SHEAR-SENSE MOTION OF MAJOR INTRA-TERRANE FAULTS WITHIN THE NASHOBA TERRANE, SHREWSBURY AREA, EASTERN MASSACHUSETTS

The Nashoba terrane is a distinct, fault-bounded lithotectonic unit in eastern Massachusetts lying between the Merrimack terrane to the west and the composite Avalon terrane of southeastern New England to the east. It consists largely of northeast-striking, northwest-dipping Ordovician arc-related volcanic rocks and sediments metamorphosed to upper amphibolite facies conditions during the mid-Paleozoic. Major faults and ductile shear zones are present throughout the terrane and suggest that these rocks now form a tectonic assemblage rather than a coherent stratigraphic succession.

During recent mapping in the Shrewsbury quadrangle, the sense of motion was determined on two of the major intra-terrane mylonite zones from different structural levels: (1) the Rattlesnake Hill fault zone, a roughly 25 km long mylonite zone that merges with the Clinton-Newbury fault zone near Clinton, MA; and, (2) the Assabet River fault zone, which roughly bisects the entire terrane and structurally truncates many of the mapped units along its 100 km trace. Both faults strike NNE to NE and dip moderately to steeply NW with stretching lineations plunging N to NNE at moderate angles. Field and microstructural petrofabric shear indicators, including δ- and σ-shaped porphyroclasts and S-C band cleavages, indicate oblique sinistral motion on these faults with the NW side moving up and southward over the rocks to the east. This sense of sinistral transpressive motion on major faults within the terrane is similar to that found for the terrane bounding fault zones (Goldstein, 1989, Kohut and Hepburn, 2004) likely indicating that they formed contemporaneously during the emplacement of the Nashoba terrane.