Thrust napping versus fold napping in the Avalon terrane of southeastern Connecticut

Revised correlations of critical units in south-eastern Connecticut contradict earlier stratigraphic correlations that led to the interpretation of major recumbent fold napping in the Avalon terrane. The revisions are required in part by the discrimination between lithodemic and lithostratigraphic units. We find no compelling evidence that schistose Putnam-Nashoba rocks exist within the Avalon terrane. On the contrary, we interpret these blastomylonitic schists and gneisses within the Avalon terrane around the Lyme dome as ductile fault rocks, derived from an Avalonian granodioritic orthogneiss. Rocks originally mapped as Brimfield Formation (Merrimack terrane) west of Old Saybrook are shown to correlate with the Middletown Formation of the Bronson Hill terrane. This new correlation makes all plagioclase gneisses of the Killingworth dome isolated from rocks of other terranes by the Middletown Formation, and provides evidence against the assignment of any plagioclase gneisses south of the Bronson Hill terrane near the Killingworth dome to the Avalon terrane. In contrast to the interpretation of fold napping, we propose that a major fault nappe exists in the Avalon terrane. The fault zone separates rocks of the Lyme dome from overlying allochthonous rocks of the Waterford Complex in the “Selden Neck block,” and fault splays isolate paragneisses of unknown age. Splays of the fault extend west, where they cut an extension of the Honey Hill fault zone, narrow slivers of Merrimack terrane rocks, and units in the Bronson Hill terrane. Important deformation in the zone probably occurred from middle Carboniferous to middle Permian, although earlier deformation is possible. Subsequently, the entire zone was folded around the Lyme dome. The tuning of the deformation suggests that it was related to the Hercynian assembly of Pangaea.