LIDAR USED TO DELINEATE MAP-SCALE FOLDS IN AMPHIBOLITE-FACIES EASTERN BLUE RIDGE ROCKS, WESTERN NORTH CAROLINA

Lidar-derived Digital Elevation Model (DEM) datasets are frequently used to identify bedrock structure in low-grade or non-metamorphosed sedimentary sequences in the Appalachian Mountains. However, few examples exist of its use to identify folds in high-grade polydeformed rocks. In this study, we utilized lidar-derived hillshade and topographic metric raster images combined with focused ground surveys to identify and compare multiple map-scale folds in the eastern Blue Ridge province of western North Carolina. These rocks consist of late Neoproterozoic to early Paleozoic metagraywacke, amphibolite, and altered ultramafic rocks of which together comprise the Ashe Metamorphic Suite (AMS). Younger Devonian to Mississippian granitoids intrude these units locally.

Geologic mapping in the vicinity of the 2020 Sparta, NC M5.1 earthquake identified a map unit several hundred meters thick consisting of amphibolite with mica schist, metagraywacke, and altered ultramafic rock that defines a map-scale, SW-plunging tight to isoclinal synform in the AMS. The rock types are interleaved at the scale of several meters with amphibolite producing high-weathering “fins” several meters long and ~1-5 m high that are distinct in hillshade images. Correlation of these fins defines bodies that can be traced for tens of km. From these images, the very tight to isoclinal nature of the fold is clearly distinguished. Mapping confirms the structure and adds additional details: intrafolial F₁ isoclinal folds occur within the layers, and these folds are tight to isoclinally folded to construct the larger map-scale F₂ structure. Limbs open to the SW and the nose of the structure is sheared for several km in the NE direction.

Near Spruce Pine, North Carolina, a similar structure located 110 km southwest of Sparta is apparent in lidar-derived images. Amphibolite of the AMS is folded into a tight synform, about 1 km in width, and plunging to the S-SW at 34°. Several m-long and m-high “fins” of amphibolite define the topographical expression of the fold limbs and nose, but in this case field mapping did not identify interlayered metasedimentary rocks. F₁ isoclinal folds within amphibolite layers were identified plunging to the NE, indicating the local preservation of an older deformation event.