EVOLUTION OF THE MURPHY SYNCLINORIUM, SOUTHERN APPALACHIAN BLUE RIDGE, GEORGIA AND NORTH CAROLINA

Near its geographic center, the structure of the S. Appalachian W. Blue Ridge allochthon is dominated by the regional Murphy synclinorium, extending for >180 km, paired with anticlinoria to the NW. These first generation, NW overturned structures, are doubly plunging, large amplitude and wavelength (>10 km) isoclinal folds contemporaneous with peak regional metamorphism, estimated to have occurred during the Visian, Neo-Acadian orogeny, between ~345 Ma and ~335. The synclinorium folds a regionally significant unconformity separating Neoproterozoic and lower Paleozoic rift (Ocoee Sg.) and drift (Nantahala/Brasstown Fms., and Murphy Marble) sequences from a younger successor basin sequence (Mineral Bluff Gp.). Strain analyses of 32 metaconglomerate samples from lithologic groups above and below the unconformity in Georgia indicate: A) similar (coaxial) low to moderate, oblate to nearly plane strain (bulk strain ratios on XZ planes range up to 4.03) in both groups, B) sub-horizontal fold axis-parallel extension (X-axis) of ~35% to 45%, and C) NW shortening of ~32% in the Z direction. Retrodeformation of the embedded regional unconformity outlines an unconformity subcrop geometry containing a very open syncline (interlimb angles >160°), slightly oblique to the superposed trend of the later Murphy synclinorium, that involved only very broad warping of the rift and drift facies units prior to development of the later synclinorium. This mild deformation is post-Early Cambrian and pre-Mineral Bluff in age. It may represent the only vestige of the dynamic effects of the Middle Ordovician Taconic orogeny to be found in this region, or alternatively, may be post-taconic and related to formation of the Mineral Bluff basin itself. The Murphy synclinorium evolved via NNW-SSE crustal shortening, combined with orthogonal NNE-SSW subhorizintal flow (stretching) subparallel to the developing regional folds. This orogen-parallel lateral escape was greater in the fold’s core (45%) than on the flanks (35%), adding to the tightening of the folds, as well as significant strike-parallel lengthening (stretching) of >60 km. Significant differences in metamorphic grade and paleodepth of the exposed synclinorium (up to10 km), seen as a result of regional cross-folding, had essentially no effect on the strain magnitudes.