STRUCTURES AND KINEMATICS OF THE CLINTON-NEWBURY SHEAR ZONE ALONG THE NW MARGIN OF THE NASHOBA TERRANE, EASTERN MASSACHUSETTS

Field mapping and structural analysis along the Clinton-Newbury shear zone, with a focus on the Tadmuck Brook Schist (TBS), indicate that high-grade sinistral movement was followed by low-grade normal movement. The Clinton-Newbury shear zone separates the Nashoba terrane, a multiply-deformed Cambrian-Ordovician arc-backarc complex, from the Merrimack terrane, a deformed Silurian-Devonian metasedimentary sequence, to the NW. The TBS is a rusty-weathering schist forming a 0.75-1.50 km wide crescent spanning from Newbury to Oxford, MA. The metamorphic grade in the TBS decreases from upper amphibolite in the southeast, adjacent to migmatitic gneisses of the Nashoba formation (NF), to upper greenschist in the northwest. The foliation in the TBS and NF dips moderately to steeply to the NW and locally steeply to the SE. In high-grade TBS, sillimanite-biotite lineations plunge shallowly to the NE and SW. There, shear sense indicators are predominantly sinistral. In the central portion of the high-grade TBS and adjacent NF, additional down-dip biotite lineations are present. In the NF gneisses, hornblende lineations exhibit patterns similar to sillimanite lineations in the TBS. In lower amphibolite facies TBS in the southwestern study area, staurolite and biotite lineations dip shallowly to the NW. In the central study area these lineations are oriented along a steeply NW-dipping girdle. In general, shear bands in lower amphibolite to upper greenschist facies rocks on the northwest side of the TBS indicate normal movement. However, in the northeastern study area of the TBS, they show normal movement with a sinistral component. Finally, folds in the mid to lower-amphibolite grade TBS typically display NW-plunging hinges and sinistral-normal oblique movement. The resultant structural pattern indicates dominant sinistral shear at high-grade, with subsequent normal (i.e. top-to-the-NW shear) motion predominating at lower grades of deformation.