TERTIARY-TO-RECENT ARCHING AND THE AGE AND ORIGIN OF FRACTURE-CONTROLLED LINEAMENTS IN THE SOUTHERN APPALACHIANS
Current estimates of the crustal thickness beneath the southern Appalachians range from about 45-55 km. Using a uniform rate of erosion of 30 m/my (Matmon et al, 2003) for the past 300 my yields an initial maximum crustal thickness of 54 -64 km. Isostatic rise of crust this thick will maintain significant topographic relief long after orogenesis has ended. At least part of the topographic relief in the Southern Appalachians is due to the thickened crust.
In addition to isostatic rise, several lines of evidence show that the high topographic relief is also a result of Tertiary-to-Recent arching of the southern Appalachian crust. The northwest-trending Cape Fear arch, the northeast-trending Selma arch, and several other smaller arches project to the highest point in the southern Appalachians (Mt. Mitchell). Stratigraphic relationships in Coastal Plain sediments from North Carolina to Mississippi along with synkinematic gravel deposits show that the arching was most active during the late Oligocene to middle Miocene. We interpret the pulse of Oligocene-Miocene sedimentation and deposition of coarse gravels to be a result of erosion of rejuvenated topography in the southern Appalachians due to an episode of crustal arching.
A series of fracture-controlled topographic lineaments in the Blue Ridge Mountains post-date Alleghanian structures and are unlikely to be related to Mesozoic rifting. Three small, shallow (<8km) earthquakes occurred near or on the east-west Laurel Creek lineament in 2005 (M=2.5, 2.8, and 3.8) and the focal plane solution for the largest quake, near Hot Springs, NC, is consistent with oblique normal motion on a fault trending about N60W (Chapman, 2005), which is parallel to the dominant fracture orientation we have measured along the lineament. We believe these fracture-controlled lineaments are related to the arching that peaked in the Tertiary but continues to be active today.