GEOMORPHOLOGIC EVIDENCE FOR A MESOZOIC(?) IMPACT IN TRANSYLVANIA AND HENDERSON COUNTIES, NORTH CAROLINA

Recognition of a 30 km diameter, sub-circular structure, defined by an annular drainage pattern, suggests the possibility of an impact feature in Transylvania and Henderson counties, SW North Carolina. A subtle radial fracture pattern appears to be present near the center, near Horse Shoe. Slightly more than 75% of the outline is defined by streams and stream segments that align in an arcuate arrangement. The most striking area is along the Little River in southern Transylvania County. The northern area is poorly defined, being situated in an area of numerous landslide deposits below the high segment of the Blue Ridge Parkway between US 276 and Mt. Pisgah. An interior arc, parallel to the south side, is present along Crab Creek/Kanuga Road between Pisgah Forest and Hendersonville.

Structural data around the circumference always include one tangential fracture orientation. Numerous linear structures crosscut the Little River displaying a marked change in topographic definition between interior and exterior of the annular structure. South of the annular structure, the topography is rugged and well defined on aerial photos but is more subdued in the interior region.

The annular structure crosscuts the Brevard Fault Zone (BFZ) along the Mills River in the northeast and at Kathy's Creek in the southwest. Bedrock geology consists of Cambrian Henderson Gneiss and Ordovician granitic gneisses southeast of the BFZ. To the northwest of the BFZ are the Proterozoic biotite schists and the Ordovician-Silurian Looking Glass and Pink Beds granitic gneisses.

The annular structure does not appear to be offset by the BFZ, suggesting a post-Alleghenian age. Recent fission track thermochronology (Naeser et al., 2004) indicates regional uniform cooling rates since the mid-Cretaceous inside and outside of the ring, suggesting that an impact would predate this time. A Permian-Cretaceous age would suggest that the impact occurred in a high, mountainous region and that all surface evidence was removed by several km of erosion so that only deep structural evidence remains.