CLEAVAGE DEVELOPMENT AND FOLDING IN THE SOUTHERN APPALACHIAN FORELAND FOLD-THRUST BELT, NE TENNESSEE: CASE STUDY OF AN EXPOSURE ON I-181 IN PART OF THE MIDDLE ORDOVICIAN SEVIER BASIN

Cleavage-bedding relationships are used to elucidate the number of deformation events at a strongly folded, faulted, and fractured outcrop of Middle Ordovician Upper Sevier Shale in Northeast Tennessee. Two types of folds are present: inclined to recumbent, tight class 2 similar folds occur in the southeastern end of the outcrop, whereas upright, class 1b parallel open folds occur in the northern end. ß-axes and measured hinges of tight and open folds, and cleavage-bedding intersection lineations have similar, shallowly plunging, 070 and 250 trends. Cleavage is fanned proportionally to fold tightness, and is axial planar only in the hinges of tight folds. A single, two-part deformation event is concluded based on cleavage-bedding relationships, similarity in structural orientations and lack of refolded folds. Cleavage formed just prior to or at the end of an early, tight folding stage based on the axial planar orientation of cleavage in the hinges of tight folds. Open folds formed later during this event, resulting in little to moderate fanning of the pre-existing cleavage. Pressure solution is the dominant cleavage-forming mechanism. An alternative hypothesis is that the difference in fold geometries reflects a decreasing strain gradient from the tightly folded southeastern part to the open folded, northwestern part of the outcrop. Higher strain at the southeastern end of the outcrop may be related to macroscale faulting immediately to the south. Principal strain axes must have rotated slightly during increments of deformation as demonstrated by fibrous calcite veins.