MOVEMENT DIRECTIONS AND AGE CONSTRAINTS ON MYLONITE ZONES NEAR THE AVALON TERRANE BOUNDARY NW OF BOSTON, MASSACHUSETTS: IMPLICATIONS FOR THE ACCRETION OF AVALON IN SE NEW ENGLAND

West and northwest of Boston, the western margin of the Avalon terrane (sensu stricto) is marked by a complex set of ductile and brittle shear zones of different ages. The most important through-going ductile shear zone, the Burlington Mylonite Zone (BMZ), forms a broad band of mylonite up to 5 km wide near the boundary with the Nashoba terrane. Recent field and microstructural studies, including the use of shear sense indicators such as s-shape porphyroclasts, S-C band cleavages and oblique foliations, indicate oblique sinistral shear in the BMZ, with the NW side (Nashoba terrane) moving up and SSW-SW over the Avalon terrane at a shallow angle. A Silurian alkaline gabbro-diorite pluton (427 ± 2 Ma) is cut by the BMZ, while a nearby Devonian pluton (378 ± 3 Ma) appears unaffected by the shearing. Thus, movement on the BMZ took place between the Late Silurian and mid-Devonian and we interpret this event as the accretion of the Avalon terrane in southeastern New England to the Nashoba terrane. Contrasting chemistries of Early Silurian plutons (calc-alkaline in the Nashoba terrane, alkaline in the Avalon terrane) provide further evidence that these terranes were not juxtaposed until the Late Silurian.

Later shearing in the area is manifested by the Bloody Bluff Fault Zone (BBFZ) and Kendal Green Mylonite Zone (KGMZ). The BBFZ is a younger, more brittle feature superimposed on the BMZ along its NW margin and marks the present terrane boundary. Movement along the BBFZ may have extended into the Mesozoic. The KGMZ is a northeast-striking, ductile shear zone that also includes cataclastic features such as pseudotachylite. The KGMZ has a dextral shear sense and may have formed at the same time as the prominent Norumbega Fault Zone to the northeast in Maine.