THE RICHARDSON MEMORIAL CONTACT AND THE DOG RIVER FAULT ZONE: EVIDENCE FOR A DUCTILE FAULT ZONE IN WOODBURY, VERMONT

The Richardson Memorial Contact (RMC) is a boundary that divides the tectonic terranes of Vermont and has been the focus of considerable debate. First described by C.H. Richardson in 1919, the structure was described as an erosional surface between the meta-conglomorates of what is now called the Shaw Mountain Formation and the phyllites of the Moretown Formation. Recent work in Vermont has described this contact to be syn-tectonic to Acadian offset, acting as a fault locally. In central Vermont, a zone of high strain has been described along the strike of the RMC, called the Dog River Fault Zone (DRFZ). New evidence from Woodbury, Vermont has identified zones of high strain along the RMC in the Shaw Mountain Formation. Garnet-grade meta-sedimentary and meta-volcanic rocks with mylonitic S-C fabrics defined by rotated garnets with pressure shadows appear to show dextral rotation along the location where rock units are juxtaposed in Woodbury, similar to the DRFZ to the south of the study area. Granitoid bodies exist on both sides of the contact and are mapped as intrusive units of the same age. Field relations suggest different emplacement styles east and west of the RMC. Mineralogy of granitoids on the west-side are dominated by alkali-feldspars making up 74% of the unit with quartz content of 24%. Trace element geochemistry supports an interpretation of pluton emplacement inboard of a subduction zone. Granitoids on the east-side of the RMC are plagioclase-rich (~50%) with lower amounts of alkali feldspar. Geochemistry supports a volcanic arc source and emplaced into crust outboard of a subduction zone. Based on detailed mapping in this area, no granitoid bodies cross the RMC. The RMC juxtaposes two different unstrained granitoid bodies that formed at different structural levels and were emplaced in geochemically unique host materials. Combined with shear stress indicators and surrounding mylonites, the unit contact mapped to the south as the RMC in this area appears to preserve evidence of stain along a ductile fault zone. These two plutons record a different geochemical signal that indicate that they intruded at structurally unique levels and were unaffected by the ductile shearing. The RMC is interpreted to have juxtaposed geochemically unique granitoid bodies, suggesting it is contiguous with the DRFZ.