Development of Cumberland Overthrust Block in Vicinity of Chestnut Ridge Fenster in Southwest Virginia

Abstract

Major folding that apparently involved the basement initiated development of the Cumberland overthrust block near Ewing, Virginia. The Pine Mountain fault later developed as a bedding-plane thrust in the incompetent Rome Formation in the western part of area and at the base of the Maynardville Limestone in the eastern part of the area. As the fault began to grow it was deflected upward by the south limb of the primary fold from the Rome and Maynardville across several thousand feet of beds to the Chattanooga Shale. Rocks stripped from the primary fold and moved toward the northwest formed the Powell Valley anticline by duplication of beds. Frictional drag on the fault caused the north limb of the nearly rootless Powell Valley anticline to fold into the Cumberland Mountain monocline. During the final phase of the orogeny the thrust plates locked in the eastern part of the area; this initiated arching of the Pine Mountain fault upward about 5,000 feet from its original position. In the western part of the area the plates did not lock, and movement continued, skewing the axis of the Powell Valley anticline northward about 1.5 miles. Displacement in the eastern part of the area is about 4 miles and in the western part about 5.6 miles. Analysis of the structure suggests that the initial dip of the Pine Mountain fault, which in the main is a bedding-plane thrust, was predetermined by the attitude of the country rock in which the fault developed. Size and shape of major folds in the moving plate were influenced by the initial fold, the stratigraphic position of the fault, the amount of displacement and frictional resistance, root structures in the stationary block, and folding in the latter stage of deformation.