AN ANNULAR DRAINAGE PATTERN IN TRANSYLVANIA AND HENDERSON COUNTIES, NC: EVIDENCE OF AN IMPACT STRUCTURE IN THE BLUE RIDGE MOUNTAINS?

Discovery of a subtle annular drainage pattern in Transylvania and Henderson counties, NC suggests the possibility of an impact structure in the heart of the Blue Ridge Mountains. The area is entirely within the upper portion of the French Broad River drainage basin, 10-15 km downstream from the headwaters. The drainage pattern presents a diameter of about 30 km, extending from the Little River in the south to Pink Beds in the north and from just west of Brevard in the west and just east of Hendersonville in the east. A less well defined partial inner ring, suggested in the Hendersonville area, is not concentric with the outer ring. Both rings are generally outlined by the roads that service this rugged area. Both rings also cross the Brevard Fault Zone but do not appear to be offset by fault movement, suggesting a post-Alleghenian origin.

The first phase of our investigation is to collect structural data around the perimeter of the circle. Joint orientations are generally tangential to the two rings but some joints parallel preexisting regional trends. Samples have been collected from the centers of both rings and are being prepared for petrographic analysis.

Rocks exposed within the circle are mostly early Paleozoic gneisses, granites, amphibolites, and schists. In our working hypothesis, we speculate that the origin of the annular pattern occurred when a roughly 1.5 km diameter bolide collided with the planet in the heart of a relatively high mountain range during late Permian or early Triassic time. All vestiges of the surface expression of this feature were lost to erosion but the deep fractures that could result from such an event preserve the circular pattern.