INFLUENCE OF INTRINSIC PROPERTIES ON DEFORMATION OF A ROCK SEQUENCE IN THE INNER PIEDMONT OF SOUTH CAROLINA

New observations in Sunset, Eastatoe Gap, and Salem 7.5-minute quadrangles of a sequence of Inner Piedmont rocks (Table Rock gneiss-TR; Poor Mountain amphibolite-PM; Chauga River Formation-CR; Henderson Gneiss-HG) have identified a suite of interrelated structural styles. Map scale asymmetric, curvilinear folds were developed in TR and PM. Mesoscopic inclined to recumbent asymmetric tight folds also were developed. Such folds are common in PM and refold older isoclinal and transposed folds. Flattened folds are preserved in different exposures. Thrusting occurs along the CR-HG contact on both map and mesoscopic scales. On the map scale, thrusting repeats a series of CR-HG horses. Duplexes also are found along thrust contacts in mesoscopic exposures.

The spatial variations in the structural styles of deformation are interpreted to reflect the spatial variations in intrinsic properties (stratigraphy and competency contrast) (Fischer and Woodward, 1992). The competent rocks (TR and PM) exhibited fold-first structural styles, whereas the less competent rocks (CR and HG) formed fault-first styles. After folding began in TR and PM rocks, flexural slip influenced fold forms and produced movement horizons within the rock sequence. Tightening locked the folds, and passive rotation in simple shear further modified the fold form. Curvilinear form, recumbent attitude, and fold flattening were subsequent products of progressive simple shear. In CR and HG rocks, thrusting was initiated along movement horizons in strain-softened HG. Increased resistance to slip at some point along this movement horizon resulted in ramping to a higher horizon. Subsequent footwall failure imbricated (possibly duplexed) the CR-HG contact. The different structural styles in the study area are related directly to the intrinsic properties of the rock sequence under regional compression, and there is no need to interpret changes in stress fields for their development.