DISTRIBUTION AND GEOMETRY OF ACADIAN DEFORMATION IN THE TACONIAN FORELAND OF WEST-CENTRAL VERMONT

The field area encompasses parts of the Green Mountain and Champlain Valley belts, which represent the hinterland and foreland of the Taconian Orogen of west-central Vermont, respectively. This region can be divided into three lithotectonic slices which are, from west to east and from structurally lowest to highest: A) the Parauthochthon, B) the Hanging Wall of the Champlain Thrust (CT), and C) the Hanging Wall of the Hinesburg Thrust (HT). Slices A and B consist of Cambrian-Ordovician sedimentary rocks of the Laurentian margin whereas Neoproterozoic-Cambrian rift clastic metasedimentary rocks and underlying Mesoproterozoic basement rocks comprise Slice C. The CT forms the tectonic boundary between A and B, whereas the HT separates B and C. Movement on both of these faults first occurred during the Ordovician Taconian Orogeny, whereas further modification of these slices occurred during the Devonian Acadian Orogeny.

From oldest to youngest, deformation phases generally include 1) map-scale asymmetric folds of bedding in slice B, 2) formation of the HT as a nappe with mylonites at the base, 3) formation of the CT, 3) development of asymmetric shear bands in Slice C as the HT moved again, 4) folding of all slices by tight to open asymmetric N-S trending, west-verging folds and 5) refolding by E-W trending folds (4B). Phases 1-3 are Taconian, phases 4 and 5 are inferred to be Acadian on the basis of style, relative timing, and correlations with structures in the GMA.

Along its ~75 km length, the HT consists of promontories (flaps) and recesses. It terminates to the north and south in tight, asymmetric, overturned, folds. Based on stratigraphic throw, relative displacement of the HT is greatest in flaps and least at the ends. In the southern flap, axial surfaces for F3 and F4 folds are orthogonal and form domes and basins. Whereas the attitudes of F3 folds remain ~constant, the trends of F4 folds vary across and along strike. At the southern terminus, F3 folds are well developed, but F4 is weak or absent.

We are investigating whether F3 and F4 folds sets developed during separate discrete events or simultaneously during constriction. The configuration of the structural margin(s), including the presence of underlying ramps, likely played a role. A large E-W trending negative gravity anomaly may reflect structural discontinuities at depth.