IS THE CHAMPLAIN VALLEY A SILURIAN MEGA-GRABEN?

The Champlain Valley is occupied by multiply-deformed Cambro-Ordovician strata abutting Proterozoic basement along a system of normal faults. Although there are generally less than 2 km of C-O strata in the valley, depth to basement is 5 to 7 km. The boundary between outcropping basement in the Adirondacks and deep basement of the Champlain Valley appears to be abrupt, probably occurring along a normal fault system.

Three normal fault systems (striking roughly E-W, NE-SW, and N-S in temporal order) are evident in Adirondacks and along the boundary. All three can be recognized locally in the Champlain Valley, but their presence is generally masked by later deformation. In the Champlain Valley, all three of these fault systems cut structures formed during the Taconic and Salinic events, but appear to be overridden or deformed by Acadian structures. The third of these fault systems appears to correlate with the primary Adirondack boundary fault. In addition, Acadian thrust patterns seem to have been affected by the presence of the Adirondack front. Therefore, an age between Salinic and Acadian seems appropriate.

Basement blocks outcrop in the Green Mountains to the east, where they have been caught up in Acadian-age thrust sheets, some of which were reactivated during the Neogene to produce the current topographic relief. It is proposed that these basement blocks represent horsts along the eastern side of the graben, with the horsts being decapitated by the Acadian thrusting.