A SUBTLE ANNULAR DRAINAGE PATTERN IN THE BLUE RIDGE MOUNTAINS OF WESTERN NORTH CAROLINA: EVIDENCE OF A LARGE BOLIDE IMPACT?

Curvilinear stream segment alignments in the Blue Ridge Mountains of southwestern North Carolina and northern South Carolina reveal a subtle, concentric, annular, drainage pattern that is ~75 km in diameter, considerably larger than previously published abstracts about this feature. A regional drainage map, originally drawn at 1:100,000 highlights the prominent 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 heads 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 center of the structure is located at I-26 Exit 37 (Skyland), south of Asheville.

An area > 11,500 km² is included by 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. An unrelated, previously published, apatite fission-track thermochronometric study suggests the feature is pre-middle Jurassic. X-ray diffraction of rock specimens from near the center suggests a minor presence of stishovite and abundant quartz but no shocked quartz was observed in thin section. A tangential fracture orientation was measured at 29 locations around the circumference of the main ring.

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 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 suggests a “low” energy impact such as a comet or a secondary impact from a larger Permian or Triassic event.

Most deeply eroded impact structures are Precambrian and are located in shield areas. Large impact features in mountainous regions are relatively unknown in the literature.