BLOCKY BEDROCK BREAKDOWN ALONG POST-OROGENIC FRACTURES SETS: A CASE STUDY FROM WESTERN NORTH CAROLINA, SOUTHERN APPALACHIANS

Much of the topography in the southern Appalachians is controlled by post-orogenic WNW-ESE-, E-W-, and N-S-striking fracture and fault zones. To illustrate the influence of these zones on the evolution of the Blue Ridge Escarpment and elsewhere in western North Carolina, we analyzed field data and lidar-derived DEMs from Graham, Polk, Swain, and Watauga counties, which span different rock types of the Blue Ridge and Piedmont geologic provinces and are crossed by topographic lineaments. There are fracture-controlled stream networks with steeply dipping orthogonal fractures that can be traced parallel to streams for hundreds of meters along slot canyons of bedrock being incised as knickpoints migrate headward when debris flows initiate along fractures. In channels where metamorphic foliation or original sedimentary bedding dips upstream and into the hillslope but intersects high-angle fractures there are well-pronounced knickpoints formed by blocky failure of the bedrock. We present numerous examples where knickpoints correspond with initiation locations of modern debris flows that carried rectangular boulders ranging from <1m to 10m in width hundreds of meters downstream. Conversely, knickzones (e.g., low angle, cascade-type waterfalls) are present where foliation or bedding dips downstream and away from the hillslope. There are some knickpoints at the same elevation as bands of fracture-controlled outcrops in the adjacent hillslopes that separate steep topography from flatter landscapes at higher elevations, suggesting that fractures influence stream and hillslope erosional processes. Intersecting vertical joints lead to large blocks of rock toppling into in the surrounding hillslopes. Fractures affect the landscape at multiple levels, ranging from outcrop-scale influences on first- and second-order stream flow directions within rectilinear drainages, to how the regional-scale Blue Ridge Escarpment is migrating more rapidly where it intersects orogen-crossing topographic lineaments controlled by minor faults and fractures.