MORE AND MORE TIMING INFORMATION INDICATE A MORE SIGNIFICANT ROLE FOR THE ALLEGHANIAN EVENT IN THE ALABAMA PIEDMONT

A fundamental advancement in our understanding of the evolution of the crystalline rocks of Alabama is the recognition that Alleghanian deformation and metamorphism was much more pervasive than we had earlier thought. Forty-years ago workers mostly favored the early-Paleozoic Taconic event to be responsible for most of the amphibolite-facies metamorphism, plutonic activity, and plastic deformation of Piedmont rocks. The evidence, however, was somewhat inferential, being based on foreland stratigraphic relationships and extrapolations with studies done in Piedmont rocks in adjacent States. During the 1970’s, isotopic studies substantiated Ordovician plutonism but persistent Devonian dates on metamorphic rocks and minerals led workers to suggest that the Acadian event may have been underemphasized. In the 1980’s, Devonian fossils found in greenschist-facies units of the Talladega Slate Belt (TSB) appeared to substantiate the Acadian as the main dynamothermal event. Throughout these three decades, effects of the Alleghanian event, though widely-acknowledged as the 'main player' in the formation of the foreland-fold-and-thrust belt, was relegated in Piedmont rocks to late-stage, brittle and plastic shear zones and localized retrogressive, greenschist-facies overprints. 40Ar/39Ar mineral cooling studies in the early 1990’s, however, indicated that much of the Inner Piedmont and all of the Pine Mountain and Uchee belts experienced amphibolite-facies metamorphism and deformation during the mid- to Late Carboniferous. At the same time, Early Mississippian plant fossils were recovered from the TSB and pushed the timing of deformation and metamorphism there back into the Alleghanian. Advances in our understanding of the Alleghanian event in the Piedmont were brought about largely due to improvements in our abilities to more precisely date the time that a metamorphic mineral cooled through its ‘blocking’ temperature. Our understanding of these processes coupled with the establishment of a suitable thermochronological data set for the Piedmont units only today allows us to begin the exciting task of refining our understanding of how sedimentation and deformation in the foreland is linked to tectonic activity in the Piedmont.