SUPERPOSED FAULT GENERATIONS AND THE ARCHITECTURE OF THE CHAMPLAIN VALLEY BELT OF NW VERMONT

The Champlain Valley Belt (CVB) of NW Vermont consists of Early Cambrian – Late Ordovician weakly-metamorphosed carbonate and clastic sedimentary rocks that record a protracted tectonic history, involving Laurentian passive margin development, thrusting during the Ordovician Taconian Orogeny, folding and fault reactivation during the Devonian Acadian Orogeny, and Cretaceous extension and magmatism. At this latitude, the CVB is bounded to the east by the Hinesburg Thrust, which separates it from the metamorphic rocks of the Green Mountain Belt (GMB), and to the west by Adirondack Mesoproterozoic rocks in New York. Internally, the CVB is disrupted by the Taconian Cumberland Head, Isle LaMotte, Champlain, and Muddy Brook thrusts. An E-W striking, shallowly north-dipping lateral ramp between Burlington and Shelburne may explain why the hanging wall of the Champlain Thrust cuts up section from north to south. The Mesozoic east-side-down St. George Fault (normal) offsets all earlier structures and is apparent at map to borehole scales. In addition to summarizing the Paleozoic tectonic events affecting the CVB, we highlight other superposed Mesozoic structures.

By supplementing detailed geologic mapping with UAV surveys and/or photogrammetry, we characterized ductile and brittle fault zones in the CVB. Two segments of the Muddy Brook Thrust were offset by NE-SW striking, steeply-dipping, en echelon fracture intensification domains (FID) that, based on slickensides and breccia zones, were determined to be faults. In both cases, bedding in massive dolostones of the Clarendon Springs Formation was transected by these faults. At the Shelburne Boat Access, steeply-dipping ~N-S and E-W striking fracture zones, which were determined by Stanley (1974) to be wrench faults presumed to be associated with the Acadian Orogeny, cut bedding in red sandstones of the Monkton Formation. An ~80 m section of outcrop of the Cumberland Head limestone along Lake Champlain at Shelburne Farms shows a series of out-of-sequence thrusts in the foot wall of the Champlain Thrust.

Our ongoing research utilizes data sets from multiple scales to build a detailed structural framework for the CVB that includes Paleozoic and Mesozoic structures. Recent geochronological data sets now provide temporal constraints for this framework.