COMPARISON OF DUCTILE STRUCTURES FROM THE SOUTHERN TERMINUS OF THE HINESBURG THRUST FAULT WITH THOSE FROM THE CENTRAL FLAP, WEST-CENTRAL VERMONT

In west-central Vermont, the Hinesburg Thrust (HT) emplaced Neoproterozoic-Cambrian rift clastic and early drift stage metamorphic rocks of the hanging wall onto Cambrian-Ordovician carbonate and clastic sedimentary rocks of the footwall, during the Ordovician Taconian Orogeny. The HT extends for ~75 km along-strike and terminates at both its northern and southern ends in map-scale, west-verging, overturned, tight folds. The fault is comprised of conspicuous promontories (“flaps”) and recesses along its length. Relative displacement along the HT is greatest in the central flaps and least along the ends.

In the field area, that lies at the southern terminus of the HT, lithologies in the hanging wall consist of phyllitic quartzites, massive metagraywackes, quartz pebble conglomerates, and marbles of the Pinnacle Fm; phyllites and phyllitic quartzites of the Fairfield Pond Fm; and phyllitic quartzites of the Cheshire Fm. Since the HT “ends” in the Cheshire Fm, other Cheshire lithogies, including stratigraphically higher massive quartzites, are also found in the footwall, as well as massive dolostones of the Dunham Fm.

In the hanging wall of the HT in the field area, two major foliations are present: 1) an older composite bedding-parallel fabric (S₁-S₂) defined by alternating quartz and mica domains and rootless reclined isoclinal folds with fold axes that plunge down dip (and collinear stretching lineations), and 2) a well-developed crenulation cleavage (S₃) that is axial planar to tight, west-verging, gently-plunging folds, with strong crenulation lineations. Although footwall structures are similar to those of the hanging wall near the thrust zone, the intensity of the older structures diminishes to the west and large-scale F₃ folds become dominant. Based on regional correlation, S₁-S₂ is Taconian and S₃ is Acadian (Devonian).

Our ongoing work seeks to define meso and micro scale structures at the southern terminus of the HT, which is a segment of minimum relative displacement. We will then compare these structures with those described by Strehle and Stanley (1986) in the central flap of the HT, a segment of maximum relative displacement. We are investigating kinematics, grain distortions, and textures to infer conditions of deformation and metamorphism at both segments.