UNDERSTANDING THE ALABAMA APPALACHIAN "INTERNIDES": WHAT HAVE THE LAST FOUR DECADES TAUGHT US?

Like other parts of the southern Appalachian metamorphic "internides," a plethora of studies during the last 40 years have focused on the Alabama Appalachian "internides" (AAI) and these studies have significantly expanded the data bases upon which tectonic understanding is based. Hindsight indicates, however, that advances in the geologic understanding of the AAI during the past 40 years were built upon a critically important foundation established during the first 6 decades of the last century by a handful of very talented geologists who made insightful advances by making keen observations via regional field studies. Much work in the AAI during the last 4 decades has simply involved refinement, extension, and confirmation of this earlier work which is still highly relevant in today's context. However, important new advances in understanding, often supported by detailed field studies, have been made more recently by scores of individual researchers through chemical, isotopic, petrologic, and structural studies. Additionally, the last 4 decades have seen modern concepts of terrane analysis (exotic terranes, terrane accretion) applied to the AAI with the recognition that many of the region's "belts" were not native to Laurentia, but have been joined to it via distinct accretion episodes along tectonically fundamental boundaries. Key fossil discoveries and petrologic and stratigraphic detail have, however, definitively linked other AAI "belts" (Talladega, Pine Mountain) to Laurentia. In addition to providing a more detailed characterization of individual rock units, major efforts during the last four decades have focused upon establishing a temporal framework for orogenic events. This has included establishment of the age of many rock units, and the relative and absolute timing of structural, metamorphic, and plutonic events. Although many details are still in debate, the developmental history of the native Laurentian parts of the AAI is reasonably well established. Perhaps the greatest advances made by students of the AAI during the past 4 decades, however, has been the characterization and the refinement in understanding of what had previously been the relatively poorly understood, enigmatic high grade terranes (ex: E. Blue Ridge, Inner Piedmont, and Uchee), even though their history is still less well understood than that of the Laurentian components.