DUCTILE AND BRITTLE STRUCTURAL CONTROL ON TOPOGRAPHIC PHOTOLINEARS IN THE SOUTHERN WORCESTER MOUNTAINS, CENTRAL VERMONT

During the summer of 2004, the Vermont Geological Survey and students from UVM and Middlebury College conducted 1:24,000 scale bedrock geologic mapping in the southern Worcester Mtns vicinity. The mapping focused on the Pre-Silurian Moretown, Stowe (includes phyllite, schist, and amphibolite members), Hazens Notch, and Ottauquechee fms. Planar and linear structural data were gathered at 146 outcrop locations; this data set includes approximately 1400 fractures. The objectives of this study were: 1) to compare the orientation of photolinears identified on air photos with that of ductile and brittle structures in the field area, 2) to synthesize plots of fracture data for the field area by formation, and 3) to compare the southern Worcester Mtns fracture data with other areas of Vermont and New England.

In general, photolinears in the valley west of the Worcester Mtns correlate (+ /– 5 degree agreement)with the orientation of the dominant foliation planes whereas photolinears in the core of the Worcester Mtns correlate with the orientation of specific fracture sets. Since the valley is primarily underlain by phyllites and the Worcester Mtns are composed of Qtz rich schist and amphibolite, we believe that lithology exerts a strong influence on the dominance of brittle vs. ductile structures in the topography.

Using frequency-azimuth rose plots and equal area nets, we observed that the dominant fracture azimuth in each formation is variable. The dominant fracture azimuths are: 1) Moretown and Ottauquechee fms. - NNW-trending; 2) Stowe Fm. phyllite - NW-trending; 3) Stowe Fm. schist and amphibolite - NE-trending, and 4) Hazens Notch Fm. - NNE-trending. For each formation, the dominant fracture set(s) may not necessarily control the dominant photolinears.

In the adjacent Montpelier Quadrangle, photolinears in Siluro-Devonian metasedimentary rocks are controlled by bedding-parallel foliations whereas photolinears in the Pre-Silurian rocks reflect brittle structures. Dominant fracture sets similar to those in central Vermont are found in metamorphic rocks from other parts of New England. Our results for central Vermont show that brittle fractures usually control photolinears in massive, quartz-rich rocks, whereas ductile structures usually control photolinears in phyllitic lithologies.