USING ACTIVE EXTENSIONAL FEATURES IN THE CENTRAL CHAMPLAIN VALLEY TO UNDERSTAND EARLIER TECTONIC EVENTS

The central Champlain valley is an active extensional regime marked by a multitude of small grabens of various sizes and extents. The active extensional faults cuts obliquely across earlier extensional and contractional structures, creating an extremely complex map pattern. However, these grabens have generally been ignored by earlier workers, allowing them to create and promulgate the inaccurate structural model of the Middlebury synclinorium.

Grabens provide a wealth of information about subsurface structures since they develop above the ramps on existing thrust faults. By correlating the structural and stratigraphic information available for the grabens, which are mostly filled with Quaternary sediment, with our existing structural knowledge of the adjoining horsts, we have been able to begin reconstruction of the subsurface network of thrust flats and ramps beneath this area.

Our current structural analysis indicates that the primary extensional detachments are thrust surfaces generated in the Acadian and reactivated with different displacement directions during Alleghanian and at least two Neogene thrusting events. Each of these events produced a distinct orientation of ramps that control the development of the current extensional structures. We believe we can demonstrate that the last of the Neogene thrusting events was ESE verging, creating the ramp structures that control the orientations of the largest modern grabens.

Depth of the detachments is still somewhat difficult to pin down precisely. In general, graben width is correlative with detachment depth, so larger grabens have deeper detachments. However, subsurface information is needed to accurately calculate depth, height, and dip of the underlying ramp. Nevertheless, it appears that all of the currently active detachment surfaces are less than 4 km deep within the central Champlain valley, meaning they lie within the Cambro-Ordovician strata and above Proterozoic basement. However, these detachment surfaces must cross the Adirondack boundary fault since the extensional structures within Proterozoic basement of the adjoining Adirondacks appear to be continuous with those of the Champlain valley.