The aeromagnetic expression of bedrock geology between the Clinton-Newbury and Bloody Bluff fault zones, northeastern Massachusetts

The pattern of mapped bedrock geology in northeastern Massachusetts bears a striking similarity to patterns of aeromagnetic anomalies in the area. The correspondence of the geology with the anomaly pattern and lineaments on aeromagnetic maps is especially well shown northwest of Boston between the Clinton-Newbury and Bloody Bluff fault zones. Here, an aluminous schist unit and an underlying series of metasedimentary and metavolcanic rocks south of the Clinton-Newbury fault zone coincide with a zone of northeast-trending positive anomalies that contrast with the broad magnetic low north of the fault. Nearly concordant granitic bodies intrude the metamorphic series and correspond to broad magnetic lows between the fault zones. Smaller narrower lows correspond to zones of amphibolite-carbonate rock. Commonly faults are indicated by lineaments that separate areas of different magnetic pattern and that offset or terminate other lineaments. The northeast-trending Spencer Brook and Assabet River fault zones and other unnamed faults are emphasized by magnetic lineaments trending slightly east of the strike of the stratigraphy.