PRELIMINARY STRUCTURAL ANALYSIS OF THE SOUTHEASTERN NASHOBA TERRANE, EASTERN MASSACHUSETTS: EVIDENCE FOR DUCTILE EXTRUSION?

The Nashoba terrane (NT) is a steeply NW-dipping fault-bounded block within the Appalachians of eastern Massachusetts (MA). The NT comprises highly deformed and metamorphosed Cambrian-Ordovician arc-backarc metavolcanic and metasedimentary rocks intruded by Silurian to Early Carboniferous plutons. The NT was metamorphosed at middle to upper amphibolite facies conditions, and is bounded by greenschist-facies rocks of the Merrimack belt to the NW and the Avalon terrane to the SE.

Recent studies suggest that during the Acadian orogeny, the partially melted NT may have been exhumed by ductile extrusion between the colder and more competent Avalon terrane and Merrimack belt. Ductile extrusion generally occurs between a reverse shear zone at the base and a normal shear zone along the top of the extrusion zone. Recent studies in MA reveal dominant normal or NW-side down asymmetric folds that formed during partial melting in the NW part of the NT, consistent with a ductile extrusion model. The purpose of this study was to test whether the SE NT preserves reverse or NW-side-up structures, consistent with the model.

There are three distinguishable styles of deformation observed in the SE NT. These include migmatitic asymmetric folds, mylonitic zones, and zones of concentrated shear that cut across foliation. Asymmetric folds within migmatitic domains and shear fabrics within steeply dipping mylonitic zones suggest NW-side-up movement in the SE NT. Stretching lineations on foliation surfaces associated with migmatitic folds and mylonitic zones plunge steeply to the NW and are consistently down-dip, indicating dip-slip movement. Zones of concentrated shear dip moderately to steeply to the NW throughout the NT and preserve sinistral fabrics and shallowly- to moderately-plunging lineations. A local zone of NW-side-down movement, indicated by asymmetric folds and shear fabrics in a migmatitic zone, occurs near the boundary with the Avalon terrane. This area may preserve a separate temporal or deformational event. The temporal and kinematic relationship between folded domains, mylonites, and zones of concentrated shear remains unclear. However, NW-side-up structures observed in the SE NT and NW-side-down structures observed in the NW NT are generally consistent with the existing model of ductile extrusion.