DECIPHERING PALEOZOIC DEFORMATIONAL HISTORY OF THE MORETOWN FORMATION, WESTERN MASSACHUSETTS

The Rowe-Moretown-Hawley Belt of Western Massachusetts contains rocks that are interpreted to mark the Taconic arc-continent suture zone. The fabrics within this zone record the tectonic history of the Taconic collision and/or reactivation by later events such as the Salinic, Acadian, and Alleghenian orogenies. However, resolving structural fabrics and tectonic events, particularly in southern New England, is hindered by the degree of transposition of earlier fabrics into the dominant N-S striking one. An important area that may assist in separating the structural fabrics is located within the Moretown Formation in the vicinity of the Hallockville Pond gneiss. The Ordovician Moretown Fm represents metamorphosed turbidite sediments inferred to be associated with the Taconic forearc basin. Its alternating schist and quartzite layers contain several generations of well-developed fabrics and microstructures. The Hallockville Pond gneiss is a tonalitic pluton (3.8km x 1.7km) within the Moretown Fm. It is interpreted as an isolated body related to the Shelburne Falls Arc (Karabinos et. al 1998). The 479 ± 8 Ma age of the gneiss (Karabinos et al. 1998) suggests that it predates most or all of the deformational fabrics in the adjacent Moretown Fm. The fabrics wrap around the pluton on the east and west, but the northern region is characterized by a complex set of folds and intersecting fabrics. This area is interpreted as a “strain shadow” where earlier structural relationships are preserved due to the competency of the pluton. At least four fabrics have been identified: S₁ preserved as inclusion trails in garnet, S₂ classified as crenulation cleavage in the schist, S₃ marked by an E-W north-dipping foliation observed in the strain shadow, and S₄ indicated by the N-S fabric. The S₃ fabric is significant because it could be associated with thrusting or reflect the underlying shape of the pluton. Field mapping and sample collection were conducted in the Moretown Fm and adjacent pluton. On-going investigation includes detailed petrography, microstructural analysis, compositional mapping, monazite dating, and thermodynamic modeling. The acquired information will be used to constrain P-T-t-D path(s) and tectonic history of the Moretown Fm and provide new insight on Paleozoic deformational history of western New England.