Doctoral Dissertations

Date of Award

12-1987

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Geology

Major Professor

Nicholas B. Woodward

Committee Members

Kenneth R. Walker, Kula C. Misra, John B. Rehder, C. Stephen Haase

Abstract

The Beaver Valley - Saltville transfer zone is a structurally complex area in the Valley and Ridge of east Tennessee, north and northeast of Knoxville, TN. Within this area, three regionally significant thrusts of the Southern Appalachians, the Copper Creek, the Beaver Valley, and the Saltville have deformed the rocks. The Beaver Valley thrust terminates within the area, but shortening remains constant.

This dissertation focuses on several problems related to the structures in this area. First, it examines the structural geometries in the area at both map (1:24,000) scale and at the mesoscopic scale. The geometries are interpreted on three balanced cross-sections. Second, a sequence of thrusting which explains the geometric evolution of the structures is presented. Finally, deformation mechanisms and twinning strains in limestones are examined and the contribution of the twinning and other features to the total strain is assessed.

The Copper Creek thrust sheet consists of four distinct areas: the Texas Valley anticline and syncline, an imbricate fan of Rome Formation and Conasauga Group lithologies, a uniformly dipping belt, and Clinch Mountain. The Beaver Valley thrust sheet is dominantly a set of folds in Middle and Upper Ordovician strata. The sheet also contains folded pieces of the Knox and Conasauga groups, and the House Mountain syncline. The Saltville thrust sheet mainly contains uniformly southeast dipping beds of the Rome Formation, and the Conasauga and Knox groups. The leading edge of the thrust however is complexly imbricated.

Three balanced cross-sections record essentially uniform shortening across the area. Shortening values are 43 %, 48 %, and 49 %. The Copper Creek thrust is interpreted as being folded at its leading edge in this area. The interpretation is supported by both surface and well data. Towards its tip point, the Beaver Valley thrust changes the position of its branch line with the Saltville thrust. The interpretation is suggested by the structures in the footwall of the Saltville thrust and it accounts for the decrease in slip on the Beaver Valley thrust.

The Beaver Valley and Saltville thrusts are interpreted as having grown toward each other laterally during emplacement. The movement of the faults was intermittant, and the times during which the faults were being emplaced overlapped. This sequence of thrusting follows from the ideas of Jones (1971) and it suggests a local reversal and "break-back" relationship between the two thrusts. The Beaver Valley thrust terminated either due to an effect of the basement, or lateral stratigraphic change, or simply the inability of the thrust to continue propagation because the Saltville thrust had already shortened strata and the Beaver Valley was no longer necessary to maintain shortening. There is no evidence for basement involvement so the stratigraphic theory combined with the thrusting sequence is favored. There are many lateral changes (especially in the Cambrian and Ordovician) in the stratigraphic column, which would argue for the stratigraphic theory. These lateral changes are of a regional scale and specific changes and their localities cannot be identified. The original stratigraphy may have strongly influenced the sequence of thrusting as well.

Deformational features recorded in limestones in the area include cleavage, stylolites and solution seams, fractures, veins, twins, breccia, and bedding slip surfaces. Strata within the thrust sheets (more than approximately 100 m from a thrust) record very low strains and appear virtually undeformed. Calculated twinning strain is low. Strains do not exceed 5 %, and most are less. No significant variation in either magnitude or orientation with structural position are observed.

A gap exists between total strain in the carbonates and total shortening indicated by the balanced cross-sections. This suggests that, although many deformational features are present in the carbonates, they contribute very little to the total strain at the scale of thrust sheets. Most of the strain occurred along fault planes or through buckling associated with map scale folds. The dominant mode of deformation, therefore was brittle failure, at very low temperatures and pressures.

This study points out the need to integrate mesoscopic and microscopic studies with studies of macroscopic structure. Examination of the rocks out of context of the larger structures would either grossly underestimate total strain in the area or attribute greater significance to grain-scale and bed-scale features than is really warranted.

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