AN ANNULAR DRAINAGE PATTERN IN THE BLUE RIDGE MOUNTAINS OF WESTERN NORTH CAROLINA: EVIDENCE OF LARGE BOLIDE IMPACT IN THE LATE PERMIAN OR TRIASSIC?

GIS drainage analysis in the Blue Ridge Mountains of southwestern North Carolina and northern South Carolina reveals a subtle, concentric, annular, drainage pattern that is ~75 km in diameter. The regional drainage map, originally drawn at 1:100,000 highlights the prominent curvilinear trends.

The strongest ring veers northeastward from Cedar Mountain, in southern Transylvania County, along the Green River, through the N-S arm of Lake Lure to Old Fort. It then trends northwestward to Mars Hill before trailing southwestward along Sandymush Creek toward the east side of Waynesville before returning southeastward over the mountains and along Kathy's Creek to follow US 276 and the Little River back to Cedar Mountain. The structure's center is located just north of I-26 Exit 37 (Skyland), south of Asheville, on radially-drained Ducker Mountain.

An area > 11,500 km2 is included by the main ring of the structure. The structural outline crosses the Brevard Fault ~4 km SW of Brevard and at Old Fort with no apparent offset, implying that it is post-Alleganian feature. A previously published but unrelated apatite fission-track thermochronometric study suggests that the annular feature is pre-middle Jurassic. Tangential fracture orientations were measured at 29 locations around the circumference of the main ring. Anomalous E-W fold axes are present on Ducker Mountain.

Our working hypothesis proposes that the annular structure resulted from the impact of a 3-4 km diameter bolide. A late Permian-early Mesozoic age suggests that the proposed impact occurred in the heart of a major mountain belt and that a minimum of several kilometers of material were subsequently removed by erosion allowing the drainage structure to follow deep fractures generated well beneath the impact surface. The absence of shocked quartz, to date, may suggest a “low” energy impact such as a comet or a secondary impact from a larger Permian or Triassic event.