Mesoscopic structures and fabric within the thrust sheets between the Cumberland Escarpment and Saltville fault

Mesoscopic structures and fabrics within the thrust sheets between the Cumberland Escarpment and the Saltville fault are analyzed and evaluated to determine the chronological history of shortening and extensional episodes of rocks involved in late Paleozoic thrusting and folding. The fabric relationships provide an accurate movement picture of the deformation in the study area that can be correlated with specific stages of thrust transport and subsequent folding of the thrust sheets.

Fabrics in the Rome Formation and Conasauga shales in the hanging walls of the Copper Creek, Beaver Valley and Southern Branch of the Saltville fault reveal the existence of an early deformational event that cannot be correlated with Nw directed thrusting. Compressional fabrics consist of low-angle thrust faults that are associated with N-S trending kink folds. Extensional fabrics consist of a conjugate system of (hkO) faults that record an oblique to regional strike extension subsequent to folding and low-angle thrusting. The age of this fabric is unknown.

Fabric within the Chattanooga and Kingston thrust sheets is suggestive of deformation that postdates arrival of these thrust sheets at the surface. The tectonic fabric consists of concentric buckle folds with subhorizontal fold axes and subvertical axial planes. . The folds are not rotated into the thrust transport direction. This suggests folding in a regime of pure shear, rather than simple shear that is correlative with active thrust transport.

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Mesoscopic structures and fabric were examined in two separate subthrust synclines in the footwall of the Whiteoak Mountain fault. Mesoscopic structural fabrics within the southwesternmost syncline suggest that a complex interplay of compressional and extensional deformation took place during growth of the syncline. At least two folding events that are separated by an extensional event are recorded in the rocks within this syncline. The first is documented by the development of the syncline itself. Folding was followed by an extension subparallel to regional strike. Fabric indicative of this extension consists of (hkO) faults at the SW end of the syncline. The second folding phase is represented by concentric folds with E-W trending axes at its SW end. This event is correlative with a late wave of horizontal compression that caused rethrusting on the Whiteoak Mountain fault.

Mesoscopic structural fabric relationships within another subthrust syncline to the NE of the first, suggest that initial folding of the footwall rocks occured before development of the Chattanooga thrust fault. Within this syncline, mesoscopic kink folds on its SE dipping limb are crosscut by a conjugate system of dip-slip faults that have been tilted into their present attitude. These structural fabrics developed when bedding was subhorizontal and were subsequently tilted to the SE by ramping of the Chattanooga fault over subhorizontal bedding. I Initial folding of the footwall rocks probably occured during ramping of the Whiteoak Mountain fault over undetached subhorizontal bedding in the subsurface.

vi Fabric within the Copper Creek thrust sheet that is correlative with a NW-SE trending stress is present in the Rome Formation in the hanging wall of the thrust sheet. It consists of left-lateral kink folds with steeply SE dipping axial planes and high angle reverse faults that are subparallel to the kink fold axial planes. The faults are not low angle thrusts that can be correlated with initial transport of the thrust sheet so that it is likely that folding and faulting postdate arrival of the leading edge at the surface.

Fabric in the Beaver Valley thrust sheet that is correlative with a Nw-SE trending stress consists of right-lateral kink folds that are interpreted to be conjugate to the basal shear plane of the thrust sheet. On a regional scale, the footwall rocks of the underlying Copper Creek thrust sheet are folded into a subthrust syncline. On a mesoscopic scale the fabric in the footwall consists of tectonic breccia.

Fabric in the hanging wall of the northern branch of the Saltville fault that is correlative with a Nw-SE trending stress consists of lowangle thrust faults that are associated with kink folds. This fabric is correlative with initial transport of the thrust sheet. Fabric in the footwall consists of tectonic breccia. On a regional scale, the footwall rocks of the underlying Beaver Valley thrust sheet are folded into a subthrust syncline.

Fabric in the hanging wall of the southern branch that is correlative with a Nw-SE trending stress consists of upright and overturned buckle folds. Buckle folds are related to folding of the Saltville fault in the subsurface and overturning is associated with subsequent rethrusting on the Saltville faults.

vii Based upon the mesoscopic structural fabric relationships within the thrust sheets of the study area, the following inferences are made regarding the nature of the deformation in this part of the thrust belt:

1. NW directed thrusting was preceded by an E-W trending layer parellel compression that resulted in low-angle E to W directed thrusting and kink folding.

2. Layer parallel shortening above the basal shear planes of the thrust sheets was minimal during initial NW directed thrusting. Most of the shortening took place either during ramping or after emplacement of the thrust sheets.

3. Macroscopic folding of bedding in the footwall of the Whiteoak Mountain fault took place during early NW directed thrusting. Macroscopic folding was accompanied by an extension parallel to the regional strike.