A STRUCTURAL FRAMEWORK FOR THE NASHOBA TERRANE IN EASTERN MASSACHUSETTS
A subvertical NE-striking composite fabric (Sn/n-1) forms the dominant structural grain of the terrane. Sn commonly deforms an older layer-parallel foliation (Sn-1) about meter- to kilometer-scale, upright to steeply inclined, NE- and SW-plunging, tight disharmonic folds (Fn). In the Nashoba Formation migmatites, Sn commonly transposes a subhorizontal Sn-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 (Ln) are locally overprinted on Sn by subhorizontal peak metamorphic to retrograde mineral lineations (Ln/n+1). Ambiguous Dn kinematics in the NE transition SW along strike to top-to-NW normal fold vergence and drag along steep north-dipping Sn axial planes and Sn - Sn+1 shear bands. Later strain (Sn+1 - Sn+2) appears to be progressively partitioned at lower grade to pre-existing Sn shear bands and discrete internal and terrane bounding fault zones which display early high-grade top-to-SE dextral or sinistral motion (Sn-1 - Sn) broadly overprinted by lower-grade top-NW movement (Sn+1).
We propose a tentative tectonic history incorporating sparse existing geochronologic and petrologic studies: Top SW Dn-1 motion coeval with ~425 Ma sill-grade metamorphism and possible accretion. Dn initiating syn ~395 Ma peak metamorphism with migmatite generation along Sn 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 Dn 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.