EVIDENCE FOR A SILURIAN ORIGIN OF THE EASTERN ADIRONDACK BOUNDARY FAULT ZONE

The eastern edge of the Adirondacks along the edge of the Champlain valley is a major normal fault system with throws of more than 3 km. Despite the dramatic topography across the border, there is not sufficient late-stage deformation in the Champlain valley to support an interpretation that this is a purely Neogene structure. In addition, the boundary is a complex system of normal faults for which a temporal order can be established.

Recent advances in our understanding of the deformational sequence affecting the Paleozoic shelf strata of the Champlain valley has allowed several episodes of extensional tectonics to be distinguished. The first three of these produce normal faults in that appear to correlate with the major normal fault systems of the Adirondack boundary. The earlier two of these events can be shown to have occurred between Taconic-Salinic and Acadian contractional deformations, the second appearing to be related to the pre-thrusting normal faults interpreted to underlie the Acadian orogenic wedge in the eastern townships of Quebec. The last of the three events can be shown to have occurred between the Acadian and Alleghanian(?) deformational events. The boundary zone does not appear to have been affected by the early Mesozoic extensional event, though the evidence for reactivation in the early Mesozoic may be masked by the reactivation related to late Quaternary (probably still active) extension.