Northeastern Section - 48th Annual Meeting (18–20 March 2013)

Paper No. 8
Presentation Time: 3:55 PM


KOPERA, Joseph P., Massachusetts Geological Survey, University of Massachusetts, Department of Geosciences, 611 North Pleasant St, Amherst, MA 01003 and MASSEY, Matthew A., Earth and Environmental Sciences, University of Kentucky, Lexington, KY 40506-0053,

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 (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.