CAN DIFFERENTIAL POST-GLACIAL REBOUND EXPLAIN THE PAIRED EAST-WEST CONTRACTION IN THE ADIRONDACKS AND EXTENSION IN THE CHAMPLAIN VALLEY?

Recent seismicity within the Adirondacks indicates active east-west contraction, whereas there is evidence of significant east-west Holocene extension in the adjoining Champlain Valley. A possible mechanism for explaining this apparent tectonic incompatibility lies in differential isostatic rebound between the Champlain valley and the adjoining mountains (Adirondacks and Green Mountains) due to removal of greatly different ice loads.

In order to evaluate this mechanism, first-order models of the differential post-glacial rebound and resulting skin-strains for the Champlain Valley and Adirondacks have been developed assuming a nearly complete rebound and a sinusoidal crustal response. The Champlain Valley is modeled as responding to the loss of approximately 400 m of extra glacial ice relative to the flanking mountain plateaus, with a width between flexural inflection points of 40 km (the valley walls are 30 km apart in the central Champlain Valley) and a 15 km deep neutral surface. This model indicates extensional skin strains up to 2.4% with total extension across the valley of more than 600 m. Using a similar approach, assuming 1 km less ice in the central Adirondacks than in the flanking Champlain and Lake Ontario lowlands and assuming a 120 km between inflection points and a 20-25 km deep neutral surface, contractional skin strains reach nearly 1% with total contraction across the Adirondacks of more than 600 m.

Thus, clearly this model can account for pairing of the adjoining contractional and extensional strain regimes. Application of this model to the west flank of the Adirondacks may explain the anomalous fracturing in the Tug Hill Plateau. Nevertheless, the observed distribution of Holocene extensional features in the Champlain valley is not entirely explained by this model, so additional neotectonic processes are assumed to be active in this area.