NEW GEOLOGIC SYNTHESIS OF THE DREHER SHOALS AND CAROLINA TERRANES, LAKE MURRAY AND SALUDA DAM, COLUMBIA, SC

Recent mapping in the Lake Murray area has generated a new data set to help make sense of the complex geologic relations. Additional mapping was done while the Saulda Dam was upgraded. During the remediation project, an extensive excavation at the foot of the dam exposed the heart of the Modoc zone, and lake levels were lowered revealing exposure seldom seen. The new data set consists of 1:120-scale maps in the dam excavation (Modoc zone and Dreher Shoals terrane) and 1:24000-scale maps of Irmo, Lake Murray East, and Lake Murray West 7.5-minute quadrangles (predominantly Carolina terrane).

Four Paleozoic ductile deformation events are recognized. D4 deformation is recognized in an east-northeast striking zone at least 20 km wide, and it shows a transition from ductile to brittle behavior, which correlates with retrograde mineral assemblages in D4 faults in the Modoc zone. D3 deformation has only been found in Carolina terrane rocks and is best described as a phase of flexural folding with attendant limb thrusts. D2 deformation is restricted to the Modoc zone and some rocks of the Dreher Shoals terrane. D2 deformation is interpreted as a shearing event. Interlayered plagiogranites and migmatites are characteristic elements of the D2 fabric. A shear-heating model is proposed for their development. D1 structures are tight to isoclinal folds of compositional layering; in places, these could be remnants of bedding. Refolded F1 isoclines have been found in both the Carolina and Dreher Shoals terranes. These fold relations may indicate the earliest connection between the Carolina and Dreher Shoals terranes, which could be interpreted as mid-Paleozoic.

The history of younger brittle deformation is equally complex. Brittle faults range from late D4 fold-faults to Mesozoic and most likely Cenozoic cross faults. Crosscutting relations give relative estimates of ages, but no absolute ages are available. Greenschist facies mineral assemblages are found in many fault zones and indicate temperatures in excess of 200ºC. One possible solution to dating faults is to correlate mineral assemblages in fault veins with a constructed temperature-time curve.