EVIDENCE FOR OFFSET OF THE NASHOBA–AVALON TERRANE BOUNDARY: CONCORD–MAYNARD QUADRANGLES, EASTERN MASSACHUSETTS

The Bloody Bluff fault separates the Avalon and Nashoba terranes in eastern Massachusetts. This brittle fault overprints the older, more ductile Burlington Mylonite zone in the western portion of the Avalon terrane. Recent EDMAP geological mapping indicates that the terrane boundary is offset by a north-to-northeast striking fault in the Concord and Maynard quadrangles. Such a fault was predicted by Castle et al. (1976) on the basis of aeromagnetic data. Offset is evident in the Burlington Mylonite zone as well as the Bloody Bluff fault. An equal amount of offset is seen in an intra-terrane mylonitic shear zone within the Sandy Pond member of the Marlboro Formation of the Nashoba terrane. This mylonite is evident at the Sandy Pond type locality in Lincoln, MA. Likewise, the Sudbury Valley Igneous Complex, southeast of the Bloody Bluff fault, is offset. The north-to-northeast striking fault also separates the Indian Head Hill Igneous Complex from the pink Sgr granite of Zen et al. (1983). No intrusive relationships between these units have been found. Field bedrock mapping shows that the host rock of the type locality of Bloody Bluff fault in Lexington, MA is a more mafic phase of the Sgr granite. The newly defined northerly-trending fault must have a component of right-lateral motion (east side moves to the south) and be younger than the Bloody Bluff fault.