Structural elements and deformational history of rocks in eastern Massachusetts

Late Precambrian stratified rocks of the Boston platform were intruded by late Precambrian granitic magma. Later they were regionally metamorphosed, folded, mylonitized, and faulted during the Acadian orogeny or an earlier event. Still later, rocks of the Boston basin were folded and faulted during the Alleghenian disturbance. Rocks along an extensive fracture system were deformed intermittently by faulting from possibly middle Devonian to Permian time. The platform rocks were deformed during three periods of deformation. The first period began early in the regional metamorphic event when folds and metamorphic schistosity (S 1 ) formed in response to ductile deformation. Possibly some cataclastic foliation (S x ) formed during this period. The second period of deformation began near the end of regional metamorphism. First, minor folds developed; later, regional north- to northeast-trending folds formed. The schistosity (S 1 ) was folded, a slip cleavage (S 2 ) developed, and cataclastic foliation (S x ) associated with deep-seated faulting also formed in the rocks during the second period. The third period began near the end of Paleozoic time, when rocks of the Boston basin were folded and faulted. A cleavage (S 3 ) formed in some rocks during this time. The Bloody Bluff is the principal fault in a complex zone that strikes northeast across a large part of eastern Massachusetts. This zone has an irregular but prominent belt of cataclastic rock associated with it. Regional relations and aeromagnetic data suggest that the Bloody Bluff fault may be part of a fracture system that trends across the Gulf of Maine into Canada.