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ABSTRACT

In a well-defined subrecess in the Appalachian thrust belt in northwestern Georgia, two distinct fold trains intersect at ~50° in the down-plunge depression of the Floyd synclinorium. A mushwad (ductile duplex) of tectonically thickened weak-layer rocks (primarily the shale-dominated Cambrian Conasauga Formation) filled the space beneath folds and faults of the overlying Cambrian–Ordovician regional stiff layer (mushwad roof). Measurements of the mushwad thickness from balanced cross sections provide the basis for three-dimensional (3-D) models. Tectonically thickened weak-layer shales in a model using a simple line-length balance of the stiff layer have a volume of ~64% of the volume in the deformed-state model, indicating that this balanced reconstruction is not appropriate. Previous work demonstrated deposition of a thick mud-dominated succession in a basement graben to balance the volume. A 3-D model incorporating a thick Conasauga Formation shale succession deposited in a basement graben yields good correspondence to the deformed-state mushwad volume. That model requires vertical separation on the graben boundary faults greater than the present small-magnitude separation; unconformable truncation of the upper part of the Cambrian–Ordovician carbonate succession documents Ordovician inversion of the graben boundary faults. In the 3-D models, the distribution of thickness in the deformed state suggests movement of weak-layer shale out of the planes of cross sections and up plunge away from the structural depression of the Floyd synclinorium. Out-of-plane tectonic translation is consistent with a relatively uniform depositional thickness of ~800 m, which allows calculation of the magnitude of vertical separation on basement faults during Conasauga Formation deposition.

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