USING DRONE SURVEYS TO INTERPRET THE GEOMETRY AND KINEMATICS OF A MESOZOIC FAULT ZONE IN DOLOSTONES OF THE CHAMPLAIN VALLEY BELT, WEST-CENTRAL VERMONT

The Champlain Valley Belt of west-central Vermont consists of Cambrian- Ordovician, weakly-metamorphosed, carbonate and clastic sedimentary rocks and is informally subdivided into 3 east-dipping lithotectonic slices, which are (from east to west): 1) the hanging wall of the Hinesburg Thrust (HWHT), 2) the hanging wall of the Champlain Thrust (HWCT), and 3) the Parautochthon. These thrust slices were juxtaposed during the Ordovician Taconian Orogeny and later modified by Acadian (Devonian) folding and Mesozoic extension. Our field area lies in the HWCT in a sliver of the Clarendon Springs Formation dolostone. This sliver is bounded by the Ordovician Muddy Brook Thrust (east) and the Mesozoic down-to-the-east St. George Fault (west).

The field area consists of a 100m x 200m rectangle of continuous outcrop in a bedrock channel of the Winooski River at the Williston/Essex border. The northeast striking (~067) and steeply-dipping fault zone was first identified in 2014-2015 through detailed field mapping and structural analysis, however, drone surveys from altitudes of 41m (base map) and 26m (2 subdomains) were necessary to evaluate the geometry and kinematics of this zone. Field mapping of fractures and bedding were conducted at multiple scales using scangrids, scanlines, and pace and compass techniques.

Photolineaments (n=715) were digitized using imagery captured by a UAV. The dominant azimuths are 067° (30%), 028° (29%), and 295° (14%). Average photolineament lengths for these groups are 3m, 2m, and 3m, respectively. By correlating photolineaments with field data, we confirmed that they represent steeply-dipping planes. The fault zone is defined by en echelon fractures with sub-horizontal slickensides that strike 067° and step to the NW. Meter-scale scangrid arrays of the major fracture sets also corroborate this stepover direction. The style of deformation in stepovers will be used to determine fault zone kinematics. With the exception of local open folds, bedding strikes NW and dips moderately eastward, suggesting minimal displacement on all faults.

Detailed mapping in other areas of west-central Vermont has identified other en echelon fault zones that strike toward 067° and cut across Paleozoic ductile structures. We suspect that they are related to motion on the St. George Fault.