TIMING AND CONDITIONS OF DOME FORMATION IN THE SOUTHERN APPALACHIANS; IMPLICATIONS FOR INTERNAL DEFORMATION OF THE BLUE RIDGE THRUST SHEET

Domes, massifs, and metamorphic complexes are ubiquitous features of mountain systems world-wide, and record a diverse set of processes that shape the upper lithosphere during orogeny. Numerous domes punctuate the Southern Appalachians; we present field, microstructural, and thermochronometric observations of the Tallulah Falls dome (TFD) of NE Georgia to determine the tectonic significance and regional context of dome formation during assembly of the Laurentian margin.

The TFD is a 25 km long open doubly plunging fold interrupting the regional strain pattern of tight upright isoclinal folds in the eastern-most Blue Ridge thrust sheet. Based on field relationships, the TFD has previously been interpreted as attaining its current structural position as a result of underplating of the Blue Ridge thrust sheet by Valley and Ridge sedimentary rocks during the latest (Alleghanian) orogenic event.

Our observations support emplacement of the TFD in its current structural position under amphibolite facies conditions between 329-312 Ma. Field observations of deformation fabrics within the TFD modify pre-existing fabrics in the upper plate and do not appear to be significantly overprinted. Microstructural observation of chessboard extinction and EBSD determined quartz c-axis orientations reflect pervasive deformation under at least amphibolite facies conditions. Published Ar-Ar in amphibole and muscovite indicate that the structural cover of the TFD cooled from ~540 °C to 340 °C from ~329-319 Ma, whereas our muscovite Ar-Ar ages show that the TFD core did not cool to below 340 °C until 312 Ma. These relative cooling ages across the dome boundary signify structural emplacement and exhumation during earliest Alleghanian time.

Our thermochronometric and microstructural observations represent relatively early and high-grade Alleghanian internal deformation of the eastern Blue Ridge thrust sheet. These conclusions suggest that a significant amount of shortening may have been accommodated by deep structures far from the Alleghanian thrust front. Moreover, if doming is indeed related to underplating by Laurentian Valley and Ridge sediments, our relative cooling ages imply at least 150 km of translation of the Blue Ridge thrust sheet upon the continental margin by early Alleghanian.