STRUCTURAL AND STRATIGRAPHIC COMPARISONS OF OUTBOARD FRAGMENTS OF THE LAURENTIAN MARGIN, SOUTHWESTERNMOST APPALACHIANS, ALABAMA AND GEORGIA

At the SW corner of the Appalachian orogen, two tectonic entities, the Talladega (TB) and Pine Mountain belts (PMB), are the most distally exposed fragments of the ancient Laurentian continental margin, each containing at least part of the Iapetus western margin drift-facies stratigraphy. Alleghanian retrodeformation of the TB places it in a palinspastic position essentially astride the current location of the PMB. This palinspastic position raises the question of the relative paleogeographic positions of the two belts prior to Alleghanian continent-continent collision. Relative restoration of the two belts is complicated by several major fault zones between the foreland thrust belt and the PMB; each has components of right-slip movement that may have shuffled terranes laterally. An additional uncertainty in attempting to restore the pre-Alleghanian positions of the PMB and TB is whether the southern Appalachian master décollement passed above or beneath the Pine Mountain window. Lithologic correlations between PMB and TSB units are speculative due to an absence of paleontologic materials from the PMB. We compare the two belts in terms of what is presently known about lithologies, lithologic sequences, unit thicknesses, depositional environments, facies relations, structures, metamorphic development and timing, and subsequent cooling. Thickness variations, facies relations, constraints of palinspastic studies in the foreland, structures controlling retrodeformation, and regional tectonostratigraphic relations mostly suggest that the TSB was rooted SE of the PMB. Such a pre-emplacement configuration has important implications for metamorphic and structural development of this part of the orogen. For example, the higher-grade PMB (kyanite-sillimanite zone) today lies outboard of the TSB (chlorite zone), but 40Ar/39Ar mineral cooling dates for the latter terrane are > 40 m.y. older than those from the former.