NEOPROTEROZOIC TECTONICS, MAGMATISM, & SEDIMENTATION IN THE CENTRAL & SOUTHERN APPALACHIANS: THE LONG ROAD TO IAPETUS

The protracted fragmentation of Rodinia during the Neoproterozoic ultimately lead to the Iapetus Ocean whose waters washed over the eastern margin of Laurentia by the early Cambrian. In the central and southern Appalachians, the pre-Iapetus record is heterogeneous, both in its preservation and overall import, but these rocks record episodes of pre-Iapetan rifting, magmatism, and sedimentation punctuated by intervals of significant erosion and uplift. However, the exact timing of these events and their relationship to global climate forcings and regional tectonics remains poorly understood.

Neoproterozoic rocks are variably exposed in the Blue Ridge province from Alabama to southern Pennsylvania. Cyrogenian magmatic rocks occur at scattered locations in the Blue Ridge and are comprised of a suite of A-type granitoid plutons with subordinate mafic rocks and minor subvolcanic to volcanic rocks. Pluton emplacement was facilitated by localized crustal extension during an early phase of rifting. The Neoproterozoic sedimentary cover sequence forms a regionally expansive stratigraphic package with a complex nomenclature and tenuous along-strike correlations. The paucity of datable volcanic rocks and an absence of fossils in this sequence has made it difficult to establish a numeric timeline with regional correlations. The oldest rocks in the Neoproterozoic cover sequence were deposited in erosional troughs (up to 1 km deep) cut into the underlying basement complex and are separated from younger strata by significant unconformities. Many of these coarse-grained rocks were deposited by glacio-marine processes during the Cyrogenian. Later erosion has removed some of these older deposits, and the regionally significant basement-cover unconformity (the Great Unconformity) is a polygenetic boundary with widely different ages along-strike.

Where preserved, the upper boundary of the Cyrogenian cover sequence also forms a significant unconformity that resulted from thermal uplift, active rifting, and renewed sedimentation/magmatism during the Ediacaran which cumulated in the opening of Iapetus. Neoproterozoic tectonics created a highly variable crustal architecture along the margin of Laurentia which, in turn, influenced the character of later Paleozoic orogenies in the Appalachians.