Northeastern Section - 37th Annual Meeting (March 25-27, 2002)

The Cambrian to Ordovician Moretown Fm. is interpreted to have accumulated in a forearc sedimentary basin associated with an island arc that was accreted to North America during the Taconic Orogeny. In western Massachusetts, the ~479 Ma Hallockville Pond Gneiss and the ~447 Ma Middlefield Granite occur within the Moretown Fm. and provide an opportunity to document the relative timing of deformation, metamorphism, and magmatism. We have conducted detailed, outcrop-scale mapping and structural analysis of the Moretown Fm. in four locations: 1) a high strain area just east of the Hallockville Pond Gneiss; 2) an apparent strain shadow north of the Hallockville Pond Gneiss; 3) within the Middlefield Granite; and 4) along Rt. 9, well away from either intrusive body. All four localities preserve distinct styles and/or orientations of fabrics that can be tied to their setting with respect to the plutonic bodies. The goal of this graduate class project is to use deformational and crosscutting relationships to provide constraints on the relative timing of regional tectonic events. Major observations include: 1) a spectacular, north-south striking, subvertical crenulation cleavage that cuts at least two older fabrics around the two plutons; 3) an east-west striking, shallowly north-dipping spaced cleavage near the northern contact with the Hallockville Pond pluton that deforms at least two older fabrics, is the dominant fabric within the pluton near the contact, and appears to decrease in intensity away from the contact; 3) abundant amphibolite layers within the Moretown Fm. that appear to have experienced all observed phases of deformation; 4) a strain gradient near the eastern contact of the Hallockville Pond pluton that may preserve the transition from primary sedimentary and intrusive contacts in the west to a sheared, transposed, and tectonically mixed variety of Moretown Fm. in the east. Structural relationships from all four localities will be presented and compared. The ultimate goal of this project is to use map scale structural relationships (regional strain partitioning), outcrop scale fabric analyses, microstructural analysis, and in situ monazite geochronology to distinguish between Acadian and Tectonic regional deformation within the Moretown Fm.