DECIPHERING THE INTERNAL STRUCTURE OF THE SILURIAN CHAMPLAIN MEGAGRABEN IN THE CENTRAL CHAMPLAIN VALLEY

The Champlain valley is a Silurian graben structure that contains the mildly metamorphosed sedimentary portions of the earlier Taconian-Salinic thrust belt. The graben structures have been overprinted by several subsequent extensional and compressional events. The Silurian normal faults cut and transpose the C-O stratigraphy and thrust belt structures, often constructing apparent stratigraphic concordance across major normal fault boundaries. This apparent concordance, combined with miscorrelations and proposed structural connections across Quaternary graben fill, have led earlier workers to generate and promulgate the false structural model of the Middlebury synclinorium.

By careful reanalysis of the stratigraphic, structural, and metamorphic signatures across the significant geologic boundaries, we have been able to distinguish a succession of roughly north-south trending horsts and grabens that pre-date Acadian and later structures. The horsts are marked by medial to inner shelf carbonate facies in late Beekmantown and younger strata and by lower metamorphic and weaker penetrative structural signatures. In contrast, the graben strata have more distal or absent correlative shelf strata, have higher metamorphic signatures, are affected by more penetrative deformations, and commonly include more exposures of lower Beekmantown and older strata.

Our analysis further indicates that the Champlain and Taconic thrust sheets and accompanying Green-Mountain-affinity thrust sheets were thrust over the eastern and central portions of the currently-exposed extensional terrain during the Acadian orogenic event, with structurally lower thrusts transecting the block faulted carbonates in the subsurface. It is these underlying thrust surfaces that are the through-cutting detachment surfaces that have been reactivated during subsequent deformations, including the current extensional event.