THE RELATIONSHIP BETWEEN BRITTLE STRUCTURES AND TOPOGRAPHIC PATTERNS IN THE KNOX MOUNTAIN GRANITE: NE VERMONT

The Knox Mt. granite pluton of the New Hampshire Plutonic Series intruded Late Silurian- Early Devonian staurolite-grade metasedimentary rocks of the Gile Mt. and Waits River fms. during the Middle Devonian. The Vt Geological Survey constructed a bedrock map of part of this pluton and surrounding rocks during the 2008 field season to serve as a framework for understanding elevated U levels in groundwater from public and domestic bedrock wells in the granite. One of the primary focuses of this study was to analyze brittle structures. The attitudes of 600+ fractures and 100+ pegmatites were measured in the field area. The topography of the granite landscape is characterized by distinct groups of steep elongate or blocky hills separated by flat low-lying areas. The purpose of this investigation is to correlate major fracture sets with topographic patterns in the field area.

For the granite portion of the field area, the dominant fractures and topographic lineaments trend to the NE, NW, and E-W. Most (~70%) pegmatites have a NW trend with subordinate groupings that strike ~E-W and ~N-S; many fractures develop along pegmatite/granite contacts. The major fracture and lineament azimuths vary geographically within the granite. Consequently, we divided the granite into six geographic domains: 1) Western Contact Domain, 2) Drew Mt. Domain, 3) Marshfield and Burnt Mt. Domain, 4) Lord's Hill Domain, 5) Owl's Head Domain, and 6) Hardwood Mt. Domain. The blocky shapes of the hills in domains 2, 3, 4, and 6 are controlled by two orthogonal “Lego” fracture sets, however, these fracture sets are not the same for each domain. The elongate and streamlined shapes of the hills in domains 1 and 5 are controlled by a single dominant fracture set. Topographic lineaments within the Gile Mt. and Waits River fm. rocks follow the NNE- trending dominant foliation (S1) whereas the most abundant fractures are roughly perpendicular to this foliation; subordinate NW and NE trending fracture sets also occur.

NNE-SSW, E-W, and NW-SE trending fracture surfaces with pronounced slickensides were observed at numerous granite outcrops. Based the presence of asymmetric steps on some of these fracture surfaces, slickenside kinematics were determined. Ongoing structural analysis will relate this late fault history to the fracture/ topgraphic lineament domains.