Tertiary Intrusions and Associated Phenomena near the Thirty-Eighth Parallel Fracture Zone in Virginia and West Virginia

Abstract

Recognition of Eocene igneous intrusions in Highland County, Virginia, by Fullagar and Bottino (1969) prompted a search for related phenomena. The temperature culmination of a group of thermal springs, the site of maximum uplift of the Schooley erosion surface, and a regional simple Bouguer gravity anomaly in excess of —80 mg occur within 20 mi of the common juncture point of Highland and Bath Counties, Virginia, and Pocahontas County, West Virginia. The Highland County intrusive rocks of Eocene age are the most recent surface expression of volcanism along a deep-seated crustal fracture that lies transverse to the Appalachians near the 38th parallel. Surficial rocks above the fracture zone contain igneous injections, flows, and pyroclastic deposits of late Precambrian, Devonian, Triassic, Jurassic, and Eocene age. Radiometric dating data now available suggest westward migration with time of intrusive centers. The fracture zone seems to have been present and sporadically active for more than 0.8 b. y. and may still be a zone of deep crustal weakness.