DISTINGUISHING ACADIAN DEFORMATIONAL SIGNATURES IN THE MULTIPLY-DEFORMED CHAMPLAIN VALLEY

Distinguishing the deformational signatures of the various tectonic events that have affected the Champlain Valley has been complicated by the overprinting of deformational features, some nearly coaxial. Within the C-O shelf strata, three general orientations of penetrative contractional structures are found in various parts of the valley, striking N25W, N70E, and roughly N-S. In addition, there are weak fold-thrust signals (without cleavage) striking nearly E-W and N15E, as well as late Neogene thrusting (N-S strike) that controls much of the present topography. These structures are cut by normal faults, primarily striking nearly N-S, with less obvious sets striking roughly E-W, N60E, N45E, and N10E; many of these have been reactivated during the current transtensional deformation.

Acadian deformation appears to be expressed in the N25W striking structures – this is consistent with pre-Alleghanian deformation found in central New York and northeastern and central Pennsylvania. N25W striking folds and associated cleavages dominate the portions of the valley east of the Logan’s line thrusts, transecting N-S striking structures where those are present. However, the N25W orientations are absent to the west, where Taconic/Salinic(?) N-S striking structures are developed right up to the Adirondack boundary normal fault in what appears to be a shadow zone created by pre-Acadian displacement along the Adirondack front. Recognition of pre-Acadian normal faults east of the Logan Line thrusts has provided solutions to a number of structural conundrums.

The N70E structures are developed zonally and post-date the emplacement of the Logan Line thrust sheets – we interpret them to be foreland deformation associated with an early phase of Alleghanian deformation seen in Pennsylvania and western New York. It should be noted that the N40E structural trend of the Mid-Hudson fold-and-thrust belt and the eastern townships of Quebec is absent in most of the Champlain valley.