A GRAVITY STUDY OF CHARLOTTE TERRANE ROOTLESS PLUTONS

The Charlotte terrane - Inner Piedmont terrane boundary southeast of Spartanburg, South Carolina is the Cross Anchor Thrust. The thrust itself does not crop out well, but appears to dip east to southeast, at 10 to 20 degrees, judging from the structures in the upper and lower sheets, and one seismic line. The Inner Piedmont terrane in the area consists of nappes of biotite gneiss, amphibolite, sillimanite schist, and minor marble; the Charlotte terrane contains metamorphosed volcanics and mafic to felsic plutons. The thrust has been hypothesized to have had Alleghanian motion; if so, many of the Proterozoic to late Paleozoic plutons in the Charlotte terrane thrust sheet are thin and rootless. In order to investigate this possibility, gravity data have been collected over a large area, which includes the Bush River and Buffalo gabbros, and the Bald Rock, Pacolet Mills, Santuc, Lowrys, and McClures Creek granites. All of the plutons have surprisingly small gravity anomalies of only a few milligals, and anomalies caused by density contrasts beneath some of the plutons interfere with the anomalies due to the plutons. Plutons such as the Bald Rock and Buffalo seem to be particularly thin, and thus are likely rootless. In contrast, plutons such as the Cuffeytown Creek and the Liberty Hill granite to the southeast in the low-grade Carolina terrane have much larger gravity anomalies, and may still be rooted. The thrust-induced telescoping of terranes and the rootless character of the plutons in the western Charlotte terrane must be considered while compiling any geographic distribution of petrologic variations in Paleozoic and Proterozic plutons. Also, the stacking of thrust sheets means that sutures have been tectonically covered and segmented, and may not be easily correlated with surface fault zones.