LEARNING FROM 50+ YEARS OF DETAILED GEOLOGIC MAPPING IN THE SOUTHERN APPALACHIAN BLUE RIDGE AND PIEDMONT

Detailed geologic mapping of a 7.5-mimute quadrangle (~92 mi²; 238 km²) involves collection of thousands of observations and rock-type identifications, relating them to known rock bodies, and recording at >1500 spatially located structural measurements, a task made easier today with GPS. Prior to the mid-1960s, there were detailed geologic maps of the Great Smoky Mountains and northeasternmost Tennessee in USGS professional papers, some state geological survey publications, and a few detailed geologic maps in the Piedmont.

From the mid-1960s onward university, state geological survey, and USGS detailed geologic maps were completed in the Carolinas, Georgia, Virginia, and Alabama. These detailed geologic maps became islands of knowledge that began to resolve the stratigraphy, structure, petrology, and fundamental tectonics of the Blue Ridge and Piedmont, identifying previously unknown tectonic boundaries and grew in tme. The advent of modern zircon geochronology provided ground truth, timing of intrusion of many plutons, timing of metamorphism, identification of previously unknown basement, separating basement from younger rocks, and provenance of unfossiliferous sequences from detrital zircons. Newly developed geochronologic methods have also yielded important discoveries. New fossil finds have provided insight into the ages and provenance of several key tectonic units.

We now know there are separable tectonostratigraphic terranes bounded by sutures (faults; not all faults are sutures). Use of structures not used before (e.g., shear-sense indicators) provided insight into the movement sense of large blocks of crust. Today we know that the Blue Ridge and western Inner Piedmont consist of sediments derived from and deposited on North American continental and oceanic crust, and that at least two new terranes, Cat Square (both North American and Gondwanan origin) and Carolina superterrane, have been identified in the southern Appalachian internides.

Today detailed geologic maps cover ~20% of the southern Appalachians, so many opportunities exist to make new discoveries to revise our concepts of southern Appalachian tectonics and further knowledge of plate tectonics and construction of orogens.