSE-WNW lineaments along its length, which form reentrants with steep valley walls. During our recent work mapping slope movements in Polk County, we focused on two of these trench lineaments, the E-W-trending Pacolet River Valley and the NE-trending Green River Gorge, where abundant near-vertical fractures in the bedrock, high topographic relief (up to 700 m), and large drainage areas above the escarpment provide favorable conditions for a variety of slope movements to initiate. Many debris flows, debris slides, rockfalls, and rock slides initiate here along high-angle joints that strike parallel to the lineaments. Debris flows often travel through rectilinear drainages that are parallel and perpendicular to these E-W- and NE-SW striking joints, or move parallel to the strike of a pervasive topographic fabric formed by metamorphic foliation that dips gently to the NNE. This foliation, which preferentially crops out on south-facing slopes, may explain the topographic asymmetry of the Pacolet River Valley and why the steeper north side of the valley is eroding by debris flows, while the less steep south side is mantled by composite debris slide, rockfall, and rockslide deposits. Some of the linear features that cross cut the escarpment are driven by fractures but others are formed from metamorphic layers and the associated differential erosional in varying lithology. The orientations of outcrop-scale fractures and foliation match lineament fabrics that are visible in high-resolution lidar and the major topographic reentrants that intersect the escarpment, where it is likely that erosion is enhanced by intersecting fracture sets and minor post-orogenic faults. Similar features found to the north such as Hickory Nut Gorge, the Swannanoa lineament, the Laurel Creek lineament, and the Boone lineament all contain orogen-crossing fractures and faults leading to increased instability and greater abundance of slope movements.