The development of slaty cleavage and its relationship to mesoscopic and macroscopic structure in the Pulaski thrust sheet near Bristol, Tennessee

A mesoscopic analysis of the Tellico-Sevier Formation in the Pulaski thrust sheet of upper east Tennessee constrains the relative timing of the deformations with respect to regional tectonic events and approximately constrains the deformational mechanisms.

There is a general increase in deformation intensity across the 7-km-wide South Holston synclinorium, which lies within the Pulaski thrust sheet. The synclinorium is in contact with a large regional thrust fault, the Holston Mountain fault, on the southeast, and also contains the southwest end of the intraplate regional thrust fault, the Lodi fault. This terminus occurs as a recumbent regional fold, the Little Jacob Creek anticline. Mesoscopic buckle folds, many with axial planar cleavage, vary from a few open, upright forms on the northwest to strongly-inclined-to-recumbent forms in the southeast. Bed-perpendicular extension fractures, mesoscopic fold-discordant faults, and tight kink folds begin to dominate the structure about 1.2 km to the northwest of the axis of the South Holston synclinorium. These structures are best developed (1) on the overturned limb of the Little Jacob Creek anticline, and (2) just beneath the Holston Mountain fault.

These relationships suggest that the dynamic control on the deformation was a continuously applied stress field which was approximately coaxial over time, and whose σ₂ direction remained approximately both parallel to and at zero degrees strike to the bedding, while the &sigma₁ and &sigma₃directions met the bedding obliquely. The principal source for the stress field was the emplacement of the Holston Mountain thrust sheet. An important secondary source of deformation was movement along the smaller Lodi fault, which overprinted the structures produced by earlier movements along the Holston Mountain fault. Movements along the underlying Pulaski fault warped the Holston Mountain thrust sheet, placing the timing of the principal deformations on this part of the Pulaski thrust sheet as earlier than the emplacement of the Pulaski fault.

In all the mesoscopic folds, layering had an active mechanical role in the folding process. Buckles generally formed under low interlayer ductility contrast. The relatively stiff layers exerted subtle control on the shapes of the folds. Relationships between angles of cleavage to bedding, fold interlimb angles, and cleavage fan angles indicate that flexural flow or slip, strongly influenced by heterogeneous pressure solution shape modification, was the likely fold mechanism. In each cleaved fold, the orientations of the cleavage traces are primarily controlled by the maximum contraction directions of local finite strain Schmidt net diagrams of folds and faults across regional strike established near coaxiality of principal strain axes for the deformations taken as a whole. Field observations, however, showed that clearly noncoaxial strain histories occurred locally.