Vermont Geological Society

10/22/16 VGS Fall Field Trip — Structural variability along and across the Taconic-Acadian margin in central Vermont
Authors: Christopher Koteas, Christopher Eddy, Kirstin Lortie, Adele Del Avellano, and Christopher DeFelice, Norwich University, Northfield, Vermont
Sponsor: Vermont Geological Society
Time: 8:30AM
Meeting Location: Public parking area behind the Village Restaurant in Hardwick, Vermont at the junction of VT Rt. 14 and VT Rt. 15
Other Logistics: Bring a lunch and wear sturdy hiking boots. The longest walk to a stop from the road is ~1/2 mile.
The Taconic-Acadian interface, often considered the Richardson Memorial Contact (RMC) has long been described as being defined as an unconformable surface (i.e. Richardson, 1919). However, this interface has also been described as preserving evidence for faulting and has been called the Dog River Fault Zone in Northfield, VT (Westerman, 1994) and Montpelier, VT (Walsh et al., 2010). Work north of Montpelier, between Woodbury and Craftsbury, has recognized an anastomosing set of upper-greenschist to lower-amphibolite grade shear zones.  These zones are sub-parallel with the RMC, but appear to operate independently and are hypothetically a continuation of the Dog River Fault Zone along-strike to the south. Field studies as well as microstructural and microgeochemical work on high-strain rocks associated with these shear zones, suggest subtle localized rheological changes that manifest as marked changes in kinematic conditions. Integrating shear strain measurements from individual strands of this distributed high-strain zone suggests minimum displacement of 25.5 +/- 0.5 km. A broad set of high-strain zones suggests that significant shortening normal to the RMC, as well as extensive elongation of minerals parallel to this interface, occurred relatively late in Acadian orogenesis.
A geochemical comparison of plutonic rocks and their contact margins, from both the east and west sides of the RMC, show rather significant changes in terms of Rare Earth Element patterns as well as igneous microtextures.  Outcrop-scale structural features suggest considerably different intrusive mechanisms across the RMC. However, the lack of strain present in igneous rocks indicates late syn- or post-tectonic Acadian emplacement. This suggests unique structural controls associated with the RMC influenced the magma mobility from east-to-west and the potential for rather different crustal levels preserved across this margin. This work highlights the complexity of this enigmatic boundary in central Vermont and this trip will focus on a selection of reasonably accessible locations to illustrate strain variability in what is interpreted here as a distributed shear zone cross-cutting the Taconic-Acadian margin as well as the differences in intrusive styles of granitoids across the RMC.
3/19-21/17 Northeast and North-Central Sections GSA Annual Meeting
Theme: Shale Gas Production: Views from the Energy Roller Coaster
Geological Society of America
Location: Pittsburgh, Pennsylvania
Details: See GSA website
3/18-20/18 Northeast Section GSA Annual Meeting
Sponsor: Geological Society of America and University of Vermont
Location: Burlington, Vermont
Details: See GSA website

Regional University Seminar Series Links

Middlebury College: (2016-2017)
University of Vermont: