PRELIMINARY STRUCTURAL ANALYSIS OF THE NASHOBA FORMATION, EASTERN MASSACHUSETTS

A detailed structural field study was carried out in the Nashoba Formation (NF) of the Nashoba terrane (NT), a moderately NW-dipping fault-bounded block within the southeastern New England Appalachians. The NT is a deformed and metamorphosed Cambrian-Ordovician arc-backarc complex intruded by Silurian to earliest Carboniferous dioritic to granitic plutons. The NW-dipping Clinton-Newbury shear zone separates the NT from the Merrimack terrane to the NW. The NF occurs in the NW part of the NT and is composed of amphibolite, schist, gneiss, and locally marble. Field work focused on schist and gneiss between I-290 and Route 2, where the unit is multiply-deformed and metamorphosed up to sillimanite+K-feldspar conditions with zones of migmatization.

Structures within the NF can be divided into a NW and a SE domain. The gneissosity generally dips between sub-vertical and 30º to the NW or SE. The foliation in the SE domain generally dips shallowly to the NW. The foliation in the NW domain dips moderately to steeply to the NW and SE, a result of tight to open outcrop-scale folding. Fold hinges within the SE domain plunge gently to the NE or SW. In the NW, fold hinges predominately plunge moderately to steeply towards the NE. In the NW domain, high grade, top-down-to-the-NW asymmetric folds are prevalent within migmatitic gneisses, possibly indicating mid-crustal flow during migmatization. Folding and migmatization are believed to be coeval, due to a scarcity of well-developed axial planar cleavage within folded leucosome layers. Migmatites are only found within the NW domain and decrease in volume towards the NE within that domain. Gneissic and migmatitic fabrics in the NW are cut by steep, top-down-to-the-NW ~0.5 m wide shear zones. Late steep top-down-to-the-NW ultra-cataclasites cut these shear zones. Late normal movement is only recognized within the NW domain and may be related to normal movement along the Clinton-Newbury shear zone.