Abstract

Recent seismic reflection profiling in western Georgia and adjacent eastern Alabama, conducted by the Consortium for Continental Reflection Profiling (COCORP), shows that reflections commonly associated with the Appalachian detachment do not continue southeastward beneath the Pine Mountain belt. Rather, these reflections terminate abruptly on the north side of the belt, along the downdip projection of the Towaliga fault. This observation is difficult to reconcile with the basement duplex interpretation traditionally applied to the Pine Mountain belt and to all other Appalachian interior basement massifs. An alternative interpretation, consistent with the reflection data and with local surface geologic relations, is that the Towaliga fault is, at least in its later evolution, a large northwest-dipping normal fault that cuts the Piedmont allochthon, Appalachian detachment, and Grenville basement. Where crossed by the COCORP profile, this fault has an inferred average dip of about 54° and offsets Grenville basement about 9 km. This interpretation is consistent with the view that the Pine Mountain belt is a structural window through the Piedmont allochthon. However, it implies that much of the structural relief on the basement exposed in the window is due to late normal faulting rather than to thrust imbrication alone. If correct, this has several important implications for Appalachian geology: (1) It implies that normal faulting of late Paleozoic and/or Mesozoic age has played a much more important role in the development of the exposed southern Appalachians than has generally been considered to date. (2) Grenville basement exposed in the Pine Mountain belt has been attached to North America since Precambrian time; it does not represent a Paleozoic accreted terrane. (3) The Appalachian detachment may be exposed around the periphery of the Pine Mountain belt and hence may be available for direct observation at this locality.

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