A STRUCTURAL FRAMEWORK FOR THE NASHOBA TERRANE IN EASTERN MASSACHUSETTS

The exhumation and tectonic significance of the migmatitic Cambro-Ordivician arc-complex of the Nashoba terrane, located between lower-grade rocks of the Avalon and Merrimack terranes in Massachusetts, has historically presented an enigma, in part, due to a lack of detailed analysis of internal structure. We propose a new terrane-scale structural model based on nearly a decade of detailed geologic mapping to provide a framework for future study.

A subvertical NE-striking composite fabric (S_n/n-1) forms the dominant structural grain of the terrane. S_n commonly deforms an older layer-parallel foliation (S_n-1) about meter- to kilometer-scale, upright to steeply inclined, NE- and SW-plunging, tight disharmonic folds (F_n). In the Nashoba Formation migmatites, S_n commonly transposes a subhorizontal S_n-1 enveloping surface into spaced meter-scale subvertical shear bands that are absent in the dominantly metavolcanic Marlborough Formation. Fold axis-parallel mineral stretching and intersection lineations (L_n) are locally overprinted on S_n by subhorizontal peak metamorphic to retrograde mineral lineations (L_n/n+1). Ambiguous D_n kinematics in the NE transition SW along strike to top-to-NW normal fold vergence and drag along steep north-dipping S_n axial planes and S_n - S_n+1 shear bands. Later strain (S_n+1 - S_n+2) appears to be progressively partitioned at lower grade to pre-existing S_n shear bands and discrete internal and terrane bounding fault zones which display early high-grade top-to-SE dextral or sinistral motion (S_n-1 - S_n) broadly overprinted by lower-grade top-NW movement (S_n+1).

We propose a tentative tectonic history incorporating sparse existing geochronologic and petrologic studies: Top SW D_n-1 motion coeval with ~425 Ma sill-grade metamorphism and possible accretion. D_n initiating syn ~395 Ma peak metamorphism with migmatite generation along S_n and progressive bulk fabric development largely complete by the intrusion of the relatively undeformed ~349 Ma Indian Head Hill granite. Exhumation can be accommodated by well-documented syn-to-post D_n regional sinistral motion combined with progressively lower grade top-NW extension along discrete structures continuing through deposition and deformation of presumed Carboniferous basin sediments along the terrane boundary.