Mechanics of vein, fault, and solution surface formation in Bays Mountain, southern Appalachians

Gregory Charles Ohlmacher, Purdue University

Abstract

Detailed mapping of structures exposed in highway cuts at Bays Mountain, Kingsport, Tenn. reveals a deformation sequence consisting of (1) early echelon thrust faulting subparallel to interfaces, (2) folding, (3) thrust faulting, (4) normal faulting, (5) strike-slip faulting, and (6) jointing. Once initiated these processes continue operating over several stages. The terminations of the early echelon thrust faults have tail cracks in the extensional quadrants and tail solution surfaces in the contractional quadrants. The echelon thrust faults form arrays with either extensional stepovers (pull-aparts) or contractional stepovers. Contractional stepovers contain pressure solution surfaces. The thrust fault arrays with pull-aparts trend parallel to bedding, and with continued slip the pull-aparts overlap forming composite pull-apart veins. Bedding-normal veins form between parallel arrays of echelon thrust faults with pull-aparts and normal to the slickensides on the bounding faults. However, bedding-parallel veins and bedding-normal solution surfaces are observed between parallel thrust fault arrays at Roundtop Hill, Md. The structures associated with layer-parallel slip near Boone, N.C. include arrays of echelon opening fractures at low-angles to the bedding interfaces. Folds have both open and chevron types. As the fold amplitude increases, new echelon thrust faults develop with different senses of slip on opposite limbs (flexural slip). The fold hinges are displaced by the intermediate thrust faults (Stage 3). During the fourth stage high-angle normal faults and opening fractures extend the rocks between postulated parallel intermediate thrust faults. Finally, vertical strike-slip faults and joints cut through the above structures. Most of these structures are distributed from Vermont to Tennessee. Displacement Discontinuity Boundary Element Models were used to explain the observed structural assemblages and their regional variations in terms of the state of stress near faults. Local stress trajectories calculated for single faults accurately match the geometries of tail cracks and tail solution surfaces. Pressure solution surfaces within contractional stepovers and pull-aparts were successfully patterned by echelon fault models. A very low frictional resistance along the faults, and a high angle ($>$45$\sp\circ$) between the remote maximum compressive stress and the fault planes is inferred from the bedding-normal veins between the parallel faults. The low frictional resistance is probably the result of nearly lithostatic fluid pressures acting on the faults. Networks of interconnected fractures controlled fluid migration during deformation. Pressure solution surfaces and short echelon thrust faults reduce the permeability of the host rock and further confining fluid migration to the vein networks.

Degree

Ph.D.

Advisors

Aydin, Purdue University.

Subject Area

Geology

COinS